Vol:.(1234567890)

Petroleum Science (2021) 18:728–741https://doi.org/10.1007/s12182-021-00567-w

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ORIGINAL ARTICLE

Effectiveness and petroleum geological significance of tectonic fractures in the ultra‑deep zone of the Kuqa foreland thrust belt: a case study of the Cretaceous Bashijiqike Formation in the Keshen gas field

Rong‑Hu Zhang1,2 · Ke Wang2 · Qing‑Lu Zeng1 · Chao‑Feng Yu1 · Jun‑Peng Wang1

Received: 12 July 2020 / Accepted: 26 November 2020 / Published online: 9 June 2021 © The Author(s) 2021

AbstractThe buried depth of the gas-producing reservoir in the Kuqa foreland thrust belt of the Tarim Basin exceeds 6000 m. The average matrix porosity of the reservoir is 5.5%, and the average matrix permeability is 0.128 × 10−3 μm2. In order to reveal the characteristics and effectiveness of ultra-deep fractures and their effects on reservoir properties and natural gas produc-tion, outcrops, cores, thin section, image logs and production testing data are used to investigate the effectiveness of tectonic fractures in ultra-deep reservoirs in the Kuqa foreland thrust zone, and the corresponding geological significance for oil and gas exploration and development are discussed. Tectonic fractures in the thrust belt include EW-trending high-angle tensile fractures and NS-trending vertical shear fractures. The former has a relatively high filling rate, while the latter is mostly unfilled. Micro-fractures are usually grain-piercing-through cracks with width of 10–100 microns. In the planar view, the effective fractures are concentrated in the high part and wing zones of the long axis of the anticline, and along the vertical direction, they are mainly found in the tensile fracture zone above the neutral plane. The adjustment fracture zone has the strongest vertical extension abilities and high effectiveness, followed by the nearly EW longitudinal tensile fracture zone, and the netted fracture zone with multiple dip angles. The effectiveness of fracture is mainly controlled by fracture aperture and filling degrees. Effective fractures can increase reservoir permeability by 1–2 orders of magnitude. The higher part of the anticline is associated with high tectonic fracture permeability, which control enrichment and high production of natural gas. The netted vertical open fractures effectively communicate with pores and throats of the reservoir matrix, which forms an apparent-homogenous to medium-heterogeneous body that is seen with high production of natural gas sustained for a long term.

Keywords Fracture effectiveness · Ultra-deep · Reservoir · Foreland thrust belt · Kuqa depression · Geological significance

1 Introduction

Tight sandstone gas is an important unconventional hydro-carbon resource (Jia et al. 2012; Guo et al. 2013; Dai et al. 2012; Zhang et al. 2014; Yang et al. 2018; Zhang et al.

2014; Li et al. 2012; Zeng 2009; Zhan et al. 2017; Zeng et al. 2012). The strong compaction effect (including both normal burial compaction and abnormal structural compres-sion) results in low matrix porosity (< 10%) and low per-meability (air permeability < 1 × 10−3 μm2 or overburden permeability < 0.1×10−3 μm2) in tight sandstone reservoirs, therefore an industrial-scale natural gas reservoir cannot be formed by the rock matrix porosity alone (Fossen 2010; Orlando et al. 2010; Solum et al. 2010; Lander and Laubach 2015; Ju et al. 2014). The presence of tectonic fractures is a very important factor for improving the physical properties of tight sandstone reservoirs and increasing natural gas pro-ductivity (Ding et al. 2015; Zeng et al. 2010a, b; Estibalitz et al. 2016; Feng et al. 2018). Therefore, the systematic study

Edited by Jie-Hao and Chun-Yan Tang

* Rong-Hu Zhang zrh_hz@petrochina.com.cn

1 PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100086, China

2 PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, Zhejiang, China

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of the effectiveness of tectonic fractures in tight sandstone reservoirs is of great significance for discovering tight sand-stone sweet spot and improving gas reservoir productivity (Kara et al. 2017; Lander and Laubach 2015; Nian et al. 2017).

