Characteristics of penecontemporaneous diagenesis in

Indian Journal of Geo Marine Sciences

Vol. 49 (12), December 2020, pp. 1819-1830

Characteristics of penecontemporaneous diagenesis in sequence and its control

over pore distribution — A case study of reef-bank sediments of upper

Ordovician in Tazhong area, Tarim basin, China

J Zheng*,a,b, Z Y Wangb, Z G Liua, Y W Zhanga & B Lia

aSouthwest Geophysical Research Institute of BGP, CNPC, Chengdu – 610 084, China bInstitute of Geosciences and Technology, Southwest Petroleum University, Chengdu – 610 500, China

*[E-mail: [email protected]]

Received 25 February 2020; revised 03 October 2020

Penecontemporaneous diagenesis of different lithologies in sequence has an uncertain correlation with pore distribution

inside the reef-bank reservoir. The article divides the Lianglitage Formation into 3 third-order sequences, 5 parasequence

sets, and 25 parasequences through the characteristics of ln(Th/U) ratio curve and sedimentary cycles, where the

parasequence thickness has an obvious correlation with sedimentary facies. The rock, mineralogical and geochemical

characteristics inside the multiple reef-bank sediments show that the penecontemporaneous diagenesis and eogenetic

seawater cementation are widely developed in it. Atmospheric diagenetic lens, formed by penecontemporaneous diagenesis,

are concentrated on the top of descending cycles of subsequence, and rich selective dissolved pores therein can contribute 3

to 6 % porosity to the reef-bank reservoir and also provide places and channels for the reconstruction of alien fluids and

tectonic movement in the late burial. The early dissolved pores formed by the penecontemporaneous diagenesis are the main

reservoir spaces and can also provide the prerequisites for the later diagenesis reconstruction, so the penecontemporaneous

diagenesis is the basic reason for the development of high quality reef-bank reservoir. The high-porosity sections are

concentrated on the top of the atmospheric diagenetic lens, revealing that the formation and distribution of selective

dissolved pores are controlled by the descending cycles of parasequences. The comprehensive analysis indicates that the

favorable development zones of selective dissolved pore as lens shape or belt shape are discontinuously distributed inside

the high energy facies belt of the platform marginal facies, which are formed under the control of steep fault.

[Keywords: ln(Th/U) curve, Parasequences, Penecontemporaneous diagenesis, Selective dissolved pores, Tazhong area

Upper Ordovician]

Introduction

As a special carbonate and complicated reservoir

system, reef-bank is one of the main target of global

oil and gas exploration1,2. More and more research

shows that the sequence interface is closely related to

the early diagenesis caused by the decrease of sea

level and plays an important role in improving the

porosity of reservoir3-7. Therefore, the identification

of sequence interface and the establishment of

sequence framework are the key to predict the

development zones of early dissolved pores8. Natural

gamma-ray spectrometry logging data show that

Th, U and K contents are related to rock type and

late transportation, sedimentation, and diagenesis9,10.

Domestic and overseas scholars have applied the

natural gamma-ray spectrometry logging data to

determine the type and content of minerals in clay11,

identify the weathering crust12, restore the

palaeoenvironment13, and sequence divisions14,15 but

have not applied the data to study sequence division

in the deep-depth and in complicated lithological

changes of reef-bank facies carbonate rock. Since

carbonate rocks have only minor changes between

their physical properties16, it is difficult to divide

sedimentary sequences and system tracts through

conventional log analysis, while the curves and ratio

curves of Th, U, and K elements in natural gamma ray

spectrometry logging are more effective and accurate

in the study of sequence stratigraphy.

The study area is located in the middle of the

central uplift belt zone of the Tarim basin, China

(Fig 1). Though large amounts of oil and gas have

been detected in the reef-bank reservoir of Lianglitage

Formation in the study area, there is still controversy

about the reasons for the formation of high-quality

reef-bank reservoirs. One view is that reef-bank

reservoir in the study area were formed by the

transformation of penecontemporaneous diagenesis17,

INDIAN J GEO-MAR SCI, VOL 49, NO 12, DECEMBER 2020

1820

while another view is that the reef-bank reservoirs

were formed by the transformation of deep fluid

with different features in the late burial18,19. However,

reef-bank reservoirs in the Lianglitage Formation

are the result of multi-factor20 and multi-period

transformations with uncertain details, in particular,

reservoir formation mechanisms by penecon-

temporaneous diagenesis lack detailed research.

