Structural deformation and its implication for hydrocarbon accumulation in the Wuxia fault belt, northwestern Junggar basin, China

3.1 Structural patterns

Structural styles refer to the general characteristics of structures closely related to shapes, distribution and mechanism, showing the general characteristics of structural deformation [38,39,40,41]. Based on seismic interpretation, strata, faults and their combinations are identified, and the structural styles and structural deformation are further analyzed. Here five typical seismic profiles are captured from the west to the east of the Wuxia fault belt (Figure 4).

Figure 4

Seismic profiles of typical structural styles in the Wuxia fault belt. (a) Profile a is the section AA′, (b) profile b is the section BB′, (c) profile c is the section CC′, (d) profile d is the section DD′, and (e) profile e is the section EE′ (see section location in Figure 2).

Profile A-A′ lies in the west of the fault belt, the structures are dominated by compressional structures, including several fault-related folds and a buckle fold in the southern area (Figure 4a). The faulted folds can be further subdivided into several fault-propagation folds, a back thrust structure. In piedmont region, the fault-propagation fold, bounded by two basement faults F1 and F2 with N-S inclination, is characterized by large fault displacement and the strata are highly uplifted and denudated. In the southern area of the fault belt, a fault-propagation fold with little displacement is developed in deep layers and a buckle fold is formed in shallow layers (T-P₁f).

From the profile B-B′, the structures consist of strike-slip structures and compressional structures (Figure 4b). A positive flower structure develops in the northern part of the fault belt, consisting of a strike slip fault with high angle and several basement reverse faults developed from bottom to top. Compressional structures are made up of some fault-related folds and a decollement fold, and the faulted folds are made up of several fault-propagation folds, a back thrust structure and an imbricate faulted fold. A fault-propagation fold with large displacement and height of uplift develops in piedmont region (Figure 4b). A decollement fault develops in the Fengcheng Formation and a decollement fold is formed in the middle fault belt. Noticeably, in the deep layers, several fault-propagation folds develop beneath the decollement fold and the fault-propagation fold to the north of fault F2 (Figure 4b). A back thrust structure develops at the frontal part of the fault belt. The decollement fold and the fault-propagation fold in shallow layers are largely thrusted above these fault-propagation folds in deep layers with large horizontal displacement.

Profile C-C′ lies in the middle of the fault belt, the main structures are strike-slip structures and compressional structures (Figure 4c). Similar to profile B-B′, in the northern part of the fault belt, a strike slip fault and several basement reverse faults give the belt an overall appearance of a positive flower structure. A decollement fault (F2) develops in the Fengcheng Formation and a north-dipping monocline appears in piedmont region. At the leading edge of the monocline, a decollement fold developments and the Fengcheng formation gets thicker at the core of decollement fold (Figure 4c). At the south limb of the decollement fold, a reverse fault (F3) develops which is assumed as a fold-accommodation fault structure. Moreover, in adjacent areas, two types of forelimb space accommodation thrusts and forelimb shear thrusts are formed where the reverse faults in the decollement fold are assumed as passive faults as a result of variation in fold deformation (Figure 5). At the frontal fault belt, two north-dipping reverse faults (F4-1 and F4-2) develop and merge into a decollement fault that formed in the Fengcheng Formation. Besides, back thrust structures and triangle zones exist in the Triassic to middle Permian strata, and these structures show shallow level and autochthonous thrusting features. At deeper depth beneath the autochthonous compressional structures, three basement reverse faults distribute in the middle Permian to Carboniferous strata and merge into a main thrusting fault (F5), which show the appearance of imbricate thrust structure (Figure 4c). Similar to other profiles, a back thrust structure is formed at the front of the fault belt. The monocline and the decollement fold in the Permian are thrusted largely superimposing on the fault-propagation fold in deeper depth.

