J. Cent. South Univ. (2020) 27: 3398-3416

DOI: https://doi.org/10.1007/s11771-020-4555-z

Sedimentary environments controlled by tectonics and induced differential subsidence: A perspective in the Permian Liangshan and Qixia Formations,northwestern Sichuan Basin, China

ZHONG Yuan(钟原)^{1, 2}, YANG Yue-ming(杨跃明)³, WEN Long(文龙)¹, LUO Bing(罗冰)¹,

XIAO Di(肖笛)^{2, 4}, TAN Xiu-cheng(谭秀成)², ZHAO Li-ke(赵立可)¹, LI Ming-long(李明隆)²

1. PetroChina Southwest Oil and Gas field Company Exploration and Development Research Institute,Chengdu 610041, China;

2. Division of Key Laboratory of Carbonate Reservoirs, CNPC, Southwest Petroleum University,Chengdu 610500, China;

3. PetroChina Southwest Oil and Gas field Company, Chengdu 610051, China;

4. School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Based on field observation, core description and well logging analysis, the tectonic-sedimentary framework of the Liangshan and Qixia Formations in the northwestern Sichuan Basin, China is deeply discussed. Two long-term sequence cycles were identified, denoted as LSC1 and LSC2, respectively. The sequence stratigraphic framework was established, suggesting the Liangshan Formation to be not isochronously deposited. Paleogeomorphy before deposition of LSC1 was reconstructed by the impression method. LSC1 was featured by thin, low-energy shoal deposits in the high topography, and thick inter-shoal sea and open sea deposits in the low topography. Meanwhile, paleogeomorphy before deposition of LSC2 was reconstructed using the residual thickness method, which was demonstrated to have primary high-energy, thick shoal deposits in the high topography, and thin inter-shoal and open sea deposits in the low topography. The results show that differential tectonic subsidence has already taken place during the Qixia Period, and thus the Dongwu Movement should occur earlier than previously expected. Meanwhile, pre-depositional paleogeomorphy has obvious controlling effects on the sequence stratigraphic filling and sedimentary facies distribution. Results of this study were expected to provide practical guidance to fine characterization of the sedimentary evolution process and prediction of high-quality reservoir distribution.

Key words: sequence stratigraphy; paleogeomorphy; sedimentary facies; Qixia Formation; Permian; northwest Sichuan Basin

Cite this article as: ZHONG Yuan, YANG Yue-ming, WEN Long, LUO Bing, XIAO Di, TAN Xiu-cheng, ZHAO Li-ke, LI Ming-long. Sedimentary environments controlled by tectonics and induced differential subsidence: A perspective in the Permian Liangshan and Qixia Formations, northwestern Sichuan Basin, China [J]. Journal of Central South University, 2020, 27(11): 3398-3416. DOI: https://doi.org/10.1007/s11771-020-4555-z.

1 Introduction

As a typical multi-cycle superimposed basin that experienced multiple episodes of tectonic movements [1-4], the Sichuan Basin was formed by ordered stacking of multiple prototype or residual basins in the three-dimensional space [5].

During the Paleozoic, there were multiple transgression-regression cycles in the Sichuan Basin, and the Permian witnessed the important transition of the Sichuan Basin in terms of sedimentary filling and tectonic evolution. Due to expansion of the Paleo-Tethys Ocean [6, 7], the Sichuan Basin was mainly in an extensional setting, with continental marginal rifting as well as intracontinental extension, depression and rifting [2, 8]. It was generally believed that the basin was rapidly transgressed in the early Middle Permian, resulting in deposition of the Liangshan Formation transitional facies clastic rocks and the Qixia Formation carbonates on the erosion surface caused by the Yunnan Movement [9-11]. Moreover, the distribution of the Qixia Formation was relatively stable in the basin. Extensional rifting activities mainly occurred in the late Middle Permian to Early Triassic, which were typically characterized by development of Guangyuan-Wangcang Trough and Kaijiang-Liangping Trough during different periods in the northern part of the basin [12-15].

