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Research Article
Relationship between geological structure and marine shale gas preservation
conditions in the western Middle Yangtze Block
*,**,***
Chen Kongquan*, Zhang Douzhong &Tuo Xiusong
Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, Hubei, 430100, China
Received 19 February 2020; accepted 25 April 2020
Available online 21 November 2020
Abstract
Lower Paleozoic dark shale is developed in the western Middle Yangtze Block, which lays a material foundation for the enrichment and
accumulation of marine shale gas. In order to ascertain the control action of geological structures on the differential preservation of shale gas and
reveal the key factors in shale gas preservation, this paper firstly analyzed the structure characteristics of this area, carried out structure pattern
recognition and structural belt division, and studied structural deformation mode and intensity. Based on this, the relationships between different
structure styles and shale gas preservation conditions were analyzed. Finally, combined with the structural deformation and the lithofacies
paleogeographic characteristics of marine shale, the favorable exploration zones of shale gas were proposed. And the following research results
were obtained. First, the western Middle Yangtze Block can be divided into four structural deformation belts, and three types of piggyback
structural patterns have been identified, including restricted type, weakly reformed type and strongly reformed type. Second, the restricted type is
located in the northwestern part of Hunan and Hubei Provinces. In this pattern, piggyback structure is incomplete and thrust belt and
Compressive fold belt are developed. Third, the weakly and strongly reformed types are located in the western parts of Hunan and Hubei, and
Wulingshan area, respectively. They both have complete piggyback structures, but the former has lower deformation intensity and has never
undergone the late superimposed reformation. Fourth, there are three structural transfer belts in the western Middle Yangtze Block, i.e. the
structural transfer belt between the East Sichuan faultefold belt and West HunaneHubei faultefold belt, the structural transfer belt between
West HunaneHubei faultefold belt and Wulingshan faultefold belt, and the structural transfer belt between the outcrop and the hinterland of
Middle Yangtze Block. The first one is structurally transformed at the Qiyueshan fault. The East Sichuan faultefold belt on the west is an
ejective fold with low fault density and formation denudation intensity, where shale gas is enriched in anticlines and slopes; while the West
HunaneHubei faultefold belt on the east is a trough-like fold with strong faulting and high formation denudation intensity, where shale gas is
enriched in residual synclines. In conclusion, shale gas preservation conditions of Upper Ordovician Wufeng FormationeLower Silurian
Longmaxi Formation in this area are the best in Zigui syncline, thrustedetachment zone and western margin of Qiyueshan fault. The favorable
exploration areas of shale gas of Lower Cambrian Niutitang Formation are distributed in the western flank of Yichang Slope, Kaixian thrust
zone, compressive fold zone and thrustedetachment zone.
©2020 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Middle Yangtze Block; Western area; Early Paleozoic; Texture and structure; Tectonic deformation; Lithofacies paleogeography; Shale gas; Preser-
vation condition; Exploration zone
*
Project supported by the National Science and Technology Major Special Project “Large Oil and Gas Fields and CBM Development”(No. 2016ZX05002), and
China Petroleum &Chemical Corporation Science and Technology Department Pilot Project “Evolution and Breakthrough Direction of Hydrocarbon Accumu-
lation in the Lower Assemblage of Western Middle Yangtze Block”(No. p16042).
**
This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn.1000-
0976.2020.04.002.
***
Peer review under responsibility of Sichuan Petroleum Administration.
*Corresponding author.
E-mail address: 30760410@qq.com (Chen KQ).
Peer review under responsibility of Sichuan Petroleum Administration.
Available online at www.sciencedirect.com
ScienceDirect
Natural Gas Industry B 7 (2020) 583e593
www.keaipublishing.com/en/journals/natural-gas-industry-b/
https://doi.org/10.1016/j.ngib.2020.04.002
2352-8540/©2020 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
0. Introduction
Shale gas reservoirs are continuously distributed generally
and may subject to dissipation and damage in the form of
diffusion, spillage, and leakage. Structural uplifting causes
caprocks to become shallow, so the sealing capacity becomes
weaker, accelerating the diffusion and leakage of shale gas
reservoirs. Structural deformation creates fault and fracture
systems, which lead to seepage and leakage of shale gas. In
southern China, marine shale accumulation was structurally
reformed intensively during the MesoeCenozoic intra-
continental tectonic process, making the adsorbed gas desorb
to generate more free gas; as a result, shale gas escaped,
spilled or even leaked. Preservation conditions are regarded as
one of the key factors determining whether marine shale gas
can be enriched and accumulated. Thus, the preservation
conditions of shale gas and the material foundation of shale
contribute to the “two-factor enrichment”of shale gas [1e3].