Figure 5

Summarized classification of structural styles in the Wuxia fault belt. Structures are divided into four types of structure styles, including fault-related fold, fold accommodation fault, fold and flower structure. (a) Fault propagation fold, a kind of fault-related fold. (b) Back thrust structure, a kind of fault-related fold. (c) Triangle zone, a kind of fault-related fold. (d) Imbricate thrust structures, a kind of fault-related fold. (e) Forelimb shear thrust, a kind of fold accommodation fault. (f) Forelimb space accommodation thrust, a kind of fold accommodation fault. (g) Buckle fold, a kind of fold. (h) Decollement fold, a kind of fold. (i) Positive flower structure, a kind of flower structure. The locations of typical seismic structure sections are shown in Figure 6.

In the east of the fault belt, several 2D seismic profiles are only available (Figure 4d and e). From profile D-D′, the structure patterns are similar to the patterns of profile C-C′, including the positive flower structure. In piedmont region, a decollement fault (F2) with great uplift and large horizontal displacement develops in the Fengcheng Formation and a decollement fold is formed which can be simplified as a monocline (Figure 4d). To the south, two sets of autochthonous fault-propagation folds are formed in the shallow strata (T-P1f) and deeper layers (P-C), respectively. One diffidence is that no decollement fold develops at the leading edge of the monocline, the second is that fewer back thrust structures are formed at the frontal fault belt. From profile E-E′, fault-propagation folds are widely distributed and show an overall feature of imbricate faulted folds (Figure 4e).

Generally, structures in the Wuxia fault belt can be classified into compressional structures and strike-slip structures (Figure 5). The compressional structures can be subdivided into fault-related folds, folds and fold-accommodation faults. Fault propagation folds, back thrust structures, triangle zones and imbricate thrust structures are widely distributed in the Wuxia fault belt. Folds in the fault belt are mainly buckle folds and decollement folds. Besides, two types of fold accommodation faults including forelimb space accommodation thrusts and forelimb shear thrusts develop in the fault belt. The strike-slip structure formed in the fault belt is positive flower structure.

3.2 Vertical and planar distribution

On the basis of the typical structure styles identified in seismic sections, vertical and planar distribution characteristics are summarized in this study. In the vertical direction, it can be found that structure styles show obvious differences in shallow and deeper depth (Figure 4). Structures in shallow depth are characterized by thrust nappe with large lateral displacement and uplift. Decollement folds and fold accommodation faults related to decollement folds above Fengcheng Formation decollement fault exist in shallow depth. Structures in deeper depth under the decollement fault are dominated by fault propagation fold in lower Permian–Carboniferous layers and generally show an appearance of imbricate structures.

Moreover, structures have a regular distribution in plane, and mainly show a zonation in S–N direction and a segmentation in E–W direction (Figures 4 and 6). In the middle part of the fault belt, a decollement fault with large horizontal displacement develops in the Fengcheng Formation such that a decollement fold develops in the upper Permian layers. In piedmont region, a fault propagation fold develops to the north of the decollement fold which possibly experienced more intense deformation than the other regions. To the south, fault related folds develop in the Triassic–Permian layers, and back thrust structures are formed at the leading edge of the fault belt.

Figure 6

Planar distribution of structural styles in the Wuxia fault belt. Note: the black circles represent structural styles of structures in shallow depth, and the orange cycles represent structures underlying the shallow structures at the frontal fault belt.

In the eastern fault belt, the region exhibited weak deformation with weak decollement, and no obvious thrust nappe exists (Figures 4 and 6). The middle-eastern part experienced intense deformation which is featured by decollement folding. The middle part also underwent intense deformation where decollement folding and thrusting both exist. In this region, decollement fold, fold accommodation faults and fault-related folds including back thrust structures and triangle zones are all widely developed (Figures 4 and 6). In the middle-western part, the deformation is also mainly controlled by decollement folding and thrusting, but the difference is that intense thrusting with large vertical displacement contributes to the formation of a fault-related fold to the north of the decollement fold. Besides, fold accommodation faults are less developed than that of the middle part (Figure 6). The western fault belt experienced weak deformation, no obvious decollement and thrust nappe exists.

