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Natural Gas Industry B 2 (2015) 406e414 www.elsevier.com/locate/ngib Research article Geological significance of paleo-aulacogen and exploration potential of reef flat gas reservoirs in the Western Depression*

Liu Shua,*, Ning Mengb, Xie Gangpinga

a Sinopec Southwest Branch Company, , Sichuan 610041, b School of Geosciences, Yangtze University, Wuhan, Hubei 430100, China

Received 19 March 2015; accepted 8 September 2015 Available online 26 February 2016

Abstract

Confirming thick hydrocarbon generation center and discovering thick porous reservoirs are two key factors to start the Permian gas exploration of the Western Sichuan Depression. In this paper, the Sinian-Cambrian structures of this area were studied by adopting the layer- flattening technology and the Lower Paleozoic thickness map was prepared in order to describe the Permian hydrocarbon generation center. Then, combined with seismic facies analysis and field outcrop bioherm discovery, the distribution of Middle Permian reef flat reservoirs were predicted. Finally, the favorable conditions for reef flat reservoir dolomitization were analyzed based on features. The study indicates that: (1) Sinian top represents a huge depression in the profile flatted by the reflecting interface of Permian bottom, with normal faults filled by thick Lower Paleozoic sediments at both sides, revealing that a aulacogen formed during the Khanka taphrogeny exists in the Western Sichuan Depression, where very thick Cambrian strata may contain hydrocarbon generation center, making Permian strata have the material conditions for the formation of large gas pools; (2) the Middle Permian strata in the Western Sichuan Depression exhibit obvious abnormal response in reef flat facies, where three large abnormal bands are developed, which are predicted as bioherm complex combined with the Middle Permian bioherm outcrop discoveries in surface; and (3) deep and large extensional faults are developed in reef flat margin, manifesting as favorable conditions for the development of dolomite reservoirs. The results show that the Middle Permian traps in the Western Sichuan Depression contain resources up to 7400 108 m3, showing significant natural gas exploration prospects. By far, one risk exploration well has been deployed. © 2016 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Sichuan Basin; Western Sichuan Depression; Middle Permian; Reef flat facies; Bioherm; Dolomite; Reservoir; Aulacogen; Khanka taphrogeny

In the Middle Permian gas reservoir, one of the major gas 2014, Well Shuangtan 1 drilled at Shuangyushi buried struc- reservoirs in the southern and Southwestern parts of the ture in the Northern Sichuan Basin and Well 1 Sichuan Basin, natural gases were once thought to reside in drilled in the Central Sichuan Basin yielded commercial gas pores, cavities and fractures, especially in the cave system flow from the Middle Permian reservoirs, the reservoir space formed in the process of Dongwu Movement [1e3], which has of which was found to be pores and fractures-pores. As per only been drilled by a few wells [2] with low production. In some further researches [4,5], the Middle Permian porous dolomite reservoirs may have high yield of gases, which ex- pands the prospect for exploration. The Middle Permian gas * “ resources in the Sichuan Basin were proved to reach Supported by National Science and Technology Major Project Clastic 12 3 reservoir identification and prediction in the Sichuan Basin” (Grant No. 1.47 10 m , and the proved reserves were only 8 3 2008ZX05002-004-003). 811.68 10 m mainly found in Southern and Eastern * Corresponding author. Sichuan Basin. Remaining gas resources were estimated to be E-mail address: [email protected] (Liu S.). 1.38 1012 m3, indicating a huge potential. Peer review under responsibility of Sichuan Petroleum Administration. http://dx.doi.org/10.1016/j.ngib.2015.09.016 2352-8540/© 2016 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414 407