Restricted by the deep-water shelf facies, dark shales are
developed in the Early Cambrian Qiongzhusi Formation and
the Late Ordovician Wufeng FormationeEarly Silurian
Longmaxi Formation in the western Middle Yangtze Block,
which lays a material foundation for the enrichment and
accumulation of marine shale gas [4e6]. However, in the
MesoeCenozoic, due to the joint effects of the
JiangnaneXuefeng paleo-uplift, the Huangling paleo-uplift,
the Dabashan orogenic belt, and the Central Guizhou paleo-
uplift [7e9], complex faultefolding and strong uplifting and
denudation occurred in the western Middle Yangtze Block,
which led to breaking of shale formations, reduction of for-
mation pressure, desorption of adsorbed gas, and escape of
free gas, thus the preservation conditions of original shale gas
reservoir were destroyed [1e3,10e13]. Differential structural
deformation caused isolated preservation conditions, making
the marine shale gas enrichment in this area no longer
continuous, but discontinuous due to the constraints of struc-
tures and preservation conditions [14e16].
The western Middle Yangtze Block is one of the key areas
for continental dynamics research, and also the target area for
marine oil and gas exploration. Previous studies have revealed
a series of findings on the basin structure [17e19], piggyback
structural deformation mode [20,21], time sequence of struc-
tural deformation [21,22], structure and hydrocarbon preser-
vation [23e25]. However, with the deepening of marine shale
gas exploration, the original results can no longer meet the
needs of understanding the preservation and enrichment laws
of marine shale gas in this area. To this end, this paper ana-
lyzes the textural and structural characteristics and the dif-
ferential structural deformation in the area. Then, this paper
clarifies the relationship between different structural styles and
shale gas preservation conditions, and reveals the key factors
for the preservation of marine shale gas, hoping to provide
support for the determination of shale gas exploration favor-
able areas.
1. Regional structural setting
In terms of regional structural location, the western Middle
Yangtze Block is separated from the JiangnaneXuefeng
paleoeuplift in the southeast by the BaojingeCili fault and
from the East Sichuan structure in the northwest by the
Qiyueshanfault,blockedbytheHuangling paleo-uplift and
opposed to the Dabashan arc structure in the north, and
connected with the Central Guizhou structure in the south.
The MiddleeUpper Yangtze Intracontinental Middle Paleo-
zoic deformation belt of over 200 km between the
BaojingeCili fault and the Qiyueshan fault is the result of the
progressive expansion and deformation of JiangnaneXuefeng
intracontinental orogeny in the northwest direction. The
structure is distributed in an arc shape, with NEEeNE
striking and NW protruding. From southeast to northwest,
it is composed of multiple NE-trending structural belts with
alternating synclinoriums and anticlinoriums, which are
dominated by thick-skinned “trough-like”structures (Fig. 1).
Since the Caledonian, the marine strata in the western
Middle Yangtze Block experienced multiple periods of
basin-forming and reworking processes, forming the
present-day good material foundation for hydrocarbon
accumulation and complex structure patterns in this area. In
the CaledonianeHercynian, the tectonic activities were
dominantly lifting and extension, giving rise to marine
craton and the tectonic pattern of alternative uplift and
depression, and forming two sets of dark organic-rich shales
in the Lower Cambrian Niutitang Formation and the Upper
Ordovician Wufeng FormationeSilurian Longmaxi Forma-
tion. Since the Mesozoic, due to the intracontinental
orogeny of the JiangnaneXuefeng paleo-uplift, strong
faultefolding occurred in the western Middle Yangtze
Block. In the Late YanshanianeEarly Himalayan, a certain
extension appeared due to the influence of the Pacific tec-
tonic system. In the Late Himalayan, regional uplifting and
denudation occurred, which contributed to the present-day
complex structural characteristics.
2. Structural characteristics
Controlled by the intracontinental orogeny of the
JiangnaneXuefeng paleo-uplift, progressive structural defor-
mation occurred from the western Middle Yangtze Block to
the Eastern Sichuan region in the Mesozoic, forming a cor-
responding regular deformation belt [26]. At the same time,
restricted by boundary conditions, different sections of various
structural deformation belts experienced differential de-
formations. Different structural deformation belts experienced
584 Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
structural transformation in the form of fault or gradual
structural deformation.
2.1. Piggyback structure
The fault folding process in the western Middle Yangtze
Block induced by the JiangnaneXuefeng paleo-uplift was
diachronously progressive, and led to piggyback structure,
which presents diverse deformation patterns under the joint
action of structural boundaries of the Central Guizhou paleo-
uplift, the Huangling paleo-uplift and the Dabashan structure.
2.1.1. Characteristics of piggyback structure
The structural deformation pattern of piggyback structure
was established for the West HunaneHubei and East Sichuan
areas. However, the piggyback structure in the West
HunaneHubei area, namely, the western Middle Yangtze
Block, has not been fully demonstrated. Sun et al. [26]
considered the West HunaneHubei area as only one deforma-
tion belt. Ding et al. [20] divided the West HunaneHubei area
into two deformation belts with the JianshiePengshui fault as
the boundary. These divisions cannot thoroughly reveal the
structural characteristics of the western Middle Yangtze Block,
and will also result in inadequate understanding of differential
constraints on shale gas preservation conditions.
