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Natural Gas Industry B 3 (2016) 515e525 www.elsevier.com/locate/ngib Research Article Sedimentary tectonic evolution and reservoir-forming conditions of the DazhoueKaijiang paleo-uplift, Basin*

Yang Yueming, Wen Long, Luo Bing*, Song Jiarong, Chen Xiao, Wang Xiaojuan, Hong Haitao, Zhou Gang, He Qinglin, Zhang Xiaoli, Zhong Jiayi, Liu Ran, Shan Shujiao

Exploration and Development Research Institute of PetroChina Southwest Oil & Gas Field Company, , Sichuan, 610041,

Received 9 August 2016; accepted 25 August 2016 Available online 8 July 2017

Abstract

Great breakthrough recently achieved in the SinianeLower Paleozoic gas exploration in the LeshaneLongnu¨si paleo-uplift, Sichuan Basin, has also made a common view reached, i.e., large-scale paleo-uplifts will be the most potential gas exploration target in the deep strata of this basin. Apart from the above-mentioned one, the other huge paleo-uplifts are all considered to be the ones formed in the post-Caledonian period, the impact of which, however, has rarely ever been discussed on the SinianeLower Paleozoic oil and gas reservoir formation. In view of this, based on outcrops, drilling and geophysical data, we analyzed the SinianeLower Paleozoic tectonic setting and sedimentary background in the East Sichuan Basin, studied the distribution rules of reservoirs and source rocks under the control of paleo-uplifts, and finally discussed, on the basis of structural evolution analysis, the conditions for the formation of SinianeLower Paleozoic gas reservoirs in this study area. The following findings were achieved. (1) The DazhoueKaijiang inherited uplift in NE Sichuan Basin which was developed before the Middle Cambrian controlled a large area of Sinian and Cambrian beach-facies development. (2) Beach-facies reservoirs were developed in the upper part of the paleo-uplift, while in the peripheral depression belts thick source rocks were developed like the Upper Sinian Doushantuo Fm and Lower Cambrian Qiongzhusi Fm, so there is a good sourceereservoir assemblage. (3) Since the Permian epoch, the DazhoueKaijiang paleo-uplift had gradually become elevated from the slope zone, where the Permian oil generation peak occurred in the slope or lower and gentle uplift belts, while the Triassic gas generation peak occurred in the higher uplift belts, both with a favorable condition for hydrocarbon accumulation. (4) The lower structural layers, including the Lower Cambrian and its underlying strata, in the East Sichuan Basin, are now equipped with a large-scale uplift with a simple structural configuration, above which a complete local structural trap was well developed with a good preserving condition. In conclusion, there are good forming conditions for hydrocarbon reservoirs in the SinianeLower Paleozoic strata in the DazhoueKaijiang paleo-uplift, East Sichuan Basin, in the higher parts of which the well-developed beach-facies reservoirs occupy an area of 8000 km2 and will be the next important gas exploration target in this basin. © 2017 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; East; DazhoueKaijiang paleo-uplift; Sinianeearly Paleozoic; Inherited lift; Reservoir formation conditions; Exploration zone