The difference of penecontemporaneous diagenesis

between different lithologies in sequence is the key to

reveal the pore distribution inside the reef-bank

reservoir. The article performs sequence division on

the Lianglitage Formation in combination with

sequence stratigraphical principles and by the natural

gamma-ray spectra analysis method, discussing the

correlation of penecontemporaneous diagenesis with

sequence interfaces as well as control over pore

formation and distribution.

Materials and Methods

The Th element curve in natural gamma ray

logging can clearly indicate the change of siliceous

mud content inside the carbonate, which can better

reflect the sedimentary cycle. Because of the high

correlation between Th element and K element, the

variation of sedimentary components in the formation

is consistent, but the correlation between U element

and Th element is poor, so the ln(Th/U) ratio curve is

further made to divide the sequence. The ratio change

of ln(Th/U) ratio curve can indicate sea-level

fluctuation or the change of depositional base-level

cycles, so the ln(Th/U) curve can be used to

accurately identify the interface of different order

sequences and system tract. The value, ln(Th/U) >

1.94 represents shallow water or an exposed

depositional environment. 0.693 < ln(Th/U) < 1.94

represents a depositional environment of shallow

sea-continental shelf alternation. ln(Th/U) < 0.69

represents a deep-water depositional environment. In

consideration of the correlation of ln(Th/U) value

with depositional environments, the article recognize

interfaces of sequences and system tracts through

ln(Th/U) curve value, range, form, contact relation,

assemblage types, carbon and oxygen isotopes,

sedimentary cycles as well as further establishes

sequence framework of the Lianglitage Formation.

Fig. 1 — Overlap distribution map of the structure and sedimentary facies of Lianglitage Formation in Tazhong area, Tarim Basin,

NW China

ZHENG et al.: THE CORRELATION BETWEEN DIAGENESIS AND PORE DISTRIBUTION

1821

Tazhong 12 well is representative for studying

sequences of Lianglitage Formation by complete

stratum development, length of coring section, and

high core yield rates.

Results

The establishment of the sequence stratigraphic framework

The Lianglitage Formation can be subdivided into

five lithologic sections. The second section is the

main development period of reef-bank facies

carbonate rock21. Previous studies surmised that the

Lianglitage Formation belongs to a secondary-order

sequence. It can be known from top-bottom ln(Th/U)

value analysis for the Lianglitage Formation of

Tazhong 12 well that the average ln(Th/U) value at

4693 ~ 4780 m and 4996 ~ 5050 m is more than 2.99

and 1.94, respectively, revealing that both the strata

are in a sedimentary environment with oxidizing

conditions. 4693 m and 4996 m can be taken as two

interfaces for third-order sequences inside the

secondary-order sequence, thus the Lianglitage

Formation is divided into 3 third-order sequences

from top to bottom. ln(Th/U) and δ13C value of the

Lianglitage Formation show decreasing trends

from top to bottom, revealing a gradual rising

trend in sea level. ln(Th/U) value of high stand

system tract of parasequence sets is slightly larger

than that of transgressive system tract. According to

the variation regularity of ln(Th/U) curve and oxygen

and carbon isotopes in the Lianglitage Formation, five

smaller levels of parasequence sets can be identified

in the whole three third-order sequences, and

25 parasequences can be further divided based on the

characteristics of lithofacies and sedimentary cycles

(Fig. 2).

Sequence stratigraphic correlation

As the third section of most of the single wells is

not drilled, the research focus of this paper is the first

section to third section. Figure 3 shows that 3

parasequence sets and 15 parasequences are identified

from first to third section of Lianglitage Formation

according to the above parasequence division

standards and single-well parasequence division, and

the wells are comparable. Nine parasequences

identified in the third section which have good

symmetry show the retrogradation/ progradation

parasequence superposition style. Four parasequences

are identified in the second section, the descending

cycles are mainly developed with grain-shoal and

organic reef sediments. The thickness of a single

subsequence clearly increases during the evolution

from open platform facies to gentle slope platform

margin facies to steep slope platform margin facies,

and the rule for the change has a close relationship to

the fact that compared with other facies belts,

there are more grain-shoal and organic reef

sediments developed in the steep slope platform

margin belt, and the sedimentation rates is faster.

Descending semi-cycles inside second section is

larger than rising semi-cycles, indicating that the sea

level continued to drop during the second section

sedimentary period, which is the main development

period for reef-bank sediments. Two parasequences

identified in the first section mainly show an

approximately symmetrical retrogradation/ progradation

parasequences superposition style.

