characteristics of penecontemporaneous diagenesis in
TRANSCRIPT
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
INDIAN J GEO-MAR SCI, VOL 49, NO 12, DECEMBER 2020
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
ZHENG et al.: THE CORRELATION BETWEEN DIAGENESIS AND PORE DISTRIBUTION
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
INDIAN J GEO-MAR SCI, VOL 49, NO 12, DECEMBER 2020
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
ZHENG et al.: THE CORRELATION BETWEEN DIAGENESIS AND PORE DISTRIBUTION
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
INDIAN J GEO-MAR SCI, VOL 49, NO 12, DECEMBER 2020
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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
ZHENG et al.: THE CORRELATION BETWEEN DIAGENESIS AND PORE DISTRIBUTION
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
4505.2 bioclastic calcarenite 1.46 -6.47
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
4891.7 bioclastic calcarenite 0.79 -6.42
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
INDIAN J GEO-MAR SCI, VOL 49, NO 12, DECEMBER 2020
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
ZHENG et al.: THE CORRELATION BETWEEN DIAGENESIS AND PORE DISTRIBUTION
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.
References 1 Al-Anzi E, Al-Mutawa M, Al-Habib N, Al-Mumen A &
Sandhu D, Positive reactions in carbonate reservoir
stimulation, Oilfield Rev, 15 (2003) 28-45.
2 Cao Y, Wang S, Zhang Y, Yang M, Yan L, et al., Petroleum
geological conditions and exploration potential of Lower
Paleozoic carbonate rocks in Gucheng Area, Tarim Basin,
China, Pet Explor Dev, 46 (2019) 1073-1306.
3 Moore C H, Carbonate Reservoirs: Porosity Evolution and
Diagensis in a Sequence Stratigraphic Framework,
(New York: Elsevier) 2001, pp. 245-287.
4 Reinhold C & Kaufmann B, Sea-level changes as controlling
factor of early diagenesis: the reefal limestones of Adnet
(Late Triassic, Northern Calcareous Alps, Austria), Facies,
56 (2010) 231-248.
Fig. 7 — Reef-bank sedimentary geomorphology and its controlling to pore distribution during the syndiagenetic stage
INDIAN J GEO-MAR SCI, VOL 49, NO 12, DECEMBER 2020
1830
5 Todaro S, Hollis C & Di Stefano P, Spongy-like porosity in
peritidal carbonates: An interaction of cyclic sea-level
oscillations, fresh water supply and sediment texture,
Sediment Geol, 333 (2016) 70-83.
6 Ying X, Xiucheng T, Zhifeng Z, Guoliang Z, Mingjie L,
et al., Middle Ordovician multi-stage penecontemporaneous
karstification in North China: Implications for reservoir
genesis and sea level fluctuations, J Asian Earth Sci, 183
(2019) p. 103969.
7 Okwara I C, Dim C I P & Anyiam O A, Reservoir evaluation
within the sequence stratigraphic framework of the Upper
Cretaceous Anambra Basin, Nigeria, J Asian Earth Sci, 162
(2020) 103708.
8 Zhang Y, Wang Z, Qu H, Luo C & Li Y, Sedimentary
microtopography in sequence stratigraphic framework of
Upper Ordovician and its control over penecontemporaneous
karstification, No. I slope break, Tazhong, Tarim block,
J Cent South Univ, 21 (2014) 735-744.
9 Aboelkhair H & Rabei M, Radioelement Mapping
and Environmental Monitoring of Surface Deposits using
Ground Gamma Ray Spectrometry of the Area Adjacent to
El-Ramlah Village, Southwestern Sinai, Egypt, Resour Geol,
62 (2012) 215-224.
10 Zhang X, Ren D H H & Sun W, Microscopic pore structure
characteristics and logging response characteristics of
different diagenetic facies reservoirs and their impact on the
distribution of high quality reservoirs, Indian J Geo-Mar Sci,
47 (2018) 567-579.
11 Aboelkhair H & Gaafar I, The Use of Gamma Ray
Spectrometry as an Aid for Uranium Exploration in Kab
Amiri Area, Central Eastern Desert, Egypt, Resour Geol, 63
(2013) 72–83.