Within the region with Sinopec mineral rights in the continental margin from the Sinian Period to the Middle Western Sichuan Basin, the Permian System has not been Triassic Epoch and regional unconformable surfaces occurred drilled by any well yet. This is because (1) large burial depths within the marine sequence due to several major tectonic around 6500 m, (2) a small volume of the cave system pre- movements [13,15]. The Caledonian orogeny gave birth to the dicted with large uncertainties in accordance with the model Central Sichuan palaeohigh (Guangxi movement). Ordovician composed of pores, cavities and fractures developed for and Silurian formations may be lost in the Western Sichuan Middle Permian gas reservoirs [1,2], and (3) stratigraphic Depression [13]. break of the Silurian System in the Western Sichuan Basin. The last one is the most important because the argillutite of the 2. Features and geologic significance of Sinian e Lower Liangshan Formation, which is Middle Permian source rock, is Paleozoic paleo-aulacogen not thick enough to generate a large amount of natural gases in such a deep zone for exploration. Therefore, gas accumulation 2.1. Geologic features of paleo-aulacogen is closely related to the existence of porous dolomite reservoirs and large-scale source rocks. As per recent studies, organic The volume of hydrocarbon accumulations is dependent on reefs may exist not only in the Changxing Formation [6] but the volume of source rocks; generally oil and gas fields lie also in the Middle Permian Series in the Western Sichuan within source beds (there is no fault) or close to source beds Depression in light of typical seismic reflection signatures of (there are faults) [16]. In the Sichuan Basin, natural gas dis- organic reefs and banks, i.e. blank to chaotic reflections with tribution in the lower assemblage is dependent on several fac- convex external forms, which were interpreted to represent tors, i.e. extensional troughs forming in the Khanka taphrogeny, organic reefs and banks in accordance with the outcrops of the palaeohighs, and basin-mountain structures [17]. In the Western Middle Permian Maokou Formation. In addition, major faults Sichuan Depression, the volume of Permian gas reservoirs is were interpreted to extend along the margin of predicted reef dominated by the scale of hydrocarbon generating center. and bank facies, which would be favorable for the generation Now at least two Sinian-Lower Paleozoic extensional of dolomite reservoirs [4,5,7e10]. Besides, a paleo-aulacogen have been confirmed to exist in the Sichuan Basin, which are has been confirmed to exist in the Western Sichuan Basin [6], Guangwang-Kaijiang-Liangping trough in the Northeastern which may function as the hydrocarbon generating center with Sichuan Basin and the extensional in the Western and thick source rocks to generate a large amount of natural gases. Central Sichuan Basin [6,17e22]. The latter occurs inside the The Middle Permian traps in the Western Sichuan Depression craton and some small gas reservoirs of reef-bank facies have were predicted to exceed 1800 km2 and corresponding gas been discovered in the margin. But there is not sufficient resources were estimated to be 7400 108 m3, more than a knowledge about their geneses. Luo Zhili [18] predicted half of the remaining resources (1.38 1012 m3). So it is another aulacogen with potential gas accumulation around possible to find large gas reservoirs in the Western Sichuan and Zhongjiang in the Western Sichuan Basin, which Basin. was confirmed in 2010 and named by Zhao Wenzhi as Pengxi- Wusheng upper shelf [19] possibly caused by continental 1. Geological features rupture and reactivation of the basement fault. Luo Zhili [20] suggested naming it as the aulacogen Mianzhu-Pengxi- The Western Sichuan Depression extends in NE direction in Wusheng aulacogen as per the concept of mantle plume and the west of the Sichuan Basin and is separated from Songpan- proposed that Mianlue fault activity during the Permian and Ganzi fold belt by Longmenshan Mountains on the west and Triassic Periods resulted in ocean basin deepening and base- from Central Sichuan palaeohigh by Longquanshan Mountains ment fault reactivation in the northwest margin of the Yangtze on the east. Micangshan fold belt and Sichuan-Yunnan struc- Plate; the latter gave birth to the aulacogen in the margin of the tural belt are on the north and south of the depression, Yangtze Continent. Li Qiufen used another nomenclature, respectively. The lateral extension basically coincides with the Yanting-Tongnan trough [21], and suggested the trough was present Cretaceous-Quaternary outcrops region (Fig. 1). generated by NW basement fault activity in the Sichuan Basin Tectonically the Western Sichuan Depression lies in the during Mianlue Ocean spreading in the Late Permian Epoch west margin of the Yangtze Block (Fig. 1) and is a foreland based on seismic and geologic studies of the depositional basin caused by tectonic compression during the Indosinian framework. Liu Shugen [17] used another nomenclature, and Himalayan orogenies [11]. It evolved from marine facies -Lezhi-Longchang-Changning extensional trough, from the Sinian Period to the Middle Triassic Epoch into as per the view point of the Khanka taphrogeny and proposed transitional facies in the early Late Triassic Epoch and finally the extensional trough experienced Tongwan movement at the into continental facies from the late stage of the Late Triassic end of Dengying depositional stage and then Maidiping Epoch to the Quaternary Period [12,13]. During the Khanka deposition and reached the climax at Qiongzhusi depositional taphrogeny from the Late Proterozoic to the Early Paleozoic stage; after Canglangpu deposition, the trough finally dis- (Pt1- 1), huge Chinese protoplatform disintegrated; Kunlun, appeared during the Longwangmiao deposition. This rift was Qinling, Longmenshan and Tianshan geosynclines outspread; confirmed by Wei Guoqi et al. [22]. the Yangtze, North China and Tarim Plates came into being An aulacogen (also known as the rift or rifted ) is [14,15]. The west margin of the Yangtze Plate was a passive defined as the geosyncline or graben developed within a craton 408 Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414