The development characteristics of reef- bank under the

control of sequence framework

The three sets reef-bank sediments that developed

in the third section are mainly composed of arene

bank sediments and sporadic developments of small-

scale organic reef sediments. During the second

section, depositional period is mainly formed by

vertical continual aggradation of high system tracts,

and four sets of reef-bank sediments developed with

large thickness of a single reef-bank sediments. The

residual area of first section is mainly formed by

vertical progradation, and two sets of reef-bank

sediments developed with both a thickness and scale

smaller than second section. In plane view, affected

by the property and intensity of fault activity, the

platform margin type evolved from a gentle slope to a

steep slope along the Tazhong No. I fault belt from

west to east direction, and the sediment types evolved

from sets of mound-bank to reef-bank, with a larger

thickness, scale, and better inter-well continuity.

Analyses of the diagenetic background in the reef-bank facies

carbonate rock

The sedimentary period of the Lianglitag

Formation is dominated by seabed diagenetic

environment to atmogenic diagenetic environment

and is characterized by atmospheric exposure

corrosion. Subsequently, the late Ordovician

diagenetic stage mainly developed shallow burial

compaction and cementation. During the Silurian to

Hercynian diagenetic stage, the pores are ruptured and

filled under the influence of the pressure solution, and

along with the oil and gas injected, tectonic disruption

and hydrothermal recrystallization occurred in the

reef-bank sediments. During the Indosinian to


1822

Himalayan diagenetic stage, sediments are mainly

developed from tectonic disruption and burial

dissolution. Due to the oil and gas injected and

accompanied by gas invasion, the different types of

reservoirs and hydrocarbon reservoirs can be

formed in the reef-bank sediments. The early selective

dissolved pores inside the reef-bank sediments

are the main reservoir spaces. According to the

comprehensive analysis, the penecontemporaneous

diagenesis is the basic reason for the development of

Fig. 2 — The sequnence division of Lianglitage Formation in Tazhong area, China


1823

high quality reef-bank reservoirs, and the

development of early dissolved pores can provide the

space and condition for fluid activity during the

further later diagenetic reconstruction.

The characteristics of diagenetic facies

The sedimentary facies control the structure and

lithology of the rocks, which can also control the

developing degree of the primary pores and greatly

affect the scale and intensity of later diagenetic

transformation. Microscopic observation shows that

the dissolved pores are developed inside the bioclastic

limestone, bioclastic calcarenite, framestone and

bioclastic bindstone, which are formed in the

medium-high energy sedimentary environment of

platform margin, and the types of dissolved pores are

mainly residual intragranular pore, moldic pore and

intergranular pore. The cryptite, cryptomonas cryptite

and micritic granula limestone, formed in the low

energy sedimentary environment of platform interior,

are characterized by the dense lithology and poor

porosity. The comprehensive analysis shows that the

granular- supported limestone is more susceptible to

the later diagenetic reconstruction than the mud-

supported limestone, especially, the granular

limestone associated with bioclast and skeletal grain

is most likely to be reformed, forming a large number

of selective dissolved pores.

Characteristics of penecontemporaneous diagenesis in

sequence framework

Diagenetic environment analysis

The third section of Lianglitage Formation

depositional period belongs to a transgressive system

Fig. 3 — Comparison of distribution of parasequence and the atmospheric diagenetic lens inside the different sedimentary

geomorphology units in Tazhong area


1824

tract. Sea level was relatively high in this period, and

the diagenetic environment mainly belonged to the

seabed underflow environment with the development

of seabed underflow cementation. The second section

depositional period belonged to high stand system

tract when sea level dropped relatively, and sediments

mainly present a progradation superposition of

sedimentary pattern. When seawater, meteoric

water and mixed water flowed towards the basin,

reef-bank sediments were transformed by a series of

diagenesis to form large effective protogenetic-

penecontemporaneous reservoir spaces and meteoric

freshwater cement. When the sea level dropped

sharply, reef-bank sediments were transformed by

atmospheric fresh water dissolution to form a large-

scale dissolution fracture-cavity system. The first

section was formed in the early period of the

transgressive system tract when a large number of

seabed cements were developed inside the seabed

diagenetic environment.

