Genesis and reservoir significance of multi-scale natural fractures in Kuqa foreland thrust belt, Tarim Basin, NW China

Genesis and reservoir significance of multi-scale natural fractures in Kuqa foreland thrust belt, Tarim Basin, NW China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 3, June 2017 Online English edition of the Chinese language journal Cite this article as: PETRO...

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PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 3, June 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(3): 495–504.

RESEARCH PAPER

Genesis and reservoir significance of multi-scale natural fractures in Kuqa foreland thrust belt, Tarim Basin, NW China LIU Chun1, *, ZHANG Ronghu1, ZHANG Huiliang1, WANG Junpeng1, MO Tao2, WANG Ke1, ZHOU Lu2 1. PetroChina Hangzhou Institute of Geology, Hangzhou 310023, China; 2. PetroChina Tarim Oilfield Company, Korla 841000, China

Abstract: Natural fractures in the deep Cretaceous Bashijiqike Formation sandstone reservoirs of Kuqa foreland thrust belt, NW China, are classified according to fracture aperture based on the data of outcrops, cores, thin sections and imaging logging, using industrial CT scanning, laser scanning confocal microscope (LSCM), cathodoluminescence (CL), electron probe, and scanning electron microscopy (SEM). The types, characteristics, genesis ages and formation sequence as well as reservoir significance of such natural fractures are examined. Four categories of fractures are classified. Category I (aperture>100 μm) is macro structural fractures, which cut single sand body to form the dominant migration pathway, helping to increase the reservoir permeability. Category II (aperture=10-100 μm) is associated micro structural fractures, which cut matrix grains to connect large matrix pores and improve the seepage performance. Category III (aperture=1-10 μm) is micro digenetic fractures at grain edges, which connect medium and small pores to improve the pore network connectivity and the gas migration and charging efficiency. Category IV (aperture<1 μm) is nano-scale matrix fissures, which connect intragranular micro pores to expand the reservoir space, thereby increasing the reserves scale. Category I and Category II fractures were developed in three stages (early, middle and late). The early- and middle-stage fractures, predominantly half-filled–filled fractures, were formed before early Pliocene when extensive oil and gas charging had not occurred. The late-stage opened fractures were formed after the Late Pliocene, they were at the same time as or later slightly than extensive oil and gas charge. The fracture network has low contribution to porosity, but it can improve the permeability by 2-3 orders of magnitudes in the parallel direction of fractures. Key words: deep reservoir; tight sandstone; natural fracture; genetic type; formation sequence; Kuqa foreland basin

Introduction In recent years, with more effort put into the exploration of deep formations in the Kuqa foreland thrust belt, gas exploration breakthroughs have been made there[13]. However, the reservoirs there are tight on the whole due to the influences of lateral tectonic compression and vertical burial compaction in the foreland thrust belt, so only when there are natural fractures acting as paths of hydrocarbon migration, can the tight sandstone gets better permeability, and forming fractured or porosity-fractured reservoirs. Therefore, fractures have become the key factor for high and stable yield in ultra-low porosity and low permeability sandstone reservoir[46]. Fracture research has become one of the essential study content in gas exploration and development of tight reservoirs. The sandstone reservoir, in the deep Cretaceous Bashijiqik Formation of the Kuqa foreland thrust belt, is more than 6 000 m deep in general, representing typical ultra-deep reservoir

with fractures in thrust belt. Previous studies show that the fractures in this area are mainly NW, NE and nearly NS trending high angle tectonic fractures[7], can be divided into tensile fracture, shear fracture and multi-stage mixed fracture according to genesis, and come in linear, fold line, complex net, wheat and broom shapes[8]; the distribution of fractures is jointly controlled by lithology, layer thickness, stratigraphic pattern and fault[913]. Different structural positions are different in the degree of tectonic deformation[3], the fractures are mainly distributed in the anticlinal limbs and near thrust faults where the tectonic stress concentrates, and there are very few fractures in the hinge area of anticline[14]. The development of micro-fractures are mainly related to buried depth and fabric of sandstone, and the contribution of fractures to production is approximately equal to that of fractures to formation permeability[15]. The porosity and permeability of fractures in the tight sandstone are strongly sensitive to stress, and have high

