Basin-Scale Sand Deposition in the Upper Triassic ... - Springer Link

Journal of Earth Science, Vol. 24, No. 1, p. 089–103, February 2013 Printed in China DOI: 10.1007/s12583-013-0312-7

ISSN 1674-487X

Basin-Scale Sand Deposition in the Upper Triassic Xujiahe Formation of the Sichuan Basin, Southwest China: Sedimentary Framework and Conceptual Model Xiucheng Tan* (谭秀成) State Key Laboratory of Oil and Gas Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China; School of Resource and Environment, Southwest Petroleum University, Chengdu 610500, China Qingsong Xia (夏青松), Jingshan Chen (陈景山), Ling Li (李凌), Hong Liu (刘宏) School of Resource and Environment, Southwest Petroleum University, Chengdu 610500, China Bing Luo (罗冰) Research Institute of Exploration and Development, PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China Jiwen Xia (夏吉文) Branch of Shunan Gas Field, PetroChina Southwest Oil and Gas Field Company, Luzhou 646001, China Jiajing Yang (杨家静) Research Institute of Exploration and Development, PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China ABSTRACT: The Upper Triassic Xujiahe (须家河) Formation in the Sichuan (四川) Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by oldlands in multiple directions including the north, east and south. Thus, the basin was generally characterized by coastal and widely open and shallow This study was supported by the Major State Basic Research

lacustrine deposition during the Late Triassic

Development Program (No. 2012CB214803), the China’s Na-

Xujiahe period. This is similar to the modern

tional

(No.

well-known Poyang (鄱阳) Lake. Therefore, we

2011ZX05004-005-03), the PetroChina Youth Innovation

investigated the framework and conceptual

Foundation (No. 2011D-5006-0105) and the Key Subject Con-

model of the Sichuan Basin during the Xujiahe

struction Project of Sichuan Province, China (No. SZD0414).

period with an analogue to the Poyang Lake.

*Corresponding author: [email protected]

Results show that the conceptual model of the

© China University of Geosciences and Springer-Verlag Berlin

deposition can be divided into transgressive and

Heidelberg 2013

regressive stages. The first, third and fifth mem-

Science

&

Technology

Manuscript received October 24, 2011. Manuscript accepted March 16, 2012.

Special

Project

bers of the formation are in transgressive stage and the deposits are dominated by shore and

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Xiucheng Tan, Qingsong Xia, Jingshan Chen, Ling Li, Hong Liu, Bing Luo, Jiwen Xia and Jiajing Yang shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have implications for understanding the large-scale sand deposition elsewhere. KEY WORDS: basin-scale sand deposition, coastal and widely open lake, sedimentary framework, sedimentary conceptual model, channel of braided river, Upper Triassic Xujiahe Formation, Sichuan Basin.

INTRODUCTION It is universally acknowledged that the deposition in continental environments is generally heterogeneous due to the common and large variations of facies and environments (Reading, 1996; Friedman and Sanders, 1978). Thus, it is rare that one type of sedimentary facies dominates the entire basin and such issues are of great scientific significance in sedimentology (Reading, 1996; Friedman and Sanders, 1978). The Upper Triassic Xujiahe Formation in the Sichuan Basin of Southwest China represents such a case (Tian et al., 2008; Hou et al., 2005). It is distinctive for the basin-scale sand deposition, which is defined as sand depositing in approximately basin scale (Long et al., 2011; Tan et al., 2011; Wang et al., 2010; Feng et al., 1994). Similar sedimentary phenomenon has been reported elsewhere such as the Cretaceous of the Songliao Basin (NE China) (Wang et al., 2002) and the Triassic of the Ordos Basin (Central China) (Zou et al., 2008; Yang et al., 2003). The interpretation for this is disputable, including large complex delta, beach and bar, and alluvial fan to braided delta (Zhu et al., 2008; Hou et al., 2005; Yang et al., 2003). This indicates the difficulty in reconstructing the basin’s prototype. Therefore, the study in the Xujiahe of the Sichuan Basin is of scientific significance for providing a complementary to better understanding the disputes. In addition, it also has practical implications as the formation is an important target for hydrocarbon exploration (Li et al., 2012; Rong et al., 2012; Xie et al., 2008). The basin-scale sand deposition of the Xujiahe Formation in the Sichuan Basin has been attributed to

many interpretations, including river, delta, lake, beach and bar, and coastal tide (Shi et al., 2008; Tian et al., 2008; Zhao et al., 2008; Jiang et al., 2007; Liu et al., 2007; Gao et al., 2005; Hou et al., 2005; Gu et al., 2004). These debates imply that the mechanism is remarkably difficult to constrain. For instance why such a basin-scale sand deposition is only developed in the second, fourth and sixth members of the formation (Ye et al., 2011). In the interpretation scheme of river-delta-lake depositions, the meandering river and its associated delta commonly forms flood plains, which, however, have not been observed in the second, fourth and sixth members of the formation. In contrast, multiple sandbodies of river channel facies superimposed directly with each other and distributed widely. For instance, the overlapping pinchout line of the second member of the Xujiahe Formation is around the Emei-Leshan-Hechuan-Yunyang areas. To northwestward around the western depression of the Sichuan Basin (i.e., wells Chuanke 1-Guanji), the sandbody thickness increases from 50 to 400 m and is dominated by sand depositions (Ye et al., 2011). This is sharply different from the typical depositional characteristics of the meandering river. In the interpretation scheme of beach and bar depositions, the sand source is hard to characterize as the basin during the Xujiahe period was typical of terrigeneous supplies. This is difficult to explain even combined with the interpretations of river and delta depositions. The sands commonly deposited in basin margins and cannot be transported to the central parts of the basin. In the interpretation scheme of coastal tide depositions, there should be a flow space for the evoporites as they