The Keshen gas field is located in the deep zone of the Kelasu tectonic belt in the Kuqa depression, the Tarim Basin, and its target formation, the Lower Cretaceous Bashi-jiqike Formation, is a typical ultra-deep (> 6000 m) natu-rally fractured tight sandstone reservoir (Wang 2014; Zeng et al. 2008; Lai et al. 2017a). Regarding the foreland thrust belt of the Kuqa depression and the tectonic fractures in Kes-hen gas field, numerous studies have carried out in various aspects before, including quantitative description and predic-tion of fractures (Zeng and Wang 2014; Zhao et al. 2014; Wang et al. 2014; Lai et al. 2019), indicative implications of regional stress field inversion provided by fractures (Zhang and Wang 2004; Zeng et al. 2004), formation mechanisms of fractures and influential factors (Zeng et al. 2010a, b; Liu et al. 2017; Chang et al. 2014; Gong et al. 2015). These stud-ies have revealed that fractures lead to obvious modification of ultra-deep tight sandstone reservoirs (Wang et al. 2018). The Cretaceous Bashijiqike Formation is found with exten-sive development of tectonic fractures, which controls high productivity of natural gas in the Keshen gas field. In this paper, on the basis of analysis of tectonic fracture types and characteristics as well as information obtained from cores, thin sections and image logs, the effectiveness of tectonic fractures is studied, with the aim to provide geological guid-ance on the efficient development of the Keshen gas field.

2 Geological settings

The Tarim Basin is the biggest petroliferous basin in China (Lai et al. 2020), and a large number of giant gas fields have been found in the Kuqa depression recently (Ju and Wang 2018). In this paper, the study area—the Keshen gas field is located in the Kelasu tectonic belt of the Kuqa depression, the Tarim Basin, which is rich in natural gas resources, with multiple faulted anticline gas reservoirs such as KeS5, KeS6, KeS2, KeS8, KeS9, and KeS13 (Fig. 1). The proven geological reserves of natural gas exceed 10,000×108 m3 (Wang 2014). The gas-bearing tar-get layer of the Keshen gas field is the Bashijiqike Forma-tion of the Lower Cretaceous, which is mainly composed of reddish brown fine sandstone, siltstone, medium sand-stone and thin mudstone, with burial depths of 5500–8500 m, an average thickness of about 300 m, matrix porosity of 1.5%–5.5%, and matrix permeability of (0.01–0.1) × 10−3 μm2 (Zhang et al. 2014). Data from cores and image

logs indicate that tectonic fractures with a density higher than 0.3  m−1 have developed in the Bashijiqike Forma-tion. The physical property measurements of fractured cores show that the permeability provided by fractures is of (0.1–10.0) × 10−3 μm2, which suggests a typical low-porosity naturally fractured tight sandstone reservoir.

3 Tectonic fracture characteristics

The observation of core and interpretation of FMI image logs indicate that there are both tensile and shear fractures developing in the Keshen gas field. The fracture surface of the tensile fracture is crooked unevenly, with an aperture of 0.5–1 mm, and the tensile fractures are mostly high-angle fractures, filled with calcite, dolomite or anhydrite. Moreover, a few of these filled fractures can be re-opened and become effective fractures due to the later tectonic stress. The shear fractures, which are mostly vertical, with flattened surfaces and long extension, often penetrate through the core with small aperture (< 0.5 mm), and most of them are unfilled. Micro-fractures are usually grain-piercing-through cracks with a width of 10–100 microns, which are often re-opened along the early filled fractures (Lai et al. 2017a; Wang et al. 2016, 2018). In FMI image logs, tectonic fractures are mainly present themselves in the group form of the parallel, echelon, broom-like, and adjustment patterns (Fig. 2), among which the parallel pat-tern is dominant and displayed the combination of multi-ple sinusoids with identical or similar shapes. The linear density of tectonic fractures interpreted by image log is generally between 0.2 and 2.3 fractures/m, with an average of about 0.6 fracture/m.

