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Citation: Zhu, W.; Mo, T.; Chen, C.;
Hu, C.; Wang, C.; Shi, C.; Shi, L.; Li, P.
Characteristics and Effects of
Laminae on a Cretaceous Reservoir
in the Bozi–Dabei Area of the Tarim
Basin, China. Processes 2023,11, 2472.
https://doi.org/10.3390/pr11082472
Academic Editor: Qingbang Meng
Received: 25 June 2023
Revised: 30 July 2023
Accepted: 7 August 2023
Published: 17 August 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
processes
Article
Characteristics and Effects of Laminae on a Cretaceous
Reservoir in the Bozi–Dabei Area of the Tarim Basin, China
Wenhui Zhu *, Tao Mo, Changchao Chen, Chunlei Hu, Cuili Wang, Chaoqun Shi, Lingling Shi and Pengzhen Li
Research Institute of Exploration and Development, PetroChina Tarim Oilfield Company, Korla 841000, China
*Correspondence: zhuwenhui08@126.com
Abstract:
Reservoir heterogeneity is an important factor in oil and gas exploration and development.
It has guiding significance for favourable target optimization because it helps clarify the formation
and development characteristics of laminae: thin, alternating layers of sediment deposited in a
repeating pattern in Cretaceous sandstone reservoirs. Reservoir heterogeneity is higher when laminae
are densely distributed. For example, laminae have a significant influence on reservoir properties
in the Kelasu structural belt in the Bozi–Dabei area, Tarim Basin, China, where oil and gas wells
have very low productivity. Hence, this study attempts to develop a classification scheme based on
laminae to identify how they influence reservoir properties. Based on an analysis of cores, thin section
and logging imaging data, laminae in this area can be classified into three types: magnetite-enriched,
iron-stained argillaceous-enriched, and grain-size change. Results show that magnetite-enriched and
iron-stained argillaceous-enriched laminae are well-developed in the BZ1 and DB10 well areas in
northern Bozi. They have much lower porosity compared to a non-laminae reservoir and their lateral
permeability is greater than vertical permeability. Grain-size change laminae are well-developed in
the southern Bozi region. For the laminated reservoirs, increasing the length of the perforation interval
and the perforation density using sand or acid fracturing is an effective method for communicating
with the vertical reservoir layers, improving permeability, and increasing single-well production.
Keywords:
Kelasu structural belt; Bozi–Dabei area; Cretaceous reservoir; iron-stained
argillaceous-enriched laminae; characteristics of laminae; reservoir heterogeneity
1. Introduction
The Kelasu structural belt in the north of the Tarim Basin is the main gas source for
China’s West–East Natural Gas Transmission Project. The western Bozi–Dabei area is the
main region for increasing natural gas reserves and production. The exploration target
in this area is the Cretaceous Bashijiqike and Baxigai formations, an ultra-deep ultra-low
porosity and ultra-low permeability sandstone reservoir at least 6000 m deep. It has mostly
faulted-anticline and anticline structure gas reservoirs. It is generally believed that the
structural high part is the low stress area, which has developed natural fractures, good
physical properties, and high potential for oil and gas production. In the oil and gas
exploration and development of Bozi–Dabei area, the productivity of some wells in the
high structural position is inferior to those in the low structural position. Take BZ 102
well for example: it is located in the eastern part of the Bozi 1 structure and characterized
by a high structural position, high porosity (6–11%), and coarse sandstone particles, with
medium sandstone accounting for more than 60%. The physical properties of the reservoir
are better than those of the adjacent high-production wells, but its tested output after
reconstruction is low: daily gas production is less than 10
×
10
4
m
3
/d. For these wells,
the laminae are usually densely developed. Besides the influence of the structure, the
sedimentary environment and structure also have great influence on reservoir quality, and
reservoir heterogeneity is strong: a hot topic in oil and gas exploration in recent years.
Predecessors have classified the types of reservoir heterogeneity according to different
Processes 2023,11, 2472. https://doi.org/10.3390/pr11082472 https://www.mdpi.com/journal/processes
Processes 2023,11, 2472 2 of 13
research scales [
1
–
7
]. This paper will study the interlayer heterogeneity in the Bozi–Dabei
area from the perspective of laminae and microstructures.
The study of laminae was first recorded in 1862 by a Swedish geologist as “hvarfig
lera”, meaning “circulatory layer” [
8
]. Laminae deposits, often measured in years, can be
applied to a lot of research, such as reconstructing paleoclimates [
9
], tracking ecosystem
responses to climate change [
10
], assessing human environmental impacts [
11
], determining
intervals between volcanic eruptions and floods [
12
,
13
], assessing regional hazards, and,
most importantly, enabling accurate dating on absolute calendar year timescales [
14
].
Laminae are the most basic and smallest constituent unit of stratification. The thickness of
a single layer is usually less than 1 mm, and the maximum is several centimetres [
15
,
16
].
The thickness positively correlates with the hydrodynamic strength and material supply
abundance [
17
]. With deeper oil and gas exploration and development, the study of laminae
has received more and more attention, mainly for fine-grained sedimentary rocks such as
carbonate and shale [
18
–
23
]. It is believed that shale rich in silty laminae usually indicates
a good shale gas production layer [
24
], and silty laminae are closely related to horizontal
fractures [
25
], which may play a role in the formation of shale gas transport and storage.
Laminated muddy limestone is considered to have good hydrocarbon generation potential
and is also a good reservoir rock [19].
