Scholarly article on topic 'Asphalt features and gas accumulation mechanism of Sinian reservoirs in the Tongwan Palaeo-uplift, Sichuan Basin'

Asphalt features and gas accumulation mechanism of Sinian reservoirs in the Tongwan Palaeo-uplift, Sichuan Basin Academic research paper on "Earth and related environmental sciences"

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{"Sichuan Basin" / "Tongwan Palaeo-uplift" / Sinian / Reservoirs / Asphalt / Hydrothermal / "Indosinian-Yanshanian period" / Palaeo-structure / "Palaeo oil reservoir" / "Natural gas accumulation"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Wei Li, Guoyi Hu, Jingao Zhou

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.

Academic research paper on topic "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) 314-322

Research article

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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 field—the Waiyuan gas field in the Sichuan Basin was discovered in the 1960s, gas exploration in Sinian has been going on without any break, but no new big breakthrough had been made until 2011 when a commercial gas flow was tapped in Sinian in Well Gaoshi 1 in the Sichuan Basin, which marks a new page of gas exploration breakthrough of this layer system, following it, another giant

* Corresponding author.

E-mail address: lwe@petrochina.com.cn (Li W.).

Peer review under responsibility of Sichuan Petroleum Administration.

Sinian gas field—Anyue gas field was discovered [1—4]. However, because of little geology information available (except for gas components and carbon isotope values, no other direct evidences), there are still disputes over the gas sources, genetic mechanisms and Sinian gas accumulation regularity etc. of the giant Anyue gas field. Hence, we tried to 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 to provide new ideas and methods for studying the enrichment 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/).

1. Asphalt features of the Dengying reservoirs in the Tongwan Palaeo-uplift

According to previous studies, the oldest palaeo-uplift in the Sichuan Basin is the Leshan-LongnUsi palaeo-uplift formed during the Caledonian Period [3,5—7]. This palaeo-uplift mainly refers to a complete joint palaeo-uplift developed in the trigonal area between Leshan-Chengdu in the western Sichuan Basin and Longniisi in the middle Sichuan Basin at the end of Silurian Period [8], with an area of 60000 km2, where formation erosion becomes stronger from the east to the west, and the formation had eroded to the Sinian Dengying Formation near the western margin of this basin at the end of Caledonian Period [3,5,7]. Based on recent exploration progress and geologic study, the oldest palaeo-uplift favorable for hydrocarbon accumulation in the Sinian Den-gying Formation is actually the one formed during the Tong-wan stage (we call it Tongwan Palaeo-uplift), not the Caledonian palaeo-uplift. The Tongwan Palaeo-uplift was generated from the karst palaeo-topography by uplifting after the deposit of the Sinian Dengying Formation. That is to say, it was generated by the evolution of the Gaoshiti-Moxi-Longnusi karst residual hill [1] east of the Deyang-Luzhou erosion valley [9] and the Leshan-Jingyan-Ziyang karst residual hill [1] west of this valley. Thus, it can be seen that the Tongwan Palaeo-uplift, quite different from the Caledonian Palaeo-uplift, is the combination of two isolated residual hills developed in the Middle-Southwestern Sichuan area. As its major part is buried below the Caledonian Leshan-Longnusi Palaeo-uplift, it is easily mistaken as the Caledonian Palaeo-uplift.

The study on asphalt of the Dengying reservoirs in the Sichuan Basin started from 1980s. Song Wenhai, Ran Longhui and Wang Lansheng et al. [5—7,10] concluded that the Sinian natural gas was the product of oil cracking, which also led to common existence of asphalt in Sinian reservoirs in the Weiyuan-Ziyang area, but no much progress has been made since then. We will mainly study the microscopic features and macroscopic regularities of the asphalt in the Sinian Dengying reservoirs.

1.1. Microscopic features of the asphalt

The asphalt in the Dengying reservoirs in the Tongwan Palaeo-uplift in the Sichuan Basin has developed not only in the Weiyuan-Ziyang area, but also in the Gaoshiti-Moxi area. Filling various pores, the asphalt can show its relationship with various diagenetic minerals and regional thermal events.