The interpretation results of image logs indicate that the tectonic fractures in the Keshen gas field include EW-trending tensile fractures and NS-trending shear fractures. Considering the structural evolution, reservoir sedimen-tary burial history and the results of temperature determi-nation based on the carbon and oxygen isotopes of tectonic fracture fillings (Ding et al. 2015; Zeng et al. 2010a, b; Wang 2014), it is considered that the EW-trending tec-tonic fracture is suggested to be related to the Cretaceous and Paleogene regional extension, the bending and arch-ing of the anticline, and the abnormal high pressure of Neogene. Such fractures are formed in the early stage and thus the filling rate is relatively high. The NS-trending tectonic fracture is mainly controlled by the compressive tectonic stress in the mid-late Himalayan Period along the NS-direction, and it occurs relatively late, which results in that most of them are not filled.

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KeS2

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4 Results

4.1 Distribution law of effective fractures

4.1.1 Differentiated characteristics of effective fractures in the thrust anticline

The effective fractures are defined as factures that can per-meate fluids effectively. In other words, they are fractures that have not been completely filled by minerals. Unfilled and semi-filled fractures are both effective fractures. Induced fractures and borehole breakouts, which appear at two sym-metrical azimuths, are easily recognized on images and to be differentiated from natural fractures (Prioul et al. 2007; Ameen et al. 2012; Etchecopar et al. 2013; Nian et al. 2016; Ju et al. 2017; Lai et al. 2021).

Through the observation of similar outcrops in the field (Fig. 3) and the analysis of downhole measurements, it is found that the fractures in the long axis of the anticline are mostly dominated by large vertical fractures penetrating

through strata. In addition, the distribution of the fractures is generally parallel to the long axis, with large fracture aper-ture, low filling degrees and high effectiveness. Along the two wings, the fracture density increases slightly, but the fracture aperture decreases gradually, accompanied by the growing filling degrees. Near the thrust fault, the fractures are mostly densely networked, which are often filled with the late-stage gypsum (outcrops) or calcite, dolomite and other minerals. The fracture effectiveness is inferior, and with the increasing distance to the fault, the fracture density decreases. In the north wing of the anticline, interlayer and oblique fractures often develop, with low filling degrees and good effectiveness. For instance, the filling rate of fractures found on cores from Well KeS8 at the higher part of the long axis of the KeS8 anticline is only 26%, the fracture aper-ture is 0.275 mm, and the linear density is 0.43 fractures/m, indicating a highly satisfactory fracture effectiveness. As for Well KeS801 located at the wing of the fold, the core frac-ture filling rate is 35%, and the aperture is 0.21 mm, while the density is 0.33 fractures/m (Fig. 4).

No.Fracture

grouppattern

Featuredescription Origin mechanism Core characteristics Imaging logging characteristicsPattern

diagram

Para

llel p

atte

rnEc

helo

n pa

ttern

Broo

m-li

ke p

atte

rnAd

just

men

t pat

tern

A group of nearly parallel preferentialfractures

A group of echelon inclined fractureswith no end-to-endconnection, and thescale of a singlefracture is relativelysmall compared tothe preferential fracture

A preferential fracture and acluster of secondaryassociated fractures,and the associated fractures are broom-shaped, ending atthe preferential fracture

Two preferential fractures and a group of associated fractures, and the associated fractures are nearly parallel and cut off between the preferential fractures

Formed under the tensile stress perpendicular to the fracture surface

Pulled open under tension, or formed during shear-slidingadjustment undertensile stresses

In the early stage, it was subjected to shear stresses, andin the later stage, it was pulled apart under tensile stresses

In the early stage, it is affected by tensile stresses, and in the later stage, it is formed by shear slidingadjustment

a

b

c

d

KS2-2-4#

Fig. 2 Tectonic fracture characteristics of the Cretaceous Bashijiqike Formation in the Keshen gas field, the Kuqa depression

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Stress field modeling analysis is commonly used to unravel mechanical development of a fault-related fold (Sun et al. 2017). The magnitudes of tectonic stress were correlated with core test (Acoustic emission (AE) analysis) stress magnitudes. The numerical simulation of the curvature and stress change of the bending anticline shows when the rock is bent and deformed into the anticline shape, due to existence of a neu-tral plane, the strata above the neutral plane are subjected to the tensile strain, where the formed tectonic fracture is mainly tensile fractures with large aperture (Fig. 5) (Sun et al. 2017). At the same time, as these parts are located at the top of the gas reservoir, less affected by formation water, the fracture filling degree is low and the effectiveness is good, yet with a small fracture line density is small. On the other hand, the strata below the neutral plane are mainly subjected to com-pressive strains, where the formed fractures are mainly netted fractures with small aperture, and the nearby formation water causes high filling degrees. Therefore the effectiveness of these tectonic fractures is low, yet associated with high fracture line density (Table 1). The thickness of the tension zone on the anticline hinge and the higher part of the hinge zone is thick, and the thickness becomes thinner towards the two wings. The thickness of the tension zone near the fault or the steep south wing is large; the thickness of the gentle wing far away from the fault is relatively smaller (Fig. 6) (Sun et al. 2017).

