BASIN ANALYSIS AS A TOOL TO REDUCE EXPLORATION RISKS ANALYSIS AS A TOOL TO REDUCE...BASIN ANALYSIS AS A TOOL TO REDUCE EXPLORATION RISKS Bucharest Geoscience Forum March 2. nd . 2007

BASIN ANALYSIS AS A TOOL TO REDUCE EXPLORATION RISKS

Bucharest Geoscience ForumMarch 2nd 2007

I) INTRODUCTION TO BASIN ANALYSIS: principles & goals

II) BASIN MODELING: basics of the risk assessment during theexploration stage

III) EXAMPLE OF 1-D MODELING: workflow application toNW-most part of Moesia

IV) FROM 1-D TO 2-D & 3-D MODELS: brief examples

SUMMARY

BASIN ANALYSIS

(only) models the reality!

It replaces in fact the real world by an alias =THE MODEL.

I) INTRODUCTION

THE MODEL employs real data such as structural / thickness / facial maps, lithology, pressures etc. as input and outputs series of parameters which must be calibrated

with actual measurements in order to achievea reasonable fit.

Generally, a MODEL

is never unequivocal! Modeling thus follows a “trial-and-error”

approach and, without accurate constraints, one cannot be able to chose from different possible scenarios the one that simulates closest the “reality”.

THE BASIN MODELING AIMS TO EITHER VALIDATEHYPOTHESIS (TESTING SCENARIOS) OR TO MAKE PROGNOSISABOUT PHENOMENA / PARAMETERS CONTROLLING THETECTONIC AND GEOFLUIDS EVOLUTION!

• VALIDATING THE “REAL”

STRUCTURAL MODEL

• RECONSTRUCTING THE BURIAL & THERMAL HISTORIESOF THE SEDIMENTARY BASIN

• ESTIMATING THE MATURITY OF THE POTENTIAL SOURCEROCKS & THEIR EVOLUTION THROUGH TIME

• ESTIMATING THE AMOUNT & TYPE OF THE HYDROCARBONSPRODUCED / EXPELLED THROUGH TIME

• INFERRING HYDROCARBON MIGRATION PATHWAYS

I) GOALS

SOURCE ROCK

SEDIMENTARYCOLUMN

DEPOSITION BURIAL

HC generation = F (T0, time)

HEAT FLOW

20 40 60HC generated(% of total C)

OIL

GAS

1

2

3

4

5

km

After Tissot & Espitalie (1975)

• kerogen type II• burial rate = 60 m / Ma• geothermal gradient = 350 / km

BIOGENIC GAS &HEAVY OIL

I) FUNDAMENTALS (1)

UPPER CRUST

20 km

LOWER CRUST

20 km

LITHOSPHERICMANTLE

110 km

ASTHENOSPHERE

INITIALSETTING

RIFTING

Rifting = thinning of the lithosphere HEATING!

100

C

13330

C

THINNING FACTOR (β) =

INITIAL THICK. / FINAL THICK.

100

C

13330

CT

HE

RM

AL

BO

UN

DA

RY

CO

ND

ITIO

NS

* * * * * * * * *

* * * * * * * * *

* * * * * * * * *

* * * * * * * * *

* * * ** * * *

SOURCE OF HEAT FLOWMOSTLY ADIABATIC ASTHENOSPHERE(assumed constant through geologic times)

RADIOGENIC PRODUCTION CONFINEDMAINLY WITHIN THE UPPER CRUST

(HEAT FLOW AFTER RIFTING / HEAT FLOW BEFORE RIFTING = β

)

I) FUNDAMENTALS (2)

STEADY STATE

–

occurs when the sedimentation rate is small (< 100 m / Ma) and when the sedimentary section is thin (a few thousands of meters); it means that the heat flow isnearly uniform throughout the sedimentary column and the thermal gradient (which is usuallyrecorded in the shallow part) varies smoothly down section and also in time.