1.1.1. Asphalt features reflected by cores

The asphalt in Sinian reservoirs in the Tongwan Palaeo-uplift has been found in intercrystal pores, intergranular pores, intergranular dissolution pores, dissolution fissures and dissolution vugs etc. Observation of cores from prospecting wells reveals that the asphalt not only exists in fissures and vugs related to fractures, but also in vugs and matrix pores formed by dissolution. For examples, in the dolomite at the

depth of 4958.40-4958.45 m in the 4th member of the Dengying Formation in Well Gaoshi 1, not only quartz, dolomite and asphalt filling the fractures and dissolved pores, but also asphalt filling dissolved pores and vugs generated after the formation of horse teeth-shaped dolosparites [11] and quartz by hydrothermal process can be seen (Fig. 1a). At the depth of 4977.81 m in the 4th member of the Dengying Formation, the dolomite and black glistening asphalt filling the dissolved pores in cores are cemented together (Fig. 1b). These phenomena can also be seen in dolomite vugs and fissures in the 2nd member of the Dengying Formation in Well Gaoke 1 (Fig. 1c), in dolomite fissures and vugs in the 4th member of the Dengying Formation in Well Anping 1 (Fig. 1d), in dissolved pores in the 4th member of the Dengying Formation in Well Moxi 8 (Fig. 1e), and in dolomite interlayer fissures in the 2nd member of the Dengying Formation in Well Moxi 11 (Fig. 1f). The asphalt content in the Dengying reservoirs is generally between 0.17% and 4.33% (over 5% in local part). For instance, the asphalt content of the 4th member of the Dengying Formation at the depth of 5055.4 m in Well Anping 1 is 6.19%; and the asphalt content of the 2nd member of the Dengying Formation in Well Wei 117 at the depth of 3157.9 m is as high as 20.73%.

The above phenomena indicate that: © hydrocarbons once filled the fissures, caverns, dissolved pores and matrix pores in the Dengying reservoirs, which left asphalt behind after oil cracking; © uneven in distribution, asphalt in these reservoirs is 1-3% of the total rock volume in most cases, but can be very concentrated in some sections; © the asphalt was formed after the Sinian regional hydrothermal diagenesis. As the first regional hydrothermal process in the Dengying Formation occurred at the early Cambrian Period when the source rocks had not generated hydrocarbons yet, the asphalt in the Den-gying reservoirs must have generated only after the Emei Taphrogeny [12] in the Permian Period.

1.1.2. Asphalt features in rock slices

The relationship between the asphalt and pore cement in the Dengying reservoirs in the Tongwan Palaeo-uplift has unique features, showing different relationships with hydrothermal events. A few researchers have studied the asphalt features in rock slices in the Dengying reservoirs, which mainly focuses on the relationship between the asphalt and the filling sequences of diagenetic minerals. For example, examining the asphalt in Sinian reservoirs in the southeastern Sichuan Basin, Huang Wenming et al. discovered the filling sequence of dolomite / asphalt / dolomite / asphalt / quartz [13]. After studying the asphalt in the Sinian reservoirs in the Caledonian palaeo-uplift, we thought that the asphalt in rock slices existed in various pores and had certain filling sequences, but apparently different from the Sinian reservoirs in the southeastern Sichuan Basin. For example, features such as asphalt filling intergranular pores, intercrystal pores, dissolution pores, dissolution caverns and fissures etc. (Fig. 1g-p), asphalt filling some intergranular pores and intercrystal pores completely (Fig. 1g and h), asphalt filling intercrystal pores together with calcite (Fig. 1i), and asphalt

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. 1j—p), show that hydrocarbons once accumulated in various pores in the Dengying reservoirs on a large scale, and asphalt was left in various pores after later thermal cracking in various degrees.