By taking the HefengeLongshan fault, JianshiePengshui
fault and Qiyueshan fault as boundaries, the western Middle
Yangtze Block and the East Sichuan faultefold belt are
divided into four deformation belts, which constitute a com-
plete piggyback structure pattern.
2.1.1.1. Thrust deformation belt. The thrust deformation belt
between the BaojingeCili fault and the HefengeLongshan
fault consists of the SangzhieShimen synclinorium and the
Fig. 1. Structural division of the western Middle Yangtze Block.
Note: 1. F1: BaojingeCili fault; F2: HefengeLongshan fault; F3: JianshiePengshui fault; F4: Qiyueshan fault; F5: NanchuaneZunyi fault; F6: Tianyangping fault. 2.
I: Restricted piggyback structure; II: Weakly reformed piggyback structure; III: Strongly reformed piggyback structure. 3. ❶Boundary between restricted piggyback
structure and weakly deformed piggyback structure; ❷Boundary between weakly reformed piggyback structure and strongly reformed piggyback structure.
585Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
YidueHefeng anticlinorium. Spatially, it is located at the
western margin of the JiangnaneXuefeng paleo-uplift. Since
the Mesozoic, the thrust deformation belt has been most
affected by compression, which led to severe shrinkage
deformation and strong thrusting of the strata. In terms of
structural type, it is mainly characterized by compressive fault
blocks and relatively tight folds. At the same time, strong
uplifting and denudation occurred. The exposed strata on the
surface are dominated by the Paleozoic, especially the Lower
Paleozoic (Figs. 1 and 2-a&b).
2.1.1.2. Compressive fold deformation belt. The compressive
fold deformation belt is bounded by the HefengeLongshan
fault and JianshiePengshui fault in the east and west,
respectively. It is composed of the Huaguoping synclinorium
and the Central anticlinorium. Because the compression is
weaker than that on the thrust belt, the faulting intensity is
relatively smaller and the folds are more developed. Compared
with the thrust deformation belt, the compressive fold defor-
mation belt demonstrates relatively wider and more gentle
folds. Specifically, in the Huaguoping synclinorium, the folds
are quasi-box-shaped. Vertically, the belt reflects weaker
uplifting and denudation strength than the thrust deformation
belt, with the Triassic strata dominantly outcropped in the
northern section and the Lower Paleozoic strata in the southern
section (Figs. 1 and 2-a&b).
2.1.1.3. Thrust-detachment transition belt. The thruste
detachment transition belt between the JianshiePengshui fault
and the Qiyueshan fault includes the Lichuan synclinorium and
the Qiyueshan anticline. The transfer belt is a regional detach-
ment transfer belt, that is, from east to west, the regional
detachment zone transforms from the Precambrian to the
Cambrian, and the detachment zone transfers the displacement
upward in a stepped manner. From the perspective of structural
deformation, this belt is located at the peak of the northwestern
edge of the Xuefeng section of the thrust nappe structure, where
the structural deformation is relatively strong, mainly presenting
as thrust and detachment. The Triassic and Jurassic are out-
cropped in the northern section, while the Paleozoic is outcropped
in the southern section (Figs. 1 and 2-b&c).
2.1.1.4. Frontal caprock fault foldedetachment belt. The belt
is located to the west of the Qiyueshan fault. The regional
detachment belt has transformed from the Precambrian to the
Cambrian, and the structural deformation is dominantly
folding under the action of regional detachment in the
Cambrian. The structural style is simple, being an ejective
fold consisting of Permian, Triassic and Jurassic. The anti-
cline is tightly closed, with relatively steep flanks and the
Permian exposed in the core. The syncline is open, with
relatively gentle flanks and the Jurassic exposed in the core.
The basement is not involved or slightly deformed (Figs. 1
and 2-b&c).
2.1.2. Types of piggyback structure
Limited by differential boundary conditions, the patterns of
piggyback structure were developed differently from area to
Fig. 2. Different patterns of piggyback structure in the western Middle Yangtze Block.
586 Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
area in the western Middle Yangtze Block. Generally, three
types of piggyback structures can be identified, i.e. restricted
type, weakly deformed type, and strongly deformed type.
2.1.2.1. Restricted type. The restricted piggyback structure is
found in the northern section of the western Middle Yangtze
Block. Restricted by the Shennongjia paleo-uplift, the Huan-
gling paleo-uplift and the Dabashan Mt. to the Xuefeng
structural domain, the NW-direction progressive structural
deformation caused by the JiangnaneXuefeng paleo-uplift
interfered with the restricted piggyback structure, forming an
“L"-shaped joint structure, which limits the systematic
development of progressive deformation. As a result, the
deformation of piggyback structure is incomplete eonly
thrust deformation belt and compressive fault fold deformation
belt (Figs. 1 and 2-a).