4.1.2 Development patterns of effective fractures

Due to the tectonic mechanics-based origin, the spatial dis-tribution of the longitudinal tensile fracture zone, adjustment fracture zone and netted fracture zone in the Keshen gas field is closely related to the high productivity of gas wells. The actual drilling and test data show that among the three types of fracture zones, the NS adjustment fracture zone has the strongest longitudinal extension ability and high effective-ness, followed by the EW longitudinal tensile fracture zone and at last the netted fracture zone with multiple dip angles. During the deployment of well sites, it is suggested in princi-ple that the location deployment and completion of appraisal wells and development wells should first select the area of the NS adjustment fracture zone that communicate with the EW longitudinal tensile fracture zone (Fig. 7a), where the fracture effectiveness is high and high-rate industrial gas streams can be obtained without reservoir stimulation or with only acidiz-ing (e.g., Well KS2-2-8). For the borehole that is confirmed later by actual drilling to encounter the relatively independent EW longitudinal tensile fracture zone (Fig. 7b), the fracture filling degree is relatively high and large-scale reservoir stimu-lation such as acidizing fracturing or propped fracturing are often required.

5 Discussion

5.1 Main factors controlling fracture effectiveness

Fracture effectiveness refers to the fluid seepage capacity through fractures, mainly represented by the fracture aper-ture and filling degrees of fractures. The effectiveness of open fractures is good, while that of filled fractures and closed fractures is poor. The comprehensive parameter of fracture effectiveness is mainly reflected by the influence of fractures on reservoir permeability, namely, fracture permeability. On the basis of the tectonic stress field simu-lation, with the constraints of outcrop fracture model and fracture parameters identified by core description, fracture ICT (industrial computed tomography) scanning (Lai et al. 2017b) and FMI image log of wells (Lai et al. 2018), com-bined with research results in lost circulation while drilling and well testing interpretation and introduction of the con-cept of fracture filling degrees, the three-dimensional spatial distribution of fracture effectiveness (fracture permeability) is determined (Eq. 1) (Song et al. 2001), with respect to the fracture density, aperture and filling degrees in the Keshen gas field, using the tectonic fracture-tectonic stress correla-tion model developed in previous studies (Wang et al. 2016). In general, the fracture permeability is in direct proportion to the fracture line density and the cube of the fracture aperture, and in inverse proportion to the filling degree. Among these parameters, the most sensitive control factor is the fracture aperture, which is mainly controlled by the structural pattern and the fracture tensile zone. The thickness of the tensile zone of the faulted anticline gradually increases with the increase in the thrust shortening rate and the decrease of the wing angle. The thickness of the transition zone and tensile zone at the southward hinge zone is larger, and the tensile zone thickness of the southern wing is larger than that of the northern wing. With the increase in the thrust shortening rate, the tensile-transition zone gradually extends from the hinge zone to both wings (Fig. 8).

where K is the fracture permeability; KfX, KfY, and KfZ rep-resent the fracture permeability in three directions, respec-tively, 10−3 μm2; KfXi, KfYi, and KfZi represent the fracture permeability component in three directions, 10−3 μm2; Dlf is the fracture line density, fractures/m; C is the fracture filling coefficient (measured quantitatively by core observation and ICT scanning). When the fracture is not filled, C = 0; when

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the fracture is completely filled, C = 1. b is the fracture aper-ture, mm; n is the permeability tensor (obtained from the fracture occurrence of the core observation or FMI logging).