After studying the relationship between asphalt and cement, we are sure that there is a filling sequence of fibrous dolomite cement / dolosparite cement / asphalt / dolomite or quartz in the Dengying reservoirs in the Tongwan Palaeo-uplift. For instance, the abundant dissolution pores and vugs in cyanobacteria powder dolomite in the 2nd member of the Dengying Formation in Well Zi 6 were partially filled with coarse dolosparite first, then the residual pores were filled with asphalt, finally cementation of hydrothermal dolomite occurred in the residual pores (Fig. 1k). In the 4th member of the Dengying Formation at the depth of 5040.3 m in Well Anping 1, dissolution fissures in the cyanobacteria cohered

dolarenite were partly filled with coarse dolosparite, then filled with asphalt, and formed asphalt shrinkage joints (Fig. 1l). In the 4th member of the Dengying Formation at the depth of 5062 m in Well Anping 1, the dissolution vugs in the cyano-bacteria cohered micritic powder dolomite were firstly filled with granular quartz, then filled with asphalt, leaving few dissolution pores; moreover, hydrothermal quartz was developed in the remained pores filled with asphalt (Fig. 1p). In the 2nd member of the Dengying Formation at the depth of 3057 m in Well Wei 117, the dissolution vugs in cyanobacteria cohered powder debris and micritic dolomite were filled in a sequence of fibrous dolomite, granular dolomite and asphalt, then asphalt shrinkage joints and tectonic fissures were developed (Fig. 1n). In the 4th member of the Dengying Formation at the depth of 4975.29 m in Well Gaoshi 1, coarse breccia dolomite filled the early dissolution pores, and then

asphalt filled intercrystal pores (Fig. 1o). In the 4th member of the Dengying Formation at the depth of 4986.57 m in Well Gaoshi 1, dissolution pores were developed in breccia micritic dolomite; dolosparite and asphalt successively filled dissolution vugs and dissolution fissures in cyanobacteria micritic dolomite, leaving massive dissolution vugs and fissures unfilled (Fig. 1m). Moreover, previous studies show that saddle-shaped dolosparite, coarse-grained dolomite and horse teeth-shaped dolomite etc. developed along pore walls of reservoirs are products of hydrothermal process [11,13—16], therefore, regional hydrothermal events happened before the generation of the Sinian reservoir asphalt, later local hydrothermal events happened.

The dissolution pores in Sinian Dengying Formation were mainly developed in the Tongwan Period and the end of Caledonian period [9,17]; while hydrothermal dolomitization might mainly happened in early Cambrian-middle Permian taphrogenesis movements [18—20]. If the hydrothermal dolosparite was formed in early Cambrian hydrothermal process, then after the Caledonian weathering and the action of massive organic acid during hydrocarbon generation process of the lower Cambrian source rocks, the dolosparite would have had apparent dissolution, but no dissolution has been found in dolosparite in all analyzed thin sections. Hence, the generation of hydrothermal dolomite in the Sinian dolomite reservoirs must have happened in the middle Permian Emei taphrogenesis movement. In other words, the hydrothermal coarse dolosparite developed along pore walls in the Sinian reservoirs must have generated in the middle Permian Emei taphrogenesis movement. Thermometry of inclusions in hydrothermal dolomite shows the homogenization temperature of inclusions in early coarse columnar dolomite in grape-flower-edge shape in the 2nd member of the Dengying Formation on Ebian-Xianfeng profile at the southern edge of the Sichuan Basin is between 95 and 99.5 °C. Based on the study results of palaeo-geotemperature in the Sichuan Basin [21], we studied the formation burial history and thermal evolution history in the Moxi area (Fig. 2), and reached the conclusion that the inclusions in grape-flower-edge shape dolomite in the 2nd member of the Sinian Dengying Formation were formed during Silurian-the end of middle Permian Maokou Formation, while there was no regional thermal event during Silurian-early Permian epoch; therefore, the dolomite of this period must have been formed at the depositional end of the Maokou Formation. Whereas the inclusions in the macrocrystalline and prismatic dolomite in the 2nd member of the Dengying Formation at 3253.9 m depth in Well Wei 117 have a homogenization temperature of 129 °C, and the high temperature of inclusions in prismatic quartz is between 129 and 169 °C, which should be the results of local strong hydrothermal activity in regional hydrothermal activity background.

