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www.elsevier.com/locate/tecto
Tectonophysics 400 (
Interpreting fracture development from diagenetic mineralogy and
thermoelastic contraction modeling
Renee J. PerezT, James R. Boles
University of California, Santa Barbara, Geological Sciences Department Room 1006, Webb Hall, Santa Barbara,
CA 93106-9630, USA
Received 1 March 2004; accepted 1 March 2005
Available online 9 April 2005
Abstract
Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene
Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during
compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 8C. The second set of
microfractures (F2) is cemented by bituminite–pyrite, formed at temperatures between 60 and 100 8C, and are associated with
kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures
(F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures
between 149 and 175 8C. These temperatures are mostly higher than maximum burial temperatures (~160 8C), suggesting that
upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward
decompression may have also caused a PCO2drop, which, at constant temperature, allowed simultaneous carbonate precipitation
into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by siderite–
hematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions
that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of
the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations
and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic
contraction, caused by the formation’s uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last
fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of
oxides and the transformation of siderite to hematite.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Sandstone diagenesis; Fracture formation; Veins; Thermoelastic contraction; Overpressure
0040-1951/$ - s
doi:10.1016/j.tec
T Correspondi
Northwest, Calg
E-mail addr
2005) 179–207
ee front matter D 2005 Elsevier B.V. All rights reserved.
to.2005.03.002
ng author. Current affiliation: University of Calgary, Department of Geology and Geophysics, 2500 University Drive,
ary, Alberta T2N 1N4 Canada.
esses: [email protected] (R.J. Perez), [email protected] (J.R. Boles).
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207180
1. Introduction
Reservoir and thin-section scale partially cemented
fractures and veins represent examples of tensile
failure and fluid/fracture interactions. Extension
fractures exhibit simple power-law scaling across
3.4–4.9 orders of magnitude (Marret et al., 1999), but
regardless of their scale, hydraulic fractures decrease
pore fluid pressure in the adjacent rock. Furthermore,
depending on their connectivity, frequency, and
hydrologic regime they may facilitate mass transfer
and heat advection. Over time, some fractures are
cemented while others remain open, causing a host-
rock scale porosity and permeability heterogeneity
that ultimately affects fluid production. Microfrac-
tures are used to determine far field stresses (i.e.,
Laubach, 1989) and magnitude, direction, and scale
of fluid movement (e.g., Laubach, 1988; Eichhubl
and Boles, 1997). Geochemical studies of cemented
fractures in sandstone are typically performed in
tectonically deformed areas such as fault zones and
folds (e.g., Hippler, 1993; Macaulay et al., 1997;
Perez and Boles, 2004; Boles et al., 2004), but
microfractures and veins are not restricted to
deformed areas.
In the absence of tectonic stresses, compaction
disequilibrium and quartz cementation can result
fluid retention and pore pressure increase (Helset et
al., 2002; Swarbrick et al., 2002), which may lead, in
turn, to hydraulic fracturing (e.g., Laubach, 1988;
Engelder and Lacazette, 1990). Uplift, erosion, and
cooling can also cause significant pore fluid changes
and thermoelastic stresses that subsequently lead to
tensile failure (e.g., Suppe, 1985; Swarbrick and
Osborne, 1998). Tensile failure produces, in turn,
pore pressure changes, allowing fluid movement, and
leading, ultimately, to vein formation (Eichhubl and
Behl, 1998). In these cases, the formation and
cementation history of veins indirectly reflect the
host rocks stress history, fluid pressure, and fluid
composition.
The Misoa Formation, the subject of our study, is
present in the subsurface on the east side of the
Maracaibo Lake, Venezuela. The Misoa Formation is
one of the most prolific siliciclastic hydrocarbon
reservoirs of the world (Higgs, 1996), and, in this
paper we show microfractures provide avenues for
aqueous and hydrocarbon flow as well as vertical
connection between sandstone horizons. The Misoa
Formation was deposited during Eocene time in a
shallow marine-deltaic environment (Gonzalez et al.,
1980), buried to more than 4 km under high
sedimentation rates, subject to temperatures higher
than 180 8C (Rodriguez et al., 1997), and uplifted
more than 2/3 of its maximum burial depth. Because
of the wide range of pressure and burial regimes, the
formation is ideal for the recognition of compaction,
and the study of inversion related fractures (Law and
Spencer, 1998). Previous diagenetic studies (e.g.,
Ghosh et al., 1990; Perez et al., 1999b) and burial-
thermal reconstruction (Rodriguez et al., 1997; Perez
et al., 1999a) of the Misoa Formation provide input
for our semi-quantitative thermo-mechanical analysis
used to interpret fracture conditions during burial and
uplift.
Our study describes the diagenesis of the Misoa
Formation sandstone and the mechanisms of fracture
development and cementation during burial and
uplift. The timing of cementation and fracture
forming-filling events are interpreted from fluid
inclusion data and the application of the quartz
precipitation kinetic model of Walderhaug (1996).
Based on petrography and diagenetic cements we
distinguish microfractures that originated during
burial subsidence (fractures sets F1 to F3) from
others possibly originated during uplift (fracture set
F4). The origin of the fourth fracture set is difficult
to ascertain. Combining, however, petrologic infor-
mation and diagenetic-mechanical modeling we
infer they have been formed and cemented in recent
in time at shallow depth, and are associated with
uplift.
Samples were collected from the Lagunillas,
Bachaquero, and Motatan onshore oil fields and a
wildcat exploration well, BA-1, which is located
approximately 2 km to the northwest of the Bach-
aquero field (Fig. 1). No information on core or
sample orientation was available. These localities
provide, however, excellent chronology of fracture
events. All fractures cements are monomineralic,
which led us to group the structures in sets, based
strictly on their mineralogy, depth of occurrence,
orientation relative to bedding, and cross-cutting
relations. The data and results that we herein present
may characterize the general diagenesis, veining
conditions, paleofluid composition, pore pressure,
Venezuela
Caribbean Sea
MARACAIBO LAKE
MIS
OA RIV
ERVE
OA
MIS
Zulia Oriental Region
N
A
LAGUNILLAS FIELD
BACHAQUERO FIELD
A'
MOTATAN FIELD
0 10 20 30 km
MARACAIBO
Fig. 1. Schematic map of the Maracaibo Lake basin depicting the Zulia Oriental Region and locations of oil fields mentioned in text. Well
coordinates and locations are proprietary information of Petroleos De Venezuela (PDVSA).
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 181
and stress regimes present during the burial and uplift
of similar siliciclastic formations in other regions.
2. Geologic setting
The studied sections of the Lower to Middle
Eocene Misoa Formation are located in the eastern
onshore region of the Lake Maracaibo basin, western
Venezuela (Fig. 1). The basin represents a stable
intracratonic terrain surrounded by active plate boun-
daries and high relief areas (Gonzalez et al., 1980;
James, 2000. The basin overlies a Mesozoic crystal-
line basement and traps Jurassic volcaniclastic, Creta-
ceous marine, and Paleogene–Neogene delta
sediments (Fig. 2; Lugo and Mann, 1995), derived
from cratonic sources as well as Caribbean and
Andean orogenic belts (Castillo et al., 1996). Late
Eocene and Miocene inversions, resulting from the
continental collision between the northern South
American border and the Lara Napes, interrupted the
subsidence of the basin (Lugo, 1991). During the
initial stage of the collision (~54 Ma) the eastern
onshore region, also called the Zulia Oriental Region
(ZOR; Fig. 1), developed into a back-arc sub-basin,
that was quickly filled by turbidite, deltaic, and
shallow marine deposits (Gonzalez et al., 1980). The
Misoa Formation is a tidal-influenced deltaic deposit
Misoa Fm.Trujillo Fm.
S-SW
21 km
A
Fig. 3. Schematic northeast–southwest structural section through Tertiary str
Formations, Members, and Zones
Bachaquero
GUASARE
CR
ETA
CE
OU
SPA
LEO
CE
NE
EA
RLY
E
OC
EN
EM
IDD
LE
EO
CE
NE
UP
PE
R
EO
CE
NE
MIO
CE
NM
IO-P
LEIS
TO-
PLI
OC
EN
E
Series
LAG
UN
ILLA
MISOA
MENE GRANDE / JARRILLAL
PAUJI
LA R
OS
A
Laguna
La Rosa
Lagunillas
Santa Barbara
?
BETIJOQUE
ISNOTU
39.5-36 to
39.
42.
44
49.
51.
54
66
84
TRUJILLO
Age in Million of
Years Ago (Ma).
Regional unconformity
Regional Seal
Studied Formation
Regional Source RocksMITO JUAN/COLON LA LUNA
Fig. 2. Schematic stratigraphic column of the Zulia Oriental Region,
western part of the Maracaibo basin. Source rocks underlies the
Misoa Formation, whereas regional shale (seal) overlies (Gonzalez
et al., 1980).
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207182
(Van Veen, 1972; Maguregui and Tyler, 1991; Higgs,
1996). Based on spontaneous potential and resistivity
logs, the sandstone to shale ratio is ~1:6. Several
normal and reverse faults cut through the Misoa strata
into Cretaceous strata, at more than 4 km subsurface
depth (Fig. 3). Reverse faults were generated by the
reactivation of Cretaceous–Paleocene normal faults
(Roure et al., 1997).
The burial history of the Misoa Formation consists
of two subsidence periods, the first from 50 to 42 Ma
and the second from 17 to 5 Ma, both interrupted by
uplifts. Much of the basin was inverted during the first
uplift between ~42 and 38 Ma. In the study area, the
second uplift occurred from 7 to 4 Ma, and was
associated with the Miocene Orogeny. Vitrinite
reflectance data indicate a variable heat flow through-
out the history of the basin (Rodriguez et al., 1997).