However, there are few related studies on laminae in sandstone, especially from the
standpoint of microstructures and reservoir physical property. Why can high oil and gas
production not be achieved in sandstone reservoirs with densely developed laminae? What
is the composition, genesis, distribution of the laminae, and their effect on the reservoir?
This paper analyzed lamina characteristics in the Bozi–Dabei area by building a scheme
to classify laminae and evaluate their influence on reservoir physical properties by using
cores, thin sections, and logging imaging data. The results of this study are also applicable
to the oil and gas exploration and development in sandstone reservoirs with dense laminae
distribution in other areas.
2. Geological Settings
The Kuqa depression began to develop from the late Hercynian period and has
experienced multiple tectonic movements. It is a Cenozoic rejuvenated foreland basin
superimposed on the Triassic peripheral foreland basin, which was developed based
on the Palaeozoic passive continental margin [
26
,
27
]. The Kelasu structural belt is the
thrust structure in the northern Kuqa Depression of the Tarim Basin [
28
,
29
], which is
divided into four sections from west to east: Awate, Bozi, Dabei, and Keshen [
30
,
31
]. The
Bozi–Dabei section of the Kelasu structural belt (Figure 1) is the main site of gas production
in the “14th Five-Year Plan” of the Tarim Oilfield [32] because of its favourable petroleum
accumulation. The hydrocarbon source rocks of the Triassic and Jurassic periods are,
respectively, lacustrine mudstones and coal-bearing strata, with large thickness, wide
distribution range, and higher maturity [
33
]. The Cretaceous Bashijiqike and Baxigai
formations are the main exploration targets for oil and gas. The lithology of the formation
is mainly fine-medium lithic feldspathic sandstone and feldspathic detritus sandstone,
interbedded with thin layers of mudstone, silty mudstone, and siltstone. The sedimentary
facies are mainly braided river delta and fan delta front deposition, which are vertically
characterized by the overlapping of a multi-stage underwater distributary channel and
mouth bar sand bodies [
34
,
35
]. The reservoir thickness is between 100 and 320 m. Thick salt
and gypsum layers of Eocene are the high-quality regional seal. The maximum thickness
is above 4000 m. The Triassic and Jurassic source rocks, Cretaceous reservoir, and Eocene
cap form an excellent source–reservoir–cap rock combination, which is favourable for the
formation of large gas fields. Laminae are widely developed in the sandstone of Cretaceous
Bashijiqike and Baxigai formations [36].
Processes 2023,11, 2472 3 of 13
Processes 2023, 11, x FOR PEER REVIEW 3 of 13
for the formation of large gas fields. Laminae are widely developed in the sandstone of
Cretaceous Bashijiqike and Baxigai formations [36].
Figure 1. Division of structural units in Kuqa Depression and the location of research area.
Since Mesozoic, the Kuqa depression presents the paern of “North Mountain and
South Basin”, influenced by multiple stages of composite uplift of the southern Tianshan
orogen [29]. This ancient geographic feature determines the distribution of Cretaceous
sedimentary facies and skeletal sand bodies. As a foreland basin [37], the ancient topog-
raphy determines the paleocurrent direction which is from north to south in the Bozi area,
with the southern Tianshan mountains as the source area. During the early Bashijiqike
formation sedimentation, the study area experienced strong tectonic activities with rapid
uplift of the orogenic belt, and significant elevation difference between sedimentary and
source area, which led to a relatively steep alluvial fan–fan delta–shallow lake sedimen-
tary system (Figure 2) with relatively low sedimental maturity. Multiple alluvial fans de-
veloped along the northern margin of the basin, connected with each other in the plane,
and the sedimentary facies exhibited changed significantly in the north–south direction.
During the middle–late of the Bashijiqike formation sedimentation, tectonic activities be-
came weak, and the uplift of the orogenic belt was slow. With the process of infilling and
levelling, the ancient topography became flaer [38–40].
Figure 2. Model of sedimentary facies in Kuqa Depression.
Figure 1. Division of structural units in Kuqa Depression and the location of research area.
Since Mesozoic, the Kuqa depression presents the pattern of “North Mountain and
South Basin”, influenced by multiple stages of composite uplift of the southern Tianshan
orogen [
29
]. This ancient geographic feature determines the distribution of Cretaceous
sedimentary facies and skeletal sand bodies. As a foreland basin [
37
], the ancient topogra-
phy determines the paleocurrent direction which is from north to south in the Bozi area,
with the southern Tianshan mountains as the source area. During the early Bashijiqike
formation sedimentation, the study area experienced strong tectonic activities with rapid
uplift of the orogenic belt, and significant elevation difference between sedimentary and
source area, which led to a relatively steep alluvial fan–fan delta–shallow lake sedimentary
system (Figure 2) with relatively low sedimental maturity. Multiple alluvial fans developed
along the northern margin of the basin, connected with each other in the plane, and the
sedimentary facies exhibited changed significantly in the north–south direction. During the
middle–late of the Bashijiqike formation sedimentation, tectonic activities became weak,
and the uplift of the orogenic belt was slow. With the process of infilling and levelling, the
ancient topography became flatter [38–40].
Processes 2023, 11, x FOR PEER REVIEW 3 of 13
for the formation of large gas fields. Laminae are widely developed in the sandstone of
Cretaceous Bashijiqike and Baxigai formations [36].
Figure 1. Division of structural units in Kuqa Depression and the location of research area.