The paleo-uplift plays an extremely important role in the large- and medium-sized gas fields is mostly related to paleo- accumulation process of hydrocarbons, and the formation of uplifts and their slope belts [1e5]. In recent years, the efficient exploration and development of the Anyue gas field in the LeshaneLongnu¨si paleo-uplift, Sichuan Basin, has drawn the * & “ attention of geologists to the large paleo-uplifts in the basin Project supported by the PetroChina Major S T Project Field Test of e e Exploration and Development Technologies for Deep Marine Carbonate again [6 11]. However, except the Longnu¨si paleo- Reservoirs in Sichuan Basin” (No.: 2014E-3208(GF)). uplift, all huge paleo-uplifts known in the basin are consid- * Corresponding author. ered to be the ones formed and finalized in the post- E-mail address: [email protected] (Luo B.). Caledonian period, such as the LuzhoueKaijiang paleo- Peer review under responsibility of Sichuan Petroleum Administration. http://dx.doi.org/10.1016/j.ngib.2017.05.001 2352-8540/© 2017 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/). 516 Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525 uplift in the Indosinian period, the Daxing paleo-uplift in the In the LeshaneWeiyuan zone with negative aeromagnetic Yanshan period, and the JiangyoueMianyang paleo-uplift in anomalies, Wells Wei 27 and 117 encountered Pre-Sinian the Yanshan period [9e16]. Did these paleo-uplifts already granite. In the LangzhongeNanjiang zone with negative exist in Cambrian or Sinian? How about their evolution fea- aeromagnetic anomalies, the field outcrop revealed a meta- tures? Could they affect the hydrocarbon accumulation in the morphic basement in the Pre-Sinian, which validated that this SinianeLower Paleozoic? These geological questions were zone is a high zone on the basement uplift. In the Longnu¨si rarely mentioned and addressed in previous researches. In the area, Well Nu¨ji encountered the Pre-Sinian basement of critical period that the Anyue gas field has been put into extrusive rocks, it is thereby speculated that the Kaijiang area overall efficient development and the strategic replacement in northeast Sichuan Basin is similar to the Longnu¨si area in resources are urgently demanded in the Sichuan Basin, it is the central Sichuan Basin, both of which are the granitoid especially important and necessary to address the above- basement with weak magnetic and shallow depth. mentioned questions as soon as possible. Accordingly, after an analysis of field, drilling and geophysical data of the 1.1.2. Layers of SinianeLower Paleozoic overlapping Sichuan Basin, the sedimentary and structural background of upward to the high positions of basement uplifts the SinianeLower Paleozoic was dissected, and the distribu- The results of field outcrop correlation in the margin of tion of reservoirs and source rocks controlled by palaeo- Sichuan Basin and the seismic stratigraphy tracing and cor- geomorphology was investigated. Moreover, on the basis of relation in the basin indicate that the basement geomorpho- structural evolution analysis, the hydrocarbon accumulation logical highs in northwest Sichuan Basin continued to develop conditions in the SinianeLower Paleozoic were identified, and in the SinianeEarly Cambrian, mainly presenting as layers of the favorable natural gas exploration zones in the SinianeLower Paleozoic overlapping upward to the high po- SinianeLower Paleozoic of the Sichuan Basin were predicted. sitions of basement uplifts. In this way, it is expected to provide a support for the further exploration of SinianeLower Paleozoic in the Sichuan Basin. 1) Field outcrop correlation in the basin margin ① The stratigraphic correlation results of the Upper 1. Features and evolution of DazhoueKaijiang paleo- Sinian Doushantuo Fm show that the Nanjiang area uplift in the north Sichuan Basin was a palaeo- in the Doushantuo Fm period. The Doushantuo Fm was Based on the study on regional geology and the analysis of absent in Changtanhe, Guimin, and other locations in field outcrop data, drilling data and interpretation results of 2D the Nanjiang area. The Upper Sinian Dengying Fm and 3D seismic data, it is considered that the Sichuan Basin unconformably contacted with the granite or meta- presented a pattern of alternate occurrence of uplifts and de- morphic rocks in the basement. In the east and west pressions during the SinianeEarly Cambrian due to the geo- sides of the palaeo-continent, the strata of Doush- morphology of basement, with the DazhoueKaijiang paleo- antuo Fm thickened, followed by the development of uplift developed in the northeast. Before the Middle conglomerate, sand-mudstone, and shale (Fig. 2). Cambrian, the DazhoueKaijiang paleo-uplift was a successive ② The stratigraphic correlation results of the Upper uplift, where the sedimentary strata in each layer of high zones Sinian Dengying Fm show that, in the Dengying Fm were relatively thin (Fig. 1). Currently, large dome anticlines period, the paleo-uplift continued to develop and the are developed in Cambrian and Sinian. high zones in the Doushantuo Fm in the Nanjiang area migrated to the NanzhengeZhenba area. Up- 1.1. Identification of paleo-uplifts wardly, the Dengying Fm overlapped to the high positions of the uplift and the content of clastic rocks According to the results of basement topographic analysis, increased, demonstrating the continuous develop- field section correlation of basin margins, and seismic strata ment of paleo-uplift before the deposition of tracing and correlation in the basin, the successive paleo- Cambrian. The NanzhengeZhenba area was seated uplifts were developed in the northeast of the basin before in subaqueous high positions of the uplift and there the Middle Cambrian and had the following obvious features. was still a palaeo-continent in the north of Nanzheng area. 1.1.1. Obvious background of basement uplifts 2) Seismic stratigraphic tracing and correlation in the basin Distribution study of the aeromagnetic abnormality in the Sichuan Basin reveals that the Sinian sedimentary basement of On the basis of paleo-uplift identification in the basin the basin presented the pattern of alternate occurrence of up- margin, the seismic stratigraphic framework section tracing lifts and depressions. Moreover, the basin included three and correlation were conducted to understand the development uplifting zones with obvious negative aeromagnetic anoma- of paleo-uplift in the basin. The results indicate that, in lies, namely, the LeshaneWeiyuan zone with negative aero- northeast Sichuan Basin, the successive uplift was developed magnetic anomalies, the Longnu¨sieDazhueKaijiang zone before Middle Cambrian. The filling structure of the strata in with negative aeromagnetic anomalies, and the Langz- paleo-uplift agreed with that in field strata, presenting layers of hongeNanjiang zone with negative aeromagnetic anomalies. SinianeLower Paleozoic overlapping upwardly to the high Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525 517

