Strike-Slip Fault Effects on Diversity of the Ediacaran Mound-Shoal Distribution in the Central Sichuan Intracratonic Basin, China

Abstract

The largest Precambrian gasfield in China has been found from the Ediacaran (Sinian) carbonates in the central Sichuan Basin. The deep ancient reservoirs were generally attributed to the high-energy mound-shoal body in the carbonate platform. However, there is still little understanding on the distribution of the mound-shoal bodies that hampers further gas exploitation from the deep subsurface. Based on the seismic data, a large strike-slip fault system was identified by new 3D seismic data in the central Sichuan Basin. Further, it was found that the strike-slip fault had some effects on the mound-shoal bodies of the Ediacaran Dengying Formation. First, the platform margin was divided by strike-slip faults into three distinct segments to show two weak-rimmed margins and one interbedded rimmed margin. Second, the platform margin could be offset or migrated with the strike-slip faults. Third, there is varied margin thickness across the strike-slip fault zone. In the inner platform, more carbonate mound-shoal bodies developed behind the weak-rimmed margin that was divided by the strike-slip fault zones. In addition, the mound-shoal bodies may be separated by faulted sag. Further, the mound-shoal bodies may have developed along the faulted higher position in one side of the strike-slip fault zone. These patterns of the mound-shoal bodies suggest that the strike-slip fault had an important role in the sedimentary microfacies’ diversity in the intracratonic carbonates. The low displacement of the strike-slip fault is chiefly responsible for a weaker controlling effect on the microfacies change in the intracratonic basin rather than other tectonic settings.

1. Introduction

With the exhaustion of shallow oil/gas resources, deep carbonate strata (>4500 m) have become a major frontier for hydrocarbon exploration in China [1,2]. However, the deep reservoirs generally have low porosity (<5%) and permeability (<1 mD), leading to low production and low economic benefit. This is a big challenge in the exploitation of the deep, tight reservoirs. In turn, the fractured reservoir is becoming one of the major targets for deep oil/gas exploration and the development of petroliferous basins [1]. Large quantities of oil and gas resources have been discovered along strike-slip fault zones in the plate marginal basins [3,4,5,6], but sparse oil/gas reserves come from the strike-slip fault-controlled reservoirs in the deep pre-Mesozoic intracratonic basins.

Recently, a series of strike-slip faults have been found in the Tarim intracratonic basin, which showed a significant controlling effect on oil/gas accumulation and distribution [7,8,9,10,11,12]. With the advance in exploitation technology of deep-fault-related carbonate reservoirs, the largest deep (>7000 m) strike-slip fault-controlled oilfield in China has been found in the central Tarim Basin [7,11]. This kind of fractured reservoir is quite different from conventional sedimentary facies- or unconformity-controlled reservoirs in reservoir distribution and oil/gas enrichment patterns. The strike-slip fault-related carbonate reservoirs are favorable for high oil/gas production, which provides a new exploration and development domain of ultra-deep strike-slip fault-controlled oil/gas reservoirs [7,11]. In the central Sichuan Basin, Ma et al. (2018) suggested that some transtensional strike-slip faults occurred in the Ediacaran–Permian carbonates [13]. Jiao et al. (2021) proposed that there are eight large strike-slip fault zones, with a total length of 1280 km in the central Sichuan Basin [14]. They suggest that the strike-slip fault plays an important role in connecting source rocks and the upper reservoirs, increasing porosity and permeability, and gas accumulation and high production in the deep Ediacaran carbonate reservoirs [13,14].

Generally, the syn-or pre-sedimentary faults have distinct controlling effects on the type, size, and distribution of the sedimentary facies [15,16,17,18]. Chen et al. (2002) proposed a case study on a distinctive depositional architecture of a platform margin that was controlled by regional tectonics with some contributions from eustasy, environmental factors, oceanographic setting, and biotic and sedimentary fabrics [15]. Basilone et al. (2016) illustrated the tectono-sedimentary evolution of a Jurassic–Cretaceous intraplatform basin that has been affected by Jurassic transtensional faults related to the lateral westward motion of Africa relative to Europe [16]. Zeng et al. (2021) proposed a seismic sedimentology analysis of fluvial-deltaic systems in a complex strike-slip fault zone in China [17]. Bonamini et al. (2022) suggested that the instability enhanced by seismic activity related to the extensional tectonics plays an important role on the sedimentary facies’ types along the slope and low-angle proximal basin [18]. However, it is thought that there is little effect of strike-slip faults on the sedimentary facies in the Tarim intracratonic basin [7,8,9,10,11,12], and there has been no such study in the Sichuan Basin. In the central Sichuan Basin, the high-energic mound-shoal body is of great importance for the Ediacaran gas exploitation. Given sparse data in the deep subsurface, little information has been used to understand the distribution diversity of the Ediacaran mound-shoal bodies.