Received date: 09 Aug. 2016; Revised date: 23 Mar. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by China National Science and Technology Major Project (2016ZX05003-001-002). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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value at the structural high and kink bands of anticline[1617]. These understandings have laid the foundation for the systematic evaluation and prediction of the ultra-deep sandstone reservoirs in the foreland thrust belt. However, previous studies take the whole fracture network as a unified single system. The effects of different genesis fractures are various due to the buried compaction and tectonic compression. In addition, different types of fractures in the fractured network system have different storage and permeability, hence different contribution to the production capacity. In this study, we classify the fractures types according to the fracture aperture, and analyze the characteristics, genesis and reservoir significance of the multi-scale fractures systematically, which provides new ideas and theoretical reference for quantitative characterization and validity evaluation of natural fractures in foreland thrust belt.

part of the Tarim Basin, NW China tectonically (Fig. 1)[18]. Since the Mesozoic, the Kuqa foreland basin has been under tectonic compression due to the uplift of the Tianshan orogenic belt. The distribution of sedimentary facies of the Cretaceous sandstone has been strongly controlled by the paleogeographic framework which is characterized by uplift of southern Tianshan and tectonic subsidence of the Kuqa foreland basin. Foreland thrust structures were widespread in the region due to strong compression since late Cenozoic[19]. The Cretaceous can be subdivided into four formations, Yageliemu Formation, Shushanhe Formation, Baxigai Formation and Bashijiqike Formation from bottom to top. The gas layers are primarily in the Bashijiqik sandstone, which is typical tight sandstone reservoir[34], with matrix porosity of generally 1.5%5.5%, and permeability of (0.010.10) × 103 μm2 generally.

1.

2.

Geological background

The Kuqa foreland thrust belt is located in Akesu area, Xinjiang autonomous region geographically, and in the northern

Fig. 1.

Classification of fractures

According to the characteristics of fractures observed in outcrops in different structural patterns of Kuqa foreland

Geological overview of Kuqa foreland thrust belt.

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thrust belt[20], the characteristics of fractures observed in core, and the classification of fractures in previous studies[21], a new classification based on fracture aperture (Table 1), named “aperture-shape-origin” classification for natural fractures there has been advanced in this study. This classification scheme involves basic attributes that can be observed and identified accurately, such as the length, aperture and extension of fracture, in the fracture network system. Stressing on the effect of multi-scale fractures on physical properties of the tight reservoirs, the classification gives a unified grading of fractures and making it easier for researchers to classify fractures in outcrops, cores, imaging logs and microscopic experiments. There are four Categorys of fractures, macro fracture (CategoryⅠ), micro fracture (CategoryⅡ), ultra-micro fracture (Category Ⅲ) and matrix crack (Category Ⅳ), in tight sandstone reservoir in Kuqa foreland thrust belt. According to origin, the fractures can be divided into nearly vertical macro tectonic fracture (aperture>100 μm), cutting across grain micro tectonic fracture (aperture=10-100 μm), along grain ultra-micro diagenesis fracture (aperture=1-10 μm) and nano-matrix inherent crack (aperture <1 μm).

3. 3.1.

The origin of multi-scale fractures Category Ⅰ-macro fractures

Macro fractures are usually visible in field outcrop, core, and Formation Micro Scanner Image of well logging (FMI), mostly high angle tension or shear fractures obliquely or vertically cutting sand body (Fig. 2). The fractures are mainly northwest-southeast or southwest-northeast trending (Fig. 3a), larger than 100 μm in aperture, generally 100250 μm and 4 mm at maximum; more than 20 mm long, generally around 500 mm (Fig. 3a), more than 5 m at maximum; and semifilled or non-filled (Fig. 3b). In FMI images, they are in linear shape, higher in convergence, closed from top to bottom, with certain continuity, high dip angle, and easy to be fit by sinusoidal curve (Fig. 2c). The fractures can end at the rock interface of varied mechanical property, such as bedding plane or interface of varied lithology (Fig. 3c), therefore CategoryⅠ fractures are strongly stratigraphic controlled, which is especially obvious in field outcrops and FMI images (Fig. 2). Table 1. Category