With continuously deepened research, the Qixia Formation in the Sichuan Basin, with a thickness of only about 100-140 m, has aroused lots of complicated scientific questions, particularly on its sedimentary environment [16] and mechanisms of karstification and dolomization [17-24]. However, no consensus has been reached, which is attributable to the poor understanding of the tectonic-sedimentary framework. Accordingly, this study comprehensively explored the existence of internal tectonic activities during deposition of the Qixia Formation based on the extensional tectonic background as that could significantly influence sedimentary filling patterns, environments and sedimentary facies distributions. As the most direct product of tectonic activities, sequence stratigraphy was undoubtedly the most reliable medium to investigate this issue. Accordingly, this study implemented fine interpretation of the Qixia Formation in the northwestern Sichuan Basin from the perspective of high-resolution sequence stratigraphy based on outcrop, drilling core (cuttings), and well logging data. Further, characteristics of sedimentary responses to the internal tectonic activities during the Qixia Period were investigated, accompanied by prediction of distribution of sedimentary facies in each episode of tectonic movement. Results of this study were expected to provide a new framework to support subsequent research on the Qixia Formation.

2 Geological setting

The study area geographically spans Mianyang City, Guangyuan City, Jiange City and Cangxi County of the Sichuan Province, China. It was generally affiliated to the transition zone of the Longmenshan Fault Belt, the Micangshan Uplift and the low-lying tectonic belt of the Guzhong Depression in the northern Sichuan Basin, with a total area of about 30500 km² (Figure 1). Influenced by the intensified Emei Taphrogeny, the northwestern Sichuan Basin was featured by uplifting in the south and northwest and subsiding in the north and northwest during the Qixia Period [25]. Such geomorphology directly resulted in the different sedimentary patterns in the space, namely tidal flat, restricted platform, open platform, platform marginal shoal, slope and basin in the northwestward direction [26]. The Qixia Formation in the northwestern Sichuan Basin was generally 100-120 m thick, and it was speculated to be mainly deposited in the open platform and platform marginal shoal facies. The lower part was more pelitic, with the main lithology of dark gray to black gray bioclast limestone, while the upper part was mainly composed of gray bioclast limestone locally intercalated with dolomite or siliceous concretion [27]. The Qixia Formation was overall rich in fossils, especially fusulinid, such as Nankinella, and Pseudofusulina. Moreover, there were plenty of fossils of corals, brachiopods, echinoderms, mollusks and algae [26].

3 Materials and methods

The study was mainly based on field observation (in northwestern Sichuan basin) and drilling and logging data analysis. Outcrop sites were selected according to the exposure completeness, measurability and comparability with underground strata. A total of four outcrop sections, including Changjianggou(CJG), Zhengyuan (ZY), Hejialiang (HJL) and Damuya (DMY) Sections, were implemented with gamma ray measurement and field observation and description. Drilling and well logging data were from 25 wells in the northwestern Sichuan Basin, including 11 cored wells, which were sufficient to support this study. With reference to the basic principles of sedimentology and high-resolution sequence stratigraphy, this study comprehensively described the selected sections and identified sequence boundaries. Subsequently, different orders of sequence frameworks were established, and calibrated by measured natural gamma ray data, which were later used to guide boundary division in the subsurface. Furthermore, the sequence filling process and the correlation between sequence and deposition were investigated based on well correlation results, followed by establishment of the conceptual geological model. Eventually, differences in tectonics during each episode of tectonic movement were characterized, accompanied by investigation into their controls on sedimentary environment as well as the sedimentary facies distribution and evolution models during each episode.

Figure 1 Location of study area and stratigraphic column

4 Results

4.1 Petrological characteristics and microfacies types

Based on the outcrop sections and core data, a variety of rock types were identified in the Liangshan and Qixia Formations. Specifically, the Liangshan Formation was mainly composed of gray-black shale, aluminous mudstone, and siltstone, while the Qixia Formation primarily consisted of limestone, dolomitic limestone and dolomite. Petrological characteristics of the Qixia Formation and the possible corresponding sedimentary environments were described below.

4.1.1 Sparry bioclastic grainstone

The type of rock was mainly composed of bioclastic granules and sparry calcite cements (Figure 2(a)). Main bioclastic granules included foraminifera, bivalves, and echinoderms, with their diameter ranging from 0.1 to 0.5 mm and overall moderate sorting. Intragranular dissolution pores and residual intergranular pores could be seen occasionally. In general, sparry bioclatsic grainstones were speculated to be products in the relatively high-energy environment, while the presence of many bioclastics reflected a shallow water area that might be close to the wave base and thus experience constant wave action, such as the platform margin and the intraplatform highlands where high-energy shoal deposits were formed.