In order to evaluate the effect of the strike-slip fault on the sedimentary microfacies, we carried out strike-slip fault interpretation in the central Sichuan Basin. Together with previous microfacies data, we analyzed the faulting effect on the high-energic mound-shoal body distribution, and then proposed the fault-related microfacies model.

2. Geological Background

Sichuan Basin is located in the northwestern Yangtze terrane in China (Figure 1). It has experienced multi-stage tectonic-sedimentary cycles, and is closely related to the opening and closing of the Proto- to Neo-Tethyan oceans [19]. Sichuan Basin is a superimposed basin of multiple prototype basins, including the faulted basin in the Ediacaran–Silurian, the Carboniferous–Middle-Triassic marine cratonic basin, the Late-Triassic–Cretaceous siliciclastic intracratonic basin, and the Cenozoic foreland basin [19,20]. In the process of multi-stage tectonic evolution, a series of fault structures formed in the periphery of the basin. The central intracratonic basin presented a broad inherited central paleo-uplift [19]. Intracratonic carbonate platform depression occurred in the Ediacaran–Cambrian in the central area, where dolomite, more than 1000 m thick, was widely deposited along the carbonate platform (Figure 1). The central paleo-uplift formed before Permian, which led to the loss of the Devonian and some residual strata of the Ordovician–Carboniferous along the paleo-uplift margin. Since then, the central paleo-uplift had stable and inherited tectonic evolution in the Permian–Cretaceous and docked finally in the Himalayan period, although the uplift axis shifted slightly around the paleo-uplift [19,20].

The Sichuan Basin has superior hydrocarbon accumulation conditions, with several sets of high-quality source rocks in the Upper Ediacaran, Lower Cambrian, Lower Silurian, Permian, Upper Triassic, and Lower Jurassic, forming multiple conventional carbonate gas reservoirs in the Ediacaran, Cambrian, Carboniferous, and Permian–Triassic [21]. In addition, multiple unconventional resources are becoming a major oil and gas exploration domain, including the Silurian–Ordovician shale gas resource, Upper-Triassic tight gas resource, and Jurassic tight oil/gas resource. In recent years, a giant Anyue Gasfield has been found in deep Ediacaran–Cambrian dolomite around Deyang-Anyue (“rift trough”) (Figure 1). The proven geological reserves of natural gas in Anyue Gasfield have exceeded 1 × 10¹² m³, revealing greater exploration potential of the deep marine carbonate rocks.

In this area, the sedimentary facies of the Ediacaran have been studied by borehole cores, seismic facies, and logging facies [22,23]. The carbonate platform has deposited carbonate, more than 800 m thick, in the second and fourth members of the Dengying Formation. The S-N-trending carbonate platform next to Deyang-Anyue (“rift trough”) is composed of a platform margin and a restrict platform. The microfacies can be divided into platform margin grain shoal and algal mound, inter-shoal sea, intra-platform grain shoal and algal mound, platform flat, and inter-shoal sea ([22,23] and references therein). The algal mound developed in the inner platform, and the grain shoal occurred across the platform margin and the inner platform. The high-energy mound-shoal body generally has higher porosity and permeability [21], and is the major target for gas exploitation in the central Sichuan Basin. However, the scale of a single mound is small, measuring ~m thick, and lacking complete microfacies. Owing to the lithology changing frequently in the vertical and lateral directions, it is generally hard to map the detailed location of the mound-shoal body in the deep subsurface. These have constrained the gas exploitation deployment and well optimization in the Ediacaran carbonate.

3. Data and Methods

More than 8000 km² 3D seismic surveys have been carried out and reprocessed since 2018 in the central Sichuan Basin. The seismic dataset has a main frequency at 20–40 Hz in the deep carbonates, which are favorable for fault mapping and carbonate platform description [14]. The main strata horizons are easily identified and tracked in seismic sections (Figure 2). In the low-resolution seismic section, it is hard to identify the strike-slip faults with a vertical displacement of less than 10 m. In addition, many vertical strike-slip faults show relatively continuous reflection or kink shapes in seismic sections. In the process of seismic interpretation, the marks of flower-shaped and high-steep antiform or syncline are used for strike-slip fault identification [13,14]. Further, we excluded the pitfalls in misinterpretation of the strike-slip faults [24]. Considering the structural complexity of the strike-slip fault style and assemblage, the planar seismic attributes are used to identify the strike-slip faults. Seismic coherence and the maximum likelihood attribute can be used to enhance the seismic imaging effect of the fault, and the analysis of fault assemblages [13,14]. In this context, we used more planar marks other than the section marks to identify the strike-slip faults.