TRASIENT STATE

–

occurs when the sedimentation rate is high*

and / or additional heat is added into the lithosphere through rifting or magmatic

intrusions. Depending upon themagnitude of these processes, the basin recovers the thermal equilibrium (i.e. steady state)in at least several millions of years, more common a few tens of millions of years.

* so-called blanketing effect - adding cold sediments in large amounts into the basin causessignificant decrease of the heat flow at depth; furthermore, cold unconsolidated sedimentsstore part of the heat (function of their heat capacity)

I) THERMAL STATE REGIME (1)

INITIAL SETTING

POST -

THRUSTING

Region where thethermal field is

perturbed

Two counteracting effects contribute to the perturbation of the thermal field:

1)

The thrust juxtaposes warmer rocks above colder ones, which promotes heating(in some cases of thick-skinned tectonics, thickening of the upper crust may causeadditional heating by duplicating the radioactive-producing layer); also, heating isexperienced initially by the rocks in footwall due to tectonic burial.

2)

The sedimentary column thickens, which promotes cooling.

I) THERMAL STATE REGIME (2)

THE RISK ASSESSMENT OF A PROSPECT / GROUP OF PROSPECTSREQUIRES THE PROBABILISTIC ESTIMATION OF SEVERAL FACTORSIN TERMS OF PRESENCE & EFFECTIVENESS, AND EQUALS THEPRODUCT OF:

•PROBABILITY OF RESERVOIR

•PROBABILITY OF TRAP

•PROBABILITY OF HYDROCARBONCHARGE

•PROBABILITY OF HYDROCARBONRETENTION AFTER ACCUMULATION

II) RISK ASSESSMENT BASICS (1)

EACH FACTOR IS RANKED ACCORDING TO THE DEGREE OF CERTAINTY BASED ON DIRECT-

SUPPORTING EVIDENCE OR BY ANALOGY WITH NEIGHBORING FIELDS.


BASIN MODELING IS BASICALY INVOLVED IN ASSESSING THE PROBABILITY OF HYDROCARBON CHARGE, WHICH IS IN TURN THE PRODUCT OF:

• PROBABILITY OF EFFECTIVE SOURCE ROCK

IN TERMS OF THE PRESENCE OF SUFFICIENT VOLUME OF MATURE ROCKS HAVING ADEQUATE QUALITY BEING LOCATED IN THE DRAINAGE AREA OF THE PROSPECT

• PROBABILITY OFEFFECTIVE MIGRATIONOF HYDROCARBONS FROMTHE SOURCE ROCKS TOTHE MAPPED PROSPECT


THIS EXAMPLE OF1-D MODELING WAS CARRIED OUT INTHE NW MOESIA.

III) EXAMPLE OF 1-D MODELING

Thanks to the Rompetrol Upstream’s Operation Manager S. Mihalache for granting permission to incorporate parts of an initially in-house presentation concerning the modeling of the Zegujani sub-basin. Also acknowledged is the V. Avram’s contribution to practical computations.

The modeling approach was applied using a preliminary structural interpretation. A more refined interpretation is pending publication this summer in a special volume “Frontiers in Earth Sciences” at Springer-Verlag entitled “Thrust belts and foreland basins” edited by O. Lacombe et al.

THE WORFLOW OF AN 1-D MODELING IS PRESENTED IN THE NEXT PROVIDING A TEMPLATE FOR THE GENERAL STEPS OF THE APPROACH AS WELL AS THE RESULTS THAT CAN BE DERIVED FROM.