Fig. 1. Regional tectonic setting of the Western Sichuan Depression. with the normal faults as its boundary. Due to mantle plume south. As is shown in Fig. 2a, the section traverses the upwelling and consequent local uplifting of overlying litho- Western Sichuan Depression and extends from Dayi in sphere, a three-branch fault system composed of normal faults the south to Yazihe in the middle and finally to Anxian- would come into being; one branch would evolve into an in the north (shown in Fig. 1). The horizon of oceanic rift and another failed rift becomes an aulacogen. The Middle Permian top is flattened to make the paleo- meeting point of three branches is called . As aulacogen stand out. In Dayi, seismic facies with per seismic reflections (Fig. 2), the rift across the Western and chaotic reflections below the Paleozoic Erathem Central Sichuan Basin demonstrates typical features of an possibly correspond to Sinian igneous rock intrusions aulacogen. It is the product of the Khanka taphrogeny [6] and overlain with Sinian sedimentary formations. In the area was reactivated during the Dongwu Movement (the Emei around , the Lower and Upper Sinian strata taphrogeny). Thick Cambrian formations may function as the exhibit continuous reflections, which are the feature of major source beds for Permian gas reservoirs. sedimentary formations. The rift is highlighted by Two seismic sections were used to characterize the paleo- wedge-like thickness variation of Upper Sinian strata, aulacogen across the Western Sichuan Basin. The overall i.e. small thickness in rift center (around Yazihe) and features are demonstrated by one section passing through large thickness at both sides of the rift. Consequently Dayi-Jiangyou (Fig. 2a) and the features of another failed rift two palaeohighs, i.e. Dayi and Jiangyou, occurred at the in the paleo-aulacogen across the Western Sichuan Basin are end of the Sinian Period. The rift, called the Western delineated by the other section across Wenjiang and Long- Sichuan aulacogen with its center in Yazihe and normal quanshan (Fig. 2b). faults at both sides, is the part of Mianzhu-Wusheng aulacogen within the Western Sichuan Basin. 1) Upper Sinian reflections show wedge-like configuration 2) Wenjiang-Longquanshan rift is another branch of the and the rift is bounded by normal faults at both sides. failed rift (Fig. 2b). The horizon of the bottom of the There may be igneous rock intrusions in Dayi in the Lower Triassic Jialingjiang Formation is flattened to Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414 409

Fig. 2. Seismic sections showing Sinian paleo-aulacogen across the Western Sichuan Basin (the locations are shown in Figs. 1 and 3).