Figure 2 Main petrological types and characteristics of Qixia Formation in northwest Sichuan basin:

4.1.2 Bioclastic packstone

The type of rock primarily consisted of bioclastic grainules (>50%) and limy matrix (Figure 2(b)). Main bioclastic granules included foraminifera, algae, bivalves, and echinoderms, with their diameter ranging from 0.1 to 2 mm and overall poor sorting. The granules presented certain abraded characteristics, which were attributed to the transportation process. The type of rock was overall in the relatively low-energy bioclastic shoal environment near the wave base.

4.1.3 Bioclastic wackestone

The type of rock was mainly composed of limy matrix (>50%) and bioclastic granules (Figure 2(c)). Main bioclastic granules included foraminifera, algae, and echinoderms, with their diameter ranging from 0.1 to 1 mm and overall poor sorting. The granules presented certain abraded and subangular characteristics, which were attributed to the transportation process. The type of rocks was overall in the relatively low-energy environment below or near the wave base, such as the inter-shoal sea.

4.1.4 Micritic limestone

The type of rocks was overall tight and consisted of mainly limy matrix (>90%) (Figure 2(d)), with a very small amount of bioclastic granules, and these features were indicators of low-energy deep-water environments. Based on previous studies on the Qixia Formation [26], the sedimentary environment should be carbonate platform or platform margin. Therefore, the possibility of a basin environment could be eliminated, and more probably the type of rock was formed in the open sea deep-water environment.

4.1.5 Dolomitic limestone

The type of rock included dolomitic bioclastic grainstone, dolomitic bioclastic packstone and dolomitic bioclastic wackestone. Replacement phenomenon could be observed in the thin sections under the microscope (Figure 2(e)). From the core, dark-colored leopard limestones were scattered in the light-colored limestone. Fabrics and types of parent rocks could be recognized in most cases (Figure 2(f)), including bioclastic grainstone, bioclastic packstone and bioclastic wackestone, with identical sedimentary environments.

4.1.6 Dolomite

Dolomite was the main reservoir rock type of the Qixia Formation in the northwestern Sichuan Basin, including medium-coarse crystalline dolomite, fine crystalline dolomite, and silty dolomite (Figures 2(g)-(j)). Its genesis had been extensively investigated, though controversies remained. In general, those layered fine, medium and coarse crystalline dolomites were thought to be products of high-energy shoal environments [28-31], with sparry bioclastic grainstones formed in the platform margin and intraplatform high- energy shoals as the parent rocks. In comparison, silty dolomites generally occurred in the tidal flat environment, with micritic-silty limestone as the parent rocks.

4.2 Sequence division

Based on the high-resolution sequence stratigraphic method, this study recognized three long-term base-level sequence cycle boundaries in the Changjianggou (CJG) and Zhenyuan (ZY) Sections, which divided the Liangshan and Qixia Formations into two long-term base-level sequence cycles, namely LSC1 and LSC2, which were subsequently calibrated on the acquired natural gamma ray curve.

4.2.1 Lithological characteristics of sequence boundaries

Sequence boundaries usually occurred as unconformities or depositional transition surfaces, representing the alteration of falling base level to rising one. Specifically, sequence boundaries of LSC1 and LSC2 were either in the form of sharp lithological changes or transitions from regression to transgression. The bottom boundary of LSC1 (SB1) was an unconformity between the Liangshan Formation and the underlying Carboniferous, Devonian and even Silurian strata (Figures 3(a) and (b)). As a transgression-regression cycle, LSC1 was featured by the transitional facies deposits of the Liangshan Formation at the bottom and the carbonate deposits of the Qixia Formation at the top. The upper boundary of LSC1 (SB2) was a transitional surface in terms of lithology and lithofacies, from the high-energy dolomitic packstone to the low-energy bioclatsic wackestone (Figures 3(c) and (d)). In comparison, the upper boundary of LSC2 (SB3) witnessed the transition from dolomitic wackestone and packstone with leopard-pattern dolomitic limestone to thin-layer wackestone without leopard-pattern dolomitic limestone, indicating rapidly rising base level and thus quickly deepening water body (Figures 3(e) and (f)).