Generally, the strata thickness and seismic facies can be used to describe the high-energy mound-shoal body in the subsurface [25,26]. First, we had detailed seismic stratigraphic interpretation of the fourth member in the Dengying Formation. Constrained by previous studies, we used the strata thickness to constrain the distribution of the platform margin and mound-shoal bodies that generally show relatively localized thicker strata in the fourth member of Dengying Formation [25,26]. Calibrated and corrected by the lithofacies of boreholes, the seismic facies, such as the external mound shape and internal chaotic reflection, were used to identify the mound-shoal body in the Ediacaran platform. Further, we carried out the 3D visualization of the strata thickness for the proxy of the paleogeomorphology of the carbonate platform. With the well data calibration, it can reveal the distribution of the mound-shoal body in the deep subsurface. Then, we added the fault surface data into the 3D paleogeomorphic map, and correlated with the mound-shoal distribution.

4. Strike-Slip Fault

In seismic sections, the strike-slip faults generally show a single slight curve rather than a planar fault, and a flower structure (negative and positive flower structure) in vertical seismic sections. These are helpful for strike-slip fault identification in the deep subsurface [14]. It is worth noting that there is generally a combined feature in the seismic section to show multiple features of strike-slip fault (Figure 2). For example, high-steep linear structures may tend to have localized inversion, and varied displacement change in different layers. Due to the low seismic resolution, it is necessary to exclude the pitfalls in fault interpretation [24].

Besides the strike-slip fault feature in seismic section, the seismic planar attributes are helpful in mapping the strike-slip fault in the deep subsurface in the central Sichuan Basin [13,14]. In combination with the methods of optimizing coherent, eigenvalue coherent, symmetry and intensity of illumination, and prestack anisotropy, the major strike-slip faults can be identified by 3D seismic data in the central 22,000 km² areas (Figure 3).There are 12 first order and 15 second strike-slip faults were identified in the Ediacaran. Most of them are NW-trending and some are NE-trending strike-slip faults to form a series of faulted blocks in the Central basin (Figure 3).

In seismic sections, there are four style types of the strike-slip faults, including positive flower, negative flower, semi-flower, and vertical types (Figure 2), showing an evolution trend from vertical type to flower type. The strike-slip faults show different styles in different tectonic layers in the Ediacaran–Silurian, Permian, and Triassic–Jurassic. The strike-slip faults developed well in the Ediacaran, and most were inherited upward to the base of the Permian, and, to some extent, to the interior of the Permian. These Permian strike-slip faults are mainly inherited linear structures and semi-flower structures. There is different scale fault activity in different tectonic layers, but the major fault zone was inherited and merged downward.

In the Ediacaran layer, the NWW-trending strike-slip faults cut across the N-S-trending Ediacaran platform margin, and they were intersected by the NE-trending strike-slip faults. Different styles and scales of strike-slip faults occurred in the Ediacaran, showing en échelon, oblique, braided, and horsetail structures. They generally show dextral transtensional faults, which is consistent with the regional oblique extension. At the base of the Lower Permian, most faults turned to transpressional faults with decreased displacement. The faults developed downward to the Lower Cambrian faults, but their linkage weakened significantly to show short isolate faults. The fault styles are dominated by linear, en échelon, and oblique structures. The fault number increased, but the maturity decreased.

It should be noted that some faults terminated at the base of the Cambrian, and had larger vertical displacement in the high and steep negative-flower structure, which indicates a syn-sedimentary faulting during the platform development period. Given that some strike-slip faults were separated by the unconformity under the Cambrian, and changed style from the Ediacaran to the Cambrian, there should be strike-slip fault activity at the end of the Ediacaran. This is consistent with the tectonic transition [27,28] and fault activity [14] at the end of the Ediacaran in the Sichuan Basin.

5. Platform Margin and Relation with Strike-Slip Fault

In this study area, the Ediacaran reservoir is mainly of the high-energy mound-shoal body in the Dengying Formation (Figure 2). The mound-shoal body is a superimposed complex of algal bioherm and grainstone shoals [29]. The mound-shoal body is generally attributed to localized paleogeomorphic heights. It was accompanied by tectonic transition from oblique extension to weak compression and sea-level change [25]. The exploration and development data also indicate the existence of lateral differences in the lithology and microfacies along the platform margin [26,27]. Thus, it is difficult to correlate lithology among wells, particularly of the algal mounds. In this way, seismic data are used in mound-shoal identification [30]. In the seismic section, the shoal-mound body often shows a distinct mound shape, which appears to be thicker at the center than at the flanks (Figure 4). Calibrated by lithofacies from the borehole, the strata isopach map is used to identify the mound-shoal bodies in the Ediacaran platform (Figure 5). A distinct strata thickness change divided the depression and carbonate platform. Together with the well and 3D seismic data analysis, mound-shoal bodies developed in the eastern platform margin with a thickness up to 400 m. The platform margin is up to 240 km in length, 10–15 km in width, and covers an area of about 2200 km².