The thermal data are interpreted to represent a heat
flow increase from 52 to 58 mW/m2, resulting in
thermal gradients between 30 and 35 8C/km from 54
to 49 Ma, followed by an exponential decay to ~48
mW/m2, from 52 Ma to present. The present day
thermal gradient varies from 25 to 28 8C/km(Gonzalez et al., 1980). The thermal interpretation
has been calibrated against apatite fission track,
present-day temperature data, and temperature logs
(Rodriguez et al., 1997). Erosion plays an important
role in our stress analysis and varies from 0.2 to ~3
km (Rodriguez et al., 1997). Based on measurements
from exploration and production wells the Misoa
N-NE
Well #LB-114
3.5 km
Pauji Fm.Undifferenciated
La Rosa-Lagunillas-Betijoque Fms.
0 km
A'
ata of the studied area. The cross section location is depicted in Fig. 1.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 183
Formation is currently at hydrostatic or sub-hydro-
static pressure due to extensive oil production
(Gonzalez et al., 1980).
3. Methods
3.1. Analytic
We collected approximately 70 sandstone core-
plug samples along 300 m of core taken from seven
wells, distributed over three oil fields and a wildcat
well in the east side of the Maracaibo Lake basin,
western Venezuela (Fig. 1). The samples belong to
unfaulted segments of the Eocene Misoa Formation
and their present depths range from 0.2 to 4.2 km,
which covers the entire formation (Fig. 2). From the
70 core plugs we made 40 thin sections and stained
them for calcium carbonate with alizarine red and
counted 300 points per slide.
We observed and analyzed the fracture mineralogy
with a conventional polarized microscope and a JEOL
scanning electron microscope (SEM) model JSM-
6300v. The SEM was set to an acceleration voltage of
15 kV and a beam current of 160 AA. An energy
dispersive analyzer (EDA Tracor Northern TN2010)
was coupled with the SEM, and used a beam current
of 250 AA. Additionally, the amount of quartz
overgrowth was estimated in 29 quartz arenites and
in 5 fractured sandstone samples with a cathodolumi-
nescence (CL) unit attached to the SEM. We obtained
15 images per sample, covering an area of ~1 cm2,
and determined quartz overgrowth percentages from
digital analysis of CL images.
The elemental composition of calcite cements is
based on electron microprobe analysis using a
CAMECA SX50, set with an acceleration voltage of
15 kV, and beam current and beam diameter of 10 nA
and 10 Am, respectively. Results were normalized to 1
mol total of Ca+Fe+Mg+Mn.
Homogenization and melting temperatures of
fluid inclusions in quartz were determined using a
Fluid-Inc., modified USGS gas-flow stage. Fluid
inclusions were present in quartz overgrowth, quartz
grain-scale fracture cement, and in quartz veins in
sandstone away from fault zones. We only used
fluid inclusions assemblages with constant liquid/
vapor ratio to avoid necking-down effects, which
change original inclusion volumes, distorting tem-
perature interpretations (c.f., Goldstein and Rey-
nolds, 1994; Roedder, 1984). Methane inclusions are
found with H2O-rich inclusion assemblages. Thus,
we did not apply pressure corrections to the
homogenization temperatures (Th) because small
CH4 concentrations cause Th to approximate true
trapping temperatures (Hanor, 1980). We calculated
salinity values from melting temperatures (Tm)
assuming an H2O–NaCl system and using Bodnar’s
(1992) revised equation of state. We used to Th’s
(1) to delimit (in time) the precipitation of quartz
overgrowth in intragranular fractures, quartz over-
growth surrounding detrital grains, quartz filling the
set of microfractures F3 using burial histories and
assuming the quartz filling occurred during burial,
and (2) to verify the quartz cementation modeling
results.
4. Modeling methods with brief theoretical
background
After determining the distribution and quantity of
quartz overgrowths, we modeled in 1-D the precip-
itation of quartz overgrowth and the stress history of
the Misoa Formation. The quartz overgrowth model
yields the amount of quartz cement and porosity
decline curves through time.
Our stress analysis yields the effective horizontal
stress through time as a function of burial depth,
while changing the rock properties. The bridge
between the diagenetic and stress model is the
porosity evolution. Experiments and empirical obser-
vations suggest that mineralogy and porosity control
the thermo-mechanical properties of sandstones (i.e.,
see review by Giles, 1997). In the Misoa sandstone,
compaction and quartz cementation mainly control
the porosity (Perez et al., 1999b). Thus, if the porosity
is calculated through time, depth, and temperature,
the horizontal effective stress, assuming the uniaxial
stress model, is indirectly a function of time, depth,
and temperature.
4.1. Quartz cementation modeling
The quartz cementation model is based on numer-
ical algorithms, which consist of empirically derived
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207184
kinetics based on observations in the North Sea basin
(Walderhaug, 1994), surface area (Walderhaug, 1996)
and a compaction decline function (c.f. Lander and
Walderhaug, 1999). This model has been tested in
basins around the world, including the Maracaibo
basin (Awwiller and Summa, 1997; Perez et al.,
1999a; Chatellier and Perez, 2000). The results of
these tests indicate that the use of the kinetic
parameters published by Walderhaug (1994) and
presented in Table 1 yield errors smaller than 3%,
which is acceptable to us. The algorithms were
numerically solved in a Microsoft Excelk spread-
sheet. The basic concepts and mathematical formula-
tions of the model are described elsewhere
(Walderhaug, 1994, 1996; Walderhaug and Bjbrkum,
2003). Hence, we include here only a brief summary.
The volume of quartz cement Vq (in cm3) precipi-
tated in a cm3 of sandstone with quartz surface area
A (cm2), during time t (in s) at constant temperature
is:
Vq ¼ MrAt=q ð1Þ
whereM is the molar mass of quartz (60.09 g/mol), r is
the rate of quartz overgrowth precipitation in mol/cm2
s, and q is the density of quartz (2.65 g/cm3). The rate r
is expressed as a logarithmic function of temperature
(Walderhaug, 1994):
r ¼ a10bT ð2Þ
where T is temperature (8C) and a and b are constants
with units of mol/cm2 s and 1/8C, respectively (Table
Table 1
Selected list of samples with thin-section scale microfractures and microv
Well BA-1 Well LS-1387
Depth (ft) Depth (m) Fracture type Depth (ft) Depth (m
1336 404.8 F4 1023.8 310.2
2503 758.5 F2–F4 6359.4 1927.1
2506 759.4 F2 7340 2224.2
3327 1008.2 F2–F4 9093.3 2755.5
3587.6 1087.2 F2 9343.3 2831.3
4169 1263.3 F2–F4 9346.7 2832.3
4748 1438.8 F2–F3–F4 9842.8 2982.7
5274 1598.2 F3–F4
6345 1922.7 F2–F3
Under cathodoluminescence, all samples contain F1 intragranular microfr
F2: bituminite–pyrite microveins.
F3: quartz or calcite microveins.
F4: open uncemented microfractures or partially cemented by iron oxides
2). When the T(t) path of the sandstone is known, T can
be divided in small linear time steps. Thus, the total Vq
precipitated from time t0 to tn is the integration of Eq.
(1) along the T–t path, that is:
Vq ¼ M
q
Xni¼0
Ai
Z tiþ1
ti
10b tciþ1þdið Þdt ð3Þ
where c is the temperature rate change or slope (8C/s)and di the initial temperature within the time step. The
surface area A is estimated as the cumulative surface
area of spheres of diameter D with the total volume
equal to the quartz fraction f times a unit volume of the
sandstone v (Walderhaug, 1996). The surface area A is
readjusted after each time step and is considered
proportional to the porosity loss through time and
depth, expressed as:
Ai ¼ 1� coatð Þ 6f v
D
�/i
/0
�ð4Þ
where /i is the porosity for the present time step and
/0 is the initial porosity at the time of deposition. The
porosity /i is also readjusted each time step because it
is considered a function of the effective vertical stress–
through a compaction function (see Eqs. (5) and (6))–
and the amount of quartz cement. The effect of grain
coating (coat parameter in Eq. (4)), which inhibits
cementation (Lander and Walderhaug, 1999), was
considered zero at all times.
Different compaction decline functions have been
coupled with the quartz overgrowth precipitation
eins
Well LB-273
) Fracture type Depth (ft) Depth (m) Fracture type
F2–F5 6468.2 1960.1 F3
F2–F5 6878 2084.2 F3
F5 7504 2273.9 F3
F3–F4 7729 2342.1 F3
F3 7904.3 2395.2 F3–F4
F3–F4 8890 2693.9 F2–F3
F2–F3
actures.
.
Table 2
Parameters used in the quartz cementation kinetic model and thermo-mechanical parameters used in the stress analysis of sandstones
Symbol Variable Units Initial
value
Final
value
Source
a Pre-exponential parameter, Eq. (2) mol/cm2 s 1.98e�22 T Walderhaug (1994)
b Exponential parameter, Eqs. (2) and (3) 1/8C 2.2e�2 T Walderhaug (1994)
C21 Exponential constant, Eq. (5) 1/MPa 8.3e3 T Chuhan et al. (2002)
C22 Exponential constant, Eq. (5) 1/MPa 8.5e3 T Chuhan et al. (2002)
C23 Exponential constant, Eq. (5) 1/MPa 8.4e3 T Chuhan et al. (2002)
E4 Young’s modulus, Eq. (10) MPa4 �1.0e3 �16.5e3 Engelder (1985)
v4 Poisson’s ratio, Eqs. (7)–(10) D 1.5e�1 3.3e�1 Bachrach et al. (2002),
Engelder (1985)
a4 Coefficient of thermal expansion, Eqs. (8) and (10) 8C�1 10.0e�6 10.8e�6 Engelder (1985)
b Biot poroelastic parameter, Eq. (7) D 1 5.6e�1 Breckels and van Eekelen (1982)
T Constant value through time.1, 2, 3 Exponential constants for IGV reduction as a function of effective stress for fine, medium, and coarse-grained sandstones, see Eq. (5) in
text.4 Assembled from various sources by Engelder (1985).