Thus, it can be seen that there were regional hydrothermal process and the formation of hydrothermal macrocrystalline dolomite and quartz at the depositional end of middle Permian Maokou Formation in the Sinian System. While the hydrothermal dolomite cementation and hydrothermal quartz

cementation occurred in the residual pores after asphalt filling were likely to be the products of hydrothermal process caused by local tectonic movement in the Indosinian-Himalayan periods after regional hydrothermal process.

1.2. Distribution features of reservoir asphalt

Though the asphalt in the Dengying Formation in Caledonian Palaeo-uplift in the Sichuan Basin is widely distributed, its content and distribution show some patterns. Firstly, the reservoir asphalt mainly distributes in Weiyuan-Ziyang and Gaoshiti-Moxi regions, but the asphalt content in peripheral regions is low (Fig. 3). For instance, the average content of volume percentage of the reservoir asphalt is around 0.8%— 1.0% in the Weiyuan-Ziyang region, higher in Well Zi 1 (1.36%); that in Gaoshiti-Moxi region is between 0.7% and 1.5%, highest in Well Anping 1 (3.61%). The average content in its peripheral regions is generally less than 0.5%. For example, the average content in Well Hanshen 1 is 0.02%, that in Gongshen 1 is 0.05%, and that in Well Pan 1 is 0.11%. Such distribution characteristics of asphalt content in Fig. 3 is similar with the features of strike direction and development location of the Sinian top structure before the deposition of Jurassic System (Fig. 4). The palaeo-structures in the Indosi-nian Period had an important controlling effect on the development of palaeo-hydrocarbon reservoir.

Secondly, the vertical distribution of asphalt in Sinian also follows some patterns, namely, mainly distributing in the upper Dengying Formation and its top. By analyzing reservoir asphalt in Sinian cores in Well Wei 117 (Fig. 5), we found that reservoir asphalt content at its upper and top is generally higher. For example, there developed a 206 m interval rich in asphalt at the upper Dengying Formation, with a reservoir asphalt content between 0.5% and 3.5%. Moreover, there is abundant asphalt in the Maidiping dolomite (0.3—1.1%). Whereas, the reservoir asphalt content in the formations below 3190 m depth in the 2nd member of the Dengying Formation is lower (generally <0.3%, only few samples >0.3%). This kind of phenomenon has been found in prospecting wells in Ziyang, Moxi and Gaoshiti regions too. Thus it can be seen that the intervals rich in reservoir asphalt above 3190 m depth in the Sinian System similar in Well Wei 117 were the locations of palaeo-oil reservoirs, and the intervals below them with lower and scattered reservoir asphalt were water layers.

According to the study on asphalt development in prospecting wells in the Palaeo-uplift, the Sinian asphalt interval and above reservoirs in prospecting wells are thicker in the central area of the Indosinian Palaeo-uplift. For instance, the thickness in Gaoshiti-Moxi-Longnusi region where the Anyue gas field is located is between 400 and 500 m, and the thickness in Weiyuan gas field and the Ziyang gas-bearing structural area is between 200 and 280 m, but the thickness in peripheral regions (such as Guang'an, Dazu, Ya'an-Leshan-Muchuan etc.) reduces sharply to less than 20 m (Fig. 4). This indicates that the Indosinian palaeo-structures in the Tongwan Palaeo-uplift controlled the asphalt development, and also

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 central area.

In conclusion, the enrichment of the Sinian reservoir asphalt on plane is mainly related to the top of the Yanshanian palaeo-structure, and its enrichment in vertical direction is mainly at upper and top Sinian reservoirs. This also indicates that giant palaeo-oil reservoirs might be developed in the Indosinian palaeo-structure background.