Fig. 1. Thickness distribution of Sinian Dengying Fm in DazhoueKaijiang paleo-uplift and its periphery.

positions of the basement uplift. From the highs to both sides SuiningeGuang'an, DazhoueKaijiang, and Hannan). The of the paleo-uplift, the layers both show an increasing ten- Hannan paleo-uplift was the highest and a palaeo-continent in the dency (Fig. 3). depositional period of the Doushantuo Fm; the remaining three paleo-uplifts had similar uplifting amplitudes and were seated in 1.2. Features and evolution of paleo-uplifts the subaqueous uplift zone in the depositional period of the Doushantuo Fm, thereby containing thin compensation thickness. Based on the data of 86 wells, 41 field geological sections, In this stage, the DazhoueKaijiang paleo-uplift was confined by 2D seismic (7300 km) and 3D seismic (5000 km2), the the PingchangeZhenba syneclise, ZhongxianeChengkou syn- thickness contours for the strata in Sinian and Cambrian were eclise, and West syneclise and separated from the Hannan prepared. The sedimentary strata thickness responses of paleo-uplift by the PingchangeZhenba syneclise. structural palaeogeomorphology can be used to reflect the morphology and the evolution features of paleo-uplifts. From 1.2.2. Before Cambrian deposition the features and evolutions of paleo-uplifts in different periods The mold thickness of Lower Cambrian Qiongzhusi Fm was (Fig. 4), it is seen that the DazhoueKaijiang area resided long employed to reflect the palaeo-geomorphological pattern before in the high positions of the uplift in the SinianeEarly the Cambrian deposition, revealing the continuous development Cambrian, and it was a successive subaqueous uplift before the of the above-mentioned four paleo-uplifts. Affected by the Middle Cambrian and evolved into a slope zone before the Tongwan movement, the DazhoueKaijiang paleo-uplift was Middle CambrianeOrdovician. uplifted much higher than the SuiningeGuang'an paleo-uplift, showing an expanded scope, counter-clockwise axial migra- 1.2.1. Before Sinian deposition tion, and a near NeS distribution. The Doushantuo Fm is a set of compensation sedimentary strata formed on the background of basements. Therefore, the 1.2.3. Before Middle Cambrian deposition mold thickness of the Doushantuo Fm can be employed to reflect Since it was in the period of intra-platform compensation the palaeo-geomorphic pattern before the deposition of Sinian. and progradation during the late Early Cambrian, the thickness Affected by the Jinning movement, the basement presented a of the Longwangmiao Fm could be used to reflect the palaeo- geomorphic pattern of alternate occurrence of uplift-depressions, geomorphological pattern before the deposition of Middle with the development of four paleo-uplifts (i.e. LeshaneWeiyuan, Cambrian. Affected by the Xingkai Movement which caused 518 Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525