D, dimensionless.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 185
model (e.g., Lander and Walderhaug, 1999). We used
new experimental compaction data based on effective
stress, rock composition, and grain size (c.f. Chuhan
et al., 2002). The advantage of using the new data set
is two-fold. First, the data reveal a correlation between
intergranular volume decrease and grain size not
revealed in other compaction studies (i.e., Pittman
and Larese, 1991). Second, the mineralogy of the
sands used in these experiments is similar to the
mineralogy of the Misoa Formation; both are quartz
arenites. We numerically fit Chuhan’s et al. (2002)
results with curves of the form:
/ ¼ C1eC2rVv ð5Þ
where C1 and C2 are constants (Table 2) and rvV is theeffective vertical stress in MPa. The latter term, rvV, isthe total overburden stress rv minus the pore fluid
pressure Pf (Terzaghi and Peck, 1948). A cross plot of
predicted vs. measured amount of quartz cement for
this specific area using the kinetic parameters pre-
sented in Table 1 is shown elsewhere (in Fig. 3 from
Perez et al., 1999b).
With the purpose of determining the origin of the
fourth fracture set, which based on diagenetic cements
is inferred to have been created recently at shallow
depth, we use a simple thermoelastic model to
understand the possible stress evolution of the Misoa
Formation.
4.2. Stress history modeling
Cementation or mineralization change the thermo-
mechanical properties of sediments (Jizba and Nur,
1990; Giles, 1997). Experimental results demonstrate
that the Poisson’s ratio, Young modulus, poroelastic
coefficient, and the coefficient of thermal expansion
vary significantly with mineralogy, porosity, and
sandstone hardness (Engelder, 1985). Thus, because
sediments are unconsolidated when buried, diagenesis
ensures that the stress path during burial is different
from that during uplift.
Our model assumes that sandstone layers are
poroelastic, homogenous, and isotropic. Thermo-
mechanical properties are assumed to (1) vary linearly
and synchronously with the amount of quartz cement
and porosity throughout the burial; (2) attain their
maximum values as porosity decreases to zero; and
(3) remain constant after the minimum porosity is
approached and during uplift (Warpinski, 1989).
Moreover, we assume that (4) the horizontal stress is
the minimum principal stress; (5) the overburden is
the maximum principal stress; and (6) that all stresses
are governed by the uniaxial strain model. Burial
histories, described before, are basic input for the
stress analysis.
In reality, our assumptions are not strictly met for
several reasons. A sandstone body may not behave
homogeneously, isotropically, and elastically due to
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207186
compaction, diagenesis heterogeneities, and stress
concentration around flaws. Moreover, some micro-
fractures may reflect regional tectonic stresses (Lau-
bach, 1989), such as the late Eocene and Miocene
plate convergence. However, based on the correlation
of fracture formations and model predictions, our
oversimplifications do not alter first-order effects of
burial and uplift on the modeled state of stress (Narr
and Currie, 1982; Apotria et al., 1994). In the
Maracaibo basin, furthermore, the present day hori-
zontal stresses are low and vary significantly from one
oil field to another (Breckels and van Eekelen, 1982)
suggesting that some oil field sub-regions, particularly
those distant from major fault and folds, may behave
as tectonically relaxed areas.
The mathematical formulations for the uniaxial
strain model are described elsewhere (e.g., Engelder
and Lacazette, 1990; Twiss and Moores, 1992).
Therefore, we will only explain briefly the theory
and numerical treatment. Assuming poroelastic
behavior the effective vertical stress rvV in the uniaxial
strain model is:
rvV ¼ rv � bPf ð6Þ
where b is the Biot poroelastic parameter (Biot, 1941)
and Pf is the pore fluid pressure. The model is based
on the premise that the stress is partially accommo-
dated by elastic grain contacts. Empirical observations
suggest that exponential functions of the porosity
reproduce b (Giles, 1997), such as:
bi ¼�
/i
/0
�0:33
ð7Þ
where /0 is the initial porosity at the time of
deposition and /i the porosity for the present time
step. The horizontal stress (rh) is a function of the
overburden (Engelder and Lacazette, 1990). When the
effective horizontal stress includes the thermal effect,
rhV is calculated as:
rhV ¼v
1� vrv � bPfð Þ þ a
E
1� v
�DT
�ð8Þ
where v is Poisson’s ratio, a is the coefficient of
thermal expansion in 8C�1, DT is the change in
temperature, and E is Young’s modulus in MPa, the
horizontal effective stress rhV sometimes decreases
faster than rvV during uplift, and may become tensile,
to the point where microfractures occur. The temper-
ature change as a function of depth Z may be
expressed as:
DT ¼ dT
dzDz: ð9Þ
If the P–T–t path (burial history) of a sandstone
body is divided in small linear time steps, the change
in the horizontal effective stress DrhV can be calculated
numerically as a function of time. The sum can be
numerically performed along a P–T–t as:
Xni¼0
rhiþ1V ¼
Xni¼0
�vi
1� viðrvi � biPf iÞ
þ�
Ei
1� vi
�aiðTiþ1 � TiÞ þ rhi
V
�ð10Þ
where i is a time step and (i+1)� i is the linear
increment. As explained above, E, v, and a are
assumed to vary linearly and synchronously with
compaction, quartz cement, and ultimately the poros-
ity, whereas b varies with Eq. (7). Breckels and van
Eekelen (1982) measured values of b on rocks of the
Misoa Formation. The initial and final values of E, v,
and a were taken directly from the literature and were
obtained experimentally from sandstone of similar
composition, but different formations (see references
in Table 2). Additional stress terms in Eq. (10), for
example those that would account for tectonic
stresses, are not included because they are not defined
in the uniaxial strain model. The pore fluid pressure Pf
varies with time. In the following section the
diagenetic evolution of quartz cement in the sandstone
of the Misoa Formation is calculated, including its
effect on porosity.
5. Host rock’s paragenesis and thermo-mechanic
evolution
The Misoa sandstone is very fine, to medium-
grained, well to moderately sorted quartz arenite to
sublitharenite. The average composition is quartz86-feldspar5lithic9 (Table 3). Early diagenesis in these
sandstones includes less than 1% of tangential grain
coating of smectite clay minerals, intergranular
framboidal pyrite, siderite cement, feldspar over-
growths, and less than 2% to 5% of kaolinite.
Table 3
Selected petrographic results
Well Depth
(m)
Quartz Feldspar Met.
lithics
Sed.
lithics
Muscovite Kaolinite Calcite Siderite Pyrite S/I Chlorite QC Bituminite P. P S. P
BA-1 692.4 53 6 1 2 2 0 29 3 0 1 0 2 1 0 1
BA-1 878.3 59 4 5 1 2 0 7 1 0 13 0 4 0 0 3
BA-1 942.8 60 6 4 3 0 1 3 2 0 10 0 6 0 1 3
BA-1 1463.9 52 5 1 3 0 0 14 11 0 0 3 3 5 0 3
BA-1 1593.9 63 4 2 2 1 2 1 0 0 0 0 17 0 8 1
LB-114 2757.6 68 1 1 1 0 0 24 0 0 1 1 0 0 0 1
LB-1387 1392.4 60 2 2 1 1 0 28 0 1 0 0 4 0 0 0
LB-1387 2244.9 58 1 3 2 0 5 1 0 0 0 2 28 0 0 1
LB-1387 2412.4 60 2 11 3 1 1 2 1 0 0 2 15 0 0 3
LB-1387 2857.3 59 1 0 2 4 0 5 3 0 0 13 10 3 0 0
LB-1387 2858.3 57 2 3 1 1 0 2 6 0 0 12 15 0 0 1
LB-1387 3854.4 55 3 7 0 1 0 4 0 1 1 0 26 0 0 0
LB-273 1978.0 53 3 3 3 4 0 7 2 0 3 6 12 1 1 0
LB-273 2040.1 67 4 1 1 0 1 0 1 0 1 0 12 1 10 0
LB-273 2100.6 56 5 6 1 4 1 0 0 2 1 5 16 0 0 2
LB-273 2103.4 59 4 6 1 1 0 3 2 0 0 3 17 3 0 0
LB-273 2293.1 61 2 6 2 2 0 11 3 0 0 0 8 1 0 3
LB-273 2363.6 49 11 6 2 3 0 1 2 0 5 0 16 4 0 1
LB-273 2511.9 56 4 8 2 2 1 3 2 1 6 15 1 0 0 0
LB-273 2694.8 61 1 5 1 2 1 0 1 0 0 0 27 0 0 0
LB-273 2718.7 62 6 4 1 4 0 2 3 0 0 8 6 1 0 3
LS-837 496.1 61 8 2 4 1 0 21 0 0 0 0 3 0 0 0
The relative % is based on 300 point counts per thin section.
Met. lithics—metamorphic lithic fragments.
Sed. lithics—sedimentary lithic fragments.
S/I—smectite/illite clay.
QC—quartz cement as overgrowth.
P. P—primary porosity.
S. P—secondary porosity.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 187
Petrographic observations suggest that kaolinite is
related to feldspar dissolution and predates quartz
cement.
Intermediate diagenesis is primarily characterized
by quartz overgrowth formation. Quartz overgrowths
postdate the aforementioned minerals and are volu-
metrically the most important authigenic phase in the
host rock. Overgrowths average 12% of the bulk rock,
but in fine-grained sandstone may reach 26% of the
bulk host rock (Table 3), based on cathodolumines-
cence (CL). Petrographic and CL observations indi-
cate the presence of two distinct generations of quartz
cement. The first quartz cement generation is volu-
metrically less than 2% and may be inherited (Ghosh
et al., 1985). The second generation of quartz cement
has a darker luminescence than the first, contains fluid
inclusion assemblages away from dust rims, and is the
main factor reducing the rock porosity. Homogeniza-
tion temperatures from two-phase liquid/vapor iso-
lated fluid inclusions, present within the last
overgrowth event, ranged from 100 to 175 8C,averaging 119 8C (Table 4, Fig. 4). Melting temper-
atures vary from �0.4 to �2.2 8C, yielding salinity
values between 0.8 and 2.7 wt.% NaCl, assuming an
H2O–NaCl system (Bodnar, 1992). Modeling results,
based on Walderhaug (1996) algorithms, indicate that
the precipitation of quartz overgrowth took place at
depths between 2.5 and 4 km. This range (2.5 to 4
km) corresponds to 10 and 18 my after deposition,
mostly during early Eocene to Oligocene time (i.e.,
Fig. 5). Based on fluid inclusion data solely it would
be tempting to presume that quartz cement also
precipitated during uplift, however, according to the
modeling results, quartz cement does not precipitate in
the sandstone beyond 18 my after deposition, even
though it is within the precipitation window, because
Table 4
Typology of fluid inclusion data analyzed on sandstones
Well Field Depth
(m)
P.D.T M.B.T. Occurrence Th range Th average Tm range Tm
average
wt.%
NaCl
LB-1182 Bachaquero 1180.3 64 117 Grain-scale quartz
cemented
microfracture (F1)
111 to 117 (n =11) 113.5F2 �1 to
�2.3 (n =2)
�1.6 2.74
MOT-4 Motatan 2545 63 N 63 Grain-scale quartz
cemented
microfracture (F1)
98–119 (n =11) 105.6F6.5 N.D. N.D. N.D.