In the 3D area, the platform of the fourth member in the Dengying Formation was divided into three segments by the strike-slip faults, F_Ⅰ9 and F_Ⅰ2 (Figure 5). The southern segment has relatively thinner strata, showing a non-rimmed platform margin (Figure 6). Large areas of thinner shoals developed along the margin, whereas there is a distinct rimmed platform margin at the central segment. The thick mounds developed to show localized geomorphic heights along the steep margin. To the northern segment, the N-S-trending margin turned to NE-trending, showing a weak-rimmed platform margin. It should be noted that large-area shoals developed in the backreef area more than in the central segment with a large area of inter-shoal sea. The microfacies change frequently in the vertical direction, but are stable laterally, even with thin interbedded inter-shoal mudstone. This feature is consistent with the borehole data in the inner platform, although there is a rapidly decreasing thickness and size of the shoals.

In addition, the carbonate margin had a rapid migration from the second member to the fourth member in the Dengying Formation (Figure 5). The migration of the platform margins was variable along the strike-slip fault zones. Across the fault F_Ⅰ2, the southern rimmed platform margin turned to a northern non-rimmed platform margin (Figure 7a). The southern margin had a westward migration by the strike-slip faults. Along the rimmed platform margin, the strike-slip fault can separate or offset the marginal mound-shoal bodies too (Figure 7b); this separation distance could be more than 1 km. Further, the thickness of the mound-shoal body has variation across the strike-slip fault zones (Figure 7c). Due to the low seismic resolution, the planar displacement is ambiguous for measurement.

In the inner platform, there are also microfacies changes along the strike-slip zones. The small mound-shoal bodies were offset or intersected by the strike-slip fault (Figure 7d). In the second member of the Dengying Formation, there is distinct thickness variation along fault F_Ⅰ6 and F_Ⅰ7 (Figure 8a). The fault escarpments are consistent with the platform margins or eroded valleys. On this basis, the mound-shoal bodies developed along the faulted heights. In the footwall, there is generally a quick subsidence to form sag (Figure 7e). There is a continuous and strong seismic reflection in the footwall, but discontinuous and weaker seismic reflection on the hanging wall (Figure 8b). Further, microfacies varied with the landform changes across the strike-slip fault zone. The depositional thickness increase along the localized heights could be consistent with isolated mound-shoal bodies (Figure 7f). The small scale of the intraplatform mound-shoal bodies are easy to be offset in the intraplatform.

6. Discussions

Generally, it is thought that a large-scale Ediacaran platform-trough system has been formed in an intracratonic rifting environment by E-W directional extension in the central Sichuan Basin [21,25,30,31]. Although the Ediacaran reservoirs are mainly grain shoals and algae mounds, they had intense heterogeneity, owing to mound-shoal diversity in multi-stage sedimentary cycles [30,31]. By the well reinvestigation, most wells penetrated fractured reservoirs along the strike-slip fault zones. In this study, the diversity of the mound-shoal body is closely related with the syn-sedimentary strike-slip fault activity (Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8). Except for the Ediacaran, eustacy plays an important role in controlling the mound-shoal body development [29,30,31], and the strike-slip fault zone affects the diversity of the high-energic mound-shoal bodies. The borehole data in the las two years support the strike-slip fault effects on the microfacies change in the Ediacaran carbonates, and form a strike-slip fault-related gas exploitation domain in the Sichuan Basin [14]. In addition, there are multiple models of the mound-shoal body by the strike-slip fault effect in the intracratonic basin (Figure 6 and Figure 7).

The syn-sedimentary normal fault has a distinct controlling effect on the carbonate sedimentary facies and distribution in the extensional basin [15,16,18], as well as the siliciclastic deposition along the strike-slip fault zones [17]. However, the platform in the Sichuan intracratonic basin was controlled by the E-W directional extension [19,20,21], and the strike-slip fault had effects on the microfacies variation in the Ediacaran platform. It is worth noting that the Yangtze terrane rotated clockwise by the subduction of the Ediacaran proto-Tethyan Ocean [27]. During the oblique subduction period, a transtensional setting led to the formation of the Anyue rift depression and subsequent accommodation of the strike-slip faults. At the same time, there were NW-trending basement weak zones in the Sichuan Basin [19,20], which facilitated to the form NW-trending strike-slip faults.

