Asphalt Features and Gas Accumulation Mechanism of Sinian Reservoirs in the Tongwan Palaeo-Uplift, Sichuan Basin

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Natural Gas Industry B 2 (2015) 314e322 www.elsevier.com/locate/ngib Research article Asphalt features and gas accumulation mechanism of Sinian reservoirs in the Tongwan Palaeo-uplift, Sichuan Basin

Li Wei*, Hu Guoyi, Zhou Jingao

Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China

Received 3 November 2014; accepted 8 April 2015 Available online 25 November 2015

Abstract

Breakthroughs have been made in natural gas exploration in Sinian reservoirs in the Tongwan Palaeo-uplift, Sichuan Basin, recently. However, there are disputes with regard to the genetic mechanisms of natural gas reservoirs. The development law of asphalts in the Sinian reservoirs may play an extremely important role in the study of the relationships between palaeo oil and gas reservoirs. Accordingly, researches were conducted on the features and development patterns of asphalts in the Sinian reservoirs in this area. The following research results were obtained. (1) Asphalts in the Sinian reservoirs were developed after the important hydrothermal event in the Sichuan Basin, namely the well- known Emei Taphrogeny in the mid-late Permian Period. (2) Distribution of asphalts is related to palaeo oil reservoirs under the control of palaeo-structures of Indosinian-Yanshanian Period, when the palaeo-structures contained high content of asphalts in the high positions of the palaeo-uplift. (3) Large-scale oil and gas accumulations in the Sinian reservoirs occurred in the Indosinian-Yanshanian Period to generate the Leshan-Ziyang and Gaoshiti-Moxi-Guang'an palaeo oil reservoirs. Cracking of crude oil in the major parts of these palaeo oil reservoirs controlled the development of the present natural gas reservoirs. (4) The development of asphalts in the Sinian reservoirs indicates that hydrocarbons in the Dengying Formation originated from Cambrian source rocks and natural gas accumulated in the Sinian reservoirs are products of late-stage cracking of the Sinian reservoirs. (5) The Sinian palaeo-structures of Indosinian-Yanshanian Period in the Sichuan Basin are favorable regions for the development of the Sinian reservoirs, where discoveries and exploration practices will play an important role in the era of Sinian natural gas development in China. © 2015 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; Tongwan Palaeo-uplift; Sinian; Reservoirs; Asphalt; Hydrothermal; Indosinian-Yanshanian period; Palaeo-structure; Palaeo oil reservoir; Natural gas accumulation

Since the giant Sinian gas fielddthe Waiyuan gas field in Sinian gas fielddAnyue gas field was discovered [1e4]. the Sichuan Basin was discovered in the 1960s, gas explora- However, because of little geology information available tion in Sinian has been going on without any break, but no new (except for gas components and carbon isotope values, no big breakthrough had been made until 2011 when a com- other direct evidences), there are still disputes over the gas mercial gas flow was tapped in Sinian in Well Gaoshi 1 in the sources, genetic mechanisms and Sinian gas accumulation Sichuan Basin, which marks a new page of gas exploration regularity etc. of the giant Anyue gas field. Hence, we tried to breakthrough of this layer system, following it, another giant answer these questions by analyzing the development mechanisms and rules of asphalt in the Sinian reservoirs, and by examining the features of the asphalt (or anthraxolite) and relationship between the asphalt and hydrocarbons, in the hope * Corresponding author. to provide new ideas and methods for studying the enrichment E-mail address: [email protected] (Li W.). Peer review under responsibility of Sichuan Petroleum Administration. regularity of gas reservoirs in this area. http://dx.doi.org/10.1016/j.ngib.2015.09.004 2352-8540/© 2015 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/). Li W. et al. / Natural Gas Industry B 2 (2015) 314e322 315