The rock and mineralogical evidence of penecontemporaneous

diagenesis

The observation of 532 m sediment cores and

identification of 381 sheets from 18 single wells show

that a large number of penecontemporaneous

diagenesis phenomenon appeared inside the reef-bank

sediments, which has the following important

identification indicators. First, the suspended

cement representing the diagenesis fabric

characteristics of atmospheric vadose zone (Fig. 4a),

and the foliated cement, horse-tooth cement, and

equiaxed fine-granular cement represented the

diagenesis fabric characteristics of atmospheric

underflow-zone are identified inside the biolithite and

grain limestone (Figs. 4b, c). The development of

atmospheric water cements are an important evidence

that reef-bank sediments are exposed to atmospheric

freshwater environments and are reformed by

penecontemporaneous diagenesis. Second, the

dissolution has obvious fabric selectivity, which is

mainly occurred inside the granulars and the first

generation of fibrous calcite, and the latter is

characterized by the dissolution of the first phase of the

seabed rim calcite cement, and it is unconformable

contact with the later granular calcite (Fig. 4d). Third,

the fabric selectivity intragranular dissolved pore

(Fig. 4e), moldic pore (Fig. 4e), intergranular dissolved

pore (Figs. 4c, e, f), and non-fabric selective dissolved

pore as well as small-sized vugs, karrens, and dissolved

fracture (Fig. 4g) are developed inside the reef-bank

sediments, and geopetal structure are shown locally

(Fig. 4h). Fourth, the penecontemporaneous diagenesis

is mainly developed in the reef-mound and grain-bank

sediments. Due to the control of multi-period sea level

changes, the multiphase intragranular dissolved pores

and moldic pores are developed inside the sediments

and are evenly overlapped together (Fig. 4i). Fifth,

cathode luminescence of intergranular dissolved pore

walls inside the grain limestone shows the non-

luminance or dark-dark orange light (Figs. 4j, k), which

is similar to that of surrounding rock, revealing that the

fluid that causes the formation of dissolved pores shall

be pore water with similar properties to normal late

Ordovician seawater. Sixth, there are occurrences of

black asphalt and hydrocarbons on the walls of early

dissolved pores (Fig. 4l), indicating an early formation

time of the reservoir space and early oil and gas filling.

Geochemical evidence of penecontemporaneous diagenesis

All the samples are tested in the relevant laboratory

at the Rock and Mineral Research Institute of the

Southwest Petroleum University. The JCXA-733

electronic probe analyzer is adopted to test trace

element of 20 calcite cement samples taken from

grain limestones. The data showed that the contents

of trace elements Sr, Fe, Mn, Na and K were lower,

with average values of 99×10-6, 89×10-6, 139×10-6,

278×10-6, and 73×10-6, respectively (Table 1). Low

content of trace element Sr is the result of meteoric

freshwater dilution. Some samples therein do not

contain trace element Fe and Mn or its content are

very low, revealing that the calcite cements inside

intragranular and intergranular dissolved pores were

formed in the near surface oxidized or weakly-

oxidized diagenetic environments.

Previous studies have indicated that the carbon

isotope of micrite matrix has been retained the original

chemical characteristics of seawater22,23, the oxygen

isotopes greatly changed by the recrystallization and

cementation of post-diagenesis24,25. The Isoprime

100 mass spectrometer is adopted to test 33 samples of

Lianglitage Formation for carbon and oxygen isotopes.

The test data shows that δ13C and δ18O value in

calcsparite grainstone are close to those in micrite,

which are both distributed in normal ranges of stable

marine facies between -8 and -4 ‰ (Table 2). The

linear correlation coefficient of δ13C and δ18O value in

sparry grainstone are R2 = 0.018 (Fig. 5). The poor

correlation indicates that carbon and oxygen isotope in

grainstone samples undergo minor alterations in post-

diagenesis, revealing that the calcite cements inside the


1825

vugs are mainly formed in the early seawater

diagenetic environment.

Control over atmospheric diagenetic lens by

penecontemporaneous diagenesis in sequence

Inter-well comparisons show that reef-bank

sediments inside descending cycles of parasequence

are easily exposed to atmogenic environments to form

atmogenic diagenetic lenses, which are rich in

selective dissolution pores and calcite cements

(Fig. 3). Rising cycle of parasequence has deep

water and a compact lithology, causing the poor

development of atmospheric diagenetic lens.