Since Mesozoic, the Kuqa depression presents the paern of “North Mountain and
South Basin”, influenced by multiple stages of composite uplift of the southern Tianshan
orogen [29]. This ancient geographic feature determines the distribution of Cretaceous
sedimentary facies and skeletal sand bodies. As a foreland basin [37], the ancient topog-
raphy determines the paleocurrent direction which is from north to south in the Bozi area,
with the southern Tianshan mountains as the source area. During the early Bashijiqike
formation sedimentation, the study area experienced strong tectonic activities with rapid
uplift of the orogenic belt, and significant elevation difference between sedimentary and
source area, which led to a relatively steep alluvial fan–fan delta–shallow lake sedimen-
tary system (Figure 2) with relatively low sedimental maturity. Multiple alluvial fans de-
veloped along the northern margin of the basin, connected with each other in the plane,
and the sedimentary facies exhibited changed significantly in the north–south direction.
During the middle–late of the Bashijiqike formation sedimentation, tectonic activities be-
came weak, and the uplift of the orogenic belt was slow. With the process of infilling and
levelling, the ancient topography became flaer [38–40].
Figure 2. Model of sedimentary facies in Kuqa Depression.
Figure 2. Model of sedimentary facies in Kuqa Depression.
Processes 2023,11, 2472 4 of 13
3. Data and Method
In the study, 51 representative wells were selected and their cores (in total 695 m)
were observed and measured to analyse the laminae characteristics. Samples were selected
from the cores with laminae and 112 casting thin sections were made. The microscopic
characteristics of laminae in thin sections were observed under microscope and their
components were identified. At the same time, the cores were also used to identify and
calibrate the laminae on the Formation MicroScanner Image (FMI) logging imaging. Based
on these analysis, the FMI imaging logging data of 18 wells without core data were used
to assist the identification of laminae. The logging interpretation data of 69 wells and the
full-diameter physical property analysis data of 23 wells were used to analyse the influence
of laminae on reservoir physical properties.
4. Results
4.1. Laminae Classification and Their Characteristics
Based on the analysis of cores, cast thin sections, and logging imaging data, laminae
in the Bozi–Dabei area were classified into three types: iron-stained argillaceous-enriched,
magnetite-enriched, and grain-size change laminae based on colour, mineral composition,
and grain size distribution (Figure 3).
Processes 2023, 11, x FOR PEER REVIEW 4 of 13
3. Data and Method
In the study, 51 representative wells were selected and their cores (in total 695 m)
were observed and measured to analyse the laminae characteristics. Samples were se-
lected from the cores with laminae and 112 casting thin sections were made. The micro-
scopic characteristics of laminae in thin sections were observed under microscope and
their components were identified. At the same time, the cores were also used to identify
and calibrate the laminae on the Formation MicroScanner Image (FMI) logging imaging.
Based on these analysis, the FMI imaging logging data of 18 wells without core data were
used to assist the identification of laminae. The logging interpretation data of 69 wells and
the full-diameter physical property analysis data of 23 wells were used to analyse the in-
fluence of laminae on reservoir physical properties.
4. Results
4.1. Laminae Classification and Their Characteristics
Based on the analysis of cores, cast thin sections, and logging imaging data, laminae
in the Bozi–Dabei area were classified into three types: iron-stained argillaceous-enriched,
magnetite-enriched, and grain-size change laminae based on colour, mineral composition,
and grain size distribution (Figure 3).
Figure 3. Characteristic of different types of laminae in core, thin section and imaging logging.
For reservoirs with iron-stained argillaceous-enriched laminae, the cores show
brown and dark brown stripes. The thickness of single-layer laminae varies from 0.2 mm
to 1.5 mm with an average of 0.5 mm. The lithology is mainly fine sandstone and the lam-
inae are well-developed near the mudstone and mud-gravel sandstone. In the cast thin
Figure 3. Characteristic of different types of laminae in core, thin section and imaging logging.
For reservoirs with iron-stained argillaceous-enriched laminae, the cores show brown
and dark brown stripes. The thickness of single-layer laminae varies from 0.2 mm to 1.5 mm
with an average of 0.5 mm. The lithology is mainly fine sandstone and the laminae are
well-developed near the mudstone and mud-gravel sandstone. In the cast thin sections
(Figure 3), silty–fine sand and iron-stained argillaceous assemble as thin layers distributed
Processes 2023,11, 2472 5 of 13
in well-developed areas of the laminae, with a high matrix content and fine grains. The
matrix composition is mainly iron-stained argillaceous, with the colour of brown and dark
brown, which is different in mudstone. In the FMI imaging logging, the laminae show-
dark stripes, and the dark stripes become bold in the location of mud gravel development.
For reservoirs with magnetite-enriched laminae, the cores show dark black stripes.
The thickness of a single-layer lamina varies from 0.2 mm to 2 mm with an average of
0.8 mm. The lithology is consistent with iron-stained argillaceous-enriched laminae, except
that there are a lot of opaque, no cleavage, granular, or irregular dark minerals. These dark
minerals can be attracted by a magnet. With the help of the microscope, the dark minerals
are identified as magnetite. In the FMI imaging logging, the laminae have higher electrical
conductivity and show black stripes (Figure 3).
For reservoirs with grain-size change laminae, the cores show no obvious colour and
composition variation. The thickness of a single layer varies from 0.5 mm and 5 mm with
an average of 1.5 mm. The lithology is mainly coarse, medium-coarse and fine sand, which
are distributed in parallel. Due to the presence of coarse particles, fine sand laminae are
compacted tightly and there are rarely pores between grains. However, in the medium-
coarse sand laminae, there are primary and secondary pores. In the FMI imaging logging,
the light and dark bands are obvious with good continuity (Figure 3).