illustrate the effect of the Emei taphrogeny on the Jiangyou palaeohigh occurs in the north. As per Sinian Permian System, as is shown in Fig. 2b. There is a thickness variation, it is inferred that the rift came into being visible salient, bounded by a normal fault, in Sinian in Pre-Sinian and continued to develop in the Sinian Period; it reflections below Longquanshan Mountains. This fault was filled in the Cambrian Period and finally leveled up active during the Dongwu Movement is a normal fault during the Caledonian orogeny. It should be the product of inside the Permian System and acts as the boundary of a the Khanka taphrogeny. In summary, this Middle Permian graben (Fig. 2b). Depression, or upper shelf [19] or trough [21], essentially evolved from the Sinian paleo-aulacogen across the Western As per seismic reflections, the Sinian paleo-aulacogen Sichuan Basin. It is bounded by normal faults and the triple could be characterized with Lower Paleozoic thickness junction may occur around Mianzhu. The drop of the rift variation. As is shown in Fig. 3, the rift extending into the exceeds 2000 m and the width is basically equal to that of the craton has two branches, i.e. Mianzhu-Wusheng aulacogen middle section of Longmenshan Mountains. The association and Mianzhu-Luodai failed rift. Dayi and Longquanshan of the Western Sichuan paleo-aulacogen and initial Long- palaeohighs occur in the south of the Depression and Lower menshan rift may be called a composite three-branch rift Paleozoic thickness at Dayi palaeohigh is only 200 m or so. [23].

Fig. 3. Paleo-aulacogen distribution in the Western Sichuan Basin. 410 Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414

2.2. Geologic significance of paleo-aulacogen It is inferred from the properties of the nappe belt that TOC content may not be very high in the Lower Cambrian Qingping As per the source control theory and the hydrocarbon Formation (i.e. the Qiongzhusi Formation). Based on the re- source system, the Western Sichuan paleo-aulacogen may sults of simulation experiments and numerical models [26,27], have thick Permian source rocks to generate sufficient gases in inverted formations and the thrust belt inside a nappe belt Permian reservoirs. Permian reef-bank facies are dominated usually stand close to the palaeohigh and is blocked by the by Mianzhu-Luodai failed rift. Normal faults reactivated palaeohigh. Therefore the Lower Cambrian outcrops observed during the Dongwu Movement in rift margins would act as the in Hanwang, Mianzhu may be blocked by Jiangyou palae- pathways for hydrothermal fluid migration, which would ohigh and be pushed out of the 's surface by the nappe facilitate the dolomitization of reservoir rocks of reef-bank belt, while it may be difficult for the Cambrian formations facies [10,18]. These normal faults may also act as hydro- within the rift to be pushed out of the earth's surface. The carbon migration pathways at the late stage. sedimentary facies inside the rift may be of larger thickness and source rocks may also be of better quality. 2.2.1. The paleo-aulacogen may become the hydrocarbon In summary, the above estimations may be less accurate generating center with thick Permian source rocks and should be corrected in the future if well drilling data are Natural gases in the Sichuan Basin were mainly generated available. But as per the data available now, the Lower in the source beds of the Lower Cambrian, Lower Silurian, Cambrian Series has already been demonstrated to be one of Middle Permian, Lower Jurassic, Upper Permian and Upper the major source beds. Triassic, especially in Lower Cambrian and Lower Silurian source beds [24]. Lower Cambrian Qiongzhusi shale is the 2.2.2. Mianzhu-Luodai failed rift dominates the distribution most important source rock for the lower assemblage in the of Permian reef-bank facies Sichuan Basin. In the Western Sichuan Basin, Silurian for- mations may be lost; Middle Permian dark argillaceous rocks 2.2.2.1. The Middle Permian Series has potential organic and argillaceous micritic limestone may generate some gases, reefs. During the ground survey of Longmenshan nappe belt which are not sufficient to form a large gas field. from 2013 to 2014, organic reef and bank outcrops of the Natural gases in Permian reservoirs may partially come Maokou Formation were found in Tongkou, Beichuan County from the Lower Cambrian Series, which has not been at the central and north sections of Longmenshan Mountains confirmed yet in the Western Sichuan Basin because there is and in Shuimo Town, Sanjiang at the south section (Figs. 1 and no well drilling data and only outcrop data are available for 4). tentative analysis. In the Western Sichuan Basin, Lower As is shown in Fig. 4, the entire Maokou Formation being Cambrian source rocks, which are composed of the deposits of more than 30 m thick could be observed on Tongkou section at sub-abysmal basin facies, littoral corcass facies or limy cor- the north part of Longmenshan Mountains (Fig. 1). The reef cass facies, crop out in such regions as Shangsi in base is composed of bioclastic calcarenite and the reef core City, Tongkou in Beichuan County, Hanwang in Mianzhu City, consists of coral limestone. The organic reef outcrop is rela- etc. and the thickness is generally larger than 100 m. Average tively intact with distinct features. TOC content of efficient source rocks is 0.61%. Source rocks The organic reef identified on Shuimo section (Fig. 1) at the with the best quality occur in Hanwang, Mianzhu City; for south part of Longmenshan Mountains is composed of sponge siliceous mudstone and siliceous source rocks in the lower reef limestone (the picture is omitted). Qingping Formation (contemporaneous with the Qiongzhusi Middle Permian organic reefs have not been drilled yet by Formation, but the sedimentary facies are different) in the wells in the Sichuan Basin (Fig. 1), but a lot of rocks of bio- Lower Cambrian Series, TOC content is 0.82% on average and clastic bank facies have been drilled. (1) In Well Guanji, bio- may reach 1.33% at most. Gas generating intensity in the clastic rocks of 6 m thick were drilled at the top of the Middle Lower Cambrian Series in the Western Sichuan Basin was Permian Maokou Formation (from 7051 m to 7057 m) and calculated to range (30e180) 108 m3/km2 with an average bioclast content ranges 10%e50%. (2) In Well Zhougong 1 of 30.02 108 m3/km2 and cumulative gas resources were drilled in the Southwestern Sichuan Basin, bioclastic rocks estimated to reach 147.3956 1012 m3 [17,25]. occupy almost the whole section from the Mao 1 Member to the Based on the data showing the Lower Cambrian source Mao 3 Member (from 3260 m to 3475 m). (3) In Well Dashen 1, rock thickness variation in the Sichuan Basin and the research bioclastic rocks occupy almost the whole section from the Mao findings from Liu Shugen [17], source rocks in the Western 1 Member to the Mao 4 Member (from 5260 m to 5580 m). Sichuan Basin are inferior to the analogues in Central and As per well drilling data, bioclastic banks are relatively Southern Sichuan Basin, but the sedimentary facies may be developed in the Southwestern Sichuan Basin and less devel- similar because they reside in the same rift [17,21,22]. The oped in the Northern Sichuan Basin. The Middle Permian depth of the rift is less than 1000 m in Gaoshiti, Central Series is of gentle slope facies on the whole with carbonate Sichuan Basin, and exceeds 2000 m in the paleo-aulacogen in rocks deposited in a warm climate [13]; the Western Sichuan the Western Sichuan Basin, which may indicate that source Depression lies between the high-energy facies in the Southern rocks in the paleo-aulacogen may be of better quality and Sichuan Basin and the low-energy facies in the Northern larger thickness. Sichuan Basin, so it is inferred that the Depression should be Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414 411