LS-1257 Lagunillas 3819.1 131 170 Quartz microvein (F3) 149–175 (n =14) 167.6F6.5 �0.4 to
�1 (n =13)
�0.9 1.57
LS-1257 Lagunillas 3819.1 131 170 Quartz overgrowth 145–160 (n =4) 151.3F6.3 �0.4 to
�1 (n =2)
�0.7 1.23
MOT-4 Motatan 2545 63 N 63 Quartz overgrowth 107–129 (n =6) 116.3F7.5 �1.6 to
�2.2 (n =5)
�0.5 0.88
BA-1 Wildcat 1953 48 N 48 Quartz overgrowth 101–128 (n =5) 110.2F10.4 N.D. N.D. N.D.
MOT-2 Motatan 2565 68 N.D. Quartz overgrowth 106–139 (n =11) 113.7F9.3 �1 to
�1.8 (n =5)
�1.3 2.24
Temperatures in Celsius.
P.D.T.—present day temperature in Celsius.
M.B.T.—maximum burial temperature based on thermal modeling.
Th—homogenization temperature.
Tm—melting temperature.
wt.% NaCl—assuming water–salt system, and using Bodnar (1992) revised equation.
N.D.—not determined.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207188
there is no intergranular space available for quartz
cement to grow.
Locally, post-quartz siderite and calcite cement
filled up to 18% of the intergranular volume, and
prevented further compaction and quartz cementation.
Based on microprobe analysis, the average composi-
tion of the late-stage calcite is Ca0.91Mg0.01(Fe+
Mn)0.08CO3. The precipitation P–T–t of the local
late-stage calcite may be inferred using results from
the quartz precipitation kinetic model (Fig. 5), specif-
ically, using intergranular volume (IGV) reduction
curves through geologic time. The method consists of
matching the available IGV left by compaction and
quartz overgrowth with the amount of late-stage calcite
cement quantified by microscopic analysis. Post-quartz
calcite cement is calculated to have precipitated
between 100 and 150 8C, at depths greater than 3 km,
between 12 and 14 my after deposition. These petro-
graphic results and cementation temperatures are
similar to other studies in the basin (e.g., Chatellier
and Perez, 2000). Other local late-stage cements
represent 2% to 6% of the host rock and consist of
siderite, chlorite, and chert (Table 3). However, the
majority of the sandstones, in which we apply the
quartz cement model, are clean, quartz-rich, and with
no other cement than quartz overgrowths.
The diagenetic model suggests that the porosity in
the clean quartz arenites decreased to almost zero
before the maximum burial depth was attained (Fig.
5C,D). Experimentally, it has been demonstrated that
the Poisson’s ratio v, Young’s modulus E, and
poroelastic parameter b of sandstone in quartz-rich
sands vary with the porosity and degree of cementa-
tion (see review by Giles, 1997). The mathematical
function describing how these properties vary with
porosity and time, i.e., either exponentially or linearly,
is to our knowledge unknown. Consequently, we
arbitrarily made v, E, and a of clean quartz arenites to
evolve linearly and synchronously with the porosity
and amount of quartz cement, from their initial values
at deposition to their final values when /c0 (Fig.
5A,B). For instance, the porosity of clean quartz
arenites in well LB-273 at 2488 m depth decreased–as
a function of quartz cementation and compaction–
from 47% to less than 2% from 0 my to 18 my after
deposition (Fig. 5C,D), respectively. Consequently,
the v, E, and b varied from 0.15 to 0.33, from
�1�103 to �16.3�103, and from 1 to 0.56,
<140 145 150 155 160 165 170 175 180 ≥1800
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
-1 -0.9 0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.10
1
2
3
4
5
6
7
8
0.0 -2.4,-2.2 -2.0-1.8 -1.6 -1.4 -1.2 -1.0-0.8 -0.6 -0.4 -0.20
1
2
3
4
5
6
7
8
0.0
90 95 100 105 110 115 120 125 135 140130 145 150
HOMOGENIZATION TEMPERATURES(in fractures and host rocks)
ICE MELTING TEMPERATURES(in fractures and host rocks)
Fre
quen
cyF
requ
ency
Fre
quen
cyF
requ
ency
Tm (oC) Tm (oC)
Th (oC)Th (oC)
LS-1257, 3819 m
Veins (F3)Overgrowth
Microfractures (F1)
MOT-4, 2545 m
Overgrowth
Microfractures (F1)
MOT-4, 2545 m
Overgrowth
LS-1257, 3819 m
Veins (F3)Overgrowth
n= 21 n= 26
n= 5n= 17
C D
BA
Fig. 4. A and B show fluid inclusion data from quartz-filled microfractures (set F1 and F3) from the Misoa Formation. Quartz precipitation
occurred over a wide temperature range, beginning at 100 8C. (C, D) Ice melting temperatures suggesting the presence of marine-diluted and
meteoric water influence in Misoa pore waters throughout the burial history.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 189
respectively, during the same period of time (Table 2;
Fig. 6A–D). The maximum burial depth was reached
20 my after deposition (Fig. 5A), 2 my after all rock
properties had attained their maximum values.
In our model the rock properties vary with time,
temperature, and diagenesis, not with depth solely,
and after the sandstone is totally cemented, when
porosity reaches zero, the mechanical properties
remain constant. Understanding the evolution of the
aforementioned parameters is important because they
are part of the basic input for our stress analysis in
determining the host-rock’s thermo-mechanical con-
ditions during uplift.
6. Analysis of microfracture morphology
Four kinds of thin-section scale fractures occur in
Misoa sandstone. The lack of core orientation forced
us to group the fractures in sets strictly based on their
mineralogy, cross cutting relations, and orientation
relative to lamination. Intragranular fractures repre-
sent the earliest set (F1) and are filled with quartz
cement. The second set of thin-section scale micro-
fractures (F2) are filled with bituminite–pyrite and are
truncated by a third set of microfractures (F3) filled by
either quartz or calcite cement. The fourth set of
microfractures (F4) is either uncemented or partially
0
500
1000
1500
2000
2500
3000
3500
4000
45000.0 10.0 20.0 30.0 40.0 50.0
Time after deposition (MY)
0
20
40
60
80
100
120
140
160
180
2000.0 10.0 20.0 30.0 40.0 50.0
Time after deposition (MY)
0
5
10
15
20
25
30
0.0 10.0 20.0 30.0 40.0 50.0
Time after deposition (MY)
0
5
10
15
20
25
30
35
40
45
50
0.0 10.0 20.0 30.0 40.0 50.0
BURIAL - THERMAL HISTORY OF HOST ROCKSD
epth
(m
)
Tem
pera
ture
(C
)
Qua
rtz
cem
ent %
Prim
ary
poro
sity
%
Present day depth
Thin section porosity based on point countMeasured
amount of quartz overgrowth
MODEL RESULTS: QUARTZ CEMENTATION AND COMPACTION OF HOST ROCK
Present day temperature
Time after deposition (MY)
A B
C D
Fig. 5. A and B represent burial and thermal history for fine-grained, clean, quartz-rich, and quartz cemented sandstone from well LB-273, at
2488 m depth (Rodriguez et al., 1997). (C) Quartz cementation kinetic model results, based on Walderhaug (1996) algorithms (Eqs. (1)–(5)),
representing the 26% of quartz cement precipitated through time. (D) Calculated porosity decreases through time as a result of compaction and
quartz cement.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207190
cemented by hematite and other concurrent iron
oxides. The latter cement type and set of micro-
fractures (F4) are observed in sandstone strata uplifted
more than 1/3 from their maximum burial depth. All
microfractures are Mode 1 (open mode), extension
fractures, and confined to host rocks rich in inter-
granular quartz cement. Their morphology and aper-
tures are described in the following text.
6.1. Intragranular fractures (F1)
A summary of the characteristics of the micro-
fractures is presented in Table 5. Intragranular
fractures–fractures within or across grains–are the
most common type of fracture in the Misoa Formation
and represent post-depositional features associated
with compaction during burial (e.g., Milliken, 1994;
Milliken and Laubach, 2000; Chuhan et al., 2002). All
intragranular fractures are Mode I, formed by a
displacement normal to the fracture wall. However,
they vary randomly in pattern, shape, and size. These
fractures generally have straight or curvilinear traces
(Fig. 7), they cut grain boundaries, and in some cases
cut overgrowth cements. Displaced fractured walls
indicate normal dilation, followed by solid-volume
increase, and repacking. In some cases, microfractures
extend across two or three grains forming trans-
granular microfractures. Wedge-shaped fractures at
grain–grain contacts commonly display triangular
shapes in a radiating pattern, and are diagnostic of
stress concentrations (Laubach, 1997). The radiating
patterns are most likely produced by compaction with
or without tectonic stresses present (Milliken and
Laubach, 2000). Many quartz-filled intragranular
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
6.0 11.0 16.0 21.0 26.0 31.0 36.0 41.0 46.0 51.0
Time after deposition (MY)
0
0.000002
0.000004
0.000006
0.000008
0.00001
0.000012
0.0 10.0 20.0 30.0 40.0 50.0
-18000
-16000
-14000
-12000
-10000
-8000
-6000
-4000
-2000
0
0.0 10.0 20.0 30.0 40.0 50.0 60.0
0
0.2
0.4
0.6
0.8
1
1.2
0.0 10.0 20.0 30.0 40.0 50.0
Bio
t Por
oela
stic
P
aram
eter
β
Poi
sson
's R
atio
V
Youn
g's
Mod
ulus
E
(MP
a)
Coe
ffici
ent o
f The
rmal
Exp
ansi
on α
(1/
C)
Time after deposition (MY)
Time after deposition (MY) Time after deposition (MY)
A B
C D
Fig. 6. Synchronous and linear variation of v, E, and a with / through geologic time (see Eqs. (1)–(5)); b varies with Eq. (7). Example for
quartz arenites from well LB-273, presently at 2488 m depth; dashed line represents the approximate time of maximum burial depth. We assume
the parameters evolve from their initial values at the time of deposition, to the final values when the primary porosity reached zero /c0 (Table
1), which is before maximum burial depth. Results are basic input for thermoelastic contraction calculations. The final b value corresponds
specifically to sandstones from the Misoa Fm. The rest are theoretical values taken from the literature, see Table 2.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 191
fractures are inherited, but these, however, have
distinctive bright luminescence under CL and blunt
terminations at grain boundaries, which distinguishes
them from quartz-filled fractures developed in situ—
in the present setting. Based on image analysis of 15
different CL photographs from three samples (five
photographs per sample) the quartz cement trapped in
microfractures averages less than 3% of the total
photograph. Qualitatively, there is a positive correla-
tion between the abundance of intragranular micro-
fractures and depth, similar to that observed in the
Gulf Coast basin (Makowitz and Milliken, 2003).