Sarmatian

& younger faults(mostly normal)

(Oligocene?) Burdigalian

faults(normal), some reactivated asstrike-slip during the Sarmatianand later

TIMOK fault

Late Badenian

& later thrustfaults (transpressive)

CERNA fault

III) TECTONIC SETTING (1)

Distribution of the OligoceneFm. (assumed the main sourcerocks)

Region undergoing possiblyOligocene extension

Region undergoing mainlyBurdigalian

extension

Wells used in modeling


Tarapoanca et al., 2007

Well North

Well West

Well Central

Well South

3001_CIOVARNASANI

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

500

SW NE

TWT(ms)

Oligocene

Lower Burdigalian

Upper Burdigalian –LowermostBadenian

(“Helvetian”)

BASE NEOGENE UNCONFORMITYPg / Mz

UNCONFORMITY

Badenian

Sarmatian

Meotian

Pontian - Pliocene

Lower Sarmatianturbidites

Burdigalian normal faults


Well South

S N

h1 h2 h4h3The software uses the McKenzie (1978) simplestretching model*

to handle the syn-

& post-rift subsidence / thermal field, which assumesthat lithospheric

STRETCHING = THINNING**.

STRETCHING ~ Σhi , h = extensional fault heave

i = 1, 2, …

We cannot provide an estimate of the amount of stretching as the

northern part wassubsequently affected by significant strike-slip tectonics. Nevertheless, the Paleogene

–Early Miocene is a time span during which major extensional tectonics took place, bothwithin the orogenic

nappe

pile***

and the foreland****.

As the total stretching remains unknown, several scenarios have been built in orderto test the impact of various amounts of rifting upon the source

rocks maturity bycomparing the modeled thermal & vitrinite

reflectance profiles with the actual data.

*** Matenco & Schmid, 1999; Fügenschuh & Schmid, 2005**** Rabagia & Matenco, 1999; Tarapoanca, 2004

Zegujani

sub-basin (schematically)

* It implies that the crust is thinned with the same amount asthe mantle lithosphere. Different thinning factors (Royden &Keen, 1980) or lateral heat dissipation (Cochran, 1983) are notconsidered.

** Which is not always true!

III) INPUT DATA (1): basics

• GENEXTM

(*) HAS BEEN USED TO CARRY OUT THE NUMERICALSIMULATIONS

• CALIBRATION***

WITH THE PRESENT TEMPERATURES HAS BEEN IMPOSED ONLY

IN ONE NUMERICAL RUN

The most sensitive parameter and, in the same time, the “biggest”

unknown, is the thermalfield evolution, which controls the maturity rank of potential source rocks. Thus, the generalapproach consists in testing several scenarios whose results are

subsequently checkedagainst actual data**

in order to obtain a reasonable fit.

** such as: borehole temperature profiles, vitrinite reflectance profiles, fission-track coolingages, pyrolysis-derived measurements: Tmax , S2 , Hydrogen Index

Unfortunately, such measurements in the Zegujani

sub-basin or neighboring areas are eitherscarce, often with small confidence degree, or totally missing. To check the modelingoutputs we used the present-day temperature profile from Well South which seems to bethe most reliable recording (best investigated by logging). Also, for one modeled well, weextrapolated vitrinite

reflectance data from a few wells drilled ~ 30 km to the SE, whichwere sampled at similar depths and stratigraphic

ages.

*** It means that following a user-imposed geothermal gradient or bottom basement heatflow (the actual one), GenexTM will iteratively adjust the temperature at the base ofthe sedimentary column in order to fit the present-day temperature profile.

III) INPUT DATA (2): approach

* Trademark of IFP (“Institut Français du Pétrole”) &BeicipFranlab

ACTUAL WELL

WELL PROLONGED TO THE OLIGOCENEBASE (assumed 70% shale, 30% sand)

EOCENE –

CRETACEOUS SEQUENCE ADDED(assumed 50% sand, 50% limestone)

8000 m (basement beneath) Average value derived from deep seismicstudies (e.g. Raileanu et al., 1994)

Actual limitsSubdivided in several unitsfor the sake of improvingthe numerical computation accuracy

FOUR WELLS HAVE BEEN MODELED; EACH OF THEM HAS BEENPROLONGED AS FOLLOWS:

III) INPUT DATA (3): wells

• Constant surface T0

= 100

C

• Source rocks: OLIGOCENE (main), SARMATIAN (some peliticsequences) with kerogen

type II

• Burdigalian

(or Oligocene? -

Burdigalian) rifting period (note that althoughmajor accommodation space was created within the basin by rejuvenation of theTimok fault system at the beginning of the Sarmatian, this deformation shouldn’tcreate significant heating as it is a transtension confined basically to the western marginof the basin)

• Paleobathimetry

= 0 m

(there is no data about this; nevertheless, it may be reasonablefor part of the Neogene succession, particularly for the Early or Middle Miocene withevaporitic layers, Meotian and Pliocene; in turn, it underestimates the Sarmatian waterdepths)

• As far as the Neozoic source rocks are concerned (particularly thepost-Eocene sequence), the Eocene –

Upper Cretaceous sequence hasbeen added only

for thermal computation purposes

• No erosion at the Neogene

or Paleogene

base, as the wells are mostlylocated beyond of unconformities seen on the seismic lines

III) INPUT DATA (4): assumptions

Two scenarios have been tested:

• mild rifting, β

= 1.1• stronger rifting, β

= 1.4

These two wells encounter a thrust fault

each; therefore, partof the sedimentary section is duplicated. The wells weremodeled twice:

• without thrusting, that is, each well has been prolonged asshown in previous slides; consequently, the results should beextrapolated laterally into non-thrust areas at similar depths

• with thrusting

(actual location); in this case, the thrust hasbeen simulated as a rapid burial by a “warm pseudo-formation”deposited in the corresponding contraction time span. Warmingis achieved by imposing 400

C as the surface temperature at thetime of “sedimentation”

instead of usually 100

C.

III) SCENARIOS

Well North

Well Central

Well South

Well West

β

= 1.1

β

= 1.4

Well North

Well Central

Well South

Well West

III) Well NORTH (no thrusting): output example

β

= 1.1 β

= 1.4

β

= 1.1

Neglecting the blanketing effect (as currently done) by extrapolating the present geothermal gradient to the past evolution of the basin leads to significant overestimatesof the maturity rank. Particularly in the Zegujani

sub-basin, two periods of high sedimentation rates (Burdigalian

& Sarmatian) should have had an important impacton the thermal balance.

Using a constant geothermal gradient throughout those periods of

rapid sedimentationimplies a considerable heat increasingly advected

to the basin bottom, which would risetwo major problems:

• the source of heat, which otherwise would overcome the estimated rifting anomaly, givingthe known formations thickness and tectonic evidences

• the thermal re-equilibration following the heating would lead to significantly greater present temperatures

III) THERMAL FIELD QUESTION (1)

• Note that using a modeled constantgeothermal gradient of 300

C / kmthroughout the basin evolution (dashed linein figure) significantly overestimates theactual thermal maturity.

• From a statistic standpoint, the onsetof the oil window (at a vitrinite

reflectanceof 0.7) within the whole Getic

Depression regardless of formation age and localtectonic deformation is not reached in anycase at depths shallower than 4 km.

• These recordings suggest that highsedimentation rates at different periodsof time have significantly influenced thethermal field of the basin.

III) THERMAL FIELD QUESTION (2)

As the sediment supplies have almost continuously cooled-down the basin during the Burdigalian-Sarmatian

times and no rifting-derived heat was added to the basin bottom in order to counterbalance the dramatic temperature decrease, a unrealistic extremely fast thermal re-

equilibration is obtained.

In other words, the temperature at the base of sedimentary column has been unrealistically risen

such that the modeled present temperature profile fits the actual one (like a massive magmatic

body has been intruded since the Latest Miocene)!

III) THERMAL FIELD QUESTION (3)To illustrate an extreme effect of neglecting the heat flow change, the modeled temperaturehistory in the Well CENTRAL is plotted beneath. No rifting

is assumed to have occurred,consequently no variation in the bottom basement heat flow took place. The actual temperaturesare calibrated

with the heat flow history through the sedimentary column.