4.2.2 Electrical characteristics of sequence boundaries

Electrical characteristics of sequence boundaries were summarized after calibration of sequence boundaries on the field sections and gamma ray curves. As an unconformity, SB1 was featured by a positive gamma ray excursion due to suddenly changed lithology, and thus it was very easy to be identified (Figures 4(a) and (b)). In comparison, as a lithological and lithofacies transition surface, SB2 witnessed the transition from high-energy rocks in the late period of base level fall to low-energy rocks in the early period of base level rise, and it was represented by the turning point where gamma ray values started to slowly increase after slow decrease (Figures 4(c) and (d)), which could also be identified in the outcrop sections and drilling curves. As the boundary between the Qixia and Maokou Formations, SB3 was featured by a positive gamma ray excursion due to the rapid transition from the high-energy Qixia Formation rocks to the low-energy Maokou Formation low-energy rocks formed during the early period of transgression (Figures 4(e) and (f)). Nevertheless, it should be noted that there were some low-energy rocks at the top of the Qixia Formation, but their leopard features made them significantly different from those low-energy rocks at the bottom of the overlying Maokou Formation. Certain errors might be inevitable due to similarity of these two types of low-energy rocks on the gamma ray curves (Figure 4(e)), which, though, were generally not more than 1 m. Therefore, from a macroscopic perspective, SB3 could also be relatively easily recognized.

Figure 3 Lithologic characteristics of long-term base-level sequence cycle boundaries:

4.3 Sequence stratigraphic characteristics

Through the sequence division scheme established according to the field sections, further sequence division was implemented in the Liangshan and Qixia Formations in each well. Lateral correlation was completed based on the selected representative section.

The selected section was composed of seven wells, including Wells HJL, K2, K3, ST2, YB6,YB7, and CS1(Figure 5), which spanned through the study area in the northwest-southeast direction, generally consistent with the early transgression direction. LSC1 was overall 10-33 m thick, with a large lateral variation. Except for Wells HJL and K2, all the other wells hosted thicker LSC than 25 m, which were dominated by relatively low-energy wackestones. In comparison, LSC2 was 75-105 m thick, with small lateral variation. High-energy deposits dominated, including packstones and sparry wackestones, and they were thicker than 98 m in Wells HJL and K2, while much thinner in the other wells.

Figure 4 Electrical characteristics of long-term base-level sequence cycle boundaries:

In general, LSC1 and LSC2 were featured by unstable lateral distribution in the section, and LSC1 was dominated by low-energy deposits while LSC2 high-energy ones, indicating gradually falling base level during deposition of Liangshan and Qixia Formations.

5 Discussion

5.1 Paleogeomorphy and sedimentary facies before deposition of LSC1

5.1.1 Non-isochronous deposition during Liangshan Formation sedimentary period

The Liangshan Formation had long been considered to be a set of transitional facies deposits formed during the early transgression on the unconformity resulting from the Yunnan Movement, and it was overlain by the Qixia Formation carbonates in the subsequent rapid transgression process. Therefore, traditionally, the Liangshan and Qixia Formations were studied independently, due to the fact that they were assumed to be isochronously deposited. However, this study speculated the Liangshan Formation and the bottom of the Qixia Formation to be contemporaneous heterotopic facies based on the following reasons.

1) In addition to absence in the Gaoshiti-Moxi Area in Central Sichuan Basin, the Liangshan Formation was deposited throughout the Sichuan Basin, with overall thin thickness and the thickest part slightly more than 20 m. Therefore, the Liangshan Formation was featured by large distribution area and thin thickness. Before deposition of the Liangshan Formation, the Yunnan Movement at the end of the Carboniferous extensively denuded the pre-existing sedimentary strata, forming a pene-plain base composed of different strata [32]. Pene-plainization generally meant flat paleogeomorphy with slight fluctuation and weakened weathering and erosion. Moreover, due to the base level rise during transgression, weathering and erosion effects were further weakened, which, together with the flat paleogeomorphy, made it difficult for the strong dynamic water flow system such as rivers to develop. Therefore, it could be indicated that the terrestrial material input should not be large in both quantity and intensity, which made it difficult to explain the wide distribution of the Liangshan Formation in the entire Sichuan Basin if it was isochronously deposited, as that was not supported by the limited material supply as well as low transportation power and thus short distance.