Basin-Scale Sand Deposition in the Upper Triassic Xujiahe Formation of the Sichuan Basin, Southwest China

are pronouncedly less developed than the underlying Middle Triassic Leikoupo Formation. These are same of the issues that have not been well settled. In practice, the deposition of the Xujiahe Formation is somewhat similar to the well-known Donghe sandstones in the Tarim Basin. They are both characterized by interbedded and stably-distributed sand and mud depositions (Tan et al., 2008). However, there are some differences. During the deposition of the Donghe sandstones from the early Late Devonian to the Early Carboniferous, the Tarim Basin received a continued transgression. This results in the sandstone deposition rising from the shore to the inland, while the mudstone thickness enhances rapidly from the inland to the shore and to the sea (Tan et al., 2007). In contrast, few such characteristics can be observed in the Xujiahe Formation, which shows a large and wide distribution sandwiched in the mudstones. The favorable reservoir sweet spots are distributed randomly in the sandstones (Zhao et al., 2010; Bian et al., 2009). Thus, the diachronnous interpretation of the Donghe sandstones cannot be extended to the Xujiahe basinscale sand deposition. Therefore, due to the existing problems with some being showed above, it is clear that there is still not an accurate understanding on the sedimentary framework and rules of the Xujiahe Formation in the Sichuan Basin. In addition, it also leads to the difficulty in establishing the predictive model of hydrocarbon reservoir evaluation, as hydrocarbons have been produced from the formation. These may be due to the hard reconstruction of the prototype of the Sichuan Basin during the Xujiahe period. The basin boundary is much wider than present (Guo et al., 1986), and the formation was eroded to certain degrees due to the uplift and erosion of surrounding mountains during the late geological evolution. These all lead to the difficulty in addressing the mechanism of the basin-scale sand deposition. In this paper, we attempt to discuss the sedimentary framework and conceptual model of the deposition. GEOLOGICAL SETTING OF TRANSGRESSION At the end of the Middle Triassic, the Sichuan Basin started an uplifting from the west to the east under the influence of the early Indosinian orogeny

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(Li et al., 2012; Wei et al., 1997). This event witnessed the termination of the general marine deposition of the Sichuan Basin. Subsequently, the basin was subject to an environment of coastal lake during the Late Triassic after relatively long uplift and erosion (Guo et al., 1986). Thus, the basin is inclined to receive transgression and the transgressive intervals are generally correlated to global high sea-levels (Yuan et al., 2006; Lin et al., 2004). The transgression took place mainly in the western basin area as it is located near the well-known Songpan-Ganzi Sea trough. This event, in turn, should impact on the general sedimentary framework. The Sichuan Basin was Connected to the Western Songpan-Ganzi Sea The Sichuan Basin is located to the east of the well-known Songpan-Ganzi Sea during the Late Triassic, providing a good chance for connection. The boundary of these two geological units is commonly believed to site from the Longmen Mountains to the Kangdian Oldlands (Wang, 1985). The termination of this Songpan-Ganzi trough is the Late Triassic in general. For instance, Zhang (1981) proposed that the residual marine basin disappeared after the Late Triassic (Dai and Sun, 2009; Yang et al., 1994). Zhang et al. (2004) believed that the time is the late episode of the Late Triassic Indosinian orogeny. On the type of the Songpan-Ganzi Basin, multiple viewpoints have been suggested including back-arc (Burchfiel et al., 1995), residual Tethys marine (Ingersoll et al., 2003), forearc (Yang, 2002) and foreland basins (Xu et al., 2003; Pan et al., 2001). The sedimentary environment and facies are generally believed to be of deep-sea turbid origin with extremely thick flysch deposition (Li J L et al., 2007; Run et al., 2007; Ingersoll et al., 2003; Du et al., 1998; Xu et al., 1992). Therefore, the Songpan-Ganzi area is most likely a residual marine basin during the Triassic and, thus, the eastern Sichuan Basin was easy to connect to this sea and receive transgression. In the outcrops of the piedmont areas of the Longmen Mountains (Wu et al., 1999), many Late Triassic marine records have been discovered including ammonite, belemnite, bryozoan, sponge, coral, encrinite, foraminifer and brachiopod (Yang et al., 2008; Wang, 1992). The outcrops include Hanwang

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Xiucheng Tan, Qingsong Xia, Jingshan Chen, Ling Li, Hong Liu, Bing Luo, Jiwen Xia and Jiajing Yang

Town of Mianzhu City, Jushui Town of An County, Shiyuan Town of Jiangyou County and several towns of Jianshe County. This implies that the environment is marine and the Longmen Mountains have not been a range of oldlands and hills. As a consequence, the western Songpan-Ganzi Sea and the eastern Sichuan Basin were connected. The basin is characterized by a coastal and offshore lacustrine deposition. The thrusting, overriding and uplifting of the Longmen Mountains took place in the late episode of the Indosinian orogeny and subsequent Yanshanian and Himalayan orogenies, leading to the formation of the western Sichuan foreland basin (Li et al., 2008).

NE-trending Longmen Mountains and approximately NS-trending Kangdian Oldland. As a consequence, the Sichuan Basin formed a semi-restricted marine basin or a large marine bay with barriers (Figs. 1 and 2), being characterized by a deposition of tidal flat to lagoon. From the west to the east, the Upper Triassic Kuhongdong and Xiaotangzi formations of marine origin were deposited from the bottom to the top. Typical marine biologies were found, including Burmesia lirata and Gervillia-Halobia bivalve assemblages. They represent the first member of the Xujiahe Formation. After the transgression, the depositional environment changes from tidal flat and lagoon to lake and delta under the influence of regression. In the eastern basin, braided river delta was developed widely on the flat tomography (Wang and Chen, 1984). This can be evidenced by abundant plant fossils, as well as Modiolus-Unionites assemblages. They are all

Paleontological Evidence for the Transgression During the early and middle stages of the Late Triassic Xujiahe period, the Sichuan Basin subsided and, thus, received sea water from the western Songpan-Ganzi Sea through the vent between the Guangyuan 80 km

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Tongzi

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Upper Lei 4³ Major provenance direction of Member the early Late Triassic

Figure 1. Paleogeological map and provenance direction prior to the Late Triassic of the Sichuan Basin.