Fig. 2. Stratigraphic correlation of Doushantuo Fm strata for Chenjiaba in GuangyuaneYangba in NanjiangeHuitan in NanjiangeXiaoyangba in Zhenjiang. the early stable uplift of the strata, in the deposition period of 2. Distribution features of reservoir facies and source the Longwangmiao Fm, the DazhoueKaijiang paleo-uplift rocks, and natural gas plays in the paleo-uplift and its evolved successively, and was seated in the evolution stage periphery of the successive subaqueous paleo-uplift. The Hannan paleo- uplift was in a greater uplifting amplitude, thus the palaeo- Based on an analysis of tectonic geomorphology in the continent was formed again. depositional period, together with the measured field outcrop data, drilling data, and seismic characterization of gypsum 1.2.4. Before Ordovician deposition rocks, an analysis was made on the influences of palae- The thickness response features of the Middle Cambrian ogeomorphology on sedimentation and on the single factors. Gaotai Fm indicate that, in MiddleeLate Cambrian, the dif- Then, the palaeogeomorphology maps for the Sinian Deng 4 ferential subsidence was obvious in northeast Sichuan Basin Member, Cambrian Longwangmiao Fm and Gaotai Fm, and affected by the increased structural movement in late Xingkai the source rock distribution maps for Doushantuo and Movement. In the DazhoueKaijiang area, the structure started Qiongzhusi Fms were prepared. Thereby, the distribution to subside and evolved from the subaqueous uplift to the intra- features of SinianeLower Proterozoic reservoirs, source platform depression zone, while the Hannan paleo-uplift in the rocks, and the natural gas plays in eastern Sichuan Basin were north continued to uplift with an increased scope. clarified.

Fig. 3. Large-scale seismic framework section in JiangeeLichuan (flare-out of Lower Cambrian Longwangmiao Fm). Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525 519

Fig. 4. Features and evolution of DazhoueKaijiang paleo-uplift before Ordovician.

2.1. Mound shoal facies were developed in Dengying highs and the mound shoal was well developed in the Deng and Longwangmiao Fms in the high positions of paleo- 4 Member, with an area of 1.5 104 km2. The Fengjiee uplifts Shizhu area was seated in the platform margin and the mound shoal was well developed in the Deng 4 Member, with an area Affected by paleo-uplift highs in the depositional period, of 2.1 104 km2. the Deng 4 Member and Longwangmiao Fm as a whole were seated in the carbonate platform depositional system and in the 2.1.2. Cambrian Longwangmiao Fm structural highs of the DazhoueKaijiang paleo-uplift, the In the deposition of the Longwangmiao Fm, the Sichuan mound shoal facies in the Dengying Fm and the granular shoal Basin was in a continuous rimed carbonate platform (Fig. 5). facies in the Longwangmiao Fm were developed on a large From west to east, the facies belts are clear e mainly mixed scale. tidal flat in the west of the basin, restricted platformesemi- restricted platform in the northecentralesouth of the basin, 2.1.1. Sinian Deng 4 Member open platform in northeast Sichuan Basinesoutheast In the deposition of the fourth member of Dengying Fm , and platform margin in the east Chongqing area. (Deng 4 Member), the Sichuan Basin was generally in the The facies gradually change to slopeebasin facies towards the facies zone of semi-restricted platform e platform margin. northeast and southwest. Affected by the DazhoueKaijiang The DazhoueKaijiang paleo-uplift was in the intra-platform subaqueous paleo-uplift highs, the intra-platform shoal was 520 Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525