The filling degree in different structural parts is an impor-tant factor influencing fracture effectiveness. Core observa-tion demonstrates that fractures on the limb always have higher filling degree than that on the hinge of anticline (Fig. 4a), especially which close to the boundary faults, where the fracture filling degree can reach to 70% (Well Keshen207).

5.2 Petroleum geological significance of effective fractures

5.2.1 Contributions/effects of fractures to/on reservoir storage space and permeability

According to the ICT experimental study of fractured core, for tight sandstone reservoirs, the porosity provided by effec-tive fractures is generally less than 0.5% (Fig. 9). Therefore, tectonic fracture usually does not exist as the main type of reservoir space, but as an efficient seepage channel, which can greatly improve the permeability of reservoir. The core measurement of Keshen gas field shows that the matrix permeability without fractures is generally (0.01–0.1) × 10−3 μm2, while the permeability with fracture is generally (1–10) × 10−3 μm2 (Fig. 10), which is 1–2 orders of magni-tude higher than the matrix permeability, and the maximum permeability of single well test is 116.5 × 10−3 μm2, proving that the effective fracture significantly improves the perme-ability of tight sandstone reservoir.

5.2.2 Influence of fracture‑pore‑throat homogeneous network on natural gas productivity

Core plug testing and well logging interpretation dem-onstrate that the matrix physical property has not signifi-cant variations in different structural belts. Therefore, the

development of effective fractures in tight sandstones will become the most important factors influencing gas produc-tion (Lai et al. 2017a; Ameen et al. 2012).

The gas reservoir of the Kuqa foreland thrust belt is all associated with a faulted anticline structure. Due to the con-centrated tectonic strain in the high part of anticline, tectonic fractures relatively extensively develop in the higher part, with larger aperture and lower filling degrees than those of tectonic fractures in other structural positions. The netted tectonic fracture has a good communication with reservoir matrix pores and throats, forming superior seepage channels, which are important guarantees for high and stable produc-tion. When the netted open fractures and vertical open frac-tures densely develop in the higher part of the structure, they communicate effectively with the reservoir matrix pores and throats (generally fine mesopores and fine throats, classi-fied in accordance with SYT 6285-1997 Evaluating Methods of Oil and Gas Reservoirs), and thus form a fracture-pore-throat seepage network system. The formation fluid pressure is balanced with the supply from fractures to the wellbore by the fluid supply from matrix rock to fracture, which leads to an apparent-homogenous to medium-heterogeneous body (Fig. 11), which can present high production and moreover long-term stable production of natural gas.

Many wells including KeS8, 801, 802, 8-1, 8-2, 8003, 8004 are of this type. The 6746-6897 m section of Well KeS8003 is recovered with a 10 mm choke after completion of acid fracturing, resulting in daily output of 113.604 × 104 m3. On November 12, 2013, it starts the production testing, with a daily natural gas production of 88 × 104  m3. After 54 days of production, the tubing pressure drops from 88.2 to 85.09 MPa, and the monthly pressure drop rate is 1.73 MPa/month, which indicates the tubing pressure is basically stable (Fig. 12).

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0.35

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Well name

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0.08

0.33 0.530.36

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0.43 0.700.28

0.26

1.22

KS801 KS807

KS804

KS800KeS8

KeS8-8KeS8-2

Wing Higher part Wing Higher part

Fig. 4 Histograms showing differences of fracture characteristics at different structural locations in Block 8 of Keshen in the Kuqa depression. a fracture filling rate; b effective fracture aperture; c image log FMI-based fracture line density. The wells marked as red, orange and yellow colors correspond to the highest, higher and high gas production wells, respectively

1.5

2.0

1.7

2.2

1.9

2.4

2.5 2.7

Mises stress, MPa

12.0 60.7 109 158 450401353304255207

Fig. 5 Finite element simulation results of tectonic stresses in anticlines with different bending degrees