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Figure 2. Depositional boundary during the early stage of the Late Triassic in the Sichuan Basin and distribution of garnet content in the second member of the Xujiahe Formation (modified after Xie et al., 2006). indicative of semi-saline lacustrine bivalves. It is the characteristics of the second member of the Xujiahe Formation. To the late stage of the Late Triassic, sea water retrograded from the basin along with the descending of the relative sea-level. As a consequence, the basin is characterized by a deposition of braided river delta to alluvial plain in general. Under such environments, plenty of plants propagated (e.g., DictyoPhyllumClathropteris assemblages). Therefore, based on the above discussion, it can be concluded that the Sichuan Basin had pathways connected to the western well-known Songpan-Ganzi Sea and, thus, received transgression during the Late Triassic Xujiahe period. Relation of the Transgression to Global Sea-Level Fluctuations The above evidences indicate that the Late Triassic Xujiahe period in the Sichuan Basin recorded transgression, which can even last to the deposition of the fifth member of the Formation according to Zhao et al. (2008) and Jiang et al. (2007). Thus, previous perspectives that the Sichuan Basin has deposited in

continental environments since the Middle Triassic Leikoupo period (e.g., Huang and Lu, 1992) should be reevaluated. In practice, this transgression is related to the global sea-level fluctuations. As shown in Fig. 3 (Haq et al., 1988), there are four global sea-level risings during the Late Triassic, i.e., 227, 223, 218 and 213 Ma. Combined with the sea-level fluctuations in the northern Himalayan areas (Shi, 2001), it can be inferred that the lake level during the Xujiahe period of the Sichuan Basin also has four risings, corresponding to the mudstones of the first, second, third and fifth members of the Xujiahe Formation. They were deposited roughly in 228, 223, 216 and 212 Ma, respectively (He, 1989). Thus, these four risings are generally consistent with the four global sea-level risings (Fig. 3), indicating a close relation between them. This further supports the above conclusion that the Sichuan Basin is coastal to the western Songpan-Ganzi Sea during the Late Triassic. SEDIMENTARY FRAMEWORK Inferences from Paleotomography and Provenance Along with the early Indosinian orogeny at the end of the Middle Triassic, the Sichuan Basin started

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Figure 3. Relation between global sea-level fluctuations and lake level fluctuations of the Sichuan Basin during the Late Triassic. to uplift from the west to the east. As a consequence, the basin was subject to an environment of coastal lake during the Late Triassic after relatively long uplift and erosion (Guo et al., 1986). Analyses from paleotomography and provenance indicate that the basin was a widely open and coastal lake in nature. As shown in the paleogeological sedimentary map of the Sichuan Basin prior to the Late Triassic (Fig. 1), large NE-trending uplifts and depressions were present under the influence of the early episode of the Indosinian orogeny. Of the uplifts, the KaijiangLuzhou paleouplift is located in the east to the southeast of the basin, whose western part is a NE-trending paleodepression. The main part of the paleodepression is in the western basin. The stratigraphic erosion was common in the uplift. Different strata overlain by the Middle Triassic Leikoupo Formation were outcropped above the Permian–Triassic unconformity. In the central area of the uplift, even the Lower Triassic Jialingjiang Formation was outcropped. In contrast, the large paleodepression in the western basin was inclined gently, with a relatively flat tomography and little erosion. Hence, the strata overlying the Middle Triassic

has a good preservation. The paleotomography of the basin is generally flat after the denudation and planation, being characterized by a westward slope-plain inclination. With respect to the provenance, the variation of heavy mineral content in sedimentary rocks can provide clues, and the direction of paleocurrent can also be implied (Xie et al., 2008; Li S J et al., 2007; Li et al., 2004). The representative heavy mineral in the Xujiahe sandstone of the Sichuan Basin is garnet, which is commonly believed to source from volcanic and metamorphic rocks. As shown in Fig. 2, the provenance can be indicated from the distribution of garnet content in the representative second member of the Xujiahe Formation. The western provenance is mainly from the northern oldland of Longmen Mountains. The northern provenance is mainly from the oldland of Micang-Daba Mountains, further including two systems of Nanjiang and Kaijiang. The southern provenance is mainly from the Qianzhong uplift and Jiangnan Oldland. The eastern provenance is mainly from the Jiangnan Oldland. In addition, there may be some provenances from the Kangdian Oldland in the


southeastern basin (Li et al., 2004). In summary, the Sichuan Basin during the Late Triassic is a terrigenous basin with multiple provenances from oldlands in the north, east and south of the basin. This is one of the basic characteristics of sedimentary framework. Analogue to Modern Poyang Lake Based on the above discussion, it can be implied that the Sichuan Basin during the Late Triassic Xujiahe period is a widely open and coastal lake in nature. Thus, the depositional features can be indicated with an analogue to a modern case, e.g., the modern Poyang Lake. The modern Poyang Lake is a fresh-water shallow lake with seasonal transgressions and regressions. The lake is characterized by multiple river systems and water supplies, mainly including five sources (i.e., the Ganjiang, Xiushui, Fuhe, Xinjiang and Raohe rivers). Of the five rivers, the Ganjiang River has the relatively biggest impacts on the formation of the delta, with an area up to 1 544 km2. It was a lake and river in nature during the high-water and low-water levels, respectively. The water in the lake was distributed flatly during the flooding time, while looked like a line during the low-water time. During the flooding time, the five rivers flowed into the lake and enlarged the lake surface. In contrast, during the low-water time in the winter and spring seasons, the lake water flows

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out and, thus, the lake looked like a river channel. Therefore, the area and volume of the lake vary sharply between the high-water and low-water time intervals. The highest water level ever recorded is 21.69 m, with area and volume being 4 647 km2 and 333×108 m3, respectively. It is the biggest fresh-water lake of China at the time. In contrast, the lowest water level ever recorded is only 5.9 m, when the area and volume are 146 km2 and 5.6×108 m3, respectively. As shown in the remote sensing map (Fig. 4), the delta formed under the influence of the Ganjiang River is fan- and flower-like in shape. The delta above the water level can be further divided into two units, i.e., the upper and lower delta plains. The upper delta plain is always above the low-water level, extending about 40 km. In contrast, the lower delta plain between the low-water and high-water levels extends only 10–20 km with an average of 15 km. Thus, a clear difference was shown between the high-water flooding time and low-water time. High-water flooding time During the high-water flooding time, the Ganjiang River divided into four distributary channels after the flow through Nanchang City (Fig. 4). The channels further forked into eight subchannels in the lower delta plain when they entered the lake. Of the channels, the farthest north main channel had the highest energy and, thus, did not divide before

Figure 4. Remote sensing maps showing natural landscape of modern Poyang Lake, which is a widely open lake. High-water flooding (left) and low-water stages (right). It was a lake and river in nature during the high-water and low-water levels, respectively.