Well CENTRAL(thrusting)

• Burdigalian

sedimentation has a distinct record on the subsidence curves, suggesting indeed a rifting -

related infill• Following thermal subsidence

is observed only for the Badenian

time

span being overprinted by the Sarmatian

transtension

(related to the Timok

fault system activity); that’s why the tectonic subsidence curves look like two successive rifting stages

• End Miocene -

Pliocene time span reflects a gradually subsidence decrease and

thechange to a large-scale uplift of the basin, coeval with that of the South Carpathians belt

III) RESULTS (1): subsidence curves

Well NORTH Well NORTH Well CENTRAL Well CENTRAL Well WEST Well SOUTH



Well WEST

Well NORTH(thrusting)

Well WEST

Well NORTH(no thrusting)

β

= 1.1


Well NORTH(thrusting)

β

= 1.4

III) RESULTS (2): expulsion & maturity

The choice of the best-fit model relies on the comparison between observed and theoreticaltectonic subsidence

curves (water-filled basin) in order to estimate as precise as possiblethe amount of rifting.

OBSERVED TECTONICSUBSIDENCE

THEORETICAL TECTONIC SUBSIDENCE

Burdigalian

riftingβ

= 1.1

Oligocene –

Burdigalian

riftingβ

= 1.1

Burdigalian

riftingβ

= 1.4

Oligocene –

Burdigalian

riftingβ

= 1.4

• Giving the supposed Oligocene thickness, the rifting appears to

cover basically theBurdigalian

time span, as pointed out by the tectonic model• Whilst β

of 1.1 seems to account for the syn-

& post-

(Badenian) rift subsidence in wellsCENTRAL & SOUTH and partly NORTH, the subsidence in well WEST is

in-betweenβ

of 1.1 and 1.4• Keep in mind that by neglecting the “true”

paleobathimetry, we have underestimated the“observed tectonic subsidence”• These values are generally supported by the actual temperatures

as well

III) DISCUSSION (1): best-fit model

Well WEST

Well NORTH Well CENTRAL

Well SOUTH

It appears that the amount of thinning increases fromS to N, from β

= 1.1 to β

= 1.25*, especially closer to theTimok

fault, which is interpreted as a transfer faultduring the Burdigalian

rifting.

*Which best accounts forthe tectonic subsidencein Well WEST

Well WESTβ

= 1.25

III) DISCUSSION (2): chosen β

values

β

= 1.25

Well WEST

Well North

Well Central

Well West

1.1

>1.11.25

Well South1.1

Well WEST, β

= 1.25


β

= 1.1

Well NORTH(thrusting)β

= 1.1

The cooling effect at mild rifting predominates for ~ 5 Ma, even

at the basin bottom.The

thermal re-equilibration (re-heating) began in Sarmatian, when the new increase in the sedimentation rate had impact only at shallow levels.

III) DISCUSSION (3): blanketing effect

A more complex thermal evolution is recorded when thrusting took place: cooling at depth is more severe and heating at shallow levels occurs for a while. Consequently, at mild rifting, the maturation advance of the Oligocene Fm. is delayed mainly due to the Burdigalian

deposition.

Stronger rifting obliterates at depth the cooling due to high sedimentation rate; thus, the Oligocene Fm.is increasingly heated during burial.

~ 7 Ma5.8 km

~ 3 Ma5.6 km

~ 6 Ma6.2 km

• Oligocene Fm. enters progressivelyearlier into the oil window from N to S

• Onset of generation is controlled by:1)

The depth of the basin originating fromthe Burdigalian

rifting;2) The Late Badenian

-

Sarmatian

thrust-related burial.

• Under the N-most oblique thrust theentrance into the oil window is expectedto be as old as the Sarmatian

(or even theLate Badenian)

• The Oligocene base entered into theoil window at depths between 5.6 km and6.2 km regardless the time. Interpolatingthese modeled values to the entire basinis not straightforward-made as it dependson the interplay between local magnitudeof rifting, thrusting and blanketing effect.