2) All previous research indicated continuous transitional sedimentation during deposition of the Liangshan and Qixia Formations. Since mixed carbonate deposition was observed at the bottom of the Qixia Formation in the Changjianggou and Zhengyuan Sections, the sedimentary assemblage from the bottom of the Liangshan Formation to the top of the Qixia Formation was speculated to consist of transitional facies clastic rocks, mixed carbonate rocks and carbonate rocks. As a set of transitional facies deposits, the Liangshan Formation was rarely correlated with the corresponding marine deposits in space. However, according to the Law of the Correlation of Facies, the continuous uninterrupted deposits in the vertical should also be adjacent in the planar distribution. Therefore, the Liangshan and Qixia Formations should have a direct contact in the plane, and thus the Qixia Formation should be the marine facies deposits in the sea side of the Liangshan Formation.

3) As shown in Figure 5, the rising half cycle in LSC1 was much larger than the falling half cycle, and areas with larger stratigraphic thicknesses hosted more lower-order cycles. All these indicated the gradual overlapping from the topographically lower parts to higher parts.

Based on the above analysis, it was suggested that the sedimentary filling process corresponding to the Liangshan Formation and the lower Qixia Formation should be different from previous understanding. Specifically, gradual overlapping from the topographically lower parts to higher parts on the unconformity occurred due to the gradual base level rise, resulting in the clastic rock deposits on the continental side and the carbonate rock deposits on the sea side. Thus, the sedimentary filling model of the Liangshan Formation and the lower Qixia Formation was established (Figure 6).

Figure 5 Long-term base-level sequence cycle correlation section through HJL, K2, K3, ST2, YB6, YB7 and CS1

5.1.2 Planar thickness distribution of LSC1 and paleogeomorphy before deposition of LSC1

Since LSC1 was deposited on a weathering surface of the Carboniferous, paleogeomorphy before deposition of LSC1 played a determinant role in the thickness distribution and sedimentary environment of LSC1. As discussed above, the lower LSC1 was ascribed to the gradual overlapping process. Meanwhile, the upper LSC1 was dominated by bioclastic packstone, with strong lateral continuity (Figure 7), which reflected stable sedimentary environments and hydrodynamics. Given the flat topography, SB2 on the top of LSC1 could be regarded as a filling and levelling up surface with strong traceability. Therefore, the impression method could be used to reconstruct the paleogeomorphy before deposition of LSC1 according to the thickness of LSC1.

Based on LSC1 thickness data from 34 localities (including field sections and wells) in the northwestern Sichuan Basin, the isopach of LSC1 (Figure 8), namely the paleo-geomorphologic map before deposition of LSC1, was made according to the tectonic confinement of the study area by the Hannan paleocontinent in the northeast and the Chuanzhong paleo-uplift in the south before the Permian [25]. Alternation of gentle depressions and uplifts could be clearly seen, including the highlands in the northwest, northeast and south of the study area, which were connected by gentle gullies and depressions, indicating a southeastward transgression direction.

5.1.3 Sedimentary environments and facies of LSC1

According to lithological characteristics and assemblages, deposition of LSC1 was featured by gradually overlapping and finally filling and levelling up. Therefore, by the impression method, smaller thickness areas should correspond to paleo- highlands, where low-energy shoals and tidal flats should be dominant in both occurrence frequency and accumulative thickness. In comparison, larger thickness areas corresponded to paleo-lowlands. As shown in Figure 9, the thickness ratio of the low- energy shoal and tidal flat deposits over the total LSC1 was plotted against the total LSC1 thickness, which demonstrated the above-stated relationships between LSC1 thickness and geomorphology. According to the main single factor mapping method [33] that 0.3 is regarded as a cutoff value for the thickness ratio of the low-energy shoal and tidal flat over the total LSC1, the total LSC1 thickness should be 29 m. Moreover, areas with the thickness ratio of the low-energy shoal and tidal flat deposits over the total LSC1 larger than 0.3 (LSC1 thickness ≤29 m) were speculated to be in the low-energy shoal or tidal flat, while those with the ratio smaller than 0.3 (LSC1 thickness >29 m) were in the inter-shoal sea or open sea. Furthermore, the favorable facies identified in the actual data and the tectonic setting were combined to map the LSC1 sedimentary facies (Figure 10). Rocks in the northwestern part of the study area might be products in the slope and basin environments, which were adjacent to the northeast-southwest extended platform margin shoal environment with weak continuity and low energy. In comparison, rocks in the northeast and south of the study area were products in the intraplatform low-energy shoal and inter-shoal sea environments. In general, the deposition of LSC1 was influenced by the paleogeomorphy with alternating uplift and depression, which resulted in restricted water body and thus low energy, accompanied by deposition in the intraplatform and platform-margin shoal, inter- shoal sea and some open sea environments.