1. Asphalt features of the Dengying reservoirs in the depth of 4958.40e4958.45 m in the 4th member of the Den- Tongwan Palaeo-uplift gying Formation in Well Gaoshi 1, not only quartz, dolomite and asphalt filling the fractures and dissolved pores, but also According to previous studies, the oldest palaeo-uplift in asphalt filling dissolved pores and vugs generated after the the Sichuan Basin is the Leshan-Longnu¨si palaeo-uplift formation of horse teeth-shaped dolosparites [11] and quartz formed during the Caledonian Period [3,5e7]. This palaeo- by hydrothermal process can be seen (Fig. 1a). At the depth of uplift mainly refers to a complete joint palaeo-uplift devel- 4977.81 m in the 4th member of the Dengying Formation, the oped in the trigonal area between Leshan-Chengdu in the dolomite and black glistening asphalt filling the dissolved western Sichuan Basin and Longnu¨si in the middle Sichuan pores in cores are cemented together (Fig. 1b). These phe- Basin at the end of Silurian Period [8], with an area of nomena can also be seen in dolomite vugs and fissures in the 60000 km2, where formation erosion becomes stronger from 2nd member of the Dengying Formation in Well Gaoke 1 the east to the west, and the formation had eroded to the Sinian (Fig. 1c), in dolomite fissures and vugs in the 4th member of Dengying Formation near the western margin of this basin at the Dengying Formation in Well Anping 1 (Fig. 1d), in dis- the end of Caledonian Period [3,5,7]. Based on recent explo- solved pores in the 4th member of the Dengying Formation in ration progress and geologic study, the oldest palaeo-uplift Well Moxi 8 (Fig. 1e), and in dolomite interlayer fissures in favorable for hydrocarbon accumulation in the Sinian Den- the 2nd member of the Dengying Formation in Well Moxi 11 gying Formation is actually the one formed during the Tong- (Fig. 1f). The asphalt content in the Dengying reservoirs is wan stage (we call it Tongwan Palaeo-uplift), not the generally between 0.17% and 4.33% (over 5% in local part). Caledonian palaeo-uplift. The Tongwan Palaeo-uplift was For instance, the asphalt content of the 4th member of the generated from the karst palaeo-topography by uplifting after Dengying Formation at the depth of 5055.4 m in Well Anping the deposit of the Sinian Dengying Formation. That is to say, it 1 is 6.19%; and the asphalt content of the 2nd member of the was generated by the evolution of the Gaoshiti-Moxi-Longnu¨si Dengying Formation in Well Wei 117 at the depth of 3157.9 m karst residual hill [1] east of the Deyang-Luzhou erosion is as high as 20.73%. valley [9] and the Leshan-Jingyan-Ziyang karst residual hill The above phenomena indicate that: ① hydrocarbons once [1] west of this valley. Thus, it can be seen that the Tongwan filled the fissures, caverns, dissolved pores and matrix pores in Palaeo-uplift, quite different from the Caledonian Palaeo- the Dengying reservoirs, which left asphalt behind after oil uplift, is the combination of two isolated residual hills cracking; ② uneven in distribution, asphalt in these reservoirs developed in the Middle-Southwestern Sichuan area. As its is 1e3% of the total rock volume in most cases, but can be major part is buried below the Caledonian Leshan-Longnu¨si very concentrated in some sections; ③ the asphalt was formed Palaeo-uplift, it is easily mistaken as the Caledonian Palaeo- after the Sinian regional hydrothermal diagenesis. As the first uplift. regional hydrothermal process in the Dengying Formation The study on asphalt of the Dengying reservoirs in the occurred at the early Cambrian Period when the source rocks Sichuan Basin started from 1980s. Song Wenhai, Ran Longhui had not generated hydrocarbons yet, the asphalt in the Den- and Wang Lansheng et al. [5e7,10] concluded that the Sinian gying reservoirs must have generated only after the Emei natural gas was the product of oil cracking, which also led to Taphrogeny [12] in the Permian Period. common existence of asphalt in Sinian reservoirs in the Weiyuan-Ziyang area, but no much progress has been made 1.1.2. Asphalt features in rock slices since then. We will mainly study the microscopic features and The relationship between the asphalt and pore cement in macroscopic regularities of the asphalt in the Sinian Dengying the Dengying reservoirs in the Tongwan Palaeo-uplift has reservoirs. unique features, showing different relationships with hydrothermal events. A few researchers have studied the asphalt 1.1. Microscopic features of the asphalt features in rock slices in the Dengying reservoirs, which mainly focuses on the relationship between the asphalt and the The asphalt in the Dengying reservoirs in the Tongwan filling sequences of diagenetic minerals. For example, exam- Palaeo-uplift in the Sichuan Basin has developed not only in ining the asphalt in Sinian reservoirs in the southeastern the Weiyuan-Ziyang area, but also in the Gaoshiti-Moxi area. Sichuan Basin, Huang Wenming et al. discovered the filling Filling various pores, the asphalt can show its relationship with sequence of dolomite / asphalt / dolomite / various diagenetic minerals and regional thermal events. asphalt / quartz [13]. After studying the asphalt in the Sinian reservoirs in the Caledonian palaeo-uplift, we thought that the 1.1.1. Asphalt features reflected by cores asphalt in rock slices existed in various pores and had certain The asphalt in Sinian reservoirs in the Tongwan Palaeo- filling sequences, but apparently different from the Sinian uplift has been found in intercrystal pores, intergranular reservoirs in the southeastern Sichuan Basin. For example, pores, intergranular dissolution pores, dissolution fissures and features such as asphalt filling intergranular pores, intercrystal dissolution vugs etc. Observation of cores from prospecting pores, dissolution pores, dissolution caverns and fissures etc. wells reveals that the asphalt not only exists in fissures and (Fig. 1gep), asphalt filling some intergranular pores and vugs related to fractures, but also in vugs and matrix pores intercrystal pores completely (Fig. 1g and h), asphalt filling formed by dissolution. For examples, in the dolomite at the intercrystal pores together with calcite (Fig. 1i), and asphalt 316 Li W. et al. / Natural Gas Industry B 2 (2015) 314e322