Type

Category I macro fractures are the result of regional tectonic compression. The tectonic evolution history shows that in the Kuqa foreland thrust belt, influenced by multiple stages of tectonic stresses toward south and north due to uplift of the Tianshan Mountains since late Cenozoic, there develop complex fault systems and rich fractures; under continuous thrusting and compression, the top of anticlines have been in tensional stress environment, so there develop E-W-trending tensile fractures; their distribution in plane-view is mainly controlled by the magnitude of stress, while their vertical depth is affected by curvature. The macroscopic distribution of this kind of fracture is consistent with regional faults, reflecting the control of regional stress and fault system over fracture distribution. 3.2. CategoryⅡ-micro fractures Micro fractures can be observed with ordinary microscopy and confocal laser scanning microscopy (CLSM) on casting thin-sections, as well as micro-CT scanning (Fig. 4). Cutting fragment grains like branches, they often appear in a group parallel with each other, some of them are two groups of fractures perpendicular to each other; and small amount of them are isolated or chaotic in plane-view. They are mostly linear, curved, some of them are in arch, bent straight lines, branched curve, irregular net, and branch shape (Fig. 4 a-b), with directionality. Micron CT scan result shows that this kind of micro-fractures mainly occur around macro fractures (Fig. 4c). They are 10100 μm in aperture, usually about 20 μm, about 220 mm long. Formed in different periods, some fractures are filled mainly with dolomite (iron), calcite, gypsum and a small amount of quartz (Fig. 5), other fractures are unfilled. Just as macro fracture, the development of the micro fractures is also controlled by formation, reflecting the control of regional stress and macro fracture system on its distribution. Both the CategoryⅡmicro fractures and CategoryⅠmacro fractures are formed due to tectonic compression, and similar in direction and distribution pattern, only different in scale. 3.3. Category Ⅲ- ultra-micro fracture This kind of fracture is mainly observed by scanning electron microscopy (SEM), confocal laser scanning microscopy

Characteristics of multiscale fractures in Kuqa foreland thrust belt

Aperture/μm Length/mm

Shape

Extensibility

Origin

Identification method

Reservoir significance

Nearly vertical

Layer controlled

Structure

Outcrop, core and imaging logging

Cutting single sand body, they are hydrocarbon migration pathways



Macro

>100

>20



Micro

10100

220



Ultra-micro

110

0.22.0

Grain edge

Grain controlled



Nano matrix cracks

0.051.00

0.000 1 0.2

Cracks

Random

Casting thin-section, Connecting macro pores, and LSCM and miimproving permeability cro-CT Connecting small pores, and Diagenesis LSCM, SEM, increasing reservoir space micro-CT, Inherent, electron probe Matrix throats diagenesis

Cutting Nearly layer Associated through grain controlled with structure

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Fig. 2.

Fig 3.

Characteristics of macro fracture in Kuqa foreland thrust belt in outcrop (a), core (b) and imaging logging (c).

Dip angle (a), filling (b) and convergence(c) frequency distribution of macroscopic fractures in Kuqa foreland thrust belt.

Fig 4. Characteristics of micro fracture in Kuqa foreland thrust belt. (a) KS301, 6 948.43 m, micro fracture in branch shape, Plane-polarized light (PPL); (b) DB304, 6932.50 m, discontinuous micro fracture, confocal laser scanning microscopy (CLSM); (c) KS207, 6 933.00 m, micro fracture image from micron-CT scan.

(CLSM), nano-CT and electron probe scanning. They are similar to detrital grain in shape, and affected by roundness of grains (Fig. 6a, 6b), lack of directionality, irregular in direction, and often extend to the grain contact point. Most of them are open, with an aperture range of 1 μm to 10 μm, mostly about 4 μm. Their lengths, controlled by the circumference and roundness of detrital grains, are generally from 0.2 mm to 2.0 mm. Formation of the ultra-micro grain edge fractures is affected by many factors, such as heterogeneous compaction between

grains, synaeresis of mudstone matrix, temperature rise of mudstone matrix during burial compaction, or release of tectonic stress during formation of macro- and micro-fractures[4]. The key controlling factor of grain edge fracture formation is inherent sandstone factors, ie, diagenesis origin (heterogeneous compaction between grains, synaeresis of mudstone matrix). There is no obvious evidence of filling in the ultra-micro grain edge fractures basically, and they are all open. Their formation time is basically the same with the open Category ⅠandⅡfractures.

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Fig. 5. Filling characteristics of micro fracture in Kuqa foreland thrust belt. (a) Well KS8003, 6 778.76 m, fracture filled by dolomite early and by gypsum later, cross-polarized light (XPL); (b) Well KS8003, 6 778.76 m, fracture filled by dolomite early and by gypsum later, electron probe; (c) Well KS501, 6 360.15 m, fracture filled by dolomite early and by gypsum later, plane-polarized light (PPL); (D) Well KS501, 6 514.74 m, fracture semi-filled by saddle dolomite, plane-polarized light (PPL); (e) Well KS501, 6 514.11 m, fracture semifilled by calcite, plane-polarized light (PPL); (f) Well DB302, 7 279.26 m, fracture filled by iron dolomite, plane-polarized light (PPL).