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entering the lake. It crossed the lower delta plain and connected with the widely open pathway of lake water. As a consequence, it can flow out from the lake. In contrast, the main body of the lower delta plain was below the lake level. Thus, the underwater distributary channel can be further divided into the ending channel system and interdistributary estuary. The ending channel systems were well developed in the delta front, similar to crevasse splay in shape. In contrast, the ending distributary channel system and crevasse splay are typical microfacies types of shallow meandering river delta (Zou et al., 2008). Low-water time During the low-water time, the flow volume of the Ganjiang River decreased. Four distributary channels were formed in the upper delta plain after flowing through Nanchang City (Fig. 4). The type of the river channel was meandering river with low sinuosity or straight river. The northern two channels had typical meandering features, with the development of point bar. In contrast, the southern channels were straight, with the development of few point bars and some core beaches. The lower distributary channel in the upper delta plain further forked, with some being abandoned to form oxbow lake. The main body of the lower delta plain was above the lake level. The ending distributary channel and crevasse splay that were formed in the high-water flooding time were well exposed, while residual lake was formed by the separation of interdistributary estuary and main lake area. In summary, the development of the Ganjiang River delta in the high-water and low-water stages reflects three characteristics of delta that is controlled by shallow river. First, the delta plain includes upper and lower parts. Second, high productive flood basin was developed in the upper delta plain, while interdistributary swamp and estuary, and residual lake were developed in the lower delta plain. Third, the lower delta plain acted mainly as the discharging of deposits and, thus, multiple-grade ending distributary channel systems were developed. The crevasse splay system was formed in the front area (Zou et al., 2008). Based on the above discussion, the general depositional features of the modern and widely open lake (i.e., the Poyang Lake) can be implied. Each facies of

the delta aggregates rapidly during the low-water stage, resulting in the enlargement of the deposition above the water and the forward extension of the distributary channel. As a consequence, the river has a relatively high energy and the deposition is composed mainly of sands with only a limited amount of suspended matters, which can be taken away by the high-energy water. In contrast, during the high-water stage, the delta regresses rapidly. Thus, the river has a relatively low energy and large amount of suspended matter deposits in the basin. Mud deposition is hence formed. The thickness of the mud is not big because the deposition of the suspended matter is slow. Furthermore, the mud cannot be preserved due to the scour and erosion in the subsequent low-water stage. Therefore, the lacustrine basin is dominated by a sand deposition in large scale even up to the entire basin. These sand depositions in the flat lacustrine basin are nearly isochronous. In addition to the above similarities, the Sichuan Basin during the Late Triassic also has differences from the modern Poyang Lake. They can be summarized into three folds. First, the basin area is largely different. The Sichuan Basin was approximately 4 2 16×10 km , as much as 100 times of that of the Poyang Lake. Second, the controlling factors on the fluctuations of water levels are different. It is global sea-level fluctuations for the Sichuan Basin as discussed above. In contrast, it is seasonal variation of meteoric water and water levels of the Changjiang River as the Poyang Lake is formed mainly by river flow. Thus, the water-level fluctuations of the Poyang Lake are more frequent than the Sichuan Basin during the Xujiahe period. Third, the Sichuan Basin during the Xujiahe period has multiple provenance systems and, thus, is easy to form braided river systems. There is a mixing between short-distance provenance from Longmen Mountains and Micang-Daba Mountains and long-distance provenance from southern Jiangnan Oldland. In contrast, the Poyang Lake has only one main provenance, i.e., the Ganjiang River, which is the main influencing factor on the river-lake sedimentary systems of the Poyang Lake. Therefore, to summarize, the Sichuan Basin during the Late Triassic Xujiahe period is generally similar to the modern Poyang Lake and, thus, these two geological units can


be analogized. CONCEPTUAL SEDIMENTARY MODEL According to the general depositional features of the modern Poyang Lake discussed above, it can be indicated that the deposition of the Sichuan Basin during the Late Triassic Xujiahe period can be divided into two stages of transgression and regression. Transgression Stage Under the influence of global periodic risings of sea-level during the Late Triassic, lacustrine transgressions took place in the Sichuan Basin during the first, third and fifth stages of the Xujiahe period. As a consequence, most of the basin was submerged by lake water and the deposition is characterized by shore-shallow lacustrine facies (Fig. 5). This is similar to the modern Poyang Lake during the high-water flooding time. Under the depositional setting of lacustrine transgression and land regression, the sedimentary facies varies from the marginal to the central basin, following the order of alluvial fan, alluvial plain,

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braided river delta and wide shore-shallow lake. In the margin of the oldlands in the north, east and south of the basin, the supply of terrigenous clastics is sufficient to form a series of alluvial plains and braided river deltas. In addition, in the east of the Longmen Mountains in the northwestern basin, alluvial fans drove directly into the basin, forming fan deltas. These facies were distributed in the basin margin in a horseshoe shape. In contrast, to the central basin, the deposition is characterized mainly by shore-shallow lacustrine facies, with less supply of coarse-grained terrigenous clastics and more supply of fine-grained clastics. Thus, mud deposition was dominated in such a low-energy environment. The sandy beach and bar occur randomly in the shore-shallow lake areas, under the reactivity of relatively high-energy lake wave and flow. Note that the terrigenous clastic deposition in the western basin has some marine characteristics as discussed above. The facies type is a semi-restricted marine tidal flat with barrier to lagoon, under the influence of marine transgression from the western Songpan-Ganzi Sea (Guo et al., 1986).