Fig. 5. Lithofacies and palaeo-geographic map of Cambrian Longwangmiao Fm in the Sichuan Basin. developed in the paleo-uplift highs, with an area of thickness of 50e150 m, a TOC content of 0.10e4.72% 0.8 104 km2; the southern part of FengjieeLichuan area was (average 2.72%), and a gas-generating intensity of seated in the platform margin shoal facies belt, with an area of 5 108e30 108 m3/km2. 2.0 104 km2. 2.2.2. Cambrian Qiongzhusi Fm 2.2. Periphery of paleo-uplifts surrounded by high- From the evaluation results of the Cambrian Qiongzhusi quality source rocks in Doushantuo and Qiongzhusi Fms Fm source rocks in the Sichuan Basin, it is concluded that the distribution of source rocks in this formation was also Controlled by geomorphological depression during the controlled by depositional geomorphology. The zones with deposition, high-quality source rocks were well developed in large thickness of source rocks are mainly distributed in the the Doushantuo and Qiongzhusi Fms in the periphery of the DeyangeAnyue intra-platform rift belt, ZhongxianeCheng- DazhoueKaijiang paleo-uplift. The source rocks have a great kou syneclise, and in western Hubei Province. Specifically, the gas-generating intensity, providing a good gas source foun- source rocks of ZhongxianeChengkou syneclise in eastern dation for the formation of giant gas fields. Sichuan Basin and in western Hubei Province contain a thickness of 100e200 m, a TOC content of 0.11e11.07% 2.2.1. Sinian Doushantuo Fm (average 1.52%), and a gas-generating intensity of In northeastern Sichuan Basin, Doushantuo Fm source 20 108e80 108 m3/km2. rocks were more developed compared with the interior basin. Controlled by geomorphological depression, the zones with 2.3. Development of direct caprocksdthick gypsolith in great thickness were mainly distributed in the PingchangeZ- Gaotai Fm henba and ZhongxianeChengkou syneclises on both sides of the DazhoueKaijiang paleo-uplift, and in western Hubei It is indicated in the analytical results of the sedimentary Province (Fig. 6). The source rocks generally contained a facies of the Gaotai Fm that the Gaotai Fm in eastern and Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525 521

Fig. 6. Thickness of Doushantuo Fm source rocks in the Sichuan Basin. southern Sichuan Bain was a restricted platform. Affected by late the SinianeLower Proterozoic in eastern Sichuan Basin is subsidence of the Xingkai Movement, the DazhoueKaijiang supposed to have evolved in three stages: (1) Late Cam- area in northeast Sichuan Basin gradually subsided from the brianeOrdovician, when the hydrocarbon generation stage paleo-uplift in the Longwangmiao Fm to the intra-platform started; (2) SilurianeEarly Triassic, when the source rocks depression, dominated by the deposits of gympso-dolomitic were in oil generation peak, recording as the formation stage lagoon. There were deposits of thick Gaotai Fm gypsum. of palaeo-oil reservoirs (with depths of source rocks between From the perspective of plays, there are two sets of favor- 2000 m and 5000 m), or Middle TriassiceJurassic, when the able sourceereservoir assemblages with reservoir rocks in the source rocks were in gas generation peak, recording as the upper part and source rocks in the lower part in the formation stage of palaeo-gas reservoirs (with depths of source SinianeLower Proterozoic, eastern Sichuan Basin, i.e. rocks between 5000 m and 5500 m); and (3) Late Jurassic Doushantuo Fm source rock þ Dengying Fm reservoir, and epresent, the late gas-generating stage. Qiongzhusi Fm source rock þ Longwangmiao reservoir. The overlying Gaotai Fm gypsum acts as the direct caprock, which 3.1.2. Structural evolution of bottoms of Cambrian and is favorable for the accumulation and effective preservation of Gaotai Fm hydrocarbons. As was mentioned above, the DazhoueKaijiang paleo- uplift developed on the Pre-Cambrian basement, and sub- 3. Tectonic evolution and current tectonic pattern sided to become a slopeeintra-platform depression in Mid- dleeLate Cambrian after the successive evolution in 3.1. Tectonic evolution in the accumulation stage SinianeEarly Cambrian. For the understanding of the evolu- tion feature and its effect on the petroleum accumulation of the 3.1.1. Accumulation stages DazhoueKaijiang paleo-uplift in the accumulation stage (after Through an analysis of the burial history and source rock Silurian), apart from the analysis of petroleum accumulation evolution history in Well Jianshen 1 and Well Chi 7 in eastern stage, the structural evolution analysis of bottoms of the Sichuan Basin, it is believed that the hydrocarbon formation of Cambrian and Gaotai Fm were conducted (Fig. 7). 522 Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525

Fig. 7. Bottom structural evolution of Cambrian and Gaotai Fm in the Sichuan Basin.