4.2. Plane Distribution of Laminae
According to the core, thin section, and FMI logging imaging analysis, laminae are
widely developed in the Cretaceous sandstone reservoir in the Bozi–Dabei area, but there
are only a few wells with a dense distribution of laminae in the entire target horizon.
Iron-stained argillaceous-enriched laminae are mainly distributed in BZ 1, DB 10, and the
northern well areas (Figure 4), which are densely developed around the wells of BZ 102, BZ
104, and DB 10, but locally developed in other wells according to cores and FMI logging
imaging analysis. The magnetite-enriched laminae are well-developed in the entire target
horizon only a few wells including BZ 102, BZ 104, and DB 10. The grain-size change
laminae are mainly developed in the southern Bozi area including wells BZ 24, BZ 8, and
BZ 901 (Figure 4).
Processes 2023, 11, x FOR PEER REVIEW 5 of 13
sections (Figure 3), silty–fine sand and iron-stained argillaceous assemble as thin layers
distributed in well-developed areas of the laminae, with a high matrix content and fine
grains. The matrix composition is mainly iron-stained argillaceous, with the colour of
brown and dark brown, which is different in mudstone. In the FMI imaging logging, the
laminae show- dark stripes, and the dark stripes become bold in the location of mud
gravel development.
For reservoirs with magnetite-enriched laminae, the cores show dark black stripes.
The thickness of a single-layer lamina varies from 0.2 mm to 2 mm with an average of 0.8
mm. The lithology is consistent with iron-stained argillaceous-enriched laminae, except
that there are a lot of opaque, no cleavage, granular, or irregular dark minerals. These dark
minerals can be aracted by a magnet. With the help of the microscope, the dark minerals
are identified as magnetite. In the FMI imaging logging, the laminae have higher electrical
conductivity and show black stripes (Figure 3).
For reservoirs with grain-size change laminae, the cores show no obvious colour and
composition variation. The thickness of a single layer varies from 0.5 mm and 5 mm with
an average of 1.5 mm. The lithology is mainly coarse, medium-coarse and fine sand, which
are distributed in parallel. Due to the presence of coarse particles, fine sand laminae are
compacted tightly and there are rarely pores between grains. However, in the medium-
coarse sand laminae, there are primary and secondary pores. In the FMI imaging logging,
the light and dark bands are obvious with good continuity (Figure 3).
4.2. Plane Distribution of Laminae
According to the core, thin section, and FMI logging imaging analysis, laminae are
widely developed in the Cretaceous sandstone reservoir in the Bozi–Dabei area, but there
are only a few wells with a dense distribution of laminae in the entire target horizon. Iron-
stained argillaceous-enriched laminae are mainly distributed in BZ 1, DB 10, and the
northern well areas (Figure 4), which are densely developed around the wells of BZ 102,
BZ 104, and DB 10, but locally developed in other wells according to cores and FMI log-
ging imaging analysis. The magnetite-enriched laminae are well-developed in the entire
target horizon only a few wells including BZ 102, BZ 104, and DB 10. The grain-size change
laminae are mainly developed in the southern Bozi area including wells BZ 24, BZ 8, and
BZ 901 (Figure 4).
Figure 4. Plane distribution of different types of laminae in the Bozi–Dabei area.
Figure 4. Plane distribution of different types of laminae in the Bozi–Dabei area.
4.3. Vertical Distribution of Laminae
The magnetite-enriched and iron-stained argillaceous-enriched laminae are mainly
developed in the upper of the normal depositional cycle with fine grain size and high
argillaceous content. The GR curves are often represented as a bell-shaped feature in the
Processes 2023,11, 2472 6 of 13
upper but a box-shaped feature at the bottom where laminae are undeveloped. The grain-
size variation laminae are mainly developed in the middle of the normal depositional cycle
with coarse grain size and low argillaceous content. The GR curves are often represented
as a high amplitude box-shaped feature.
5. Discussion
5.1. The Influencing Factors of the Laminae Formation
Previous studies have suggested that the formation of laminae is influenced by var-
ious factors, such as climate, lake bottom topography, water properties, the amount of
terrestrial debris, and biological processes [
41
–
44
]. From the sedimentary facies plan, in the
depositional stage of the Bashijiqike formation, the paleocurrent direction in most of the
Bozi area was from north to south. However, it was from north to east and south in the area
of the Bozi 8 and Bozi 9 wells. In the Bozi 12 well area, the paleocurrent is from south to
north. The sedimentary facies near the southern Tianshan Mountain is braided river delta
plain and it gradually becomes braided river delta front southward (Figure 5). Stable heavy
minerals in the study area are mainly composed of magnetite, hematite, ilmenite, zircon,
garnet, tourmaline, etc., with magnetite content being the highest, accounting for about 60%.
From the comprehensive analysis of sedimentary facies and heavy minerals, it is believed
that with the increase in transport distance, the energy of the water body weakens, the flow
rate slows down, the content of stable heavy minerals and fine-grained sediment increases,
resulting in the increase in iron-stained argillaceous content and the layer distribution of
magnetite in the Bozi 1, Dabei 10, and Dabei 6 well areas at the braided river delta front.
The content of iron-stained argillaceous is high, and the magnetite-enriched layers are
well-developed. In the southern Bozi–Dabei area, the sedimentation of the Cretaceous
Bashijiqike formation is influenced by the paleocurrent from north to the south and east
(Figure 5), with a coarser grain size, sufficient grain sorting, low argillaceous content, and
alternating coarse and fine-grained sedimentation, resulting in the development of grain
size change laminae in the southern Bozi area.