3.4.

Category Ⅳ-matrix cracks

Matrix cracks are mainly fissures or cleavages in grains (Fig. 7a-c), which can only be observed by scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and nano-CT scan. They are random in shape, without prior orientation and often end at the edge of the grain. The fractures are unfilled, with an aperture range of generally 501 000 nm, and their length is controlled by long axis and short axis of the mineral grain, generally from 0.1 μm to 200 μm. The matrix cracks have two origins: (1) The difference of grain sizes caused various pressure during rapid burial period, larger grains born higher pressure, so cracks formed in them, for example crush fracture in quartz or lithic grains (Fig. 7a); (2) Primary transformation of detritus grains, for example, feldspar matrix cracks resulted from dissolution of feldspar cleavage surface (Fig. 7b-f). The mineral cleavage is the key to the formation of matrix cracks, if the minerals have no cleavage, such as quartz, matrix cracks are generally formed by the stress difference; if the minerals have cleavage, such as gypsum, feldspar, matrix cracks are formed during sediment deposition and burial diagenesis.

4. 4.1.

Fracture formation stage and sequence Formation stage of structural fracture

According to the cross relationship of structural fractures in core scale, the macro fractures generally have three filling states, full filled, low-partially filled, and non-filled. Based on their cross relationship and filling state, it is concluded that

the tectonic fractures are formed in three stages: (1) early stage fracture fully filled; (2) middle stage fracture low-partially filled, and cutting the early fractures, (3) late stage fracture low-non-filled and cutting the early and middle stage fractures. Observation of cores shows that in the same structure, the early stage fractures are fully filled, irregular in orientation, and usually no more than 10 cm long, 0.52.0 mm in aperture, and mainly filled with iron calcite and dolomite. Generally, the early fractures are in the lower part of Bashijiqike sandstone, with a dip angle of less than 30°. The second stage fractures are low-partially filled, mostly high in angle, generally in the range 30°60°, long in extension (generally no less than 20 cm), and 0.51.0 mm in aperture. The filling minerals are mainly coarse grains of calcite, dolomite, gypsum and mud. Generally, the second stage fractures occur in the middle-upper part of Bashijiqike sandstone. The late stage fractures are mostly non-filled vertical fractures, longer in extension, generally more than 1 m, and from 0.1 mm to 3.0 mm in aperture. They mainly occur in the upper-top part of Bashijiqike sandstone. In summary, during formation of fold deformation, the fractures in the lower part of the anticline are formed earliest, while fractures in top part of the anticline are formed latest, and the early stage fractures are more likely to be filled. 4.2.

Formation sequence of fracture network

Kuqa foreland thrust belt have three fracture formation periods based on tectonic development history, the intersection relationship and filling mineral types of structural fracture: the

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Fig. 6. Characteristics of ultra-micro fracture in Kuqa foreland thrust belt. (a) Well KS501, 6 3667.25 m, grain edge ultra-micro fracture (green), CLSM; (b) Well KS501, 6 366.87 m, micro-pores at the edge of the grains in discontinuous linear shape, CLSM; (c) Well KS201, 6 511.26 m; (d) Well DB304, 6 925.20 m, SEM; (e) Well DB304, 6 876.20 m, grain edge ultra-micro fracture, EP; (f) magnification of Figure (e) in red box, EP; (g) Well KS12, 7 293.40 m, grain edge ultra-micro fracture, FEI-SEM; (h) Well KS902, 7 972.64 m, grain edge ultra-micro fracture, FEI-SEM; (i) Well KS904, 7 931.95 m, grain edge ultra-micro fracture, FEI-SEM.