3.3 Deformation recovery analysis

Balancing and restoration of seismic sections is conducted by stripping off each layer and restoring faults and folds. We restored one length-balanced section across the fault belt in Figure 7 with 3D Move software in this study (section BB′, see section location in Figure 2). In Early Permian (P1f), several basement faults were active and some fault-related folds developed in the Carboniferous–Permian Fengcheng Formation. In the middle Permian (P2x-P2w), the fault belt experienced intense deformation with high shortening rate, 38% in the P2x period and 17% in the P2w period. Basement faults were still active and extended upward to middle Permian strata. In this period, the fault propagation fold in piedmont region was highly uplifted, and a decollement fault developed in the Fengcheng Formation in the P2x period. In the Late Permian, the fault belt was still in a compressional field and experienced a deformation with a shortening rate of 22%. The decollement fault extended upward and a decollement fold developed in the Permian strata. In this period, the decollement fold and the fault propagation fold to the north were highly uplifted and underwent intense denudation to create an erosional angular unconformity at the base of the Triassic, meanwhile back thrust structure was formed at the leading edge of the fault belt. Up to the Late Permian, the present-day structure features of the fault belt were fundamentally formed where a decollement fold above Fengcheng Formation superimposed on the underlying basement fault-related folds. The balanced section shows that the Triassic–Cretaceous sequences overlap the fault belt and no intense thrusting activities existed during that time. However, from seismic sections in the middle part of the fault belt (Figure 4c), it can be found that local thrusting and uplifting movement developed, the faults in the fault belt reactivated but the structural pattern remains relatively stable. The basement faults and decollement fault have been active for several times in the Jurassic and Cretaceous to cut unconformities at the base of the Jurassic and Cretaceous (Figure 4). However, they did not contribute significantly to the formation of the Wuxia fault belt.

Figure 7

A length-balanced section from Well HS2 to Well FN4 constructed in the Wuxia fault belt (see location and orientation of the section in Figure 2).

4.1 Tectonic evolution of northwestern Junggar basin

The tectonic setting of northwestern Junggar basin since Late Carboniferous remains a controversial issue. However, in recent years, researchers tend to accept that the northwestern margin of Junggar basin was in a subduction-related tectonic setting in the Late Carboniferous [8,42,43]. At the end of Late Carboniferous, the western Junggar orogenic belt was formed by the collision between the Junggar plate and the Kazakhstan plate [44,45,46]. However, the tectonic setting of northwestern Junggar basin in the Permian still remains a hot debate, especially in the Early Permian. In the Early Permian (P1j and P1f), some researchers proposed that the northwestern Junggar basin experienced a tectonic transition from the subduction in Late Carboniferous to the foreland basin [17,18,24,42,47]. Some researchers argue that the western Junggar basin entered the post-collisional extension stage by analyzing the geochemical characteristics of magmas [8,45], and some geoscientists propose that a rift basin developed in the northwestern margin by analyzing soft-sediment deformation structures in the Fengcheng Formation [3,48]. According to our research, the structural deformation in the Wuxia fault belt owns distinctive features of a foreland basin (Figures 4 and 5). Besides, when the Early Permian Fengcheng Formation was deposited, several reverse faults developed and thrusting activities continued until the end of the Permian, during which several strong activities occurred (Figure 7). This suggests that the collision between the Kazakhstan plate and the Junggar plate remained active until the Late Permian. Moreover, in the Wuxia fault belt, the development of detachment folds and fold accommodation faults verify that the extremely thick mudstones of Fengcheng Formation lay the rock material foundation for long-distance thrusting and detachment, and this may be the reason why soft deformation in a foreland basin develops in the Fengcheng Formation.