Fig. 1. Photos and thin slice identiﬁcation charts of the asphalt in cores from the Dengying Formation in the Tongwan Palaeo-uplift.

filling some dissolution pores (Fig. 1jep), show that hydro- dolarenite were partly filled with coarse dolosparite, then filled carbons once accumulated in various pores in the Dengying with asphalt, and formed asphalt shrinkage joints (Fig. 1l). In reservoirs on a large scale, and asphalt was left in various the 4th member of the Dengying Formation at the depth of pores after later thermal cracking in various degrees. 5062 m in Well Anping 1, the dissolution vugs in the cyano- After studying the relationship between asphalt and cement, bacteria cohered micritic powder dolomite were firstly filled we are sure that there is a filling sequence of fibrous dolomite with granular quartz, then filled with asphalt, leaving few cement / dolosparite cement / asphalt / dolomite or dissolution pores; moreover, hydrothermal quartz was devel- quartz in the Dengying reservoirs in the Tongwan Palaeo- oped in the remained pores filled with asphalt (Fig. 1p). In the uplift. For instance, the abundant dissolution pores and vugs 2nd member of the Dengying Formation at the depth of in cyanobacteria powder dolomite in the 2nd member of the 3057 m in Well Wei 117, the dissolution vugs in cyanobacteria Dengying Formation in Well Zi 6 were partially filled with cohered powder debris and micritic dolomite were filled in a coarse dolosparite first, then the residual pores were filled with sequence of fibrous dolomite, granular dolomite and asphalt, asphalt, finally cementation of hydrothermal dolomite then asphalt shrinkage joints and tectonic fissures were occurred in the residual pores (Fig. 1k). In the 4th member of developed (Fig. 1n). In the 4th member of the Dengying the Dengying Formation at the depth of 5040.3 m in Well Formation at the depth of 4975.29 m in Well Gaoshi 1, coarse Anping 1, dissolution fissures in the cyanobacteria cohered breccia dolomite filled the early dissolution pores, and then Li W. et al. / Natural Gas Industry B 2 (2015) 314e322 317 asphalt filled intercrystal pores (Fig. 1o). In the 4th member of cementation occurred in the residual pores after asphalt filling the Dengying Formation at the depth of 4986.57 m in Well were likely to be the products of hydrothermal process caused Gaoshi 1, dissolution pores were developed in breccia micritic by local tectonic movement in the Indosinian-Himalayan pe- dolomite; dolosparite and asphalt successively filled dissolu- riods after regional hydrothermal process. tion vugs and dissolution fissures in cyanobacteria micritic dolomite, leaving massive dissolution vugs and fissures un- 1.2. Distribution features of reservoir asphalt filled (Fig. 1m). Moreover, previous studies show that saddle- shaped dolosparite, coarse-grained dolomite and horse teeth- Though the asphalt in the Dengying Formation in Caledo- shaped dolomite etc. developed along pore walls of reser- nian Palaeo-uplift in the Sichuan Basin is widely distributed, voirs are products of hydrothermal process [11,13e16], its content and distribution show some patterns. Firstly, the therefore, regional hydrothermal events happened before the reservoir asphalt mainly distributes in Weiyuan-Ziyang and generation of the Sinian reservoir asphalt, later local hydro- Gaoshiti-Moxi regions, but the asphalt content in peripheral thermal events happened. regions is low (Fig. 3). For instance, the average content of The dissolution pores in Sinian Dengying Formation were volume percentage of the reservoir asphalt is around 0.8%e mainly developed in the Tongwan Period and the end of 1.0% in the Weiyuan-Ziyang region, higher in Well Zi 1 Caledonian period [9,17]; while hydrothermal dolomitization (1.36%); that in Gaoshiti-Moxi region is between 0.7% and might mainly happened in early Cambrian-middle Permian 1.5%, highest in Well Anping 1 (3.61%). The average content taphrogenesis movements [18e20]. If the hydrothermal in its peripheral regions is generally less than 0.5%. For dolosparite was formed in early Cambrian hydrothermal example, the average content in Well Hanshen 1 is 0.02%, that process, then after the Caledonian weathering and the action in Gongshen 1 is 0.05%, and that in Well Pan 1 is 0.11%. Such of massive