Processes 2023, 11, x FOR PEER REVIEW 6 of 13
4.3. Vertical Distribution of Laminae
The magnetite-enriched and iron-stained argillaceous-enriched laminae are mainly
developed in the upper of the normal depositional cycle with fine grain size and high
argillaceous content. The GR curves are often represented as a bell-shaped feature in the
upper but a box-shaped feature at the boom where laminae are undeveloped. The grain-
size variation laminae are mainly developed in the middle of the normal depositional cy-
cle with coarse grain size and low argillaceous content. The GR curves are often repre-
sented as a high amplitude box-shaped feature.
5. Discussion
5.1. The Influencing Factors of the Laminae Formation
Previous studies have suggested that the formation of laminae is influenced by vari-
ous factors, such as climate, lake boom topography, water properties, the amount of ter-
restrial debris, and biological processes [41–44]. From the sedimentary facies plan, in the
depositional stage of the Bashijiqike formation, the paleocurrent direction in most of the
Bozi area was from north to south. However, it was from north to east and south in the
area of the Bozi 8 and Bozi 9 wells. In the Bozi 12 well area, the paleocurrent is from south
to north. The sedimentary facies near the southern Tianshan Mountain is braided river
delta plain and it gradually becomes braided river delta front southward (Figure 5). Stable
heavy minerals in the study area are mainly composed of magnetite, hematite, ilmenite,
zircon, garnet, tourmaline, etc., with magnetite content being the highest, accounting for
about 60%. From the comprehensive analysis of sedimentary facies and heavy minerals,
it is believed that with the increase in transport distance, the energy of the water body
weakens, the flow rate slows down, the content of stable heavy minerals and fine-grained
sediment increases, resulting in the increase in iron-stained argillaceous content and the
layer distribution of magnetite in the Bozi 1, Dabei 10, and Dabei 6 well areas at the
braided river delta front. The content of iron-stained argillaceous is high, and the magnet-
ite-enriched layers are well-developed. In the southern Bozi–Dabei area, the sedimenta-
tion of the Cretaceous Bashijiqike formation is influenced by the paleocurrent from north
to the south and east (Figure 5), with a coarser grain size, sufficient grain sorting, low
argillaceous content, and alternating coarse and fine-grained sedimentation, resulting in
the development of grain size change laminae in the southern Bozi area.
Figure 5. Sedimentary facies of the Cretaceous Bashijiqike Formation in the Bozi–Dabei area.
Processes 2023,11, 2472 7 of 13
5.2. Effects of Laminae on Reservoir Properties
Analysis of 112 cast thin sections from 32 wells in the Bozi–Dabei area shows that the
lithology of the reservoir with iron-stained argillaceous-enriched laminae is mainly com-
posted of fine and medium sandstone, with a significant increase in iron-stained argillaceous
in the filling materials, which are distributed in thin layers and intermittently parallel to the
bedding (Figure 6A–C). The reservoirs always have a strong cementation and low porosity.
The magnetite-enriched laminae are similar to the iron-stained argillaceous-enriched lami-
nae, with an increased content of magnetite, which is accumulated and distributed in thin
layers (Figure 6C,D). For the reservoirs iron-stained argillaceous-enriched and magnetite-
enriched laminae undeveloped, their grain size is relatively coarse, mainly composed of fine
to medium sandstone, mainly cemented by calcite, with a small number of intergranular
pores, intergranular dissolution pores, and intragranular dissolution pores, and the surface
pore rate is obviously higher than that with laminae developed. The grain-size change lam-
inae show a laminated distribution of very fine sand, fine sand, and medium-coarse sand
under the microscope, with particles arranged in an oriented manner. The filling materials
are mainly composed of iron-stained argillaceous and calcite. A small number of primary
intergranular pores, intergranular dissolution pores, intragranular dissolution pores, and
clay micropores can be seen in the rock, mainly distributed in the medium-coarse-grained
laminae (Figure 6E,F).
Processes 2023, 11, x FOR PEER REVIEW 7 of 13
Figure 5. Sedimentary facies of the Cretaceous Bashijiqike Formation in the Bozi–Dabei area.
5.2. Effects of Laminae on Reservoir Properties
Analysis of 112 cast thin sections from 32 wells in the Bozi–Dabei area shows that the
lithology of the reservoir with iron-stained argillaceous-enriched laminae is mainly com-
posted of fine and medium sandstone, with a significant increase in iron-stained argilla-
ceous in the filling materials, which are distributed in thin layers and intermiently par-
allel to the bedding (Figure 6A–C). The reservoirs always have a strong cementation and
low porosity. The magnetite-enriched laminae are similar to the iron-stained argillaceous-
enriched laminae, with an increased content of magnetite, which is accumulated and dis-
tributed in thin layers (Figures 6C,D). For the reservoirs iron-stained argillaceous-en-
riched and magnetite-enriched laminae undeveloped, their grain size is relatively coarse,
mainly composed of fine to medium sandstone, mainly cemented by calcite, with a small
number of intergranular pores, intergranular dissolution pores, and intragranular disso-
lution pores, and the surface pore rate is obviously higher than that with laminae devel-
oped. The grain-size change laminae show a laminated distribution of very fine sand, fine
sand, and medium-coarse sand under the microscope, with particles arranged in an ori-
ented manner. The filling materials are mainly composed of iron-stained argillaceous and
calcite. A small number of primary intergranular pores, intergranular dissolution pores,
intragranular dissolution pores, and clay micropores can be seen in the rock, mainly dis-
tributed in the medium-coarse-grained laminae (Figure 6E,F).