4.2 Implications for hydrocarbon accumulation

Permian Fengcheng Formation is considered the main source rock in the Mahu sag and adjacent areas. Intense faulting and folding in the fault belt has changed the original distribution of the source rocks. Previous literature have investigated the sedimentary paleo-environment of the Early Permian strata, and the Fengcheng Formation is assumed as the product of a semi-deep to deep alkaline lake [3,8,29,49]. Feng et al. (2018) demonstrated that the Paleo-Junggar lake may have reached at least the Darbut area in the Early Permian, or even the northern Heshitologa basin. The balanced section constructed in this study also indicates that large lateral displacement and intense uplift exist in the northwestern Junggar basin, the distribution of source rocks is greatly affected and reconstructed in the Wuxia area. With the discovery of shale oil and tight oil in the Mahu sag, the Wuxia fault belt may be the potential region for further exploration of unconventional petroleum. In recent years, an exploration well FC3 was drilled at the hinge of the decollement fold, and thick dolomitic mudstones or siltstones of Fengcheng Formation with a thickness of nearly 700 m were encountered during the drilling process (Figure 8). This further proves that the Fengcheng Formation is thickened by decollement folding. Moreover, intense thrusting in the Wuxia fault belt contributes to the superimposition of Fengcheng Formation on the autochthonous source rocks (Figure 8). The thrusting also results in the uplift and denudation of Fengcheng Formation in piedmont region.

Figure 8

Schematic diagram of the distribution of unconventional oil and gas in the Fengcheng Formation in the Wuxia fault belt (see location and orientation of the section FF′ in Figure 2).

Fractures are crucial for shale oil and tight oil exploration. Faulting and folding both contribute to the development of fractures, and the fractures are mainly developed in areas near faults, the hinge and core of the fold [50,51,52,53]. In the Wuxia fault belt, fault-related folds, decollement folds and fold accommodation faults are widely developed. Due to continuous intense faulting and folding, fractures are also widely developed in the Fengcheng Formation, especially in areas near faults (Figure 8). In addition, some tensile fractures may develop at the hinge of the decollement fold, which is beneficial for improving the quality of reservoirs. Moreover, drilled wells reveal that the main storage spaces of the Fengcheng Formation are dissolved pores, residual inter-granular pores and micro-fractures. The porosity of the Fengcheng Formation in the fault belt is assumed to be larger than that of the Mahu sag.

Fengcheng Formation is matured and begins to generate hydrocarbons in the Late Permian [11,54]. At this time, hydrocarbons are accumulated in Permian strata, but are damaged by the intense uplift and denudation in the fault belt. As a result of the thrusting, the autochthonous source rocks of Fengcheng Formation underlying the decollement fault are buried rapidly and may begin to generate large amounts of hydrocarbons, and hydrocarbon generation of source rocks on the thrust belt may be stagnated. At the end of the Triassic, the source rocks of Fengcheng Formation in the Mahu sag and the source rocks of the autochthonous Fengcheng Formation underlying the decollement fault generate large amounts of hydrocarbons. Then, oil and gas migrate upward through faults and ultimately accumulate in favorable traps in the fault belt, such as X69 oil-gas reservoir, FC3 oil reservoir, HS2 oil reservoir etc. FC3 oil reservoir is characterized by dark heavy oil with a density of 0.959 g/cm³. After the Triassic, the fold accommodation faults developed near Well FC3 may all contribute to the damage of reservoir. Generally, faults developing in the thrusting activities provide a conduit for hydrocarbon migration from autochthonous source rocks to the fault belt. Besides, intense uplift and denudation caused by tectonic activities may have a destructive effect on the accumulated hydrocarbon.

In the Wuxia fault belt, oil reservoirs of the Fengcheng Formation in different structural styles are superimposed vertically, showing a tridimensional oil-bearing characteristic (Figure 8). Great discovery of unconventional petroleum has been found in the Mahu sag, and recent exploration reveals that the oil reservoirs in shallow depth may be damaged. The oil reservoirs may exist in the Fengcheng Formation in the deeper detachment fold and the autochthonous Fengcheng Formation in the fault propagation fold. These oil reservoirs are the most favorable regions for further exploration.