Based on the structural analysis of the strike slip faults, it is found that the strike-slip faults formed in the late Ediacaran (Figure 2) [14]. The strike-slip fault displacement is generally less than 300 m, which had a weak effect on the sedimentary facies distribution, as the syn-sedimentary N-S-trending normal faults led to the E-W-trending platform-trough system (Figure 8a). This is inconsistent with the case studies with a strong fault-controlling effect on the sedimentary facies [15,16,17,18,32]. According to the analysis of the relationship between high-energic mound-shoal bodies and strike-slip faults of the Dengying Formation, there are multiple patterns of mound-shoal bodies along the strike-slip fault zones (Figure 6, Figure 7 and Figure 8). These suggested that the distribution diversity of the Ediacaran mound-shoal bodies is closely correlated with the strike-slip faults in the central basin.

During the development of the fourth member of the Dengying Formation, the platform margin migrated eastward to the fault escarpment of the rejuvenated NE-trending strike-slip faults (Figure 5). The microfacies migrated with the development of the syn-sedimentary strike-slip faults. As syn-sedimentary fault activity, the NWW-trending strike-slip faults intersected the N-S-trending platform and affected geomorphic changes that led to the sedimentary microfacies variation along the platform strike [32]. This led to a distinct segmentation of the platform margin (Figure 5 and Figure 6) that showed a different lithofacies distribution, with relatively independent high-energy mound-shoal bodies. In addition, the northern weak-rimmed margin presents contemporaneously intraplatform shoals; however, the central margins developed a wide-rimmed margin and relatively fewer intraplatform shoals (Figure 6). Due to the small displacement of the strike-slip fault zones (<200 m), the large-scale Ediacaran platform margin, generally, has not been offset completely by the strike-slip faults. However, the small offset of the margin can lead to the varied thickness and distribution of the mound-shoal bodies (Figure 7a–c). Although it is hard to discriminate syn- or post-sedimentary offset, some of the dislocation along the strike-slip faults were syn-sedimentary separation or post-sedimentary intersection by the faults during and after the Dengying Formation deposition, respectively.

In the interior of the platform, landform changes are favorable to form localized heights along the strike of the strike-slip fault zone. On this basis, the high-energic mound-shoal bodies were prone to take shape in the localized heights (Figure 7e). In this way, microfacies and their thickness varied with the landform changes across the strike-slip fault zone, as well as changes along the fault strike. In addition, the small-scale intraplatform mound-shoal bodies can feasibly be offset in the intraplatform. This can lead to forming independent shoals along two fault walls, or the inter-shoal sea separating the mound-shoal bodies (Figure 7d). This is mainly controlled by the difference of the uplift between the two walls of the strike-slip fault zone. Due to the influence of the transtensional faults, the falling sag that formed along the fault zone is favorable for tight mudstone deposition separating the high-energy mound-shoal body (Figure 8b). On the other hand, this results in microfacies differentiation in the intraplatform. When a fault barrier is formed between the adjacent trantensional fault zones, a faulted horst is favorable for geomorphic height development, resulting in interlacing of the mound-shoal and inter-shoal sea (Figure 7e).

Consequently, the weak intraplate strike-slip fault plays an important role in the microfacies diversity in the intracratonic platform. These fault-related mound-shoal models are favorable in the strike-slip fault-related gas exploration and development in the central Sichuan Basin, as well as insights in similar fault-related reservoir exploitation.

7. Conclusions

Although there are complicated depositional microfacies and strike-slip faults in the deep subsurface, the integrated analysis of the Ediacaran mound-shoal bodies and strike-slip faults in the central Sichuan Basin presented the following conclusions:

(1)

A large strike-slip fault system has been identified, and had a correlation with the Ediacaran mound-shoal distribution in the central Sichuan Basin.

(2)

The fourth member of the Ediacaran platform margin was divided into three distinct segments by strike-slip faults, and could be offset or migrated with the strike-slip faults, as well as the varied thickness across the strike-slip fault zone.

(3)

The strike-slip fault zone influenced the distribution of the carbonate mound-shoal bodies, and separated or offset the mound-shoal bodies in the inner platform.

(4)

The diversity of the Ediacaran mound-shoal body is closely related to the syn-sedimentary weak strike-slip fault activity in the Sichuan Basin.

(5)

This case study provides insight in the strike-slip fault effects on the diversity of the carbonate platform microfacies in the intracratonic basin.