In Fig. 7, the Gaotai Fm bottom in eastern Sichuan Basin the structural evolution of Cambrian bottom has the features has basically similar regional structural patterns in all stages. similar to the pattern of Gaotai Fm. First, affected by the Guangxi Movement, the northeast- Based on the structural evolution features in the accumu- ernenorthern Sichuan Bain was uplifted again, and the Daz- lation stage, it can be concluded that, after Ordovician, the houeKaijiang area was seated in the south of the uplifted DazhoueKaijiang area was uplifted from slopes again and highs at the end of Silurian. In these stages, the Gaotai Fm gradually evolved into uplift zones. It was in the secondary bottom presented as structural highs. Second, affected by the slopeenose shaped uplift zone in the Permian oil peak stage. Yunnan Movement in the end of Carboniferous and the In the Triassic gas peak stage, it was seated in the gentle uplift Dongwu Movement in the end of Early Permian, the zone, being favorable for the accumulation of hydrocarbons. LeshaneLongnu¨si uplift zone was greatly uplifted at the end of Permian, and the Gaotai Fm bottom in the Daz- 3.2. Effect of current structural pattern on hydrocarbon houeKaijiang area generally presented as a secondary slo- accumulation peenose shaped uplift zone. Third, affected by Indosinian Movement, in the DazhoueKaijiang area, the Gaotai Fm 3.2.1. Vertical structures bottom in the Middle Triassic uniformly presented as sec- In the eastern Sichuan Basin, there are three sets of struc- ondary uplift highs. At the end of Late Triassic, the uplifting tural layers vertically which are significantly different in range expanded, forming a gentle uplifting zone. Generally, structural features. (1) In the upper structural layer Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525 523

(JurassiceMiddle Triassic Leikoupo Fm), the fold deforma- analysis of structural conditions of hydrocarbon accumulation tion becomes gentle in syncline, with a dip of more than 40, and preservation, it is suggested that the lower structural layer or even upright and reversed, while the core of the anticline is is most favorable for the accumulation of large oil and gas severely denudated, with worse preservation conditions. (2) In reservoirs vertically. the middle structural layer (Lower TriassiceMiddle Cambrian), the faults are well developed, with complex 3.2.2. Lateral structures structures and small traps, and the thick argillunite developed From the Dabashan front in the margin of the Sichuan in the Silurian acts as a secondary detachment layer with faults Basin to the basin center, there is a Dabashan thrust fold belt, a in different periods containing obvious stratification. (3) In the steep barrier high fold belt in eastern Sichuan Basin and a lower structural layer (Lower Cambrian and the strata below), gentle tectonic belt in central Sichuan Basin successively. the strata show relatively weak deformation, and the structure From the margin to the center of the basin, the tectonic stress is stable under the background of large uplifts, with relatively gradually decreases and there is weak development of tectonic simple structural morphology and no faults (Fig. 8). From the deformation and decreased development of faults. In eastern

Fig. 8. Present structural pattern in DazhoueKaijiang palaeo-lift zone and its periphery. (Data from the Geophysical Exploration Branch of CNPC Chuanqing Drilling Engineering Company Limited, 2016). 524 Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525