2.1.2.2. Weakly deformed type. The weakly deformed piggy-
back structure is located to the south of the restricted piggy-
back structure and to the north of the
FulingeWulongePengshui belt. After the formation of the
NE-trending primary structure in the Mesozoic, no strong
tectonic reworking occurred, so a relatively complete thrust
nappe structure was available. From the JiangnaneXuefeng
paleo-uplift to the Eastern Sichuan region, the thrust defor-
mation belt, compressive fault fold deformation belt,
thrustedetachment transition belt, and frontal caprock fault
foldedetachment belt can be identified. Particularly, the thrust
deformation belt and compressive fault fold deformation belt
have similar characteristics to the restricted piggyback struc-
ture (Figs. 1 and 2-b).
2.1.2.3. Strongly deformed type. Compared with the weakly
reformed piggyback structure in the West HunaneHubei area,
the structures in the Wulingshan area to the south of the
FulingeWulongePengshui belt are more abundant in shapes.
After the formation of the NE-trending Xuefeng structural
domain in the early stage, it was superimposed with the NNW-
trending structure in late stage. The strata were intensely
denudated and the outcropped strata are dominantly the Lower
Paleozoic, with relatively tight folds and strong faulting action
(Figs. 1 and 2-c).
2.2. Structural transfer belts
Structural deformation zones have different reformation
strengths and deformation patterns, and transfer belt of such
strength and pattern exists between the structural deformation
zones. There are three structural transfer belts in the study
area.
2.2.1. Structural transfer belt between the West
HunaneHubei faultefold belt and the Wulingshan
faultefold belt
This structural transfer belt is a transfer belt between
weakly reformed piggyback structure and strongly reformed
piggyback structure, with different structural traces and
uplifting and denudation degree in the south and north. The
spatial distribution and combination of the structures are
described below.
To the north of the transfer belt, the most important and
significant structural line is composed of folds and their asso-
ciated faults roughly trending in NNE. These folds are arranged
alternately and parallel to each other, with a length of tens to
hundreds of kilometers, forming the primary structure of the
West HunaneHubei faultefold belt. Moreover, the folds are
wide and open, with the axis plane plunging in an ES direction.
To the south of the transfer belt, in addition to NE-trending
structures, there are also NS-trending structures. As a result,
some NNE-trending structures are interfered by or combined
with SN-trending structures to form arc-shaped structures. Be-
sides, there are some inconspicuous NNW-trending folds and
their associated faults, which are superimposed on the NNE-
trending structures and restricted by the NNE-trending struc-
tures. These three groups of structures interfere, intersperse,
restrict, and unite with each other, which are attributed to
different times, stress fields or boundary conditions. The folds in
the area are relatively tight, with relatively steep flanks.
2.2.2.Structural transfer belt between the West
HunaneHubei faultefold belt and the East Sichuan
faultefold belt
The Qiyueshan fault is the fold transfer belt of horizontal
structural deformation pattern between the West
HunaneHubei faultefold belt in the east and the East Sichuan
faultefold belt in the west. Particularly, the box-shaped anti-
cline type on the west side of the Jiaoshiba section of the
Qiyueshan fault zone implies the occurrence of intermediate
fold type as the trough-like fold transforms to ejective fold.
Therefore, fold combination from the Middle Yangtze Block
to the Upper Yangtze transfer belteQiyueshan fault margin is
not a simple model of transfer from trough-like fold to ejective
fold; instead, there are interludes of box-shaped structures,
such as the Jiaoshiba box-shaped anticline and the Wulong
box-shaped syncline.
Spatially, the Qiyueshan fault is the frontal zone of the
Precambrian regional detachment fault. To the west of the
Qiyueshan fault, the regional detachment fault is developed
in the Cambrian. Therefore, the Qiyueshan fault appears as
a thrust as a whole. However, due to the differences in the
components of the Qiyueshan fault system, different
structural transfer models appear in different sections. In
the Shizhu section to the north of Jiaoshiba, the system
controlled by the Qiyueshan fault is reformed to thrust
faultesubtle thrust þfault-propagation fold. Thrusting
occurred in the major Qiyueshan fault, being a high-angle
strong thrust, which resulted in strong uplifting and denu-
dation of strata in the east, while subtle thrusting took place
in the frontal branch faults, controlling the fault-
propagation folds, such as the Jiannan anticline (Fig. 3a).
In the Jiaoshiba section, it appears as thrust fault - subtle
thrust þfaultedetachment folds, that is, the front branch of
the Qiyueshan fault is detached along the strata in the
Precambrian, resulting in the development of box-shaped
587Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
anticline in the caprock (Fig. 3b). The fault structure in the
Nanchuan section to the south of Jiaoshiba is a thrust
faultesubtle thrust þfault-propagation fold, and the front
branch faults control the Pingqiao fault-propagation anti-
cline (Fig. 3c).
2.2.3. Structural transfer belt between outcrop and
hinterland
Bounded by the Tianyangping fault, the JiangnaneXuefeng
structural domain is located in the west, and the Qinling
structural domain in the east; the Tianyangping fault is
covered in the transfer belt between such two domains. The
fault is interpreted as a basement fault on the seismic profile,
and shows mainly the features of compression, as well as some
signs of strike-slipping, with the characteristics of multiple
periods of activities.