III) CONCLUSION (1): oil-window onset

~ 70 %

~ 30 %

~ 60 %

• The cumulative expelled HCs

rangebetween 30 % and 70 % in the modeledwells

• Most of the Oligocene Fm. is presentlybeing into the oil window, with a peak ofoil generation around the Well WEST

• Under the N-most oblique thrust, theOligocene Fm. is probably generatingcondensate / wet gas, possibly dry gasfarther north, as the depth apparentlyincreases in this direction

• Short-range up-dip HC migration is likelyto have taken place towards S and SW

III) CONCLUSION (2): expelled HCs

• The main misfit may arise from the heat flow at the bottom of the sedimentary column.Despite of the high sedimentation rates and the good fit with the present temperature profile,the deep temperatures may have been higher particularly during the Tertiary. A thinnerpre-Oligocene sequence, a higher radioactive heat production (taking

into account theimportant granitic

bodies of the nearby outcropping Danubian Autochthonous) and / or athinner lithosphere would increase the heat flow, therefore the geothermal gradient.Consequently, the maturity rank of the potential Oligocene source rocks may be higher andthe entrance in the oil window may have occurred earlier than shown in the previous slide.

• On the other hand, there may have been some counteracting factors, as lateral dissipationof the rifting heat, or further cooling by the Pontian

high sedimentation (according to recentpaleomagnetic

datings*, the Pontian

spans only ~ 1 Ma, thus implying a sedimentation ratealmost three times higher than considered herein).

• Uncertainties related to the variation of the paleobathimetrry

through time could require alarger amount of rifting, therefore the thermal anomaly produced

by thinning could be greater

• Further uncertainties are related to the original configuration

of the Burdigalian

basin as thesubsequent strike-slip displacements have modified the northern margin.

• The thickness of the main potential source rocks (Oligocene) is

possibly underestimatedespecially to the north (around Well NORTH); if true, it implies

that a thicker sequence hasentered into the oil window and started to generate earlier than

the Late Pontian.

* Vasiliev et al., 2005; Dupont-Nivet et al., 2005

III) UNKNOWNS & POSSIBLE MISFITS

IV) 2-D & 3-D MODELING EXAMPLES (1.1)ALBERTA FOOTHILLS -

CanadianRocky Mountains & foreland

Figures of this 2-D modeling study are from Faure et al., 2004.

Modeled geological transect

First step is the structural restoration (balancing) of the selected transect (up) to the top of passive margin sequence –

base Cretaceous-

(down).

IV) 2-D & 3-D MODELING EXAMPLES (1.2)

Vitrinite

sampling data (up) and 1-D modeling in (pseudo) wells in foreland (down) help to constrain the amount of erosion taking place both in the deformed orogenic

wedge and the undeformed

foredeep

basin.


The initial state resulting from balancing is then input in the 2-D forward modeling* which couples kinematics with thermal field evolution such that the temperature history as well as the evolution of the maturity rank of potential source sequences

can be eventually accurately predicted.

* THRUSTPACK software developed by IFP (“Institut Français du Pétrole”)

Several time snapshots of the progressive shortening.


IV) 2-D & 3-D MODELING EXAMPLES (2)Both figures are from Johanessen et al. (2002) using TEMIS 3D* package.

NORTH SEA –Brent structure

* Trademark of IFP (“Institut Françaisdu Pétrole”) & BeicipFranlab

MODELING MEANS

TO SEE THE FOREST,

NOT THE TREES!

NOTA BENE!

Documents

BASIN ANALYSIS AS A TOOL TO REDUCE EXPLORATION RISKS ANALYSIS AS A TOOL TO REDUCE...BASIN ANALYSIS AS A TOOL TO REDUCE EXPLORATION RISKS Bucharest Geoscience Forum March 2. nd . 2007