736 Petroleum Science (2021) 18:728–741

1 3

Tabl

e 1

Zon

al c

hara

cter

istic

s of f

ract

ures

in re

vers

e th

rust

antic

line

Stre

ss se

ctio

nRe

sisti

vity

, Ω·m

Aco

ustic

tim

e di

ffere

nce,

μs

·ft−

1

Max

imum

pr

inci

pal s

tress

, M

Pa

Hor

izon

tal s

tress

di

ffere

nce,

MPa

Frac

ture

com

bina

tion

char

acte

ristic

sFr

actu

re d

evel

opm

ent

degr

eeRe

serv

oir m

icro

ch

arac

teris

tics

Com

mon

eng

inee

ring

com

plex

issu

es

Tens

ile se

ctio

n13

–40

56–6

913

7–18

520

–35

Sim

ilar s

hape

s, do

mi-

nate

d by

hig

h-an

gle

eche

lon

frac

ture

s, w

ith lo

cal a

ssoc

i-at

ed fr

actu

res

Low

, goo

d eff

ectiv

e-ne

ssH

igh

surfa

ce p

oros

ity,

the

rese

rvoi

r spa

ce

is m

ainl

y co

mpo

sed

of in

ter-

and

intra

-gr

anul

ar d

isso

lutio

n po

res,

and

dis-

solu

tion

is re

lativ

ely

com

mon

Loss

of c

ircul

atio

n

Tran

sitio

n se

ctio

n22

–60

55–6

015

5–19

025

–42

Dom

inat

ed b

y hi

gh-

angl

e fr

actu

res,

and

the

asso

ciat

ed fr

ac-

ture

s with

low

ang

le

grad

ually

incr

ease

Low

to m

ediu

m,

mod

erat

e eff

ectiv

e-ne

ss

Bet

wee

n te

nsile

sec-

tion

and

com

-pr

essi

ve-to

rsio

nal

sect

ion

Loss

of c

ircul

atio

n,

drill

ing

tool

stuc

k, b

it bo

unci

ng, d

eman

ding

fo

r fish

ing,

logg

ing

cabl

e stu

ckC

ompr

essi

ve-to

rsio

nal

sect

ion

38–1

0550

–57

160–

190

34–4

5D

omin

ated

by

low

an

gle

asso

ciat

ed

frac

ture

s, w

ith a

sm

all n

umbe

r of

high

-ang

le fr

actu

res

in lo

cal a

reas

Med

ium

to h

igh,

poo

r eff

ectiv

enes

slo

w su

rface

por

os-

ity (<

1%),

and

the

dire

ctio

nal a

rran

ge-

men

t of p

artic

les i

s co

mm

only

obs

erve

d in

the

field

-em

issi

on

scan

ning

ele

ctro

n m

icro

scop

y (S

EM)

and

cast

thin

sec-

tions

737Petroleum Science (2021) 18:728–741

1 3

Tensile zone

Compressive-torsional zone

Regional Zone

Syn-foldingfractures

Pre-foldingfractures

Pre-folding fractures

0 1 km

Fault Fractures Extensional area Transition area Shortening area

NDamage Zone

Regional Zone

Pre-folding fractures

Syn-folding fractures Post-foldingfractures

Finite neutral lineIncremental neutral line

Transition zone

Fig. 6 Zonal fracture development patterns of the faulted anticline of the Keshen gas field, the Kuqa depression

D B

B

D

C

C

B

S N

A

A

A B C D

Early EW tensilefractures

Late EW tensilefractures

NS adjustmentfractures

Early nettedfractures

Late nettedfractures

Adjustment fracturedevelopment zone

Longitudinal tensile fracture development zone

Netted fracture development zone

Fracture underdevelopedzone

Fault line

Tensile zone

Transition zone

Compressive-torsional zone

Fracture bandwidth

Fracture bandwidth

40 m 30 m 20 m

Fracture bandwidth

Fig. 7 The fracture zones and the development pattern of fracture effectiveness in the Bashijiqike Formation in the Keshen gas field in the fore-land thrust zone of the Kuqa depression

738 Petroleum Science (2021) 18:728–741

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6 Conclusion

(1) Tectonic fractures in the thrust belt include EW-trend-ing high-angle tensile fractures and nearly NS-trending vertical shear fractures. The former is related to the Cretaceous and Paleogene EW regional extension and the bending and arching of the anticline, and the frac-ture filling rate is relatively high. The latter is related to the NS tectonic compression in the mid-late Himalayan

Period, and most of these fractures are unfilled. Micro-structural cracks are usually grain-piercing-through cracks with width of 10–100 microns. In FMI image log, tectonic fractures are mainly arranged in parallel.