6.2. Cemented thin-section scale fractures (F2–F4)
Other microstructures present in Misoa sandstone
are microfractures that extend across thin sections.
They occur either isolated or in parallel sets. Again,
we grouped these microstructures in three different
sets based on their mineralogy and relative timing of
formation, not orientation as lack of oriented cores
prevented us from determining fracture direction and
from further testing our grouping method. Never-
theless, the three sets, in order of occurrence, consist
of (1) microfractures partially or fully cemented by
bituminite–pyrite (F2), (2) microfractures fully
cement by either quartz or calcite (F3), and (3)
microfractures either open or cemented by hematite–
iron oxides (F4). All fracture filling cements display
finer crystals than the host rock, except for fractures
filled by calcite cement. The following three sets of
microfractures can be described as separate events,
because their mineralogy suggests that they formed at
different geochemical conditions and geological time.
6.2.1. Second set of microfractures (F2)
The microfractures of the second set are filled by
bituminite (residual oil) and pyrite cement (Fig. 8A–
Table 5
Characteristics of vein generations including timing, fracture mechanisms, and possible temperature range
Basin event Fracture
generation
Microvein
cements
Microvein
secondary cement
Fracture
termination
Orientation
relative
to bedding
lamination
Host rock
intergranular
cement
Mechanism
that lead to
fracturing
Fluid flow, and
mass transport
mechanism
Temperature
(8C)
Subsidence F1 Quartz Quartz cement Triangular,
tabular,
radiating
pattern
Random Quartz arenites Compaction
Stress
Grain scale silica
diffusion
~ 100+
Oil generation–
migration-charge
F2 Bituminite Pyrite, siderite Triangular Sub-parallel Quartz cemented
quartz arenites
CD, kerogen
maturation
Kilometer scale from
underlying source rocks
50–100TT
Maximum burial F3 Quartz None Abrupt/blunt High angle Quartz cemented
quartz arenites
CD, kerogen
maturation
Local diffusion or less
than a kilometer scale
upward flow
140–170+
Maximum burial Calcite None Abrupt/blunt High angle Quartz cemented
quartz arenites
CD, kerogen
maturation
Local diffusion or less
han a kilometer scale
upward flow
140–170+
Uplift F4 Uncemented Pyrite, Hematite Triangular Random,
high angle
Quartz cemented
quartz arenites
Thermal
stresses
Small scale diffusion
and meteoric water influx
b 50T
Temperatures derive from fluid inclusion data.
T Hydrocarbon generation–migration from Rodriguez et al. (1997).+ Inferred temperature from IGV of host rock, petrology, and burial-thermal curves.
TT Inferred from petrology and thermoelastic contraction modeling.
R.J.
Perez,
J.R.Boles
/Tecto
nophysics
400(2005)179–207
192
QC
QG
QG
QGQC
F1
200 microns
Fig. 7. Sample from well LB-1182, 1172 m depth; F1 intragranular
quartz-filled microfractures; in black is quartz cement QC, in gray
the detrital quartz grain QG. The figure depicts the solid volume
increase, normal dilation, and stress concentration at grain contacts.
The triangular fracture shape suggests compaction origin but we
have no information on orientation.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 193
D). These fractures are 0.2 to 0.5 mm wide (aperture)
and less than 0.5 cm long. They occur at a high
angle with respect to bedding lamination, in parallel
sets. They are sub-laminar and follow grain bounda-
ries (Fig. 8B). These observations suggest that
fracture propagation and bituminite infiltration were
both controlled by fluid percolation through a
compacted, but not fully cemented, host rock.
Fracture terminations are triangular tips, depicting
simple tapering. Fracture spacing is usually less than
0.5 cm. Bituminite-filled microfractures have shrink-
age cracks filled by quartz cement (Fig. 8B)
indicating quartz precipitation in the presence of
hydrocarbons. In the absence of quartz crystals,
bituminitic microfractures are also filled by concur-
rent siderite and pyrite.
Based on petrographic observations, siderite cor-
roded adjacent quartz grains and percolated in the
host rock adjacent to the vein. Based on SEM-EDX
pyrite, in contrast, precipitated within the bituminitic
filled space. The bituminite cement in microfractures
is in all cases less than 2% of the total rock. The
presence of bituminitic microfractures suggests that
the fracture event took place after hydrocarbon
generation-expulsion or synchronously with hydro-
carbon charge.
6.2.2. Third set of microfractures (F3)
The third set microfractures is filled by quartz and
calcite cement, they are orthogonal or cut at high
angle to the set filled by bituminite–pyrite (Fig.
8C,D). Quartz and calcite microfractures are present
in samples that are currently at the same depth range.
They are grouped in sub-parallel sets, at a high angle
with respect to bedding lamination. They also post-
date quartz cement or occur within the intergranular
quartz cementation window. The microfractures of the
third set are 0.3 to 0.5 mm wide (aperture) and up to 2
to 3 cm long. In several samples, F3 microfractures
overlap, adjoining different segments. Fracture termi-
nations are abrupt, and the fracture spacing is
relatively close, ranging from 1 to 3 per 2 cm2 of
thin section.
In core samples, calcite crystals in these micro-
fractures are euhedral. Under CL, the crystals are dull-
orange to non-luminescent and lack zonation. The
sandstone adjacent to the fracture-filling space com-
monly consists of irregular zones of calcite cement,
0.1 to 0.2 mm in width. These rims may have formed
by host rock displacement through filling by calcite,
indicating that part of the fracture cement grows
outward into the host rock matrix, as well as inward
into the vein center. The euhedral crystal terminations
are indicative of growth into open space, whereas the
cements diffuse contact with the host rock is
indicative of dissolution/precipitation or matrix
recrystallization.
Quartz-cemented microfractures are sub-perpen-
dicular to the set F2, which is filled with bituminite–
pyrite (Fig. 8D,E). Euhedral quartz crystal termina-
tions indicate growth into open fracture space. Quartz
cemented fractures occur along grain boundaries.
Under CL, the quartz cement in veins has dark
luminescence and lacks zonation (Fig. 8F), indicating
a diagenetic origin. We suggest that the mechanism
responsible for the F3 microfracture event took place
after hydrocarbon migration, based on the fact that the
F3 quartz and calcite set cut F2 bituminitic micro-
fractures.
6.2.3. Fourth set of microfractures (F4)
The microfractures from the fourth set consist of
transgranular, uncemented, and partially cemented
microfractures (Fig. 9A–C), that are isolated or cut
the previously described microfracture sets F2 and F3,
Bituminite microvein
F2
Bituminite filling microfractures
F2
Quartz-filled microfracture
F3
2mm0.1mm
A D
Quartz-filled microfracture
F3
Bituminite-filled microfracture
F2 Quartz bridge
2mm0.02mm
B E
Quartz-filled microfracture
F3
Bituminite-filled microfracture
F2
Bituminite-filled microfracture
F2
1mm 1mm
Calcite-filled microfracture
F3
Quartz overgrowth preceeding calcite
cement
C F
Fig. 8. Representative photomicrographs of microfracture sets F2–F4; (A) sample from well LS-1387, 2755.4 m depth with microfracture filled
by bituminite–pyrite (set F2); (B) detail of quartz filled cracks, same sample as above. (C) Sample from well BA-1, 1450 m depth, calcite
cemented microfracture (F3) postdating (cross cutting) bituminite filled microfractures (F2) intergranular quartz cement. D to F are from well
LB-1387, 2755.5 m depth, (D) F2 displaced by set F3; (E) conjugated microfractures filled by quartz cement (F3) in quartz arenite sandstone;
(F) cathodoluminescence reveals no zonation, possibly (but not conclusively) suggesting a single precipitation event in F3 quartz-filled
fractures.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207194
and occur in sandstone that are presently buried to
depths less than 0.5 km deep. These sandstone bodies
have been uplifted more than 2.5 km. These micro-
fractures are Mode I in origin, as the rest of the sets,
and are 0.2 to 0.5 mm wide (aperture) and less than
0.5 cm long. Fracture terminations are blunt, triangu-
lar tips, and cut grains, suggesting a high degree of
cementation and cohesion of the host rock at the time
of fracturing. Irregular crystals of siderite, pyrite, and
patches of hematite line the fracture walls (Fig. 9B).
0.5mm
P P
Reopened Microfault
Iron Oxide filled microfracture
F4
P
P
2mm
Open uncemented microfractures
F4
2mm
A
B
C
Fig. 9. Three representative F4 photomicrographs; (A) Well BA-1,
1922 m depth uncemented tensile fracture; (B) well BA-1, 1008.2 m
depth, partially cemented fractures by iron oxides and pyrite; (C)
well LS-1387, 1925 m depth, microfaults are common features and
we interpret reopening associated to uplift, based on calculations in
text and cementation. P is pore space.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 195
Furthermore, this set (F4) can also be associated with
the dilatancy and reopening of microfaults (Fig. 9C).