Figure 6. Thin section characteristics of different types of laminae. (A) BZ13 well, 7005.02 m, iron-
stained argillaceous-enriched laminae; (B) BZ22 well, 6279.38 m, iron-stained argillaceous-enriched
laminae; (C) AW5 well, 3203.9 m, iron-stained argillaceous-enriched laminae and magnetite-en-
riched laminae; (D) DB6 well, 6861.7 m, magnetite-enriched laminae; (E) BZ24 well, 7227.65 m,
grain-size change laminae; (F) BZ9 well, 7680.3 m, grain-size change laminae.
Several wells were selected to analyse the effects of laminae on the reservoir’s poros-
ity. It was found that the logging interpretation effective porosity of the reservoirs with
iron-stained argillaceous-enriched and magnetite-enriched laminae ranged from 4.0% to
6.0%, with an average of 5.2%, while the effective porosity of the non-laminated reservoirs
ranged from 5.0% to 8.0%, with an average of 6.0%. The logging interpretation effective
porosity of the reservoirs with grain-size change laminae ranged from 4.0% to 7.0%, with
Figure 6.
Thin section characteristics of different types of laminae. (
A
) BZ13 well, 7005.02 m, iron-
stained argillaceous-enriched laminae; (
B
) BZ22 well, 6279.38 m, iron-stained argillaceous-enriched
laminae; (
C
) AW5 well, 3203.9 m, iron-stained argillaceous-enriched laminae and magnetite-enriched
laminae; (
D
) DB6 well, 6861.7 m, magnetite-enriched laminae; (
E
) BZ24 well, 7227.65 m, grain-size
change laminae; (F) BZ9 well, 7680.3 m, grain-size change laminae.
Several wells were selected to analyse the effects of laminae on the reservoir’s porosity.
It was found that the logging interpretation effective porosity of the reservoirs with iron-
stained argillaceous-enriched and magnetite-enriched laminae ranged from 4.0% to 6.0%,
with an average of 5.2%, while the effective porosity of the non-laminated reservoirs ranged
from 5.0% to 8.0%, with an average of 6.0%. The logging interpretation effective porosity of
the reservoirs with grain-size change laminae ranged from 4.0% to 7.0%, with an average
of 5.8%, while the effective porosity of the non-laminated reservoir mainly ranged from
6.0% to 9.0%, with an average of 7.1% (Figure 7).
Processes 2023,11, 2472 8 of 13
Processes 2023, 11, x FOR PEER REVIEW 8 of 13
an average of 5.8%, while the effective porosity of the non-laminated reservoir mainly
ranged from 6.0% to 9.0%, with an average of 7.1% (Figure 7).
Figure 7. Histograms of logging interpretation effective porosity of the laminae and non-laminae
reservoir. (A) The logging interpretation effective porosity of the reservoir with iron-stained argil-
laceous-enriched and magnetite-enriched laminae. (B) The logging interpretation effective porosity
of iron-stained argillaceous-enriched and magnetite-enriched laminae undeveloped reservoir. (C)
The logging interpretation effective porosity of grain-size change laminated reservoir. (D) The log-
ging interpretation effective porosity of grain-size change laminae undeveloped reservoir.
Overall, the porosity of the reservoir with laminae developed is obviously lower than
that of laminae undeveloped. The existence of laminae reduces the reservoir porosity. Ad-
ditionally, the porosity of the reservoir with grain-size change laminae is higher than the
reservoir with iron-stained argillaceous-enriched and magnetite-enriched laminae.
Due to the plan distribution of laminae and its strong vertical heterogeneity, the small
plug permeability is not representative which is significantly affected by the sample size.
Therefore, the evaluation of permeability is based on the analysis of full-diameter perme-
ability results. For the three types of laminae, core samples from the well-laminated sec-
tions were selected for full diameter petrophysical analysis. Analysis shows that the aver-
age permeability in the vertical direction of the magnetite-enriched laminae (referred to
as vertical permeability, denoted as K_vertical) is 0.227 mD, and in the parallel direction
of laminae (referred to as lateral permeability, denoted as K_lateral) is 0.318 mD, with
vertical permeability significantly lower than lateral permeability. The average vertical
permeability for the iron-stained argillaceous-enriched laminae is 0.569 mD, and the lat-
eral permeability is 1.372 mD, with vertical permeability significantly lower than lateral
permeability. For the laminae with grain-size change, the average vertical permeability is
1.671 mD, and the lateral permeability is 1.378 mD, with lile difference between vertical
and lateral permeability (Figure 8). Overall, the existence of iron-stained argillaceous-en-
riched and magnetite-enriched laminae tends to result in lower vertical permeability.
Figure 7.
Histograms of logging interpretation effective porosity of the laminae and non-laminae reser-
voir. (
A
) The logging interpretation effective porosity of the reservoir with iron-stained argillaceous-
enriched and magnetite-enriched laminae. (
B
) The logging interpretation effective porosity of iron-
stained argillaceous-enriched and magnetite-enriched laminae undeveloped reservoir. (
C
) The
logging interpretation effective porosity of grain-size change laminated reservoir. (
D
) The logging
interpretation effective porosity of grain-size change laminae undeveloped reservoir.
Overall, the porosity of the reservoir with laminae developed is obviously lower than
that of laminae undeveloped. The existence of laminae reduces the reservoir porosity.