organic acid during hydrocarbon generation pro- distribution characteristics of asphalt content in Fig. 3 is cess of the lower Cambrian source rocks, the dolosparite similar with the features of strike direction and development would have had apparent dissolution, but no dissolution has location of the Sinian top structure before the deposition of been found in dolosparite in all analyzed thin sections. Hence, Jurassic System (Fig. 4). The palaeo-structures in the Indosi- the generation of hydrothermal dolomite in the Sinian dolo- nian Period had an important controlling effect on the devel- mite reservoirs must have happened in the middle Permian opment of palaeo-hydrocarbon reservoir. Emei taphrogenesis movement. In other words, the hydro- Secondly, the vertical distribution of asphalt in Sinian also thermal coarse dolosparite developed along pore walls in the follows some patterns, namely, mainly distributing in the Sinian reservoirs must have generated in the middle Permian upper Dengying Formation and its top. By analyzing reser- Emei taphrogenesis movement. Thermometry of inclusions in voir asphalt in Sinian cores in Well Wei 117 (Fig. 5), we hydrothermal dolomite shows the homogenization tempera- found that reservoir asphalt content at its upper and top is ture of inclusions in early coarse columnar dolomite in grape- generally higher. For example, there developed a 206 m in- flower-edge shape in the 2nd member of the Dengying For- terval rich in asphalt at the upper Dengying Formation, with mation on Ebian-Xianfeng profile at the southern edge of the a reservoir asphalt content between 0.5% and 3.5%. More- Sichuan Basin is between 95 and 99.5 C.Basedonthestudy over, there is abundant asphalt in the Maidiping dolomite results of palaeo-geotemperature in the Sichuan Basin [21], (0.3e1.1%). Whereas, the reservoir asphalt content in the we studied the formation burial history and thermal evolution formations below 3190 m depth in the 2nd member of the historyintheMoxiarea(Fig. 2), and reached the conclusion Dengying Formation is lower (generally <0.3%, only few that the inclusions in grape-flower-edge shape dolomite in the samples >0.3%). This kind of phenomenon has been found 2nd member of the Sinian Dengying Formation were formed in prospecting wells in Ziyang, Moxi and Gaoshiti regions during Silurian-the end of middle Permian Maokou Forma- too. Thus it can be seen that the intervals rich in reservoir tion, while there was no regional thermal event during asphalt above 3190 m depth in the Sinian System similar in Silurian-early Permian epoch; therefore, the dolomite of this Well Wei 117 were the locations of palaeo-oil reservoirs, and period must have been formed at the depositional end of the the intervals below them with lower and scattered reservoir Maokou Formation. Whereas the inclusions in the macro- asphalt were water layers. crystalline and prismatic dolomite in the 2nd member of the According to the study on asphalt development in pro- Dengying Formation at 3253.9 m depth in Well Wei 117 have specting wells in the Palaeo-uplift, the Sinian asphalt interval a homogenization temperature of 129 C, and the high tem- and above reservoirs in prospecting wells are thicker in the perature of inclusions in prismatic quartz is between 129 and central area of the Indosinian Palaeo-uplift. For instance, the 169 C, which should be the results of local strong hydro- thickness in Gaoshiti-Moxi-Longnu¨si region where the Anyue thermal activity in regional hydrothermal activity gas field is located is between 400 and 500 m, and the background. thickness in Weiyuan gas field and the Ziyang gas-bearing Thus, it can be seen that there were regional hydrothermal structural area is between 200 and 280 m, but the thickness process and the formation of hydrothermal macrocrystalline in peripheral regions (such as Guang'an, Dazu, Ya'an-Leshan- dolomite and quartz at the depositional end of middle Permian Muchuan etc.) reduces sharply to less than 20 m (Fig. 4). This Maokou Formation in the Sinian System. While the hydro- indicates that the Indosinian palaeo-structures in the Tongwan thermal dolomite cementation and hydrothermal quartz Palaeo-uplift controlled the asphalt development, and also 318 Li W. et al. / Natural Gas Industry B 2 (2015) 314e322