Microfaults, when present in uplifted intervals of the
Misoa Formation, reveal dilatancy and reopening, and
have small sub-euhedral quartz crystals growing
inward into the opened space, suggesting a contrac-
tion event and late-stage precipitation.
7. Interpretation of fracture cements from
petrologic, microthermometric, and probe data
7.1. Generation of the first set of microfractures
Fluid inclusions occur in quartz-filled, intragranu-
lar microfractures. The inclusions occur in small
clusters without preferred orientation and contain
two visible fluid phases at room temperature. In most
cases, the homogenization temperature (Th) in intra-
granular microfractures is lower than the Th in
overgrowths. These observations clearly suggest that
quartz grain fracturing and re-sealing preceded quartz
overgrowth (Fig. 10). Moreover, fluid inclusion data
suggest that quartz cement precipitated initially within
microfractures and subsequently surrounded grain
surfaces, as observed under CL in other studies
(e.g., Milliken, 1994; Laubach, 1997; Makowitz and
Milliken, 2003). The observations also indicate that
fluid salinity remained relatively constant during
quartz precipitation (Fig. 10).
7.2. Generation of the second set of microfractures
A compositional analysis of residual hydrocar-
bons present in microfractures is beyond the scope
of our research. Based, however, on burial-thermal
histories and the most current oil generation-migra-
tion model, the Misoa Formation was charged with
hydrocarbon at temperatures between 60 and 100
8C (Rodriguez et al., 1997). Thus, bituminite–pyrite
microveins represent a fracture event that occurred
within the same temperature range or later than the
oil generation-migration event. Furthermore, geo-
chemical experiments demonstrate that when resins
and asphaltenes are reduced between 60 and 100
8C, they yield H2S, CO2, and viscous black tar (see
review by Machel, 1987), similar to that observed
in pipelines.
-2.5
-2
-1.5
-1
-0.5
0
90 100 110 120 130 140 150 160 170 180
Homogenization Temperature Th (°C)
Quartz-filling microfractrures set F3
Quartz cement in fracture set F1
Quartz overgrowth in grains
Ice
Mel
ting
Tem
pera
ture
Tm
(°C
)
Legend Depth of occurrence
Fig. 10. Homogenization vs. ice melting temperatures from quartz cements in overgrowths, intragranular fractures, and quartz-filled
microfractures indicating the relative depth and sequence of occurrence.
CaCO3
(Fe+Mn)CO35%
100%MgCO3
5%
n=18
Fig. 11. Normalized molar compositions, based on microprobe
analysis, of calcite cements present in microfractures showing
relatively pure composition.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207196
7.3. Generation of the third set of microfractures
The third set of microfractures is filled by either
quartz and or calcite. Calcite and quartz micro-
fractures are grouped together because they both cut
bituminite-filled fractures in an orthogonal direction
(Fig. 8C,D), but they do not occur in the sample nor at
the same depth. Homogenization temperatures in
quartz cements present in the third set of micro-
fractures F3 range from 149 to 175 8C suggesting a
late stage of fracturing, and quartz precipitation (Table
5). Matching these temperatures with burial histories,
it is possible that the cementation occurred after
hydrocarbon emplacement, consistent with F3 cross
cutting of F2. Melting temperatures range between �1
and �0.4 8C (Fig. 10), yielding salinity values
between 1.7 and 0.5 wt.% NaCl assuming an H2O–
NaCl system (Bodnar, 1992). The salinity range
indicates dilute marine and meteoric fluid sources
were present during F3 fracture cementation.
Based on microprobe analysis, calcite crystals in
calcite microfractures averages Ca98(Fe+Mn)1.6Mg0.4CO3, have a Fe/Mg ratio of about 3, and have
from 1 to 3 mol% Fe+Mg+Mn substitution for Ca
(Fig. 11). Deep late-stage carbonate precipitation has
been reported in the Misoa Formation (e.g., Perez et
al., 1997) as well as in other basins of the world (i.e.,
Boles 1998). In general, calcite becomes less soluble
with increasing temperature—i.e., at greater depths. In
addition, fracturing causes decompression of fluids
and gases, which decreases PCO2. The PCO2
drop at
constant temperature increases the carbonate satura-
tion, which can lead to carbonate precipitation. If the
Fe/Ca ratio of the pore fluid is relatively low (Boles
and Ramseyer, 1987; Eichhubl and Boles, 1998)
calcite would precipitate upon decompression, if high,
siderite would dominate. Petrographic study indicates
that F3 calcite microveins formed after oil emplace-
ment, cutting F2 bituminite–pyrite microveins (Fig.
8C). Thus calcite and quartz filled microfractures may
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 197
have occurred within the same temperature range, i.e.,
~150 to 175 8C, even though they do not coexist in
the same sample. It is important to denote that some,
but not all, fractures filled with calcite cement show
quartz lineaments or quartz grains growing into the
fracture, predating the calcite cement (see detail in
Fig. 8C). This type of relationship could be associated
to synkinematic quartz cement, and possibly, post-
kinematic calcite cement (Laubach, 2003).
7.4. Generation of the fourth set of microfractures
The fourth set of microfractures are transgranular,
extensional, Mode I, and are partially cemented by
pyrite–hematite and other concurrent iron oxides.
Pyrite is the main fracture cement, whereas hematite
is secondary but still fills the fracture space. The
presence of late-stage hematite and iron oxides
supports a diagenetic event associated with uplift
and exposure to meteoric fluids. Hematite is stable in
reducing environments with pHN4.6, however, its
free energy is greater under oxidizing conditions and
may result from oxidation of pyrite or siderite (Garrels
and Christ, 1965). Pyrite and siderite have small
stability fields, and should precipitate under strongly
reducing conditions at pHN7 (Garrels and Christ,
1965). Thus, dissolution of pyrite or siderite and
precipitation of hematite is a reaction that can take
place under oxidizing conditions during influx of
meteoric water associated with uplifting of the Misoa
Formation at temperatures less than 50 8C.
8. Microfracture and mass transfer mechanisms
Compaction and repeated fracturing in the Misoa
Formation led to four microfracture sets (F1 to F4;
Fig. 12; Table 5). Intragranular microfractures (set F1)
were the first set to occur and compaction stresses are
usually interpreted as the main fracture mechanism for
these (e.g., Milliken, 1994). Compaction experiments,
performed under hydrostatic pressures and using
sands of similar composition and grain size to
Misoa’s, demonstrate that intragranular fractures
could initiate at 6 MPa and increase in number and
size at stress levels up to 20–30 MPa (Chuhan et al.,
2002). Average Th’s in F1 fractures are 105.6F6.5
and 113F2 8C (Table 4) suggesting that cementation
took place after fracture formation (Fig. 10), at depths
~3 km corresponding to rvV from 25 to 30 MPa—
assuming a ~25 8C/km thermal gradient and hydro-
static pressure gradient, respectively.
Based on the shape, mode, fracture shape, and
relative orientation to bedding (Table 5; Figs. 8 and 9),
we interpret microfractures F2 to F4 to be extensional
and to have formed perpendicular to the minimum
confining stress r3. Based on their inferred depth of
formation and the absence of nearby faults, micro-
fractures F2, F3, and F4 may have formed when Pf
was higher than r3, so that the minimum effective
stress r3V became tensile and higher than the fracture
toughness To. In other words, generations F2 to F4
may have occurred hydraulically, meaning r3V=r3�b d PfbTo, where To is the tensile strength of
the rock (Fig. 13). The presence of Pf causes a rock to
behave as if the confining pressure were decreased by
an amount equal to Pf (Twiss and Moores, 1992). At
small differential stress, an increase in pore pressure
drives the effective stress toward the tension field
along the normal stress axis, where it meets the
criteria for tension failure (Fig. 13A). At large
differential stresses (large r1V–r3V), an increase of pore
pressure would shift the stress toward the failure
envelope in the Coulomb failure criteria, causing
shear.
Hydraulic fractures by themselves are, by no
means, indicative of overpressures. However, within
our geologic context, burial depths greater than 3 km,
the presence of an oil-gas source rock, the hydraulic
microfractures F3 strongly, but not conclusively,
suggest the presence of paleo-overpressures, meaning
(a) the pore fluid (Pf) pressures was higher than
hydrostatic, and (b) the r3V was higher than the
hydrostatic stress, higher than the fracture toughness,
and lower than the overburden stress.
Overpressure is created by a pore fluid volume
increase with minimal change in porosity and at a rate
that does not permit dissipation (Swarbrick et al.,
2002). Several mechanisms may have caused Pf to
increase above hydrostatic in the Misoa Formation.
The first to occur in the burial sequence and most
important is compaction disequilibrium, which is
caused by compaction exceeding rates of pore fluid
loss. Clay dehydration, smectite–illite transformation,
kerogen transformation, gas generation, hydrocarbon
buoyancy, quartz cementation, and osmosis are other
500.0
1000.0
1500.0
2000.0
2500.0
3000.0
3500.0
4000.0
4500.0
0.0 10.0 20.0 30.0 40.0 50.0
Time from deposition (MY)
D
epth
(m
)
A
?
Present day
0.0
FR
AC
TU
RE
PA
RA
GE
NE
SIS
(1) Intragranular microractures F1
(2) Quartz cement overgrowth within intragranular fractures
(3)Quartz cement overgrowth around detrital grains
(5)Bituminite filling fractures
(10)Quartz filling completely F3 microfractures
(8)Synkinematic quartz overgrowth into fractures
(7)Calcite cement filling microfractures set F3
(9)Postkinematic calcite cement filling fractures
(13)Iron Oxides fills set F4
(4)Occurrence of set F2
(6)Occurrence of set F3
(11)Occurrence of set F4
(12)Meteoric water invasion
Fig. 12. Paragenetic sequence of fracture events present in the Misoa Formation. *The occurrence of F4 contraction fractures varies from field to
field, depending upon the burial history. **Iron oxides precipitated in sandstones units that were uplifted to depths less than 0.5 km from the
surface. (1) Based on Chuhan et al. (2002) experiments; (2) based on fluid inclusions data (FI); (3) based on FI data and Walderhaug, 1996
model; (4) inferred from petrography; (5) based on timing of oil migration (Rodriguez et al., 1997); (67) inferred from petrography; (89) based
on quartz lineaments; (10) based on FI data and petrography; (11) based on thermoelastic contraction model; (1213) based on the presence of
iron oxides.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207198
causes that occur at a relatively same temperature
window (see review by Swarbrick and Osborne,
1998). Several of the mechanisms above may have
occurred in isolation, simultaneously, or sequentially,
triggering overpressure and ultimately microfracture
in the Misoa Formation.