Figure 6 Sedimentary filling model of Liangshan Formation and lower Qixia Formation

Figure 7 Medium-term base-level sequence cycle and sedimentary facies correlation section through HJL, K2, K3, ST2, YB6, YB7 and CS1

Figure 8 Paleogeomorphologic map before deposition of LSC1

Figure 9 Correlation between thickness ratio of low- energy shoal and tidal flat deposits over total LSC1 and the total LSC1 thickness

5.2 Paleogeomorphy and sedimentary facies before deposition of LSC2

5.2.1 Planar thickness distribution of LSC2 and paleogeomorphy before deposition of LSC2

Different from LSC1 in terms of sedimentary characteristics, LSC2 was characterized by a large number of relatively high-energy bioclastic grainstones with sparry cements (Figure 7). Moreover, the proportion of low-energy shoal facies packstones was significantly larger, which indicated more open water body during deposition of LSC2 than that of LSC1. According to division and comparison of mid-term cycles within LSC2, these mid-term cycles were continuous and stable (Figure 7). Furthermore, the LSC2 thickness was correlated with the thickness of deep-water low-energy deposits in the inter-shoal sea and open sea environments (Figure 11(a)). It could be seen that the proportion of the deep-water low-energy deposits decreased as the LSC2 thickness grew, which suggested good matching between the carbonate rock depositional rate and the accommodation space growth rate. In general, the sedimentary thickness was mainly controlled by the paleogeomorphy before deposition, with larger thicknesses of high-energy deposits and the whole strata in the topographically higher parts where the shallow water body and strong hydrodynamics were accompanied by exposure and development of tidal flat deposits. In comparison, smaller thickness of low-energy inter-shoal sea and open sea deposits and the whole strata were seen in the topographically lower parts, which resulted from the slow sedimentary process. Therefore, paleogeomorphy before deposition of LSC2 could be reconstructed according to its thickness (Figure 12). Slight difference was observed between paleogeomorphy before deposition of LSC2 and that of LSC1, including the emergence of two NE-SW-extended paleo-geomorphic highlands in the central and northwestern parts of the study area. The southern and northeastern parts of the study area presented no significant differences in terms of paleogeomorphy before deposition of LSC1 and that of LSC2, and they always stayed in the topographically higher parts, while the others parts were in the depressions and narrow valleys between highlands.

Figure 10 Sedimentary facies plan of LSC1

5.2.2 Sedimentary environments and facies of LSC2

LSC2 mainly consisted of sparry (dolomitic) grainstone, bioclastic (dolomitic) packstone, bioclastic (dolomitic) wackestone, granular dolomite (with grainstone as the parent rock), and silty dolomite (with micritic-silty dolomite as the parent rock), and these rock types indicated relatively open water body during the deposition, and the sedimentary level should be near or above the wave base.

Figure 11 Correlation between thickness ratios of low-energy and high-energy deposit over the total LSC2 and the total LSC2 thickness:

Except for the lowest mid-term cycle that hosted inter-shoal sea deposits, all the other mid-term cycles were featured by shoal deposits, including both high-energy and low-energy ones (Figure 7). Thicknesses of both high-energy deposits and the total LSC2 in Wells HJL and K2 were relatively high, which were ascribed to their topographically high positions at that time. In comparison, thicknesses of the total LSC2 in Wells YB6 and YB7 were relatively low, and low-energy deposits took the dominance, which indicated the second highest paleo-geomorphic positions.

Figure 12 Paleogeomorphologic map before deposition of LSC2

As mentioned above, the thickness ratio of the deep-water low-energy deposits (inter-shoal sea and open sea facies) over the total LSC2 decreased as the LSC2 thickness grew (Figure 11(a)). According to the dominant facies principle, 0.3 was selected as the cutoff thickness ratio, which corresponded to a LSC2 thickness of 81 m. In general, the inter-shoal sea and open sea deposits had a thickness ratio larger than 0.3 and a LSC2 thickness no greater than 81 m, whereas the tidal flat and shoal deposits had the opposite features (Figure 11(a)). Furthermore, the thickness ratio of shallow-water deposits (high-energy shoal and tidal flat facies) over the total LSC2 was plotted against the total LSC2 thickness in the scattering form (Figure 11(b)), which presented good positive correlation. Subsequently, another cutoff thickness ratio of 0.4, corresponding to a total LSC2 thickness of 89 m, was used to differentiate high-energy shoal and tidal flat deposits (ratio >0.4) and low-energy shoal deposits (ratio ≤0.4).