12 Shaaban F F, A new approach for recognizing subsurface
breaks in undifferentiated sequences using natural
gamma ray spectrometry and high resolution dipmeter logs,
2nd International symposium on Geophysics, 92 (2001)
202-213.
13 Chen Z H, Zha M & Jin Q, Application of Natural Gamma
Ray Logging and Natural Gamma Spectrometry Logging to
Recovering Paleoenvironment of Sedimentary Basins,
Chinese J Geophys, 47 (2004) 1145-1150.
14 Gao D, Lin C, Hu M & Huang L, Using Spectral Gamma ray
Log to recognize High-frequency Sequences in Carbonate
Strata: A case study from the Lianglitage Formation from
Well T1 in Tazhong area, Tarim Basin, Acta Seismol Sin, 34
(2016) 707-715.
15 Sérgio L R S, Luís V D, Alcides J S C P & Ricardo L S,
Field gamma-ray patterns and stratigraphic reinterpretation
of offshore well-log data from Lower Jurassic organic-rich
units of the Lusitanian Basin (Portugal), Mar Pet Geol, 98
(2018) 860-872.
16 Buckovic D, Martinuš M, Kukoč D & Tešović B C, High-
frequency sea-level changes recorded in deep-water
carbonates of the Upper Cretaceous Dol Formation (island of
Bra, Croatia), Geol Carpath, 61 (2016) 29-38.
17 Zheng J, Wang Z, Yang H, Su D, Zhang Z, et al., The
Control Mechanism of Microtopography in the High
Frequency Sequence Framework to Reef-Bank Complex
Reservoir:An Example from the Upper Ordovician
Lianglitage Formation in Eastern Tazhong Area, Acta Geol
Sin, 89 (2015) 942-956. ( in Chinese with English abstract)
18 Shan X, Zhang B, Zhang J, Zhang L, Jia J, et al., Paleofluid
restoration and its application in studies of reservoir forming:
A case study of the Ordovician in Tarim Basin, NW China,
Pet Explor Dev, 42 (2015) 301-310.
19 Qu H, Wang Z, Yang H, Zhang Y, Yu H, et al.,
Karstification of reef-bank facies carbonate rock and its
control on pore distribution: A case study of Upper Ordo-
vician Lianglitage Formation in eastern Tazhong area, Tarim
Basin, NW China, Pet Explor Dev, 40 (2013) 552-558.
20 Zheng J, Wang Z, Zhong Z, Zhai N & Liu Y, Development
pattern and reservoir-formation mechanism of reef-bank
complex in Late Ordovician Lianglitage Formation, Tazhong
area, Tarim Basin, China, Indian J Geo-Mar Sci, 47 (2018)
269-280.
21 Liu Y & Xie R, Sedimentary Facies of Lianglitag Formation
in Tazhong Area, Tarim Basin, Chin's Manganese Industry,
34 (2016) 38-40. ( in Chinese with English abstract)
22 Metzger J G & Fike D, Techniques for assessing spatial
heterogeneity of carbonate δ13C values: Implications for
craton-wide isotope gradients, Sedimentology, 60 (2013)
1405-1431.
23 Liang H, Xu F, Xu G, Yuan H, Huang S, et al., Geochemical
characteristics and origins of the diagenetic fluids of the
Permian Changxing Formation calcites in the Southeastern
Sichuan Basin: Evidence from petrography, inclusions and
Sr, C and O isotopes, Mar Pet Geol, 103 (2019) 564-580.
24 Ando A, Kaiho K, Kawahata H & Kakegawa T, Timing and
magnitude of early Aptian extreme warming: Unraveling
primary δ18O variation in indurated pelagic carbonates
at Deep Sea Drilling Project Site 463, central Pacific Ocean,
Palaeogeogr Palaeoclimatol Palaeoecol, 260 (2008)
463-476.
25 Zhao M Y & Zheng Y, A geochemical framework
for retrieving the linked depositional and diagenetic
histories of marine carbonates, Earth Planet Sci Lett, 460
(2017) 213-221.
26 Li A, Ju L & Zhang L, Relation between hydrocarbon
accumulation and Paleo-Mesozoic tectonic evolution
characteristics of Gucheng Lower Uplift in Tarim Basin,
J Jilin Univ (Earth Sci Ed), 48 (2018) 545-555. ( in Chinese
with English abstract)