At depths shallower than 2 km there is generally
little potential for overpressure generation. However,
in basins characterized by sedimentation rates
between 0.5 and 1 km/my and restricted pathways
for fluid loss, such as the Central North Sea basin and
Nile Delta (Law and Spencer, 1998) as well as the
Coulomb fracture criterion
Tensile fracture criterion
σn−σn σ1 σ1 σ1 σ3σ3σ3 =Το
Normal stress
Shear stress
τ
−τ
^
Tension Compression
Applied stress
Overburden
Pf
Pf increase leads to tensile failure
Effective stressHydraulic fracture
Coulomb fracture criterion
Tensile fracture criterion
σn−σn σ1
σ1σ1σ1 σ3 σ3 σ3σ3 =Το
Shear stress
τ
−τ
^
^^
Tension Compression
Overburden
Hydraulic fracture
Initial state of stress
Final state of stress after cooling
Initial state of stress
Final state of stress after Pf
increase
Mohr Envelope
Mohr Envelope
Normal stress
A
B
Fig. 13. Mohr circle sketch diagram representing various possible stress stages of the Misoa Formation; (A) burial cycle. Pore fluid pressure Pf
reduces the consolidation. Tensile fractures will form if the minimum stress is equal of higher than the fracture toughness, i.e., r3zTo. During
burial, increasing pore pressure shifts the differential stress to the tensile field leading to hydraulic fracturing; (B) uplift cycle. During uplift the
initial state of stress, as a result of erosion and cooling, may increase the differential stress and shift it to the tensile field. As we have shown the
net effect of load decreased and thermal contraction could be extensional.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 199
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207200
Maracaibo basin, pressure profiles reveal overpressure
at depths shallower than 1.3 km.
Clay dehydration also occurs in the Maracaibo
basin (Perez et al., 1997) and its effect may add to the
overpressure caused by compaction disequilibrium
(Swarbrick and Osborne, 1998). As explained,
increased pore fluid pressures shift the minimum
effective stress r3V to the tensile field at values equal
to, or higher than, the fracture toughness To (Fig. 13A).
Microfractures filled by bituminite–pyrite cement,
formed at depths greater than 2 km, based on oil
migration and generation models. We interpret these
microstructures to reflect hydraulic microfractures
associated with overpressure caused by kerogen
maturation, oil expulsion/gas generation, and gas to
oil cracking—in addition to disequilibrium compac-
tion. Kerogen maturation was most likely absent in
the Misoa Formation due to its low TOC, high O2
index, and low oil generation potential (Jaffe and
Gardinali, 1990). Thermobarometry and organic
maturation modeling, however, suggest the presence
of near-lithostatic pressures in the underlying source
rock La Luna Formation (Fig. 2) during oil gener-
ation/expulsion (Sweeney et al., 1995; Vrolijk et al.,
1996a and 1996b). These high pore pressure con-
ditions may have been transferred upward to lower
sections of the highly cemented Misoa. Upward
transfer of pressure is a common mechanism and is
a main control of overpressure distribution in basins
such as the central North Sea and Mahakam Delta
(Swarbrick and Osborne, 1996).
Upward transfer of excess pore fluid pressure may
have created a pressure gradient that could explain
hydrofracture, upward flow, and the addition of
dissolved carbonate and silica into microfractures.
Microfractures filled with quartz cement (F3) have
Th’szmaximum burial temperatures and contain
abundant oil fluid inclusions. Thus, they might
indicate upward transport of hydrocarbons and silica,
by decompressing fluids into fractures, from intervals
that were more advanced in burial diagenesis and
temperature. The quartz lineaments in some calcite-
filled microfractures (Fig. 8C) strongly suggest the
presence of synkinematic quartz overgrowth and post-
kinematic calcite precipitation.
Calcite filling microfractures (F3) could also reflect
decompression and upward fluid flow. Fracturing
increases the total pore volume locally, decreasing
the pore pressure. At constant temperature, conse-
quently, the PCO2drop after hydraulic fracturing
increases carbonate precipitation. Moreover, if the
temperature is decreased during upward flow, the
PCO2decrease caused by vertical fluid drainage could
outweigh the temperature effect, and also favors
carbonate precipitation into fractures (Boles and
Ramseyer, 1987; Eichhubl and Boles, 1998). Further-
more, hydraulic fracturing could occur under adiabatic
(constant heat) irreversible conditions–due to the low
thermal conductivity of rocks–leading to temperature
increases caused by the internal work generation of
the fluid phase during decompression (Wood and
Spera, 1984). The increase in temperature, accompa-
nied by PCO2drop, may decrease carbonate solubility
even further, leading to faster carbonate precipitation.
Additionally, geochemical experiments suggest
that shale in the Misoa and La Luna Formations
release cyclic and acyclic organic acids at %Ro 0.33–
0.77 and %Ro 0.5–1.8, respectively (Jaffe and
Gardinali, 1990). In general, the thermal breakdown
of kerogen yields directly CO2, CH4, H2S, and organic
acids (Lundegard and Land, 1986). At high concen-
trations (i.e., N 1.4 M acetic acid at 60 8C) organicacids act as a buffer to added CO2 resulting in
carbonate precipitation (Surdam and Crossey, 1985).
At lower organic acid concentrations, carbonate would
be precipitated with PCO2drops, presumably caused
during veining and its associated decompression and
upward fluid flow (Eichhubl and Boles, 1998).
Based on correlations between the amount of
quartz cement and concurrent fractures in rocks
lacking any other tectonic driver (Laubach, 1988), it
has been proposed that the rapid rate of porosity
reduction due to quartz overgrowth precipitation may
be another primary factor in generating overpressures
(Helset et al., 2002). Given the high cementation rates
in Misoa’s sandstones, it is not unreasonable to
assume quartz cementation was an important factor
in elevating fluid pressures. However, a much more
investigation is needed on this particular topic. For
example, Swarbrick and Osborne (1998) disregarded
the importance of quartz cementation as an over-
pressure generator based on Darcy’s Law.
The origin of open uncemented microfractures,
such as the F4 set, is difficult to ascertain. Open
microfractures may result from stress during drilling,
core handling, sample preparation, or unloading
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 201
during coring (Nelson, 1981; Laubach, 1997). We
interpret some F4 fractures, however, as the result of
basin unroofing, similar to those described by
Walderhaug (1992) based on the presence of hematite
cement, which suggest a late stage of oxidation event.
Precipitation of hematite and iron oxides takes place
under oxidizing conditions, possibly during meteoric
fluid invasion associated with uplift of the Misoa
Formation. Present fluids of non-treated reservoirs in
the Misoa Formation have an Na/ClN1, TDS ~1500
mg/l, and Ca/Mg ratio averaging 1.5, clearly suggest-
ing meteoric influence (Vasquez, 1998).
Moreover, a stress analysis of the formation during
uplift supports our argument. The net effect of uplift,
which implies erosion and cooling, is that the
horizontal stress becomes the minimum compressive
stress and the effective horizontal stress becomes
tensile even at depths greater than 1 km. We believe
that this mechanism, also called thermoelastic con-
traction, may have been an important factor during the
generation of the last microfracture event.
9. Modeling results in support of thermoelastic
contraction
To support our argument that late stage fractures
(F4) are associated with uplift, we analyzed the stress
conditions for fracturing during uplift assuming a
uniaxial strain model. In the uniaxial strain model, if
the rocks behave elastically, the Poisson effect
predicts that a decrease in the vertical load causes
expansion in the vertical direction, and contraction in
the horizontal direction. Thus, the horizontal stress at
any given depth may be quite different during burial
relative to uplift.
In our first simple approximation we calculated the
effect of burial and uplift on stress on a representative
P–T curve of a Misoa horizon (Fig. 5A,B; Eq. (9)).
From the assumption of tectonic stress being absent,
we assume the maximum and minimum horizontal
normal stresses are equal (DrH(max)=DrH(min)), a
function of the vertical stress, and temperature. For
simplicity, we chose a geobaric and thermal gradient
of 25 MPa/km and 25 8C/km, respectively. The
calculations were done at three points: at the surface
(0 m), at maximum burial (4 km), and at minimum
burial after uplift (1.5 km). In this calculation, the
initial values of v, E, and a (Table 2) are assumed
constant during burial. Subsequently, after the max-
imum burial depth is reached v, E, and a are at final
values shown in Table 2, and remain constant there-
after. Results indicate that the horizontal stress is
compressive at maximum burial (rh 40 MPa) and
tensile during uplift (�11 MPa). The fracture tough-
ness is generally between �5 and �20 MPa for
sandstone (Suppe, 1985), and is exceeded after ~1/2
of the thickness of the strata is gone (Fig 14A). Hence,
the appropriate conditions for fracturing formed
toward the end of the uplift. Subsequently, fractures
were filled with iron oxides at shallow subsurface
conditions, less than 0.5 km depth, and presumably at
hydrostatic fluid pressure conditions.
In our second approximation, equations for effec-
tive horizontal stress (Eqs. (3)–(10) are integrated
numerically throughout the time–depth path of the
Misoa sandstone (Fig. 5A,B). Even though F2 and F3
microveins strongly suggest an excess of fluid
pressures throughout the Misoa Formation during
deep burial, we assume minimum conditions for
fracturing, i.e., near-hydrostatic fluid pressures, where
k=Pf /rv varied from 0.45 to 0.51. Clearly this
assumption is an underestimation. Any increase in
Pf over hydrostatic shall accentuate the stress effect,
leading to a faster or deeper thermoelastic contraction.