Figure 5. Conceptual sedimentary model of the Sichuan Basin during the lacustrine transgressive Xujiahe period.

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Regression Stage Compared with the transgression stage, the Sichuan Basin in the lacustrine regression stage during the Late Triassic Xujiahe period was subject to combined influences of global periodic declines of sealevel and shrinking of water flow pathway in the western basin margin. As a consequence, the lake water in the basin flowed out from the pathway. The lacustrine regression generally took place in the second, fourth and sixth stages of the Xujiahe period. Consequently, most of the basin turned to the environment of continental plain, being characterized by the development of continental alluvial plain and braided river delta plain (Fig. 6). This is similar to the modern Poyang Lake during the low water-level stage. Under the depositional setting of lacustrine regression and continental aggression, the lake water retreated to the western basin. The sedimentary facies from the marginal to the central basin follow the order of alluvial fan and plain, braided river plain, braided river

delta front, and shore-shallow lake. A series of alluvial fans were developed in the margin of the basin’s northern, eastern and southern oldlands. It is distinctive in the wide development of braided river delta plain, which crossed in various directions in the basin. As a consequence, a braided network of sand deposition was formed, along with the lateral migration and forward moving of the river channels. The lake area in the western basin margin influenced by marine transgression turned to an environment of delta front, also being characterized by a sand deposition. The fine-grained sediments flowed out from the basin to the Songpan-Ganzi Sea. Therefore, the second, fourth and sixth members of the Xujiahe Formation were dominated by sand depositions. GEOLOGICAL IMPLICATIONS The sedimentary framework and conceptual model of the Sichuan Basin during the Late Triassic Xujiahe period established above can provide clues to

Figure 6. Conceptual sedimentary model of the Sichuan Basin during the lacustrine regressive Xujiahe period.


some of the disputes in the study of the Xujiahe Formation, including stratigraphic correlation, sedimentary framework and type, and basin-scale sand deposition. (1) Influenced by the global periodic risings and declines of sea-level during the Late Triassic, the Sichuan Basin was coastal and lacustrine in nature accordingly. Three periodic cycles of lacustrine transgressions and regressions were developed. Of the transgression time including the first, third and fifth stages of the Xujiahe period, mud deposition dominated. In contrast, the second, fourth and sixth stages of the Xujiahe period witnessed the sand deposition along with the lacustrine regression. Thus, each member of the Xujiahe Formation is roughly isochronous with little stratigraphic diachronism. This indicates that the present stratigraphic studies are reasonable, in which the Xujiahe Formation comprises six members and the sandstones and mudstones are well correlated in stratigraphy. (2) The sedimentary framework and conceptual model of the Sichuan Basin during the Xujiahe period were established with an analogue to the modern Poyang Lake. The widely open lake is coastal in nature, and has multiple water systems and provenances. During the lacustrine regression stage, the basin is characterized by wide braided rivers and delta plains. The cross network of sandy river-channels migrated laterally and moved forward. As a consequence, the sand deposition occurred widely in the flat basin, forming the distinctive basin-scale sand deposition in the second, fourth and sixth members of the Xujiahe Formation. (3) According to the model, the lake-water level declined in a relatively long time due to the lacustrine regression. The continental braided river and delta plain aggregated rapidly and, thus, thick sand deposition was formed predominantly. Delta front was only developed locally in the northwestern basin. The superimposed style of sand bodies varies in facies. Of the widely-distributed braided plain facies, finingupward sequences are common (Fig. 7). In contrast, the lake water rose rapidly during the lacustrine transgression stage. Hence, the lake water flooded in most of the basin, which is typical of shallow lake. The lake wave and flow reactivated on the primary sediments

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of braided plain, leading to a reverse grain size order in the top of the sand body. In addition, the delta-front sand body appeared in the basin margin, while most of the basin is characterized by relatively thin mud layers (Fig. 7). When the next relatively long lacustrine regression took place, the mud deposits would be scoured and eroded by the rapidly moving of braided plain channels. They were preserved mainly as residual deposition. Thus, the multiple cyclic alterations of short and rapid lacustrine transgression and relatively long lacustrine regression may be one of the important reasons responsible for the development of basin-scale sand deposition. (4) Of the basin-scale sand deposition, the main channel of the braided river generally has a high depositional energy, thus commonly forming coarse-grained sand body with relatively large scale and good sorting. It is favorable for the development of good sandstone reservoir, with porosity >5% and -3 2 permeability >0.1×10 μm in general. In contrast, the other channels have relatively low depositional energy. Under such conditions, the sand body is commonly fine-grained, with relatively poor sorting and low maturity. These are not favorable for reservoir development, even when being reactivated by short lake wave and flow. The porosity and permeability are commonly less than 3% and 0.05×10-3 μm2, respectively. In addition, the sand body of the main channel has lenticular and banded distributions when perpendicular and parallel to the water flow, respectively. Hence, the good reservoir occurs randomly in the sandstone layers with low porosities and permeabilities (Fig. 8). This is why the reservoir in the Xujiahe Formation is developed heterogeneously, and the main channel and its extension area is favorable for the development of reservoir. CONCLUSIONS In this study, we established the sedimentary framework and conceptual model of the Lower Triassic Xujiahe Formation in the Sichuan Basin, with an analogue to modern cases, i.e., the Poyang Lake. The results lead to a further discussion on the development of favorable reservoir and provide clues to address some disputes in the study of the Xujiahe Formation. (1) The sedimentary framework during the

100

Xiucheng Tan, Qingsong Xia, Jingshan Chen, Ling Li, Hong Liu, Bing Luo, Jiwen Xia and Jiajing Yang B: Short lacustrine transgression

Sea level

Songpan-Ganzi trough

Longmen Mountains island chain

Lake level

Front of braided delta

Lake

Braided delta plainAlluvial fan alluvial plain

Oldland

Alluvial fan

Oldland

A: Long lacustrine regression

Sea level

Songpan-Ganzi trough

Longmen Mountains island chain

Lake level

Alluvial fan

Lake


Braided delta plain-alluvial plain

Braided delta plainalluvial plain


Lake

Early sand body altered by lake wave and flow

Figure 7. Sketch map showing superimposed style of sand bodies and variation of lake level, the Upper Triassic Xujiahe Formation of the Sichuan Basin.