Fig. 2. Formation burial history and thermal evolution history in the Moxi area, Sichuan Basin. giant palaeo-oil reservoirs must have been developed in their kerogen, it should have lighter or similar isotope value than central area. that of kerogen. According to analysis results, the carbon In conclusion, the enrichment of the Sinian reservoir isotope value of the Sinian reservoir asphalt is between asphalt on plane is mainly related to the top of the Yanshanian À37.0‰ and À34.5‰, and that of the Cambrian kerogen is palaeo-structure, and its enrichment in vertical direction is between À33.0‰ and À28.5‰, showing bigger difference. mainly at upper and top Sinian reservoirs. This also indicates The carbon isotope value of kerogen in the Cambrian Maid- that giant palaeo-oil reservoirs might be developed in the iping Formation and the Qiongzhusi Formation is between Indosinian palaeo-structure background. À36.5‰ and À31.5‰, similar with and heavier than the carbon isotope value of reservoir asphalt in the Dengying 2. Relationship between reservoir asphalt and Formation, showing a genetic relationship between them. hydrocarbon accumulation Moreover, Xu Guosheng et al. also thought that the asphalt in the Sinian reservoirs in the central Sichuan Basin came from It can be seen from the above analysis that there is a close the Cambrian System [25]. From this, we can conclude that relationship between the development of the Sinian reservoir the reservoir asphalt in the Dengying Formation mainly comes asphalt and the palaeo-oil reservoirs in the Sichuan Basin. from the source rocks in the Lower Cambrian Maidiping Whether there is a close relationship between the development Formation to Qiongzhusi Formation. That is to say that the of the Sinian reservoir asphalt and the present natural gas hydrocarbons in the Sinian palaeo-oil reservoirs mainly come distribution is a crucial question. According to our analysis, from Lower Cambrian source rocks. the distribution of reservoir asphalt has relationship with the palaeo-oil reservoir cracking and the source rocks. 2.2. Relationship between reservoir asphalt and palaeo- oil reservoirs 2.1. Relationship between reservoir asphalt and source rocks The distribution of asphalt at the Tongwan Palaeo-uplift indicates that giant palaeo-oil reservoirs were once devel- The carbon isotope values of the reservoir asphalt in the oped at the palaeo-uplift. A lot of previous researches show Dengying Formation show good correlation with Lower that the reservoir asphalt is mainly related to palaeo-oil res- Cambrian source rocks. As most hydrocarbons are generated ervoirs [26e29]. By studying the reservoir asphalt in Sinian by thermal cracking of organic matter in formations, according cores in more than 20 wells, we found some intervals rich in to the isotope fractionation principle, the hydrocarbons reservoir asphalt: the 4th member top of the Dengying For- released early from organic matter have lighter isotope value mation in Well Laolong 1 (16 m thick), the 2nd member of the than that of kerogen, and the hydrocarbons released later from Dengying Formation at the Sinian top in Well Zi2 (260 m organic matter have higher isotope value [22e24]. Though the thick), the 2nd and 4th members of the Dengying Formation at asphalt is the remains of cracked crude oil, separated from the Sinian top in Well Wei 117 (180 m and 26 m thick, Li W. et al. / Natural Gas Industry B 2 (2015) 314e322 319