(2) In the planar view, the effective fractures are concen-trated in the high part and the wing zones of the long axis of anticline, and along the vertical direction they mainly develop in the tensile fracture zone above the neutral plane. The adjustment fracture zone has strong longitudinal extension abilities and high effectiveness,

Transition belt (neutral plane)

Transition belt (neutral plane)

-0.80 -0.67 -0.53 -0.40 -0.27 -0.13 0 0.13 0.27 0.40 ×107

Transition belt (neutral plane)

Transition belt (neutral plane)

Shortening rate 5% — Angle between two wings1600

Shortening rate 15% — Angle between two wings1200

Shortening rate 10% — Angle between two wings1400

Shortening rate 20% — Angle between two wings1000

Fig. 8 Simulation diagram of dynamic evolution of the tension zone and transition zone of the Keshen gas field in the foreland thrust belt of Kuqa depression

Well name

Poro

sity

, %

Fracture porosity0.5

0.4

0.3

0.2

0.1

0

Fracture porosity (filled)

KeS2

05

KeS2

-2-3

KeS2

-2-4

KeS2

-2-5

KeS2

-2-8

KeS2

-2-1

4

KeS2

07

KeS2

08

0.38

0.26

0.16

0.06

0.100.08

0.12

0.03

0.07

0.02

0.07

0.03

0.11

0.22

0.39

0.20

Fig. 9 Histogram of fracture porosity of the Keshen gas field in the foreland thrust zone of Kuqa depression

KeS8

02,

7238

.65~

7238

.75

KeS8

003,

6777

.23~

6777

.36

KeS8

003,

6778

.46~

6778

.57

KeS8

003,

6778

.9~6

779.

01

KeS8

003,

6779

.22~

6779

.31

KeS2

02,

6765

.25~

6765

.35

KeS2

02,

6797

.73~

6797

.84

KeS2

06,

6707

.52~

6707

.64

Sampling depth, m

Perm

eabi

lity,

10-3

µm2

100.000

10.000

1.000

0.100

0.010

0.001

0

1.093

0.117

8.260

3.740

0.426

0.003

1.153

25.768

0.427

0.983

2.717

0.0530.028

0.127

0.026

0.001

Matrix permeabilityMatrix permeability with fractures

Fig. 10 Matrix permeability and fracture permeability of the Keshen gas field in the foreland thrust zone of the Kuqa depression

739Petroleum Science (2021) 18:728–741

1 3

followed by the nearly EW longitudinal tensile fracture zone, and the netted fracture zone with multiple dip angles.

(3) Fracture effectiveness is mainly controlled by fracture density, aperture and filling degrees, among which the fracture aperture is the most critical and most sensi-tive factor. High fracture aperture is mainly seen in the fracture tensile zone. The tectonic fracture can directly increase permeability of the reservoir matrix by 1–2 orders of magnitude, and increase the porosity by less than 0.5%. The higher part of the thrust anticline is characterized by concentrated tensile strains, high frac-ture density, large aperture, low filling degrees, and thus good fracture effectiveness. Such zones are asso-ciated with high tectonic fracture permeability, which controls enrichment and high production of natural gas.

(4) When the netted open fractures and vertical open frac-tures densely develop in the high part of the structure, they communicate effectively with the reservoir matrix pores and throats, which forms a fracture-pore-throats seepage network system. The formation fluid pressure is balanced with the supply from fractures to the well-bore by the fluid supply from matrix rock to fracture, which leads to an apparent-homogenous to medium-heterogeneous body that is seen with high yield and long-term stable production of natural gas.