Fig. 4. Organic reef outcrops of the Maokou Formation, Tongkou section at the north part of the Longmenshan Mountains. rich in bioclastic banks. The Middle Permian Series has been 1) Faults F1, F2, F3 and F4 constitute a graben formed in demonstrated to have potential organic reefs according to the the Permian Period. Four normal faults, i.e. east-inclined organic reef outcrops of the Maokou Formation (Fig. 4). F1, F2 and F3 and west-inclined F4, are interpreted on Therefore, it is inferred that Middle Permian abnormal seismic the section. They are all normal faults, especially F3 and facies may correspond to organic reefs and banks. F4. The graben is the depression between those oppo- sitely inclined faults. 2.2.2.2. The aulacogen dominates the distribution of Permian 2) Middle Permian thickness changes abruptly at the organic reefs and banks. An organic reef is shown as internal hanging walls of F3 and F4, which indicates the control chaotic reflections with a convex external form thinning at of faults on Middle Permian deposition. Therefore, it is either side; continuous reflections with onlap at both sides of inferred that these faults are the product of the Dongwu the reef could be observed [28]. These reflection patterns have Movement. proved effective in exploration practice [29]. A lot of organic 3) Faults F1 and F4 occur in the margin of the paleo- reefs and banks of the Changxing Formation [19,21] have been aulacogen, which means that these faults were caused found to coexist along the margin of Mianzhu-Pengxi- by marginal fault reactivation. Wusheng aulacogen in the Central Sichuan Basin. 4) Changxing reefs and banks basically coincide with As per data available, the Middle Permian Series exhibits Middle Permian reefs and banks, which indicates that distinct reflections (Figs. 2b and 5) that could be interpreted as Middle Permian lithofacies palaeogeography dominates organic reef and bank facies larger than Changxing anomalies. the deposition of the Changxing Formation. The east part of Wenjiang-Longquanshan section (Fig. 2b, the east part of BeB0 section) is zoomed in and shown in Fig. 5 to In summary, paleo-aulacogen reactivation during the exhibit the seismic facies around Xindu and Longquanshan Dongwu Movement (the Emei taphrogeny) gave birth to the Mountains. The bottom reflection of the Jialingjiang Forma- horsts and in the early Permian Period; Middle tion (T1j ) is flattened to make the control of the paleo- Permian and Changxing reefs and banks were deposited in the aulacogen on the Permian System stand out. Within the re- margin of the grabens. Therefore, it is concluded that the gion around Xindu, there are two reef-bank anomalies inside paleo-aulacogen dominates the deposition of Permian reefs the Middle Permian Series (enclosed by two boxes in Fig. 5). and banks. Convex 1 with distinct draping and oblique reflections is interpreted to be of bank facies and convex 2 with internal 2.2.3. Normal faults at both sides of the rift act as the chaotic reflection is interpreted to be of reef facies. Seismic pathways for hydrothermal fluid and hydrocarbon anomalies below Longquanshan fault-fold belt (circled by an migration ellipse in Fig. 5) are interpreted to be of reef facies. As per Only dolomitized reefs and banks may have good reservoir outcrop observations (Fig. 4), Middle Permian seismic facies properties. The Dongwu Movement (the Emei taphrogeny) at are predicted to be of composite reef-bank facies. the end of the Early Permian Epoch has a great impact on In accordance with further analysis, the predicted Middle Permian gas reservoirs [20]. Permian and Changxing reef-bank facies geographically coin- The Middle Permian Series is a major gas pay zone in the cide with the paleo-aulacogen, as is shown in Figs. 2b and 5. Southern and Southwestern Sichuan Basin, the reservoir space 412 Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414