Additionally, the porosity of the reservoir with grain-size change laminae is higher than
the reservoir with iron-stained argillaceous-enriched and magnetite-enriched laminae.
Due to the plan distribution of laminae and its strong vertical heterogeneity, the
small plug permeability is not representative which is significantly affected by the sample
size. Therefore, the evaluation of permeability is based on the analysis of full-diameter
permeability results. For the three types of laminae, core samples from the well-laminated
sections were selected for full diameter petrophysical analysis. Analysis shows that the
average permeability in the vertical direction of the magnetite-enriched laminae (referred to
as vertical permeability, denoted as K_vertical) is 0.227 mD, and in the parallel direction of
laminae (referred to as lateral permeability, denoted as K_lateral) is 0.318 mD, with vertical
permeability significantly lower than lateral permeability. The average vertical permeability
for the iron-stained argillaceous-enriched laminae is 0.569 mD, and the lateral permeability
is 1.372 mD, with vertical permeability significantly lower than lateral permeability. For
the laminae with grain-size change, the average vertical permeability is 1.671 mD, and
the lateral permeability is 1.378 mD, with little difference between vertical and lateral
permeability (Figure 8). Overall, the existence of iron-stained argillaceous-enriched and
magnetite-enriched laminae tends to result in lower vertical permeability.
Processes 2023,11, 2472 9 of 13
Processes 2023, 11, x FOR PEER REVIEW 9 of 13
Figure 8. The relationship between lateral permeability and vertical permeability of core in different
laminae.
5.3. Effects of Laminae on Fracture
In rocks, the change in grain size and compositions of debris particles often form a
weak stress plane, and the density, size, and dip angle of fractures near the weak stress
plane will suddenly change in thick tight sandstone after being subjected to stress [45,46].
The cores shows that fractures in the laminated area often cut in arc or terminate at the
laminae bedding plane, and the fracture density in the laminated sections is significantly
lower than that in the non-laminated sections. From the FMI logging imaging data, we
identified the distribution of fractures. Taking the BZ 17 well as an example, the core is
located in the depth of 6059.9–6067.9 m of the Cretaceous Bashijiqike formation. For the
core in the depth of 6059.9–6061.9 m, multiple high-angle fractures are developed and the
core is broken. The laminae are undeveloped. Four high-angle parallel fractures are iden-
tified on the logging image, with an average linear density of two fractures/m. For the core
in the depth of 6062.9–6064.9 m, the core is relatively intact, with two high-angle nearly
parallel fractures developed. Iron-stained argillaceous-enriched laminae are developed.
The logging image shows alternating light and dark stripes, and one high-angle fracture
is identified with an average fracture line density of 0.5/m (Figure 9). Overall, the debris
particle size, composition, and other changes in the development of lamination are signif-
icant, and the elastic properties of rocks change greatly, which to some extent prevents
the expansion of fractures in the longitudinal direction.
Figure 8.
The relationship between lateral permeability and vertical permeability of core in
different laminae.
5.3. Effects of Laminae on Fracture
In rocks, the change in grain size and compositions of debris particles often form a
weak stress plane, and the density, size, and dip angle of fractures near the weak stress
plane will suddenly change in thick tight sandstone after being subjected to stress [
45
,
46
].
The cores shows that fractures in the laminated area often cut in arc or terminate at the
laminae bedding plane, and the fracture density in the laminated sections is significantly
lower than that in the non-laminated sections. From the FMI logging imaging data, we
identified the distribution of fractures. Taking the BZ 17 well as an example, the core
is located in the depth of 6059.9–6067.9 m of the Cretaceous Bashijiqike formation. For
the core in the depth of 6059.9–6061.9 m, multiple high-angle fractures are developed
and the core is broken. The laminae are undeveloped. Four high-angle parallel fractures
are identified on the logging image, with an average linear density of two fractures/m.
For the core in the depth of 6062.9–6064.9 m, the core is relatively intact, with two high-
angle nearly parallel fractures developed. Iron-stained argillaceous-enriched laminae are
developed. The logging image shows alternating light and dark stripes, and one high-angle
fracture is identified with an average fracture line density of 0.5/m (Figure 9). Overall,
the debris particle size, composition, and other changes in the development of lamination
are significant, and the elastic properties of rocks change greatly, which to some extent
prevents the expansion of fractures in the longitudinal direction.
Processes 2023,11, 2472 10 of 13
Processes 2023, 11, x FOR PEER REVIEW 10 of 13
Figure 9. Comprehensive histogram of BZ17 well of Cretaceous Bashijiqike formation and fracture
and laminae development characteristics of core. (A) BZ17, 6060.4 m, core, a group of parallel frac-
ture. (B) BZ17, 6061.3 m, core, a group of high angle fracture. (C) BZ17, 6064.2 m, core, eight laminae
can be identified.