respectively), the 2nd and 4th members of the Dengying Formation in Well Gaoke 1 (160 m and 348 m thick, respectively), the 2nd and 4th members of the Dengying Formation in Well Moxi 8 (160 m and 350 m thick, respectively), and richer asphalt at the 4th member top of the Dengying For- mation in Well Guangtan2 (about 22 m thick). Based on the analysis of palaeo structural maps and palaeo-burial depth before the deposition of the Jurassic System, we drew a palaeo-oil reservoir proﬁle according to the palaeo-oil reservoir height inferred from palaeo asphalt, which shows that these proﬁles have extreme consistency in oil/water interface (Fig. 6), proving that the present development and distribution of the reservoir asphalt are the results of palaeo-oil reservoir cracking before the deposition of the Jurassic System. The tectonic features at the end of the Indosinian Period in the Sichuan Basin lasted to the middle Yanshanian Period [8,30e33], thus the development of the reservoir asphalt may be related to the palaeo-structures from the end of Indosinian Period to Jurassic Period. It can be seen from Fig. 4 that the Sinian palaeo oil reservoirs at the end of the Indosinian Period Fig. 3. Plane variation of the Sinian asphalt content in the Tongwan Palaeo- had two major giant oil reservoirs in Ziyang-Weiyuan and uplift. Gaoshiti-Moxi regions, with oil accumulation reserves of 111 Â 108e133 Â 108 t and 389 Â 108e467 Â 108 t, respectively; moreover, there were some smaller palaeo-oil reservoirs in Guang'an, Laolongchang and Hebaochang etc.