Sand body reservoir formed in the main braided river channel Sand body of sub-braided river channel

Figure 8. Conceptual model of good sandstone reservoir developed in the main channel of braided river, the Upper Triassic Xujiahe Formation of the Sichuan Basin. Late Triassic Xujiahe period is generally a widely open and coastal lake in nature, which is surrounded by oldlands in the north, east and south of the basin. To the west, there is a flow pathway connecting with the Songpan-Ganzi Sea. Thus, it is a terrigenous clas-

tic basin with multiple water systems and provenances. (2) The general sedimentary framework is similar to the modern Poyang Lake. Thus, the mechanism of the sand deposition in the Xujiahe period was discussed with an analogue to the modern Poyang Lake. During the lacustrine regression stage, the basin was characterized by a wide development of braided river and delta plain. The cross network of sandy river-channels migrated laterally and moved forward. As a consequence, the sand deposition occurred widely in the flat basin, forming the distinctive basin-scale sand deposition in the second, fourth and sixth members of the Xujiahe Formation. (3) The multiple alternations of short and rapid lacustrine transgression and relatively long lacustrine regression are significantly responsible for the basinscale sand deposition. The main channel of the braided river and its extension area are favorable for the development of reservoir. (4) The study here is generally macro-scale in investigating the sedimentary framework and the


established model is preliminary and conceptual. Thus, considering the sedimentary heterogeneities of continental lacustrine basins, more middle to micro scale studies are needed to improve the accuracy of researches.

101

with English Abstract) Friedman, G. M., Sanders, J. E., 1978. Principles of Sedimentology. Jon Wiley & Sons Press, New York Gao, Z. Y., Luo, P., Zheng, R. C., et al., 2005. The High-Resolution Sequence Stratigraphy Analysis of the Upper Triassic Xiangxi Formation in the Moxi-Longnusi

ACKNOWLEDGMENTS We would like to thank senior engineers Benqiang Du and Fei Long of the Shunan Gas Field Branch, PetroChina Southwest Oil and Gas Field Company, for their help to improve the article in scientific communications. Editors and anonymous reviewers are thanked for constructive comments. Dr. Xiaohui Mou is thanked for help in drawing some figures. This work was jointly funded by the Major State Basic Research Development Program (No. 2012CB214803), the China’s National Science & Technology Special Project (No. 2011ZX05004-005 -03), the PetroChina Youth Innovation Foundation (No. 2011D-5006-0105) and the Key Subject Construction Project of Sichuan Province, China (No. SZD0414).

Structure of Sichuan, China. Journal of Chengdu University of Technology (Science & Technology Edition), 32(6): 597–603 (in Chinese with English Abstract) Gu, J. L., Zheng, R. C., Luo, P., et al., 2004. Sequence Stratigraphic Framework and Source-Reservoir-Cap Rock of Xujiahe Formation in West Sichuan Depression. Journal of Chengdu University of Technology (Science & Technology Edition), 31(3): 282–290 (in Chinese with English Abstract) Guo, Z. W., Deng, K. L., Han, Y. H., et al., 1986. The Formation and Development of Sichuan Basin. Geological Publishing House, Beijing (in Chinese) He, L., 1989. Seismic-Stratigraphic Division and Correlation Project of the Upper Triassic System in Sichuan Basin. Oil & Gas Geology, 10(4): 439–446 (in Chinese with English Abstract) Haq, B. V., Hardenbol, J., Vail, P. R., 1988. Mesozoic and Ce-

REFERENCES CITED

nozoic Chronostratigraphy and Eustatic Cycles. SEPM

Bian, C. S., Wang, H. J., Wang, Z. C., et al., 2009. Controlling

(Spec. Pub.), 42: 71–108

Factors for Massive Accumulation of Natural Gas in the

Hou, F. H., Jiang, Y. Q., Fang, S. X., et al., 2005. Sedimentary

Xujiahe Formation in Central Sichuan Basin. Oil & Gas

Model of Sandstone in Second and Fourth Members of

Geology, 30(5): 548–555 (in Chinese with English Ab-

Xiangxi Formation in the Upper Triassic of Sichuan Basin.

stract)

Acta Petrolei Sinica, 26(2): 30–37 (in Chinese with Eng-

Burchfiel, B. C., Chen, Z., Liu, Y., et al., 1995. Tectonics of

lish Abstract)

Longmenshan and Adjacent Regions, Central China. In-

Huang, Q. S., Lu, S. M., 1992. The Primary Studies on the

ternational Geology Review, 37: 661–735, doi:10.1080/

Palaeoecology of the Late Triassic Xujiahe Flora in East-

00206819509465424

ern Sichuan. Earth Science—Journal of China University

Dai, Z. M., Sun, C. M., 2009. Shrinking History of Archipelagic Island-Arc-Basin System in the Plate Junction in

of Geosciences, 17(3): 329–335 (in Chinese with English Abstract)

the West Songpan-Ganzi Orenic Zone during the Meso-

Ingersoll, R., Dickinson, W. R., Graham, S. A., 2003. Remnant-

zoic. Acta Geologica Sichuan, 29(Suppl.): 8–13 (in Chi-

Ocean Submarine Fans: Largest Sedimentary Systems on

nese with English Abstract)

Earth. In: Chan, M. A., Archer, A. W., eds., Extreme

Du, D. X., Luo, J. N., Hui, L., 2008. Sedimentary Facies and

Depositional Environments: Mega End Members in Geo-

Palaeogeography of the Triassic Sedimentary Basins in the

logic Time. Geological Society of America (Special Paper):

Bayan Har Mountainous Area: An Example from the

191–208

Aba-Zoige Basin, Sichuan. Sedimentary Facies and Pa-

Jiang, Z. X., Tian, J. J., Chen, G. J., et al., 2007. Sedimentary

laeogeography, 18(1): 1–18 (in Chinese with English Ab-

Characteristics of the Upper Triassic in Western Sichuan

stract)