5.4. Effects of Laminae on Reservoir Reconstruction
A large number of fine-grained laminae in the sandstone reservoir seriously intensify
the heterogeneity of the reservoir [47,48]. For the well sections with magnetite-enriched
and iron-stained argillaceous-enriched laminae, the reservoir porosity and vertical per-
meability are significantly reduced, and the fracture size and density are small, resulting
in poor vertical connectivity of the reservoir, impairing oil and gas flow, and reducing the
exploitation effect. Conventional acid fracturing has limitations: firstly, the vertical exten-
sion of artificial fractures is limited due to the existence of laminae, resulting in a small
range of reservoir communication. Secondly, the acid fracturing fluid is lost along the
laminae bedding plane, which reduces the fracturing effect. Thirdly, there is no obvious
impact of acid on the iron-stained argillaceous-enriched laminae and magnetite, affecting
the acid fracturing effect. After reservoir reconstruction, the single well productivity can-
not be fully released, so it is necessary to increase the length of the perforation interval
and perforation density in the laminated sections and choose sand fracturing to maximize
the vertical connectivity of the reservoir and improve the flow capacity; thereby, enhanc-
ing single well production capacity (Table 1). For well sections with grain-size change
laminae, although the fractures are not significantly developed, the reservoir property is
good and laminae have lile effect on vertical permeability. Most wells can achieve high
productivity through conventional testing, with relatively low requirements for recon-
struction method. The production capacity will be further improved after sand fracturing.
Figure 9.
Comprehensive histogram of BZ17 well of Cretaceous Bashijiqike formation and fracture
and laminae development characteristics of core. (
A
) BZ17, 6060.4 m, core, a group of parallel fracture.
(
B
) BZ17, 6061.3 m, core, a group of high angle fracture. (
C
) BZ17, 6064.2 m, core, eight laminae can
be identified.
5.4. Effects of Laminae on Reservoir Reconstruction
A large number of fine-grained laminae in the sandstone reservoir seriously intensify
the heterogeneity of the reservoir [
47
,
48
]. For the well sections with magnetite-enriched
and iron-stained argillaceous-enriched laminae, the reservoir porosity and vertical per-
meability are significantly reduced, and the fracture size and density are small, resulting
in poor vertical connectivity of the reservoir, impairing oil and gas flow, and reducing
the exploitation effect. Conventional acid fracturing has limitations: firstly, the vertical
extension of artificial fractures is limited due to the existence of laminae, resulting in a
small range of reservoir communication. Secondly, the acid fracturing fluid is lost along
the laminae bedding plane, which reduces the fracturing effect. Thirdly, there is no obvious
impact of acid on the iron-stained argillaceous-enriched laminae and magnetite, affecting
the acid fracturing effect. After reservoir reconstruction, the single well productivity cannot
be fully released, so it is necessary to increase the length of the perforation interval and
perforation density in the laminated sections and choose sand fracturing to maximize the
vertical connectivity of the reservoir and improve the flow capacity; thereby, enhancing
single well production capacity (Table 1). For well sections with grain-size change laminae,
although the fractures are not significantly developed, the reservoir property is good and
laminae have little effect on vertical permeability. Most wells can achieve high productivity
through conventional testing, with relatively low requirements for reconstruction method.
The production capacity will be further improved after sand fracturing.
Processes 2023,11, 2472 11 of 13
Table 1. Completed testing of representative wells where laminae develop.
Well Completion
Testing Formation Well Section
(m)
Nozzle
(mm)
Oil Pressure
(MPa)
Daily Gas
Production
(104m3/d)
Daily Oil
Production
(m3/d)
Laminae Type
BZ102
routine test
K1bs 6760–6879
3 33.80 3.23 5.54
magnetite-
enriched type,
iron-stained
argillaceous-
enriched
type
acid test 4 38.38 10.66 10.44
sand fracturing
test 6 31.53 16.28 18.63
BZ104
routine test
K1bs 6757–6850
3 43.39 3.74 0.98
sand fracturing
test 7 81.29 51.19 36.38
BZ9
routine test
K1bs 7677–7760.5
5 86.64 22.39 /
grain-size variation
type
sand fracturing
test 8 94.35 70.55 167
BZ24
routine test
K1bs 7320–7390
4 42.98 11.78 12.3
sand fracturing
test 6 69.64 28.71 40.8
6. Conclusions
The results show that the development of laminae has a significant effect on reservoir
heterogeneity. After an oil and gas well is drilled, it is necessary to identify whether the
laminae are developed according to the core, thin section, and logging imaging data. For
wells with laminae developed, the type of the laminae should be further identified, and the
oil testing method should be further determined according to the laminae type in order to
improve the oil and gas productivity. For the Bozi–Dabei area, some understandings are
listed as follows:
Laminae are widely developed in Cretaceous sandstone reservoirs in the Bozi–Dabei
area and can be divided into three types based on colour, composition, and grain size:
magnetite-enriched, iron-stained argillaceous-enriched, and grain-size change laminae.
Influenced by the sedimentary environment, the iron-stained argillaceous-enriched and
magnetite-enriched laminae are mainly distributed in BZ 1, DB 10, and their northern well
areas, while the grain-size change laminae are mainly developed in the southern Bozi area.
The existence of laminae tends to lower the reservoir porosity. The lateral permeability
of reservoirs with magnetite-enriched, iron-stained argillaceous-enriched laminae is signifi-
cantly greater than the vertical permeability while there is little difference between lateral
and vertical permeability of reservoirs with grain size change laminae, and the pores are
mainly distributed in medium-coarse-grained laminae.
For the laminated reservoirs, increasing the length of perforation interval and per-
foration density and using sand fracturing or acid fracturing are effective methods for
communicating with the vertical reservoirs, improving the flow capacity, and increasing
the single-well production.
Author Contributions:
Writing—original draft preparation, W.Z.; writing—review and editing, T.M.;
investigation, C.C. and C.H.; resources, C.W.; data curation, C.S.; validation, L.S. and P.L. All authors
have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the major science and technology project of the China National
Petroleum Co., Ltd. (2018E-1801).
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
Processes 2023,11, 2472 12 of 13
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