Fig. 4 — The macroscopic and microcosmic characteristics of diagenesis inside the Lianglitage Formation in Tazhong area: a) suspended

calcite cement, algae calcarenite, TZ16 well, 4620 m, plainlight; b) foliaceous isopachousrim, and equiaxialgranular-blocklike calcite

cement, TZ161 well, 4476.28 m, plainlight; c) horse tooth shape calcite cement, intergranular dissolution pore, bioclastic calcarenite,

TZ83 well, 5457.61, red casting; d) the first phase of the seabed rim calcite cement is dissolved, and it is uncomformable contact with the

later granular calcite, algae calcarenite, TZ161 well, 4227.3 m, plainlight; e) intragranular dissolution pore, calcsparite calcarenite TZ721

well, 5036.44 m, red casting; f) intergranular dissolved pore, oospararenite, TZ30 well, 5025.12 m, red casting; g) dissolved fracture and

saccate cave full-filled with vadose silt and mud, calcarenite, TZ24 well, 4453.1, core; h) geopetal structure, micrite calcarenite, TZ16

well, 4251.25 m, plainlight; i) intragranular and moldic dissolution pore, bioclastic limestone, TZ72 well, 4968 m, core; j & k) The value

of multiple trace elements inside the clitellum A, B, C are very low, the cathodeluminescence image show a lowering light, TZ54 well,

5755.31 m, cathodeluminescence; l) intragranular dissolution pore, the organic matter exists in the wall of the dissolution pore, TZ44,

4829.5 m, plainlight


1826

Therefore, parasequence interface controls the

distribution and scale of atmospheric diagenetic lens,

which has a concentrated distribution on the upper of

descending cycles in parasequence. In the facies belt

direction from the open platform to the platform

margin, the number of atmospheric diagenetic lens

evolves from 1~2 sets to 3~8 sets and shows obvious

increase of single-set thickness. Along the direction of

the Tazhong No. I fault zone, the high-steep platform

margin of the eastern section of No I fault zone has

more sets and larger thickness of atmospheric

diagenetic lens than the steep slope platform margin

of the central section of No 1 fault zone and gentle

platform margin of the western section of No 1 fault

zone, and the fact is there is a close relationship

between higher palaeogeomorphology, longer

exposure time of reef-bank sediments, and a higher

intensity of penecontemporaneous diagenesis. The

thickness of atmospheric diagenetic lens generally

ranges from 7 to 35 m, taking up about 30 % of the

thickness of the parasequence. The residual effective

dissolved pores inside atmospheric diagenetic lens

can contribute 3 ~ 6 % porosity to reef-bank

reservoirs and provide places and conditions for

dissolution in the later stage of burial, forming high-

quality reservoirs.

Influence of penecontemporaneous diagenesis in sequence on

pore distribution

Microscopic observations of 325 cast thin section

from the Lianglitage Formation show that the

Table 1 — The trace elements test results of different types of calcite cements inside grain limestone from the Lianglitage Formation in

Tazhong area, China

Well Depth

(m)

Cement type Cathodelu-minescence Trace element test results (×10-6)