Sichuan Basin, which is located in the high and steep fold area, providing favorable trap conditions for hydrocarbon belts, the deep structural layer in the DazhoueKaijiang uplift accumulation. Based on the analytical results of the paleo- zone has relatively simple morphology and no faults, uplift background, sedimentary reservoirs, source rocks, demonstrating superior hydrocarbon preservation conditions to structural evolution, and trap conditions, it is considered that the basin margin. in the eastern Sichuan Basin the SinianeLower Paleozoic can be divided into two large favorable exploration zones (Table 4. Petroleum exploration prospects of SinianeLower 1), i.e. the DazhoueKaijiang paleo-uplift favorable area, and Paleozoic in eastern Sichuan Basin the FengjieeLichuan platform margin favorable zone. The former has a favorable exploration area of 0.8 104 km2 and 4.1. Control of paleo-uplifts on hydrocarbon the latter 1.3 104 km2. accumulation To sum up, in eastern Sichuan Basin, the SinianeLower Paleozoic exploration zone, with a large area and a good According to the above-mentioned research, the develop- prospect, is a new prominent field for the next natural gas ment and evolution of large-scale DazhoueKaijiang paleo- exploration in the Sichuan Basin. Accordingly, the exploration uplift play a prominent role in controlling the hydrocarbon of the SinianeLower Paleozoic in this area should be accumulation of SinianeLower Paleozoic in eastern Sichuan enhanced, and priority of drilling should be given to the su- Basin. First, before SinianeMiddle Cambrian, the shoal res- perposition highs of current and palaeo-highs in the Daz- ervoirs were developed in the highs of the successive Daz- houeKaijiang uplift. Efforts should be made to facilitate the houeKaijiang paleo-uplift; in the periphery sags of the paleo- ascertaining of structural traps in the FengjieeLichuan plat- uplift, there were Doushantuo and Qiongzhusi Fm source form margin and select the traps with a good preservation rocks. Second, in MiddleeLate Cambrian, the paleo-uplift condition for drilling. subsided into intra-platform depression with the develop- ment of thick gypsum rocks which act as the direct sealing for 5. Conclusions the two sets of advantageous sourceereservoir assemblages (i.e. Doushantuo Fm source rock þ Dengying Fm reservoir, 1) Before Middle Cambrian, the successive Daz- and Qiongzhusi Fm source rock þ Longwangmiao Fm houeKaijiang uplift was developed in northeastern reservoir). They laid a good foundation for the formation of Sichuan Basin. In the high positions of paleo-uplifts, large gas fields. Third, after the Ordovician, the Daz- there were shoal reservoirs. In the periphery sags of houeKaijiang area was uplifted from a slope zone again, and paleo-uplifts, thick Doushantuo and Qiongzhusi source gradually evolved into an uplift zone. In the Permian oil peak, rocks were developed, providing favorable sour- it was seated in a secondary slopeenose-shaped uplift zone. ceereservoir assemblages. In the Triassic gas peak, it was seated in the low gentle uplift 2) In MiddleeLate Cambrian, the paleo-uplift subsided to zone. The good superposition of current and palaeo-structure become an intra-platform depression with the develop- is favorable for the accumulation and preservation of ment of thick gypsum rocks, which act as the direct hydrocarbons. sealing for the two sets of favorable sourceereservoir assemblages (i.e. Doushantuo Fm source 4.2. Petroleum exploration prospects rock þ Dengying Fm reservoir, and Qiongzhusi Fm source rock þ Longwangmiao Fm reservoir), presenting In eastern Sichuan Basin, the Sinian Dengying Fm and favorable sourceereservoirecaprock conditions for the Lower Paleozoic Longwangmiao Fm have the basic geological formation of large gas fields. conditions for the large-scale accumulation of natural gas. 3) After Ordovician, the DazhoueKaijiang area was Combined with the confirmation of SinianeLower Paleozoic uplifted from a slope zone again and gradually evolved structures in eastern Sichuan Basin, it is considered that the to an uplift zone. In the Permian oil peak, it was seated deep structure was well developed with a large closed trap in a secondary slopeenose-shaped uplift zone. In the

Table 1 Favorable exploration zones in SinianeLower Proterozoic in the eastern Sichuan Basin. Exploration zone DazhoueKaijiang FengjieeLichuan Favorable (1) Intra-platform shoal facies development (1) Developmental zone of dolomitic shoal facies conditions zone in Longwangmiao Fm in Longwangmiao Fm platform margin (2) In the sub-salt Gaotai Fm (2) Reservoir development validated by drilling (3) Adjacent to hydrocarbon-generating centers of (3) Seated in Gaotai Fm sub-salt Qiongzhusi, Doushantuo Fms in northeastern Sichuan Basin (4) Seated in hydrocarbon-generating center of (4) Development of deep structural traps Qiongzhusi Fm, and adjacent to hydrocarbon-generating (5) Superimposition with developmental zone center of Doushantuo Fm of mound shoal facies in Dengying Fm (5) Development of deep structural traps Zone area 8000 km2 13000 km2 Yang YM. et al. / Natural Gas Industry B 3 (2016) 515e525 525

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