To conclude, when the LSC2 thickness varied in the range of ≤81 m, 81-89 m, and >89 m, the dominating facies were inter-shoal sea and open sea, low-energy shoal, and high-energy shoal and tidal flat, respectively. Accordingly, these facies at different localities, along with tectonic background information, were plotted in the LSC2 isopach, which resulted in the sedimentary facies planar distribution map (Figure 13). The northwestern part of the study area was still dominated by slope and basin deposits, and there were two rows of NE-SW-extended platform margins towards the platform. Compared with the paleogeomorphy during deposition of LSC1, that of LSC2 was featured by topographic transition of Well ST1 from the low inter-shoal sea environment to the high platform-margin high-energy shoal environment. The northeast and south of the study area were dominated by intraplatform low-energy shoal and high-energy shoal deposits, while the other parts were dominated by inter-shoal sea and open sea deposits. In general, during the deposition of LSC2, the study area was seen with differential tectonic uplifting, relatively open water body, and relatively high energy, accompanied by dominant development of platform-margin deposits and intraplatform high-energy and low-energy shoal deposits as well as occasional occurrence of inter-shoal sea and open sea deposits.

5.3 Depositional models for Liangshan and Qixia Formations under tectonic control

According to above analysis, paleogeomorphy before deposition of the Liangshan and Qixia Formations had obvious controlling effects on the sedimentary facies of the Liangshan and Qixia Formations. Moreover, different characteristics were seen during different periods. In general, the topography tended to be levelled up after deposition of LSC1, and it then was featured by alternating uplifts and depressions before deposition of LSC2, which indicated differential subsidence and controlled the sedimentary facies types and distributions of LSC2. Accordingly, the sedimentary evolution models of the Liangshan and Qixia Formations in the northwestern Sichuan Basin under the tectonic control was established (Figure 14).

The sedimentary environment of LSC1 was mainly controlled by the paleogeomorphy formed by the Yunnan Movement. Overlapping gradually took place during the northwestward slow transgression in the early Middle Permian, which contributed to the contemporaneous heterotopic facies of the Liangshan and Qixia Formations. In general, the deposition of LSC1 was associated with a restricted environment with low water energy,absence of sparry grainstones, and dominance of bioclastic packstone and wackestone. Regression occurred at the end of LSC1 deposition, which tended to level up the topography, resulting in the low-energy shoal deposits in the highlands and high-energy inter-shoal sea deposits in the lowlands as well as dominance of shoal deposits in the southern, western and northeastern parts of the study area.

Figure 13 Sedimentary facies plan of LSC2

Figure 14 Sedimentary evolution model of Liangshan (a) and Qixia (b) Formations in northwestern Sichuan Basin

Located in the northwestern margin of the Upper Yangtze Platform, the study area was in the passive continental marginal area where rifting activities occurred frequently during the Permian. Before deposition of LSC2, episodic tectonic movements resulted in a series of NE-SW-extended half-grabens in the west of the study area, which was close to the open sea. Fault blocks could be subjected to rigid rotation in the tectonic activities, with one side tilted while the other side inclined. Since the northern part of the study area was relatively far from the Mianlue Ocean, near NW-SE-extended grabens were formed as responses to the extension of the oceanic basin. Such differential tectonic subsidence along with the transgression in the early depositional period of LSC2 resulted in enhanced difference in topography and smooth water circulation. Specifically, two rows of NE-SW-extended high-energy platform margins were developed at the tilting position of the uplifted fault block. In the south and northeast of the study area, high-energy intraplatform shoals were dominant in the higher parts such as the horst, while inter-shoal sea and open sea deposits took the dominance in the lower parts of the titled fault blocks and the grabens. Specifically, the open sea area in the southeast of the study area was speculated to be the precursor of the Guangyuan-Wangcang Basin during the deposition of the Maokou Formation.

According to the built depositional model of the Liangshan and Qixia Formations in the northwestern Sichuan Basin, extensional activities might have already occurred during deposition of the Qixia Formation, which brought forward the timing of the Dongwu Movement in the northwestern Sichuan Basin. Moreover, those platform-margin and intraplatform high-energy shoal deposits formed under the tectonic control were linearly arranged in the belt-like distribution form, and they were regarded as favorable sedimentary facies for reservoir development, which should be highlighted in the future exploration.