Fig. 4. Tectonic feature of the Sinian top and its relationship with asphalt development and gas reservoirs in the Tongwan Palaeo-uplift (Before the deposition of Jurassic System). 320 Li W. et al. / Natural Gas Industry B 2 (2015) 314e322

It can be seen from the previous analysis that though the development of reservoir asphalt is closely related to palaeo- oil reservoirs, it has considerable difference from the distribution of present gas reservoirs. For instance, not all the palaeo-oil reservoirs in peripheral regions of the palaeo-oil reservoirs far away from the Indosinian palaeo-structures have formed natural gas reservoirs (especially the palaeo- structures including Well Guangtan 2 and Well Laolong 1 with reservoir asphalt). Why does such phenomenon occur? After investigation, we ﬁnd that this is mainly because the later cracked products of small palaeo-oil reservoirs were not enough to accumulate into gas reservoirs. According to the pyrolysis experiments with low mature marine oil in the Ordovician System in Well Yingmai201 in the Tarim Basin, and natural gas isotope assay and analysis of thermal evolution process of the Sinian geological history, we have found the evolution curve of carbon isotope of methane from crude oil cracking in various accumulation stages during the Sinian geological history (Fig. 7). The carbon isotope value in methane of cracking gas from crude oil accumulated in the Sinian reservoirs underwent the following processes: they gradually became lighter during Triassic to the early stage of early Jurassic Period, then became heavier during the late stage of early Jurassic Period to the end of Jurassic Period, i.e., decreased from À43.5‰ during Triassic Period to À55.2‰ during the early stage of early Jurassic Period, then increased to À23.5‰ at the end of Jurassic Period. In contrast, the analysis data of carbon isotope values of methane in natural Fig. 5. Distribution of the Sinian asphalt content in Well Wei 117, Sichuan À ‰ Basin. gas in the Sinian Dengying Formation is between 32.3 and À32.5‰, which is similar to the carbon isotope value of later products of crude oil cracking (later Jurassic Period). This Thus, we can see that most of the present gas reservoirs are conclusion coincides with the Sinian burial history and ther- the development area of large palaeo-oil reservoirs, but the mal evolution history mentioned above. The Sinian crude oil areas with smaller palaeo-oil reservoirs fail to form natural gas in the Moxi region also reached the end of thermal cracking in accumulation after crude oil cracking. late Jurassic Period. Hence, the natural gas accumulated in the Sinian System should be the products of cracking of oil in 2.3. Reservoir asphalt and formation of natural gas palaeo-Sinian oil reservoirs, and this period should be in late ﬁelds Jurassic, being the product of later oil cracking. In summary, the development of reservoir asphalt in the The above discussion shows that in the Tongwan Palaeo- Dengying Formation not only shows that hydrocarbons in uplift, natural gas could only accumulate in the areas with Sinian palaeo-oil reservoirs mainly comes from Lower rich and thick asphalt layers, which is mainly related to the Cambrian source rocks, but also proves that the development later gas accumulation produced by crude oil cracking in giant and distribution of current reservoir asphalt are the result of palaeo-oil reservoirs. the palaeo-oil reservoir cracking before the deposition of the Previous studies show that most crude oil started cracking Jurassic System, and the natural gas accumulated in the Sinian into natural gas at 160 C, and was completely cracked at System must be the product of late palaeo-oil reservoir about 200 C [34e36]. Former graph of burial depth history cracking. Gas reservoir development is generally related to and thermal evolution history (Fig. 2) of the Moxi region also large palaeo-oil reservoir development, while smaller palaeo- shows that in early Triassic Period, the formation paleo- oil reservoirs couldn't form gas reservoirs by crude oil temperature of the Dengying Formation was between 150 cracking. and 170 C, which was the initial stage of oil cracking; in middle Triassic-middle Jurassic periods, the formation paleo- 3. Conclusions temperature of the Dengying Formation was between 160 and 200 C, which was the peak of massive oil cracking; in 1) The asphalt in the Dengying Formation in the Tongwan late Jurassic-early Cretaceous periods, the Dengying Forma- Palaeo-uplift of the Sichuan Basin exists in various tion, between 190 and 210 C in formation paleo-temperature, pores, such as intergranular pores, intercrystal pores, entered the end of oil cracking. dissolution pores, dissolution vugs and dissolution Li W. et al. / Natural Gas Industry B 2 (2015) 314e322 321

Fig. 6. Palaeo oil reservoir proﬁle of the Sinian System in the Tongwan Palaeo-uplift (Before the deposition of Jurassic System).