Res

ervo

ir P

rodu

ctio

n Te

stin

g C

hara

cter

istic

s O

pen

flow

test

resu

lts,

104 m

3 ·d-

1

Sta

tic c

hara

cter

istic

s of

rese

rvoi

r stru

ctur

esR

eser

voir

prop

ertie

s

Rep

rese

ntat

ive

wel

ls

Res

ervo

irco

ntro

lfa

ctor

sW

ell

perfo

rman

ce ty

pes

Hom

ogen

eity

of

doub

le-p

oros

itym

edia

Ty

pica

l pro

duct

ion

test

ing

curv

esFr

actu

rech

arac

teris

tics

Mat

rix p

ores

Mat

rixth

roat

Frac

ture

por

e th

roat

conf

igur

atio

n ty

peS

truct

ural

mod

el (l

inea

rfra

ctur

e, s

tar-

flake

por

e)P

oros

ity, %

Per

mea

bilit

y,10

-3 μ

m2

Hig

h yi

eld

with

stab

le tu

bing

pres

sure

s (Ia

)

The

fract

ures

are

wel

l dev

elop

ed,

and

the

supp

ly o

fm

atrix

rock

s to

fract

ures

mat

ches

the

supp

ly o

ffra

ctur

es to

the

wel

lbor

e

>200

Den

se n

ette

dop

en fr

actu

res

Mai

nly

mes

opor

es

Mai

nly

Mic

roth

roat

Hig

hly

effic

ient

and

appa

rent

-hom

ogen

eous

,w

ith n

ette

d fra

ctur

esan

d m

ediu

m h

ole

mic

ro th

roat

Wel

l log

ging

:6.

0–10

.0

Wel

l log

ging

:0.

06–0

.08

Test

ing:

>100

KeS

2-1-

6,2-

1-14

,80

03,

801,

8004

,80

2etc

Frac

ture

,di

ssol

utio

n,lit

hofa

cies

Hig

h yi

eld

yet

with

slo

wde

clin

e (Ib

)

Frac

ture

s ar

ede

velo

ped,

the

supp

ly c

apac

ityof

mat

rix ro

cks

to fr

actu

res

isle

ss th

an th

atof

frac

ture

s to

wel

lbor

e

150–

200

Den

se n

ette

dan

d ve

rtica

lop

en fr

actu

res

Mai

nly

fine-

to-m

ediu

mpo

res

Mai

nly

fine

thro

at

Med

ium

-het

erog

eneo

us,

with

ver

tical

frac

ture

s,fin

e m

esop

ores

, and

fine

thro

ats

Wel

l log

ging

:6.

0–8.

0

Wel

l log

ging

:0.

05–0

.08

Test

ing:

10–1

00

KeS

2-2-

4,2-

2-8,

3,20

1,20

3,20

6,8,

8-1,

8-2

Frac

ture

,di

ssol

utio

n,lit

hofa

cies

Dai

ly p

rodu

ctio

n

Tubi

ng p

ress

ure

100 0

100

012

Test

tim

e, m

Production rate, 104m3/d

Tubing pressure, MPa

0

Dai

ly p

rodu

ctio

n

Tubi

ng p

ress

ure

100 0

100

012

Test

tim

e, m

Production rate, 104m3/d

Tubing pressure, MPa

0

Fig. 11 Contributions of the reservoir fracture-pore-throat network system to hydrocarbon production in the Keshen area of the foreland thrust belt of the Kuqa depression

▸

740 Petroleum Science (2021) 18:728–741

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Acknowledgements This work was supported by the National Key Research and Development Project (No. 2019YFC0605501) and the National Science and Technology Major Project (2016ZX05003001). The authors would like to thank PetroChina Tarim Oilfield Company for their assistance in providing data. We thank the editors of Petroleum Science and the reviewers, whose constructive comments improve the paper.

Open Access This article is licensed under a Creative Commons Attri-bution 4.0 International License, which permits use, sharing, adapta-tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.

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Highly heterogeneousbody with high yieldand rapid decline

Matrix homogeneousbody with low andstable production

KeS1

KeS101

KeS3

KeS202KeS201

KeS205KeS203

KeS204

KeS207

KeS206KeS102

keS301

KeS2KeS2-1-5

KeS2-2-5

KeS2-2-3

KeS2-2-4KeS2-2-8KeS106

KeS2-1-3

KeS105 KeS2-2-14KeS2-1-7

KeS2-1-11KeS2-1-1

KeS2-2-1KeS208KeS2-1-12

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KeS3-1

KeS2-2-18

0 10 km

Equivalent apparent-homogeneous body with

high and stable production

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Fig. 12 Distribution of high-production stable-production zones in Block 2, the Keshen area, the foreland thrust belt, the Kuqa depression

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