The southern section of the Tianyangping fault to the south
of Honghuatao has not been broken to the surface. The
Paleogene and Quaternary strata are exposed on the surface
and are in angular unconformity contact with the Cambrian
and the Ordovician in the west. The profile shows that the fault
is concealed under the Cretaceous with a dip of SSW, and
thrust detachment occurred. Small fault-propagation folds are
developed on the hanging wall and the deformation of top
unconformity strata in Cretaceous and above is incompatible
with it, indicating that the fault was formed before the depo-
sition of the Cretaceous and was a product of the
JiangnaneXuefeng movement. The footwall of the fault is flat
and gentle, with weak structural deformation, indicating that
the JiangnaneXuefeng orogeny died out here to the west of
the Tianyangping fault and did not affect the structural
deformation in the footwall in the east (Fig. 4).
Fig. 3. Seismic interpretation profile of different sections of the Qiyueshan fault.
588 Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
The fault is characterized by multiple periods of tectonic
activities. Since the Mesozoic, under the action of the
JiangnaneXuefeng paleo-uplift, the fault was resurrected in
the Indosinian, and suffered a strong SWeNE thrust in the
Early Yanshanian, which also controlled the north flank of the
Changyang anticline. In the Late Yanshanian and Early Hi-
malayan, affected by the Pacific tectonic domain, the fault was
negatively reversed and at the same time it controlled the
CretaceousePaleogene deposits and the emplacement of the
granite belts in Shishou and Jianli in the southern section of
the fault.
3. Structural deformation pattern and strength
Structural deformation pattern and strength are different
depending on structural zones/belts, which leads to the sepa-
rability of shale gas preservation conditions.
3.1. Structural deformation pattern
Restricted by the intracontinental orogeny of the
JiangnaneXuefeng paleo-uplift, the structural deformation
pattern in the western Middle Yangtze Block is evidently
different by belts.
3.1.1. Structural deformation pattern of thrust deformation
belt
The thrust deformation belt is connected with the
JiangnaneXuefeng paleo-uplift via the BaojingeCili fault.
Due to strong compression since the Mesozoic, the strata were
intensively thrust. For example, a strong thrust occurred in the
Middle Ordovician in the western Cili County, giving rise to a
series of fault blocks (Fig. 5). Under the strong thrust, the
SangzhieShimen synclinorium was tightly closed and the
YidueHefeng anticlinorium popped up severely, making the
Sinian exposed. The structural styles are dominated by thrust
structure, imbricate structure, fault-propagation fold, and
backthrust structure (Fig. 6).
3.1.2. Structural deformation pattern of compressive fold
deformation belt
The structural deformation of compressive fold deforma-
tion belt is relatively weak. The induced folds are mainly box-
shaped. Moreover, fault-propagation folds are developed as a
result of fault-propagation action (Fig. 7). The overall uplifting
and denudation strength is smaller than that of the thrust belt,
and the pop-up amplitude of the fault is small, which only
leads to relatively weak backthrust action. The structural styles
are mainly fault-propagation structure, backthrust structure
and ramp structure.
3.1.3. Structural deformation pattern of thrustedetachment
transition belt
The thrustedetachment transition belt is a thrust front belt
of the regional detachment fault in the Precambrian, with
stronger structural deformation than the compressive fold belt.
The front edge suffered a strong thrust. Affected by multi-level
detachment, interlayer detachment may easily occur and a
large number of fault-propagation folds are formed. At the
same time, with the development of small backthrust faults,
pop-up structure is formed (Fig. 8). The overall uplifting and
denudation strength is relatively small. The Mesozoic outcrop
predominates, and the youngest strata exposed are the Jurassic.
The structural styles are mainly fault-propagation structure,
backthrust structure and faultedetachment structure.
Fig. 4. Seismic interpretation profile of Tianyangping fault.
Fig. 5. Middle Ordovician thrust block at Point D01 in the west of Cili County.
Fig. 6. Seismic interpretation profile of Line SZ2014 in the western Middle
Yangtze Block.
Fig. 7. Seismic interpretation profile of Line hgp2012-03-2013hf-l21 in the
western Middle Yangtze Block.
589Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
3.2. Structural deformation strength
The differences in structural deformation strength are re-
flected in the lateral shrinkage rate and vertical denudation
strength. Analysis of these two aspects can effectively reveal
the differential structural deformation in the western Middle
Yangtze Block since the Mesozoic (Table 1).
3.2.1. Differential deformation strength in lateral direction
Among the three structural belts in the western Middle
Yangtze Block, the thrust deformation belt near the
JiangnaneXuefeng orogenic belt has the strongest
shrinkage deformation, followed by the thrustedetachment
transition belt, and the compressive fold deformation belt
shows the weakest shrinkage deformation. In each structural
belt, the deformation strength also varies in different sec-
tions. Specifically, the northern and southern sections of the
compressive fold deformation belt exhibit the weakest
shrinkage deformation. As shown in Fig. 2b, in the
compressive fold deformation belt, the folds are mostly
box-shaped, and faults are not developed with relatively
weak deformation. The structural style is mainly thrust fold.