fissures etc. The reservoir asphalt was formed after the the Sinian reservoirs should be the product of late Emei Taphrogeny and hydrothermal event in the middle- palaeo-oil reservoir cracking. later Permian Period. 4) The palaeo-structures of the Dengying Formation in the 2) The distribution of reservoir asphalt in the Dengying Sichuan Basin during the Indosinian-Yanshanian period Formation in the Tongwan Palaeo-uplift of the Sichuan were favorable regions for the development of palaeo-oil Basin is mainly related to the palaeo-structures in the reservoirs. Looking for regions with large-scale hydro- Indosinian-Yanshanian Period. The high position of the carbon accumulation in the Sinian System in this period palaeo-structures in this period has higher asphalt con- is of great significance for opening up new areas of tent. Large-scale hydrocarbon accumulation in the Sinian natural gas exploration. Sinian System in the palaeo-uplift occurred in the Indosinian-Yanshanian period, and formed Leshan- Ziyang and Gaoshiti-Longnu¨si-Guang'an palaeo-oil References reservoir development belts. Crude oil cracking in the major parts of these palaeo-oil reservoirs control the [1] Yang Yu, Huang Xianping, Zhang Jian, Yang Guang, Song Jiarong, development of current natural gas reservoirs. Song Linke, et al. Features and geologic significances of the top Sinian 3) The development of reservoir asphalt in the Dengying karst landform before the Cambrian deposition in the Sichuan Basin. Nat Gas Ind 2014;34(3):38e43. Formation in the Sichuan Basin not only shows that [2] Luo Bing, Zhou Gang, Luo Wenjun, Xia Maolong. Discovery from hydrocarbons in palaeo-oil reservoirs in the Dengying exploration of lower paleozoic: Sinian system in central Sichuan palaeo- Formation mainly come from Lower Cambrian source uplift and its natural gas abundance law. China Pet Explor rocks, but also prove that the natural gas accumulated in 2015;20(2):18e29. [3] Du Jinhu, Zou Caineng, Xu Chunchun, He Haiqing, Shen Ping, Yang Yueming, et al. Theoretical and technical innovations in strategic discovery of huge gas fields in Longwangmiao formation of central Sichuan paleo-uplift, Sichuan Basin. Pet Explor Dev 2014;41(3):268e77. [4] Zou Caineng, Du Jinhu, Xu Chunchun, Wang Zecheng, Zhang Baomin, Wei Guoqi, et al. Formation, distribution, resource potential, prediction and discovery of Sinian-Cambrian super-giant gas field, Sichuan Basin. Pet Explor Dev 2014;41(3):278e93. [5] Song Wenhai. Research on reservoir-formed conditions of large-medium gas fields of Leshan-Longnu¨si paleohigh. Nat Gas Ind 1996;16(S1):13e26. [6] Ran Longhui, Xie Yaoxiang, Wang Lansheng. Understanding exploration of marine carbonate reservoirs in South China through Sichuan Basin. Oil Gas Geol 2006;27(3):289e94. [7] Li Guohui, Li Xiang. The Sinian gas reservoir control factors of the Caledonian ancient uplift in Sichuan Basin. Oil Gas Geol 2000;21(1):80e3. [8] Li Wei, Yi Haiyong, Hu Wangshui, Yang Geng, Xiong Xuan. Tectonic evolution of Caledonian paleohigh in the Sichuan Basin and its relationship with hydrocarbon accumulation. Nat Gas Ind 2014;34(3):8e15. Fig. 7. Evolution pattern of carbon isotope in methane in gas produced by [9] Wang Zecheng, Jiang Hua, Wang Tongshan, Lu Weihua, Gu Zhidong, crude oil cracking in Sinian geological history. Xu Anna, et al. Paleo-geomorphology formed during Tongwan 322 Li W. et al. / Natural Gas Industry B 2 (2015) 314e322

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