Fig. 5. Permian graben- reflections and reef-bank distribution (the location of section shown in Figs. 1 and 3). of which is composed of dissolved pores, cavities and frac- oil and gas generated in source rocks could migrate along tures. But stratified dolomite reservoirs were drilled in Wells those normal faults produced by the Dongwu Movement, an Zhougong 1, Nanchong 1 and Shuangtan 1. There may be two extensional movement. Compared with Yuanba, the Permian reasons for dolomitization. System in the Western Sichuan Basin has better reservoir forming conditions due to the existence of the aulacogen 1) Hydrothermal sub-basin induced dolomitization. This [28,29]. concept was presented by Li Yi et al. [4] to explain the origin of Maokou dolostone in the Eastern and Central 3. Gas exploration potential of the Middle Permian reef- Sichuan Basin. South China witnessed severe volcanic bank facies activities in the Permian Period, which began at the late stage of the Middle Permian Maokou Formation, ended Typical seismic facies of the organic reef and bank are at the end of the Late Permian Epoch and reached a characterized by internal blank to chaotic reflections with climax at the turn from the Middle Permian to the Late convex external forms, while the basin facies features low Permian. Some faulted basins occurred in the Eastern energy of deposition, continuous and stable reflections and Sichuan Basin due to the growth of contemporaneous strong amplitude (Fig. 5). Therefore, the amplitude attribute faults. Magmatic hydrothermal fluids migrating upward extracted along the top of the Middle Permian Series could be along these deep faults altered the physical and chemical used to delineate the distribution of Middle Permian reef-bank conditions of the basins to generate hydrothermal sub- facies (Fig. 6). basins in a hydrothermal sedimentary environment. Fig. 6 shows the seismic amplitude extracted along the top Therefore, the Maokou Formation was mainly deposited of the Middle Permian Series in the Western Sichuan with dolostone. Wang Hua et al. [5] proposed a new Depression. In general, weak amplitude corresponds to the point of “hydrothermal sub-basin microfacies” in the reef and bank facies and strong amplitude corresponds to the Middle Permian Series in the Sichuan Basin, which basin facies. But for the fractured nappe belt at the foreland of stated within the range of a hydrothermal sub-basin, Longmenshan Mountains, weak amplitude is the reflection of dolostone is hot-water deposits distributed along the faulted zone instead of reef-bank facies. In accordance with basement fault due to the effect of hydrothermal water. the amplitude attribute and the seismic profile (Fig. 5) as well 2) Tectonic hydrothermal fluid induced dolomitization as data quality evaluation (for example, data quality of 3D [8e10]. Most researchers are in favor of this hypothesis, Yuquan survey is poor), three plays with large Middle Permian which states deep hydrothermal fluids (which may be reef-bank facies were predicted across Western Sichuan. originated from or atmospheric water after deep circulation) travelled upward along a lot of extensional 1) The play with reef-bank facies around Luodai and Xindu strike-slip faults to the top of the Qixia Formation and (indicated by two boxes in Fig. 5). Trap area was esti- caused dolomitization of the surrounding rocks. Mean- mated to be 700 km2 and organic reefs were predicted to while, dissolved pores were generated due to chemical be 200e300 m thick. As per the exploration practice in erosion. Thus hydrothermal dolomite reservoirs formed the Northeastern Sichuan Basin and drilling data of at the top of the Qixia Formation and at the bottom of Wells Zhougong 1 and Dashen 1, the net pay was pre- the Maokou Formation. sumed to be 50 m. In accordance with Middle Permian resources abundance for the Central Sichuan Basin, the In spite of the differences, the above two hypotheses have resources were estimated to be 3000 108 m3. something in common; that is deep-seated faults would avail 2) The play with reef facies around Longquanshan and to generate porous dolostone. Dolomitization is likely to occur (indicated by the ellipse in Fig. 5). Trap area in the Western Sichuan Depression (Figs. 2b and 5) because was estimated to be 370 km2 and the resources were Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414 413