The middleenorthern section of the thrustedetachment
transition belt is relatively weak in shrinkage and defor-
mation, and the structural style is dominantly fault-
propagation fold.
3.2.2. Differential deformation strength in vertical direction
Vertically, the thrust deformation belt, except for the
SangzhieShimen synclinorium, is dominated by the outcrop of
Lower Paleozoic, while the compressive fold deformation belt
and the northern section of the thrustedetachment transition belt
mainly demonstrate the outcrop of Mesozoic, and the southern
section of the thrustedetachment transition belt shows the
outcrop of Paleozoic. The denudation strength suggests that the
thrust deformation belt is most severely denudated and the
compressive fold deformation belt and the thrustedetachment
transition belt are relatively less denudated.
4. Relationship between geological structure and shale gas
preservation conditions
4.1. Response of preservation to structure
The control of structure on hydrocarbon accumulation is a
fundamental feature of complex altered residual basin.
Different structural deformations in the Mesozoic and Ceno-
zoic caused different preservation conditions between struc-
tural deformation zones and between spatial strata in the same
structural deformation zone. Different hydrocarbon accumu-
lation laws and models are the direct reflections of different
hydrocarbon accumulation elements. The control of structure
on preservation conditions is one of the fundamental causes
for differential hydrocarbon accumulation [27].
Since the Mesozoic, in terms of structure space, the western
Middle Yangtze Block has presented as an intersection of
multi-action boundaries, and superposition and variation of the
interactions between different levels of structures in the tec-
tonic pattern. In terms of evolution, it exhibits a continuity and
superposition transformation of structures in the same period,
and also a superposition of structures of different natures
caused by profound transformation (reversal) of structures in
different periods, thereby resulting in differences in structure
and deformation strength of different regions. Structure is a
Fig. 8. Detachment structure in Lower Triassic at Point D02 in the northern
section of the Lichuan synclinorium
Table 1
Data of differential structural deformation in western Middle Yangtze Block.
Structural unit Structural style IndosinianeMiddle
Yanshanian
denudation/m
Late Yanshaniane
Himalaya
denudation/m
Deformation time Shrinkage
rate
Thrust deformation
belt
Northern section Thrust block, torsional
structure
1000e3000 1000e2000 IndosinianeEarly
Yanshanian
13.38%
Middle section Thrust block, torsional
structure
1000e3500 1000e2000 IndosinianeEarly
Yanshanian
19.92%
Southern section Thrust block 2000e3500 1000e2000 IndosinianeEarly
Yanshanian
14.20%
Compressive fold
deformation belt
Northern section Thrust fold, ramp
structure
1000e2000 2000e2500 Early Yanshanian 3.08%
Middle section Thrust fold 1000e2000 2000e2500 Early Yanshanian 8.11%
Southern section Thrust fold 1000e3000 1000e2500 Early Yanshanian 3.90%
Thrustedetachment
transition belt
Northern section Fault-propagation fold 1000e3000 2000e2500 EarlyeMiddle Yanshanian 8.12%
Middle section Fault-propagation fold 1000e3000 2000e2500 EarlyeMiddle Yanshanian 14.01%
Southern section Fault-propagation fold 1000e3000 1000e2500 EarlyeMiddle Yanshanian 12.00%
590 Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
product of deformation, and it can effectively reflect the
strength and pattern of deformation. Based on a static analysis
of present-day structure, the strength of deformation is
analyzed structurally, so that the coupling relationship be-
tween structure and shale gas preservation can be established.
Regionally, in contrast to the strongly reformed zone, the
weakly reformed zone has weaker fault folding, lower fault
density, and fewer periods of structural reformation. Generally,
it reflects weaker structural deformation, and better preserva-
tion conditions. In other words, the preservation conditions in
the West HunaneHubei area are better than that in the
Wulingshan area.
4.1.1. Piggyback structure characteristics and shale gas
preservation
The thrust deformation belt, the compressive fold defor-
mation belt, the thrustedetachment transition belt and the
frontal caprock fault foldedetachment belt demonstrate a
descending order of structural deformation strength (Fig. 2)
and structural uplifting amplitude (Table 1), corresponding to
improved preservation conditions of shale gas.
The thrust deformation belt suffered a strong thrusting,
giving rise to the structures of relatively closed anticline and
syncline, and it was also reformed by a strike-slipping process,
leading to the development of a fault fixture block. Since the
intensive uplifting of closed anticline exposed the target layer
and the tightly closed syncline increased the depth of the target
layer, the fault occurrence is large and the fault fixture block
demonstrates poor preservation conditions. Therefore, the
thrust deformation belt exhibits poor shale gas preservation
conditions, and no exploration achievement has been made at
present.
The compressive fold deformation belt has experienced a
compression fault fold process, so the faultefold belt and fault
block are highly uplifted and the structure is highly involved,
affecting the preservation conditions of shale gas. Neverthe-
less, the overall structure is relatively complete, especially in
the northern and central sections, where there are box-shaped
folds and few large faults and the rocks are weakly reworked
by faults, providing conducive conditions for the preservation
of shale gas. In the southern section, especially, faults are
relatively developed and the rocks are truncated by faults into
blocks, contributing relatively poor preservation conditions.