6 Conclusions

1) The Qixia Formation in the northwestern Sichuan Basin was mainly deposited in the open platform and platform margin environments, with main deposits of high-energy shoals (in both intraplatform and platform margin), low-energy shoals, inter-shoal sea and open sea.

2) There were two long-term base-level sequence cycles (LSC) in the Liangshan and Qixia Formations, namely LSC1 and LSC2. Both cycles had greatly variable planar distributions. In general, LSC1 was dominated by low-energy deposits, and the water energy was gradually enhanced during deposition of LSC2, accompanied by the gradually falling base level.

3) The Liangshan Formation was not an isochronous stratigraphic unit. Deposition of the Liangshan Formation and the Lower Qixia Formation was ascribed to the layer-by-layer overlapping process on a continental weathering surface due to the gradual base level rise, forming the Liangshan Formation clastic rocks in the continental side and the Qixia Formation carbonate rocks in the sea side, which were referred to as contemporaneous heterotopic facies.

4) The deposition of LSC1 was roughly a levelling-up process for the previous paleogeomorphy. The impression method was used to reconstruct the paleogeomorphy based on the LSC1 thickness. As suggested by the correlation between the thickness ratio of shallow water deposits over the total formation and the total formation thickness, there was a good matching relationship between the depositional rate and the accommodation space growth rate. Moreover, larger LSC2 thicknesses indicated higher positions in the paleogeomorphy before deposition, based on which the paleogeomorphy before deposition was reconstructed using the residual thickness method. Finally, planar distributions of sedimentary facies of LSC1 and LSC2 were prepared by the main single factor mapping method.

5) The depositional model for the Liangshan and Qixia Formations of the northwestern Sichuan Basin under the tectonic control was established. Specifically, deposition of LSC1 was controlled by the unconformity-related geomorphology caused by the Yunnan Movement, with main deposits of intraplatform and platform margin low-energy bioclastic shoals and some deposits of inter-shoal sea and open sea. Episodic tectonic activities before the Dongwu Movement resulted in a series of semi-grabens, horsts and grabens before deposition of LSC2, which led to differential subsidence of the sedimentary base and thus controlled the deposition process of LSC2. During deposition of LSC2, the dominant facies were high-energy shoals (in both intraplatform and platform margin), low-energy shoals, inter-shoal sea and open sea, and there were two rows of platform margins extended in the northeast-southwest direction.

Contributors

ZHONG Yuan provided the idea of the study, developed the overarching research goal, and led the research activity planning and execution. YANG Yue-ming made great contribution to the improvement of manuscript after the initial draft finished. Other co-authors offered some valuable suggestions for the contents of the manuscript and polished the language of the manuscript. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

All authors declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

构造及其差异沉降对川西北部地区梁山-栖霞组沉积环境的控制作用

摘要：在野外观察、岩心描述及测井分析的基础上，对川西北部地区梁山组-栖霞组的构造-沉积格架进行深入探讨。共划分出2个长期旋回LSC1、LSC2，建立了川西北部梁山-栖霞组层序地层格架，分析认为梁山组并非等时沉积。采用印模法恢复出LSC1沉积前古地貌、LSC1在古地貌高部位沉积厚度较薄的低能滩，而在低部位以厚度较大的滩间海、开阔海沉积为主。同时，采用残厚法恢复出LSC2沉积前古地貌，地貌高部位以较厚的高能滩相沉积为主，地貌低部位为较薄的滩间海、开阔海沉积。结果表明栖霞期已经发生了不同程度的构造差异沉降，东吴运动发生时间可能在目前认识上进一步提前，沉积前古地貌对层序地层充填和沉积相分布具有明显的控制作用。研究结果对精细刻画沉积演化过程和预测优质储层分布具有实际指导意义。

关键词：层序地层；古地貌；沉积相带；栖霞组；二叠系；四川盆地西北部

Foundation item: Project(41802147) supported by the National Natural Science Foundation of China; Project(2016ZX05007-004) supported by the National Major Science and Technology Projects of China

Received date: 2020-02-16; Accepted date: 2020-07-18

Corresponding author: ZHONG Yuan, PhD; Tel: +86-15202858323; E-mail: zhongyuan2018@petrochina.com.cn; ORCID: https://orcid. org/0000-0001-5223-7470