Foreland Basin. Journal of Palaeogeography, 9(2):

Feng, Z. Z., Wang, Y. F., Liu, H. J., et al., 1994. Sedimentology of China. Petroleum Industry Press, Beijing (in Chinese

143–154 (in Chinese with English Abstract) Li, J. L., Run, Z., Yu, L. J., et al., 2007. Sedimentary and

102

Xiucheng Tan, Qingsong Xia, Jingshan Chen, Ling Li, Hong Liu, Bing Luo, Jiwen Xia and Jiajing Yang Structural Characteristics of the Ruoergai-Songpan Fore-

the Mid-Upper Triassic Sedimentary Facies in the

land Basin. Acta Petrologica Sinica, 23(5): 919–924 (in

Ruoergai-Songpan Area. Progress in Natural Science,

Chinese with English Abstract)

17(2): 196–204 (in Chinese)

Li, Q. F., Wang, Z. C., Li, J., et al., 2012. Discovery of

Shi, X. Y., 2001. Triassic Sequence Stratigraphy and Sedimen-

Yanting-Tongnan Trough of Late Permian in Sichuan Ba-

tary Evolution in the Qomolongma Area, Southern Xizang

sin and Its Significance. Journal of Earth Science, 23(4):

(Tibet)—From Epicontinental Sea to Basin. Acta Ge-

582–596, doi:10.1007/s12583-012-0276-z

ologica Sinica, 75(3): 292–302 (in Chinese with English

Li, S. J., Shi, Y. H., Wang, Q. C., et al., 2007. Changes of De-

Abstract)

trital Heavy Minerals’ Composition in the Kuqa Depres-

Shi, Z. S., Yang, W., Jin, H., et al., 2008. Study on Sedimentary

sion from Cretaceous. Chinese Journal of Geology, 42(4):

Facies of the Upper Triassic in Central and South Sichuan

709–721 (in Chinese with English Abstract)

Province. Acta Sedimentologica Sinica, 26(2): 211–220 (in

Li, Y., Zhou, Y. Q., Yan, S. Y., et al., 2008. Establishment of

Chinese with English Abstract)

Tectonic Evolution Pattern of Longmenshan Orogen.

Tan, X. C., Chang, Y., Wang, Z. Y., et al., 2007. Depositional

Journal of China University of Petroleum (Edition of

Framework and Evolution of Bachu Formation in Tarim

Natural Science), 32(2): 12–20 (in Chinese with English

Basin. Journal of Southwest Petroleum University, 29(4):

Abstract)

39–43 (in Chinese with English Abstract)

Li, Z., Wang, D. X., Lin, W., et al., 2004. Mesozoic–Cenozoic

Tan, X. C., Liu, H., Li, L., et al., 2011. Primary Intergranular

Clastic Composition in Kuqa Depression, Northwest

Pores in Oolitic Shoal Reservoir of Lower Triassic

China: Implication for Provenance Types and Tectonic At-

Feixianguan Formation, Sichuan Basin, Southwest China:

tributes. Acta Petrologica Sinica, 20(3): 655–666 (in Chi-

Fundamental for Reservoir Formation and Retention


Diagenesis. Journal of Earth Science, 22(1): 101–114,

Lin, C. S., Wang, Q. F., Xiao, J. X., et al., 2004. Sequence

doi:10.1007/s12583-011-0160-2

Stratigraphic Construction and Filling Response Model of

Tan, X. C., Xia, Q. S., Luo, B., et al., 2008. The Sedimentary

Cretaceous Sequence in Kuqa Sag. Science in China Se-

Facies and Reservoir Characteristic of the Upper Triassic

ries D: Earth Sciences, 34 (Suppl. I): 74–82 (in Chinese)

Xujiahe Formation in the Danfengchang-Liangdongmiao

Liu, J. H., Zhang, S. Q., Sun, Y. T., et al., 2007. Correlation and Evolution of the Upper Triassic Xujiahe Formation in the West Sichuan Basin. Journal of Stratigraphy, 31(2): 190–196 (in Chinese with English Abstract)

Area. Internal Technical Report (Southwest Petroleum Institute), Chengdu (in Chinese) Tian, J. J., Jiang, Z. X., Li, X. Z., et al., 2008. Genesis of the Thick Sandstones of Xujiahe Formation Member II of

Long, S. X., Wu, S. X., Li, H. T., et al., 2011. Hybrid Sedimen-

Upper Triassic and the Influences on Reservoir and Gas

tation in Late Permian−Early Triassic in Western Sichuan

Pool in Chuanzhong Region. Petroleum Geology and Oil-

Basin. Journal of Earth Science, 22(3): 340–350,

field Development in Daqing, 27(2): 43–46 (in Chinese

doi:10.1007/s12583-011-0186-5

with English Abstract)

Pan, G. T., Wang, L. Q., Li, X. Z., et al., 2001. The Tectonic

Wang, G. Z., Liu, S. G., Ma, Y. S., et al., 2010. Characteristics

Framework and Spatial Allocation of the Archipelagic Arc

of Subaerial Karstification and Late Reconstruction in the

Basin Systems on the Qinghai-Xizang Plateau. Sedimen-

Dengying Formation, Sichuan Baisn, Southwestern China.

tary Geology and Tethyan Geology, 21(3): 1–26 (in Chi-

Journal of Earth Science, 21(3): 290–302, doi:10.1007/


s12583-010-0093-1

Reading, H. G., 1996. Sedimentary Environments: Processes, Facies and Stratigraphy. Blackwell Press, Oxford-Boston

Wang, H. Z., 1985. Atlas of the Paleogeography of China. Cartographic Publishing House, Beijing. 94 (in Chinese)

Rong, H., Jiao, Y. Q., Wu, L. Q., et al., 2012. Effects of

Wang, P. J., Schneider, W., Mattern, F., et al., 2002. The Char-

Diagenesis on the Acoustic Velocity of the Triassic Oolitic

acters of Transgressive Sequence of Terrigenous Basin:

Shoals in the Yudongzi Outcrop of Erlangmiao Area,

Correlation between the Triassic in Central European Ba-

Northwest Sichuan Basin. Journal of Earth Science, 23(4):

sin and the Cretaceous in Songliao Basin of China. Jour-

542–558, doi:10.1007/s12583-012-0274-1

nal of Mineralogy and Petrology, 22(2): 47–53 (in Chi-

Run, Z., Yu, L. J., Li, J. L., et al., 2007. The Characteristics of


Basin-Scale Sand Deposition in the Upper Triassic Xujiahe Formation of the Sichuan Basin, Southwest China Wang, X. C., Chen, L. X., 1984. Sedimentary Environment and Seam Erosion in the Xujiahe Formation in the East Sichuan Basin. Coal Geology & Exploration, 2(2): 12–17 (in Chinese) Wang, Y. S., 1992. The Earlier Late Triassic Ammonites from Longmen Mountains. Journal of Chengdu College of Geology (Science & Technology Edition), 19(4): 28–35 (in Chinese with English Abstract)

103

nology (Science & Technology Edition), 35(4): 455–462 (in Chinese with English Abstract) Yang, Z. R., 2002. The Formation and Evolution of the Songpan-Garze Fore-Arc Basin, Western Sichuan. Sedimentary Geology and Tethyan Geology, 22(3): 53–59 (in Chinese with English Abstract) Ye, T. R., Li, S. B., Lü, Z. X., et al., 2011. Sequence Stratigraphic Framework and Sedimentary System Distribution

Wei, K. S., Xu, H. D., Ye, S. F., 1997. Sequence Stratigraphic

of the Xujiahe Formation in the Sichuan Basin. Natural

Characteristics of Sichuan Basin. Oil & Gas Geology,

Gas Industry, 31(9): 51–57 (in Chinese with English ab-

18(2): 151–157 (in Chinese with English Abstract)

stract).

Wu, S., Zhao, B., Gou, Z. H., et al., 1999. The Structural

Yuan, W. F., Chen, S. Y., Zeng, C. M., 2006. Study on Marine

Framework and Its Evolution in the Middle South Section

Transgression of Paleogene Shahejie Formation in Jiyang

of Longmen Mountains. Journal of Mineralogy and Pe-

Depression. Acta Petrolei Sinica, 27(4): 40–44 (in Chinese

trology, 19(3): 82–85 (in Chinese with English Abstract)

with English Abstract)

Xie, J. R., Li, G. H., Tang, D. H., 2006. Analysis on

Zhang, G. W., Guo, A. L., Yao, A. P., 2004. Western

Provenance-Supply System of Upper Triassic Xujiahe

Qinling-Songpan Continental Tectonic Node in China’s

Formation, Sichuan Basin. Natural Gas Exploration and

Continental Tectonics. Earth Science Frontiers, 11(3):

Development, 29(4): 1–3 (in Chinese with English Abstract)

23–32 (in Chinese with English Abstract) Zhang, Q. W., 1981. The Sedimentary Features of the Flysh

Xie, J. R., Zhang, J., Li, G. H., et al., 2008. Exploration Pros-

Formation of the Xikang Group in the Indosinian

pect and Gas Reservoir Characteristics of Xujiahe Forma-

Songpan-Garze Geosyncline and Its Geotectonic Setting.

tion in Sichuan Basin. Journal of Southwest Petroleum

Geological Review, 27(5): 405–412 (in Chinese with Eng-

University (Science & Technology Edition), 30(6): 40–45

lish Abstract)

(in Chinese with English Abstract)

Zhao, W. Z., Wang, H. J., Xu, C. C., et al., 2010. Reservoir-

Xu, Q., Pan, G. T., Jiang, X. S., 2003. Songpan-Ganze Belt:

Forming Mechanism and Enrichment Conditions of the

Forearc Accretion or Backarc Collapsing? Journal of

Extensive Xujiahe Formation Gas Reservoirs, Central Si-

Mineralogy and Petrology, 23(2): 27–31 (in Chinese with

chuan Basin. Petroleum Exploration and Development,

English Abstract)


Xu, Z. Q., Hou, L. W., Wang, Z. X., 1992. The Orogenetic

Zhao, X. F., Lü, Z. G., Zhang, W. L., et al., 2008. Paralic Tidal

Process of Songpan-Ganzi Orogenic Zone in China. Geo-

Deposits in the Upper Triassic Xujiahe Formation in

logical Publishing House, Beijing. 1–190 (in Chinese)

Anyue Area, the Sichuan Basin. Natural Gas Industry,

Yang, F. Q., Yin, H. F., Yang, H. S., et al., 1994. The Songpan-


Garze Massif: Its Relationship with the Qinling Fold Belt

Zhu, R. K., Zou, C. N., Zhang, N., et al., 2008. Diagenetic Flu-

and Yangtze Platform and Development. Acta Geologica

ids Evolution and Genetic Mechanism of Tight Sandstone

Sinica, 68(3): 208–218 (in Chinese with English Abstract)

Gas Reservoirs in Upper Triassic Xujiahe Formation in

Yang, H., Fu, J. H., Yu, J., 2003. Oil Reservoir Enrichment

Sichuan Basin, China. Science in China Series D: Earth

Patterns of Large Delta Systems and Application of Ex-

Sciences, 51(9): 1340–1353, doi:10.1007/s11430-008-

ploration Techniques in Shanbei Area. Acta Petrolei Sinica, 24(3): 6–10 (in Chinese with English Abstract)

0102-8 Zou, C. N., Zhao, W. Z., Zhang, X. Y., et al., 2008. Formation

Yang, R. J., Liu, S. G., Wu, X. C., et al., 2008. Distribution

and Distribution of Shallow-Water Deltas and Central-

Characteristics and Controlling Factors of Upper Triassic

Basin Sandbodies in Large Open Depression Lake Basins.

Ma’antang Formation in the Front of Longmen Mountians,

Acta Geologica Sinica, 82(6): 813–825 (in Chinese with

Sichuan, China. Journal of Chengdu University of Tech-

English Abstract)