first period fractures were formed in late Miocene, and are mainly filled by iron-bearing dolomite and mud; the second period fractures were formed in early Pliocene, and semi- to partly filled by calcite, dolomite or quartz; the third period fractures were formed after the end of Pliocene, in the same time of large-scale natural gas charge or a little later. The third period fractures, almost unfilled due to the inhibition of hydrocarbon, act as the most effective fractures in Kuqa foreland thrust belt. In summary, three periods of structural fractures developed because of continuous tectonic compression of the southern Tianshan Mountains since Neogene. Based on diagenesis and tectonic evolution in the study area, the formation sequence of multi-scale fracture network is as following: Firstly, the matrix cracks (Category Ⅳ) in grains were formed along cleavage because of dissolution during pene-

contemporaneous-early diagenetic stage. When the Bashijiqike Formation entered into the late period of rapid and deep burial, the matrix cracks in quartz, feldspar and lithic grains were formed due to mechanical compaction. Secondly, when the Bashijiqike Formation entered into the deep burial stage (burial depth>3 500 m), macro fractures (CategoryⅠ)and micro fractures (Category Ⅱ) of different stage and scale were formed due to several periods of uplift and compression during late Himalayan movement. Thirdly, during formation process of macro fractures (CategoryⅠ) and micro fractures (Category Ⅱ), ultra-micro fractures (Category III) developed along the edge of grains in sandstone due to release of tectonic stress, temperature drop and the shrinkage effect between grains and matrix. Category I-IV fractures constitute the complex fracture network system of the Cretaceous Bashijiqike tight sandstone in the Kuqa foreland thrust belt.

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Fig. 7. Characteristics of matrix cracks in Kuqa foreland thrust belt. (a) Well KS501, 6 351.85 m, matrix crack in feldspar grains, plane-polarized light (PPL); (b) Well KS6, 5 622.93 m, feldspar cleavage crack, CLSM; (c) Well KS8003, 6 778.76 m, feldspar cleavage crack, SEM; (d) Well KS902, 7 972.64 m, feldspar cleavage crack like book, FEI-SEM; (e) Well KS904, 7 934.30 m, cleavage crack between calcite cement crystal, SEM; (f) Well KS12, 7 293.40 m, cleavage crack like book, FEI-SEM.

5. Reservoir significance 5.1. Geological significance of multi-scale fractures The fractures of different scales have various effects on porosity and permeability of tight sandstone reservoir. Based on description and characterization of fracture system in the Kuqa foreland thrust belt, the geological significance of the multi-scale fractures in this region has been analyzed as follows: The CategoryⅠmacro-fractures cutting across sand body, serve as dominant nature gas migration paths, and improve the permeability of reservoir. Observation of outcrops and well cores shows that these macro-fractures originated from tectonic deformation or faulting are generally vertical or obliquely intersected with bedding surface at high angle (Fig. 8a, 8b). The fractures terminate at mechanically varied interface, such as layer interface or lithology change interface, showing fractures strong formation control feature. The fractures, mostly not filled, have similar effect as thrust faults, allowing the large scale natural gas migration rapidly along the direction of fracture. The Category Ⅱ micro fractures cutting across the matrix grains, connect the large pores in matrix, and improve the fluid permeability of porous tight sandstone. According to the microscopic observation of a large number of thin sections and scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and nano-CT scan results, the

micro-fractures often connect large pores in matrix along the direction of fracture, hence the pores were charged preferentially by natural gas, and improve the reservoir permeability (Fig. 8c). The formation of micro fracture-large pore network has greatly improved the connectivity of pores. Therefore, some previously isolated dead pores are connected by fractures and full charged by natural gas. The Category Ⅲ ultra-micro-fractures connect the medium- small pores, and improve the pore connectivity network and efficiency of gas injection. Mainly occurring at grain edge, they connect medium-small pores (Fig. 8d), improve pore structure, and reduce gas injection pressure, so that the gas can charge into the tight sandstone reservoir earlier, and make the original isolated "dead pores" connect with other pores, greatly improving the gas saturation in tight sandstone. The Category Ⅳ matrix cracks connect the micro-pores in the tight reservoir, enlarge the space of reservoir and increase the scale of reserve. A large number of intragranular micro-pores occur in deep tight sandstone reservoir in the Kuqa foreland thrust belt[22], increasing the connectivity of micro-pores (Fig. 8e), adding reservoir space, increasing the reserve scale. 5.2. Effect of multi-scale fractures on porosity and permeability The contribution rate of fractures to porosity is low and negligible. Through identification of fractures and analysis of

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Fig. 8. Sequence characteristics of multi-scale fracture-pore network in Kuqa foreland thrust belt. (a) Well KS8003, 6 779.40 m, multi-scale fracture network in core; (b) full-size scanning of (a), micro-CT; (c) Well KS8003, 6 779.40 m, micro fracture-pore network cutting through grains, CLSM; (d) Well KS201, 6511.26 m, ultra-micro fracture-pore network along feldspar grain edge, CLSM; (e) Well KS8003, 6 778.76 m, matrix crack-micropore network in grain, SEM.