For example, the pressure coefficient of Well Pengye 1 in the
Sangzheping syncline in Pengshui County is 0.98.
The thrustedetachment transition belt contains the struc-
tures of wide and gentle synclines and low-angle monoclines,
without faults. Therefore, the overall preservation conditions
are good. For example, the pressure coefficient of Well
Longye 1 in the Wulong syncline is 1.05.
The frontal caprock fault foldedetachment belt shows good
overall preservation conditions. The shale strata shallower
than 4500 m in depth are mainly distributed at the western
margin of the Qiyueshan fault. The structural styles are box-
shaped anticlines, arc-shaped anticlines and monoclinic
structures. The structures are characterized by weak defor-
mation, complete morphology, and weak faulting intensity.
The overall preservation conditions are good. For example, the
pressure coefficient of Well Jiaoye 1 in the Jiaoshiba area is
1.55 and that of Well Shengye 1 in the Dongsheng structure in
the Nanchuan area is 1.35.
4.1.2. Structural transfer belt and shale gas preservation
The structural transfer belt between the West
HunaneHubei faultefold belt and the East Sichuan
faultefold belt (bounded by the Qiyueshan fault) is the
boundary of the thrust deformation belt, the compressive fold
deformation belt, the thrustedetachment transition belt and
the frontal caprock fault foldedetachment belt, and also an
extremely important boundary for hydrocarbon preservation
(shale gas). In the area to the east of the belt, the structural
deformation is strong, the structures are complex, faults are
developed, and the effects of uplifting and denudation are
huge, corresponding to overall deterioration of shale gas
preservation conditions. To the west, namely, in the Sichuan
Basin, the structural deformation is weak, and the effects of
uplifting and denudation are small, contributing to good shale
gas preservation conditions. Shale gas discoveries are mainly
distributed to the west of this belt.
To the northwest of the structural transfer belt between the
West HunaneHubei faultefold belt and the Wulingshan
faultefold belt, due to the activity of the Central Guizhou
uplift, the NE-trending Xuefeng structural domain was formed
and superimposed with the NNW-trending structure in the late
period. The complex structure, presence of faults, and great
uplifting and denudation have resulted in the overall deterio-
ration of shale gas preservation conditions.
To the southwest of the structural transfer belt between the
outcrop and the basin hinterland (bounded by the Tia-
nyangping fault), there is the JiangnaneXuefeng structural
domain, where strong structural deformation, presence of
faults and large uplifting amplitude cause poor shale gas
preservation conditions. To the northeast, there is the Qinling
structural domain, where affected by the Huangling paleo-
uplift, the Yichang slope is structurally stable and the faults
are not developed, so shale gas preservation conditions are
good.
4.2. Structural types favorable for shale gas preservation
4.2.1. Positive structure at the western margin of Qiyueshan
fault
Positive structure is developed at the western margin of the
Qiyueshan fault, with complete anticline structure and weak
fault alteration. At the same time, the anticline structure
transits to the East Sichuan structure in the form of monocline
structure. For example, the Jiangdong slope of the Jiaoshiba
structure, with low dip angle, appropriate burial depth, com-
plete structure, and weak structural deformation, shows good
preservation conditions.
591Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
4.2.2. Residual syncline in thrustedetachment transition
belt
The residual syncline in the thrustedetachment transition
belt is wide, with low dip angles in strata on both sides,
moderate burial depth, and fewer faults. Therefore, the overall
preservation conditions are good, such as the Wulong syncline
[28]. This type of structure is worthy of more exploration
efforts.
4.2.3. Box-shaped structure in compressive fold belt and
thrustedetachment belt
The compressive fold belt and thrustedetachment belt have
lower shrinkage rate than the thrust belt and show the devel-
opment of structural types with favorable preservation condi-
tions such as box-shaped anticline, small faultefold anticline,
and monocline. Favorable preservation conditions are mainly
found in the central and northern sections of the Huaguoping
synclinorium and the Lichuan synclinorium. This type of
structure needs to be explored.
4.2.4. In-situ rock mass in thrust belt
Under the nappe of the BaojingeCili fault, there is an in-
situ rock mass in the footwall of the fault, with weak defor-
mation, and large wide and gentle folds, implying good
preservation conditions. This type of structure needs to be
explored.
4.2.5. Yichang slope in the east of Huangling paleo-uplift
At the southeastern margin of the Huangling paleo-uplift,
under the influence of the Huangling paleo-uplift, mono-
clinic structures are developed in the periphery of the paleo-
uplift and the monoclinic structure is stable with low dip
angle, and relatively fewer faults, which imply good preser-
vation conditions for shale gas. For example, the pressure
coefficient of Well Eyiye 2 is 1.03.