Sr Mg Fe Mn Na K

TZ24

4690.8

Inte

rgra

nula

r ca

lcit

e ce

men

t

Fibrous calcite disphotic 0 1720 113 0 120 48

cone shaped crystal powder calcite disphotic 0 1170 0 0 0 0

The center part of cryptomerous calcite disphotic 161 1289 0 0 240 17

The frontier part of cryptomerous calcite Dark orange

Light

340 1065 0 0 390 40

mesocrystalline zone calcite (zone A) Dim light 0 1428 452 502 227 59

mesocrystalline zone calcite (zone B) Orange light 0 1090 190 325 521 73

mesocrystalline zone calcite (zone C) Dim light 273 1371 0 250 416 0

mesocrystalline zone calcite (zone D) Dark orange

Light

328 1580 0 30 0 97

Ingotism calcite Dark orange

Light

60 1320 40 0 361 0

TZ82

5441.8

Granular calcite Dim light 0 1594 172 224 185 0

Bladed calcite Dark orange

Light

172 1792 72 0 0 51

mesocrystalline zone calcite (zone A) Dim light 0 1284 194 421 273 42

mesocrystalline zone calcite (zone B) Dim light 249 1627 0 311 0 0

mesocrystalline zone calcite (zone C) Dim light 127 1164 58 296 163 31

TZ44

5017 Acinose calcite disphotic 0 770 0 0 10 0

Bladed calcite disphotic 0 1056 163 0 79 0

Bladed calcite Dim light 89 983 0 429 142 0

TZ26

4281.6

coarse bladed calcite disphotic 186 1000 28 0 0 0

Canine tooth-Like calcite Dim light 0 1327 121 0 0 39

Acinose calcite disphotic 0 994 193 0 69 52

TZ24

4693

Intr

agra

nula

r ca

lcit

e ce

men

t Fibrous calcite disphotic 58 1427 0 21 117 0

cryptomerous calcite Dark orange

Light

241 989 0 0 320 28

TZ44

5017

Foliaceous calcite Dim light 269 1380 73 156 361 0

Acinose calcite disphotic 0 1694 0 0 219 27

TZ26

4281

cryptomerous calcite Dim light 0 1426 0 69 0 41

granular-blocklike calcite Dim light 94 1410 94 148 104 0

TZ72

5011 Fibrous calcite Dark orange

Light

0 1081 37 0 69 71

Foliaceous calcite disphotic 204 1478 94 216 108 0


1827

Table 2 — The carbon and oxygen isotopes test results of

different lithology from the Lianglitage Formation in Tazhong

area, China

Well Depth

(m)

Lithology δ13C-

VPDB(‰)

δ18O-

VPDB(‰)

TZ24 4456.3

Gra

in s

upport

ed l

imes

tone

Calcsparite

calcarenite

0.75 -6.48

4468.1 bioclastic calcarenite 1.23 -5.80

4470.4 Calcsparite

calcirudite

1.02 -6.13


4512.4 Biolithite 0.94 -4.92

TZ72 5007.5 bioclastic calcarenite 0.82 -6.28

5025.6 calcarenite with

oncolithe

2.17 -4.94

TZ86 6272.2 oolitic limestone 1.39 -5.38

6583.9 Calcsparite

calcarenite

0.94 -6.12

TZ82 5365.1 Biolithite 2.21 -5.81

5443.5 calcarenite with

oncolithes

1.77 -5.94

5485.2 Calcsparite

calcarenite

0.99 -6.24

5448.1 algal limeston 1.29 -6.52

TZ44 4856.8 Biolithite 1.35 -6.02


4850.2 Biolithite 1.17 -5.92

TZ451 6111.2 oolitic limestone 1.96 -5.91

6114 Calcarenite with

oolite

2.21 -6.37

TZ44 4852.4

mud s

upport

ed l

imes

tone

Cryptite 1.28 -5.34

4923.8 Cryptite 1.09 -5.19

5017.1 Cryptite 2.15 -4.83

TZ30 5076.5 Cryptite 2.41 -4.69

5092.9 Cryptite 1.89 -5.19

TZ82 5381.2 Cryptite 1.09 -6.36

5434.6 Cryptite with

bioclastic

1.12 -4.62

5458.9 Cryptite 0.98 -5.85

5490.7 Cryptite 0.82 -6.36

TZ72 4962.5 Argillaceous cryptite 1.37 -7.21

5021.4 Cryptite 1.19 -6.25

5041.9 Cryptite 1.52 -6.01

5089.3 Cryptite 1.27 -6.51

TZ162 4321.6 Cryptite 1.10 -5.78

4426.1 Cryptite 0.95 -5.43

TC1 4005.3 Cryptite 1.75 -4.38

4021.8 Cryptite 2.32 -4.62

4034.2 Cryptite 1.28 -4.79

contribution rate of intergranular and intragranular

dissolved pores was 63 % to the plane porosity.

Although microfractures have a high rate of

occurrence, it contributes less than 12 % to the plane

porosity. Moldic pores contribute 9.5 % to the plane

porosity, and non-fabric selective dissolved pores and

other dissolved pores contribute 15.5 % to the plane

porosity. Thus, selective dissolved pores formed by

penecontemporaneous diagenesis are the contributor

of matrix pore. By comparing ln(Th/U) and porosity

change rules of the Lianglitage Formation in TZ62

well block, 6 parasequence cycles are identified

within a range of 60 m (Fig. 6). The thickness of a

single parasequence is distributed in the range of

8 ~ 14 m, showing a large coincidence with the

change rules of porosity. The high-porosity section

is mainly distributed in the upper part of the

descending cycles of subsequence. Inter-well

comparative analysis shows that other wells also have

similar characteristics, revealing that sea level drop

periods of parasequence cycles have a close

relationship to the development of high-quality

reservoirs rich in selective dissolved pores.