Fig. 6. Seismic amplitude extracted along the top of the Middle Permian Series and the predicted organic reefs and banks in the Western Sichuan Depression.

predicted to be 1200 108 m3 with the same 4. Conclusions parameter. 3) The play with bank facies around Zhongjiang and 1) An aulacogen formed in the Khanka taphrogeny exists in Fenggu (the section is omitted). Trap area was estimated the Western Sichuan Depression. Inside the craton, two to be 750 km2 and the resources were predicted to be branches across the Western Sichuan Depression and 3200 108 m3 with the same parameter. initial Longmenshan rift form a composite three-branch rift, which is inferred to come into being in Pre-Sinian In the above predictions, small facies and the zones with and continue to develop in the Sinian Period. It may poor data quality (e.g. Yuquan) have not been included. be filled in the Cambrian Period and leveled up during Cumulative trap area exceeds 1800 km2 and total resources the Caledonian orogeny. This composite rift may be a reach 7400 108 m3 based on the same parameter for the hydrocarbon generating center with thick Lower Paleo- Central Sichuan Basin. The burial depth on the east of Xindu zoic deposits. is less than 7300 m. Changxing organic reefs have been 2) The Lower Cambrian Series may be one of the major delineated with relatively high confidence [6] and also basi- source beds for Permian gas reservoirs in the Western cally coincide with the region with Middle Permian reef-bank Sichuan Depression. Normal faults at either side of the facies (Fig. 5). For the exploration with Middle Permian reef- rift, which were reactivated during the Emei taphrogeny, bank facies as the primary target and Changxing reef-bank would facilitate dolomitization and act as hydrocarbon facies as the secondary target, the drilling risk may be reduced. migration pathways. In summary, Middle Permian gas reservoirs in the Western 3) A typical organic reef and bank is characterized by in- Sichuan Depression may be economically promising. A risk- ternal blank to chaotic reflections with convex external taking exploratory well has been deployed and drilled by forms on seismic sections, based on which three plays Sinopec Southwest Oil & Gas Company. with large-scale seismic anomalies were predicted. As 414 Liu S. et al. / Natural Gas Industry B 2 (2015) 406e414

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