4.3. Favorable preservation zone for shale gas
By focusing on the WufengeLongmaxi Formation and
Niutitang Formation shale strata, favorable shale gas explo-
ration zones are selected depending on the lithofacies paleo-
geographic characteristics and structural deformation.
4.3.1. Favorable shale gas zone in WufengeLongmaxi
Formation
The lithofacies paleogeography and structural deformation
of the WufengeLongmaxi Formation show that most of the
thrust belt and compressive fold belt are in shallow shelf
facies, while a small part in the northern and southern sections
of the compressive fold belt, the thrustedetachment belt, and
the eastern side of Yichang slope are deposited in deepwater
shelf facies, with good material foundation in shales. Based on
the structural deformation characteristics and shale sedimen-
tary facies of the belts, it is believed that the Zigui syncline,
thrustedetachment belt and the western edge of the Qiyueshan
fault have the best preservation conditions.
4.3.2. Favorable shale gas zone in Niutitang Formation
In the Niutitang Formation period, except for the western
edge of the Qiyueshan fault that were in a shallow shelf envi-
ronment, other areas in the western Middle Yangtze Block were
basically in a deepwater shelf environment, with a good material
foundation. The Niutitang Formation of the Zigui syncline is
deeply buried, and the Zigui syncline is not evaluated herein.
According to the structural deformation characteristics and
shale sedimentary facies of the belts, the favorable shale gas
zones of the Niutitang Formation are distributed in the Kaixian
ramp zone, the compressive fold zone, the thrustedetachment
belt and the west side of the Yichang slope.
5. Conclusions
1) With the HefengeLongshan fault, JianshiePengshui
fault and Qiyueshan fault as boundaries, the piggyback
structures in the West HunaneHubeieEast Sichuan area
are divided into four deformation belts. The thrust
deformation belt close to the JiangnaneXuefeng
orogenic belt has the strongest contraction and defor-
mation, with strong thrusting, which led to tightly
closed folds, highest vertical uplifting and denudation
strength, and the structural style of closed folds and
fault blocks. The compressive fold deformation belt has
the weakest shrinkage and deformation, with the box-
shaped folds in dominance, low denudation strength,
and the outcrops of Triassic and Upper Paleozoic. The
thrustedetachment transition belt is dominated by fault-
propagation folds, with weak shrinkage deformation and
low denudation strength, and with the Jurassic, Triassic,
and Paleozoic outcrops in dominance and tightly closed
folds. The frontal caprock fault fold-detachment belt is
mainly affected by fault propagation, with trough-like
folds, small vertical denudation strength, and the
dominance of Jurassic outcrops, as well as wide and
gentle syncline.
2) In the western Middle Yangtze Block, there are three
pattern types of piggyback structure: restricted, weakly
reformed, and strongly reformed. The restricted type is
found in the northern section of the western Middle
Yangtze Block, with the development of thrust defor-
mation belt and compressive fold deformation belt due
to the blockage of the Huangling paleo-uplift and the
Dabashan Mt., and jointly affected by nearly EW- and
NE-trending structures. The weakly reformed type is
mainly located in the West HunaneHubei area, with a
complete piggyback structure; the NE-trending structure
was weakly reformed in the late period, and the trans-
formation is mainly uplifting and denudation. The
strongly reformed type is mainly located in the
Wulingshan area, with a relatively complete piggyback
structure; the NE-trending structure was severely
reformed in the late period, while the NNW-trending
structure superimposed the early NE-trending structure,
resulting in strong uplifting and denudation.
592 Chen KQ et al. / Natural Gas Industry B 7 (2020) 583e593
3) There are three structural transfer belts in the western
Middle Yangtze Block. The West HunaneHubei fold belt
and the East Sichuan fold belt transfer structurally at the
Qiyueshan fault. The West HunaneHubei fold belt in the
east is a trough-like fold with strong faulting and intense
denudation, and shale gas is enriched in the residual
syncline. The East Sichuan fold belt in the west is an
ejective fold, with low fault density and low denudation
strength, and shale gas is enriched in anticlines and slopes.
The weakly reformed and strongly reformed piggyback
structures experience structural transfer along the
FulingeWulongePengshui belt. The weakly reformed
structure has weak deformation, relatively wide folds,
relatively weak uplifting and denudation strength, and the
strongly deformed structure shows strong deformation
with high fault density; the folds are closed or even
inverted and the stratum denudation is strong. The Tia-
nyangping fault is the boundary of the structural transfer
belt between the West HunaneHubei fold belt and the
Yichang slope The Yichang slope is structurally stable,
with low fault density and low rock dip.
4) Based on structural and lithofacies paleogeographic
characteristics, it is believed that the Wufeng
FormationeLongmaxi Formation shale gas favorable
areas are distributed in the Zigui syncline, the West
HunaneHubei thrustedetachment belt and the western
margin of the Qiyueshan fault. The Niutitang Formation
favorable shale gas zones are located in the Kaixian
ramp belt, the West HunaneHubei compressive fold
zone and the thrustedetachment belt, as well as the
western side of Yichang slope.
Conflicts of interest
The authors declare that there is no conflicts of interest.
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