Discussion

The Tarim basin is formed by multi-episodic

tectonism26, the transformation mechanism for

reef-bank reservoirs by tectonism and karstification

of different periods and properties after the

sedimentation period are the key to study on reservoir

formation mechanism. Microscopic observations

show that there is basically no development of

selective dissolved pores inside TZ45-TZ451-TZ86

well block at the gentle slope platform margin in the

western section of the Tazhong No. I fault zone. More

selective dissolved pores develop inside the TZ54-

TZ826-TZ82 well block at the steep slope platform

margin in the middle section, but most dissolved

pores are filled by calcite cements. The selective

dissolved pores inside the TZ72-TZ44-TZ24 well

Fig. 5 — The correlation of carbon and oxygen isotopes inside

various lithology from Lianglitage Formation in Tazhong area


1828

block at high and steep platform margin have the best

development degree and effectiveness, with the

effective porosity of dissolved pores reaching 10 %.

Therefore, the development degree and later

preservation degree of selective dissolved pores differ

with different positions of fault zones and present a

large plane difference.

Microscopic observations show that some well

with a large number of effective early dissolved

pores are distributed in high and steep platform

margin, while other wells with early dissolved

pores filled by calcite are distributed in gentle

slope platform margin. It is analyzed that the

platform margin which is formed by the control

of the steep fault presents a high and steep

sedimentary landform with low water salinity,

which is favorable for the development of selective

dissolved pores, and the early dissolved pores are

well preserved because of the injection of early

hydrocarbon. The platform margin controlled by

relatively low-gentle faults, or faults with weak

activity presents low, wide, and gentle sedimentation

landforms with high water salinity, which suffered a

strong transformation of submarine cementation and

later cementation, resulting in most of the early

dissolution pores are filled and plugged by calcite

cement (Fig. 7). It is inferred from the space and

time differences of activities of the Tazhong No. I

fault zone that further refinement is required to

study controlling sedimentation and diagenesis by

faults. The high and steep platform margin is a

favorable reservoir development zone. Further

study is needed to ascertain whether the formation

mechanism of effective reservoirs is related to

burial dissolution in the area in regards to no

development of selective dissolution pores inside

TZ45-TZ451-TZ86 well block at the low and gentle

platform margin.

Fig. 6 — The comparison of parasequence cycle and high porosity section in Lianglitage Formation, Tazhong eastern areas


1829

Conclusion

Based on the comprehensive analysis of the

ln(Th/U) curve, carbon and oxygen isotopes, and

sedimentary cycle characteristics, the Lianglitage

Formation in Tazhong area can be divided into 5

parasequence sets and 25 parasequences. Compared

with the open platform facies, the thickness of

parasequence inside the steep slope platform margin

facies is larger. The descending hemicycle of the

single parasequence of the second section is obviously

larger than the rising hemicycle and refers to the main

development period of the reef-bank sediments.

Penecontemporaneous dissolution pores are the main

contributors of matrix porosity for the reef-bank

reservoir of the Lianglitage Formation, with the

effective porosity up to 3 ~ 6 %, and these pores also

provide dissolution conditions and places for later

burial dissolution, thus playing an important

controlling role in the output of effective reef-bank

reservoirs. Atmospheric diagenetic lens is

concentrated development on the upper part of the

descending cycles of parasequence. The effective

pores are mainly distributed on the top of the

atmospheric diagenetic lens. Comprehensive analysis

show that there is the development zone of selective

dissolution pores inside high and steep platform

margins in TZ54-TZ826 well block and steep

platform margins in TZ44-TZ24 well block, and these

areas represent the main area of oil and gas

exploration and development.

Acknowledgements

Professor Xiucheng Tan and Professor Fanghao Hou

proposed some valuable amendments to this paper, and

the Exploration and Development Research Institute of

Tarim Oilfield provided strong support to core

observation, lamina observation and sampling.

Geochemical analysis was done under the help of

Southwest Petroleum University Minerals Isotope

Laboratory and Electron Microprobe Laboratory;

Authors express sincere thanks to all of them together.

Conflicts of Interest

The authors certify that there is no conflict of

interest with any individual/organization for the

present work.

Author Contributions

JZ & ZYW designed the research ideas for this

manuscript. JZ & ZGL designed the experiment and

completed the collection of experimental samples and

discussed the control mechanism of fault and

geomorphology on diagenesis. YWZ & BL carried

out experiments and the analysis of experimental

results. JZ wrote the manuscript.

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