We assumed a Poisson’s ratio of 0.15 for uncon-
solidated sand (Bachrach et al., 2002) and 0.33 for
well-cemented sandstone (Engelder, 1985). The bulk
overburden density increases from 2 (rock+pores) at
the time of deposition to 2.65 g/cm3 (highly quartz
cemented sandstone) at maximum burial depth. Hence
the total vertical stress is greater during uplift–due to
higher rock density–than at the corresponding depth
during burial. The poroelastic parameter decreases
from 1 to 0.56, following Eq. (6). As we explained, v,
E, and a are assumed to vary linearly and synchro-
nously with the porosity and degree of cementation
during burial (Fig. 6A–C). Because the porosity
approaches zero before the maximum burial depth,
the parameters remain constant at their final values
before maximum burial depth is achieved and during
uplift (Fig. 6A–D), although high fracture densities
may change the bulk density. Various rock properties
and burial history scenarios from wells MB-1, LB-
1387, LS-1257, LB-114, mostly around the Bach-
aquero field (Fig. 1) were incorporated into the stress
Stress History Analysis
0
500
1000
1500
2000
2500
3000
3500
4000
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Effective Horizontal Stress (MPa)
Bur
ial D
epth
(m
)
Tension Field
CompressionField
0
500
1000
1500
2000
2500
3000
3500
4000
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Effective Horizontal Stress (MPa)
Bur
ial D
epth
(m
)
CompressionField
Burial
Burial
Uplift
Uplift
Calculation in 3 time steps
Calculation in 30 time steps. Based on Equations 1 to 10.
(See text)
Tension Field
A
B
Fig. 14. Effective horizontal stress as a function of burial depth assuming uniaxial strain model. (A) Calculation performed in three steps
assuming thermo-mechanical parameters constant through time (Table 2), based on Eqs. (8)–(10). (B) Calculation performed in 30 time steps
assuming that thermo-mechanical parameters vary through time (Table 2), based on Eqs. (1)–(10). Results from both calculations show that the
effective horizontal stress rhV becomes lower than Pf at ~3.7 km depth and becomes tensile 3 Ma, at less than 2.5 km depth. Shaded area
represents typical values of fracture toughness To.
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207202
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207 203
history calculation and consistently demonstrated that
uplift reduces rhV to the tensile field (Fig. 13B).
It is noteworthy to recall that the Biot coefficient
was originally defined to accommodate the elastic
deformation of mineral grains under pressure. How-
ever, at temperatures above 100 8C the rock’s volume
reduction is also partially controlled, among many
other parameters, by chemical compaction and not by
mechanical squeezing solely (Bbjrkum, 1996). Under
these circumstances the use of the Biot coefficient in
Eqs. (6)–(10) perhaps would not adequately describe
the decreased effect of the fluid pressure on the
effective vertical stress. Nonetheless, during burial,
chemical compaction, although inelastic and irrever-
sible, will also decrease the net effect of the vertical
stress (rock overburden load) decreasing consequently
the effective vertical stress. In view of the above
discussion, we decided to perform simulations in
which the Biot coefficient was considered constant
and equal to 1 at all times. In other words, neglecting
its effect. The results are illustrated with dashed lines
in Fig. 14B. All calculations seem to indicate that, if
the Biot coefficient is dropped or not considered, it
would be easier to obtain a tensile horizontal effective
stress at the end of the uplift period, supporting further
our interpretation.
Summarizing, under our assumptions, for increas-
ing values of rhV during burial, Pf exceeds rhV leadingto conditions favorable for hydraulic fractures only if
Pf is above hydrostatic. However, during uplift, even
under hydrostatic pressure and at depths greater or
equal to 1 km the effective horizontal stress is
ultimately reduced to the tensile field. The Poisson
ratio contributes to lower stresses, but the slope of rhVduring uplift is dominated by the Young’s modulus in
the thermal stress term. In other words, thermal
contraction of the rock associated with uplift over-
whelms the Poisson effect and leads to tensile failure.
In addition to the horizontal effective stress, the fluid
pressure must exceed the fracture toughness To in
order to propagate a fracture (Fig. 14B). For the
purpose of this discussion, this term is not included in
our stress modeling. It is noteworthy that sandstone
fracture toughness is typically between �5 and �20
MPa (Suppe, 1985), and rhV in our model reaches
between �5 and �25 at the end of the uplift,
depending upon the burial history analyzed, and the
inclusion or exclusion of the Biot coefficient.
10. Discussion
Models as simple as the one we are presenting
cannot express accurately the complex stress, burial or
pressure history of Misoa sandstone for various
reasons. For instance, observations from geologic
structures and in situ measurements suggest that a
state of stress in which r1=r3 is uncommon (e.g.,
Engelder and Geiser, 1980). Pressure-solution and
fracturing change the basic assumption of elasticity
(Narr and Currie, 1982). Several faults that are present
in the studied oil fields can accommodate radial
contraction of the strata during uplift, however there is
no correlation between the occurrence of micro-
fractures and these faults. And last, but not least, we
neglected tectonic stress because its identification and
quantification is difficult, subjective, and prone to
error (Narr and Currie, 1982).
Thermoelastic contraction modeling has been used
to explain fracture genesis in several sedimentary
basins. For instance, Narr and Currie (1982) devel-
oped a stress-history model to explain joints in Utah’s
Uinta basin. Similar to us, their elastic model included
effects of overburden, pore pressure, temperature, and
tectonic strains, and they suggested that high pore
fluid pressure was responsible for the joint system
during uplift. Engelder (1985) studied loading paths
and joint propagation of the Appalachian Plateau
during different tectonic cycles including uplift. The
study also concluded that failure propagated by
contraction during uplift. Warpinski (1989) developed
a mathematically complex model for estimating stress
states in reservoirs for the Piceance basin assuming
elastic and viscoelastic rock behavior. The model
included pore pressure, temperature gradients, and
consolidation, but lacking, however, diagenetic
effects. The model concluded that stresses in sand-
stone are fairly accurately represented with an elastic
analysis similar to ours. Apotria et al. (1994) studied
the fracturing and stress history of the Antrim Shale in
the Michigan basin, and concluded that unloading
provided an important mechanism for fractures con-
sistent with the contemporary stress field of the area.
We believe that the calculations, herein presented,
are useful in deciphering physical effects and geologic
processes that are important in controlling the state of
stress in quartz-rich sandstone during different stages
of burial and uplift. We also believe that, in spite of
R.J. Perez, J.R. Boles / Tectonophysics 400 (2005) 179–207204
the oversimplifications, the model reproduces first
order effects of burial and uplift. Finally, although we
cannot prove that open uncemented microfractures
were caused by tensile stresses during uplift, they are
consistent with our first order predictions of stress
conditions resulting from thermoelastic contraction
within the Misoa Formation.
11. Conclusions
Intragranular quartz-filled microfractures (set F1)
in the quartz-rich Misoa Formation are the most
common microstructures and represent post-deposi-
tional structures associated with compaction stresses.
Th’s in F1 fractures are lower than in overgrowths,
indicating that quartz grain fracturing and annealing
predates quartz overgrowth. Quartz overgrowth is
the most important authigenic phase in the fracture
host rocks, averaging 12%, but may locally reach up
to 26%. Quartz precipitated between 100 and 175 8Cduring burial. Modeling and fluid inclusion data
suggest that the porosity in quartz arenite decreased
to almost zero before the maximum burial depth.
Sequentially, four sets of microfractures formed in
the Misoa Formation. Bituminite–pyrite (set F2) as
well as quartz and calcite filled microfractures (set
F3) suggest periods of hydraulic extension that
could be associated with overpressure. Compaction
disequilibrium in the Misoa may have been the
primary overpressure mechanism, however, it may
have been augmented by an upward pressure
gradient generated by underlying oil source rocks
at the time of kerogen maturation and by the rate of
porosity reduction due to quartz cementation.
Homogenization temperatures in quartz-filling micro-
fractures (F3) range between 149 and 175 8C,whereas maximum burial temperatures are around
160 8C. The vertical upward transfer of pore fluid
pressure explains the formation and addition of silica
into F3 fractures from intervals more advanced in
diagenesis. Microfractures filled by calcite (set F3),
which postdate those filled by bituminite–pyrite, may
reflect hydrofracture, decompression, and a subse-
quent PCO2drop, which may have outweighed any
temperature effect on solubility, leading to carbonate
precipitation. Synkinematic quartz cement growing
into the fracture space predates calcite cement,
suggesting that calcite may have a post-kinematic
origin. As discussed previously, hydraulic F3 fractures
by themselves are not indicative of overpressures. As
we explained, however, within the Misoa’s geologic
context, they suggest the presence of pressures above
hydrostatic.
We interpreted open uncemented and pyrite–
hematite partially cemented microfractures (F4) to
have formed late in the diagenetic history as a result of
meteoric water incursion during the late stage of
uplift. Based on our calculations, we conclude that the
Misoa Formation had conditions favorable for tensile
failure during the uplift. We also conclude that a total
denudation, uplift, or surface exposure does not
constitute a requirement to drive the stress to the
tensile field. Contraction fractures can occur even at
several kilometers depth, if tectonic stresses are
absent. Thermal elastic contraction of sandstones
may have been the dominant, but not the only,
mechanism responsible for late stage, fourth set of
fracturing. Our analysis indicates that there is an
intimate interplay between host-rock diagenesis, over-
pressure, hydraulic fracturing, mass transport, and
veining mechanisms that should be examined as
coupled transformation processes.
Acknowledgments
We thank E. Perez and Dr. Arthur Sylvester for
their editorial and critical reviews. Early comments by
Drs. Jean-Yves Chatellier, Santosh K. Ghosh, and
Thomas L. Dunn were very useful. Suggestions by the
journal referees Drs. Kevin Furlong, Olav Walder-
haug, and Steven Laubach greatly improved the
content of the paper. PDVSA-Exploration provided
samples and the U.S. Department of Energy (DOE)
funded our research under Grant No. 444033-22433.
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