fracture parameters from electric imaging logging of water-based mud well, we estimated the fracture apparent porosity is 0.02%0.05%, which accounts for only 0.15%2.1% (on average 0.51%) of total porosity. The fracture porosity measured from core is mainly from 0.01% to 0.04%, with maximum value of 0.66%, and the number of core samples with porosity >0.10% only account for 18%. The effect of fractures on permeability is obvious in the direction parallel to trending of fractures. Full diameter permeability of core samples with or without fractures indicates that the permeability can be increased by 2 to 3 orders due to development of fractures, the average permeability increased from (0.010.10)× 103 μm2 for samples without fractures to (1100) ×103 μm2 for samples with fractures. For example, CT scan data shows that the permeability of fractured sandstone is 13.84×103 μm2, but the measured matrix permeability is only 0.014×103 μm2 for samples from 6 513.356 513.50 m in Well KS201; and the matrix permeability in Well KS8 is generally (0.001 0.500)×103 μm2, but that is (0.12725.380)×103 μm2 in core samples with fractures. However, the effect of fracture on permeability is not obvious in the direction perpendicular to the trending of fractures (Fig. 9). In addition, the numerical simulation of yield and fracture permeability shows that wells with more fractures have higher open-flow capacity than wells with few fractures in the same structure (Fig. 10). Combined with the exploration practice, the multi-scale fractures have little contribution to the porosity of sandstone reservoir, but have remarkable improvement for

Fig. 9. Comparison of permeability vertical and parallel to fracture direction in full diameter tight sandstone.

sandstone reservoir permeability in the study area. Without fractures, it would be difficult to form large-scale effective reservoirs. Therefore, high and stable yield of natural gas is the result of the matching of multi pore networks and multi fracture system networks in the Kuqa foreland thrust belt. When macro fractures are developed, but other small scale fractures are less-developed, the well usually have high but unstable gas yield due to low permeability of tight sandstone. In contrast, when matrix cracks and ultra-micro fractures are well developed, but macro fractures and micro fractures are less-developed, the gas yield is mainly from percolation through matrix pore-throat, so the productivity is stable but

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Fig. 10. Numerical simulation of fracture permeability and open-flow capacity of wells in Keshen 8 gas field.

low. The reservoir of high and stable production in the Kuqa foreland thrust belt is often characterized by development of both multi scale fracture system (macro, micro and ultra-micro fracture and matrix) and multi scale pore system (macro pores, medium-small pores and micro pores).

6.

References

Conclusions

The multi-scale fracture network in the Kuqa foreland thrust belt can be divided into macro tectonic fracture (aperture>100 μm), micro tectonic fracture (aperture=10100 μm), ultra-micro diagenesis fracture (aperture=110 μm) and nano matrix crack (aperture <1 μm). The CategoryⅠmacro tectonic fractures cut across sand body, form dominant hydrocarbon migration pathways, and increase permeability. The CategoryⅡ micro tectonic fractures cut matrix, connect matrix large pore and improve the permeability. The Category Ⅲ ultra-micro grain edge diagenesis fractures connect medium-small pores, and improve network connectivity and the natural gas charging efficiency. The Category Ⅳ nano matrix cracks connect intragranular micro pores, expand the reservoir space, and increase reserves. The contribution rate of multi scale natural fracture system to porosity of reservoir is low, but in the direction parallel to the trending of fractures, the permeability can be increased by 2 to 3 orders. Macro and micro tectonic fractures in the Kuqa foreland thrust belt have three formation stages, the first and the second stage fractures formed before early Pliocene when the large scale hydrocarbon migration had not begun. The third stage fractures formed since late Pliocene in the same time or slightly later than the mass hydrocarbon accumulation. High and stable gas production of tight sandstone reservoir in the Kuqa foreland thrust belt is the result of matching of multi-scale fracture (macro, micro and ultra-micro and matrix) and the multi-scale pore system (large pore, medium-small pore and micro pores).

Acknowledgments The authors thank Sun Xiongwei and Tang Yangang from Tarim Oil field Company for their constructive help during the research. The authors are particularly grateful to Qin Yujuan, Hu Yuanyuan and Wei Dongxiao from Key Laboratory of Carbonate Rock of PetroChina for helping during experimenting.  503 

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