2. Relationship between reservoir asphalt and hydrocarbon accumulation

It can be seen from the above analysis that there is a close relationship between the development of the Sinian reservoir asphalt and the palaeo-oil reservoirs in the Sichuan Basin. Whether there is a close relationship between the development of the Sinian reservoir asphalt and the present natural gas distribution is a crucial question. According to our analysis, the distribution of reservoir asphalt has relationship with the palaeo-oil reservoir cracking and the source rocks.

2.1. Relationship between reservoir asphalt and source rocks

The carbon isotope values of the reservoir asphalt in the Dengying Formation show good correlation with Lower Cambrian source rocks. As most hydrocarbons are generated by thermal cracking of organic matter in formations, according to the isotope fractionation principle, the hydrocarbons released early from organic matter have lighter isotope value than that of kerogen, and the hydrocarbons released later from organic matter have higher isotope value [22-24]. Though the asphalt is the remains of cracked crude oil, separated from

kerogen, it should have lighter or similar isotope value than that of kerogen. According to analysis results, the carbon isotope value of the Sinian reservoir asphalt is between —37.0%o and — 34.5%o, and that of the Cambrian kerogen is between —33.0% and —28.5%, showing bigger difference. The carbon isotope value of kerogen in the Cambrian Maid-iping Formation and the Qiongzhusi Formation is between —36.5% and —31.5%, similar with and heavier than the carbon isotope value of reservoir asphalt in the Dengying Formation, showing a genetic relationship between them. Moreover, Xu Guosheng et al. also thought that the asphalt in the Sinian reservoirs in the central Sichuan Basin came from the Cambrian System [25]. From this, we can conclude that the reservoir asphalt in the Dengying Formation mainly comes from the source rocks in the Lower Cambrian Maidiping Formation to Qiongzhusi Formation. That is to say that the hydrocarbons in the Sinian palaeo-oil reservoirs mainly come from Lower Cambrian source rocks.

2.2. Relationship between reservoir asphalt and palaeo-oil reservoirs

The distribution of asphalt at the Tongwan Palaeo-uplift indicates that giant palaeo-oil reservoirs were once developed at the palaeo-uplift. A lot of previous researches show that the reservoir asphalt is mainly related to palaeo-oil reservoirs [26-29]. By studying the reservoir asphalt in Sinian cores in more than 20 wells, we found some intervals rich in reservoir asphalt: the 4th member top of the Dengying Formation in Well Laolong 1 (16 m thick), the 2nd member of the Dengying Formation at the Sinian top in Well Zi2 (260 m thick), the 2nd and 4th members of the Dengying Formation at the Sinian top in Well Wei 117 (180 m and 26 m thick,

Fig. 3. Plane variation of the Sinian asphalt content in the Tongwan Palaeo-uplift.

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 Formation 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 [8,30—33], 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 had two major giant oil reservoirs in Ziyang-Weiyuan and Gaoshiti-Moxi regions, with oil accumulation reserves of 111 x 108—133 x 108 t and 389 x 108—467 x 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).

Fig. 5. Distribution of the Sinian asphalt content in Well Wei 117, Sichuan Basin.

Thus, we can see that most of the present gas reservoirs are the development area of large palaeo-oil reservoirs, but the areas with smaller palaeo-oil reservoirs fail to form natural gas accumulation after crude oil cracking.

2.3. Reservoir asphalt and formation of natural gas fields

The above discussion shows that in the Tongwan Palaeo-uplift, natural gas could only accumulate in the areas with rich and thick asphalt layers, which is mainly related to the later gas accumulation produced by crude oil cracking in giant palaeo-oil reservoirs.

Previous studies show that most crude oil started cracking into natural gas at 160 °C, and was completely cracked at about 200 °C [34—36]. Former graph of burial depth history and thermal evolution history (Fig. 2) of the Moxi region also shows that in early Triassic Period, the formation paleo-temperature of the Dengying Formation was between 150 and 170 °C, which was the initial stage of oil cracking; in middle Triassic-middle Jurassic periods, the formation paleo-temperature of the Dengying Formation was between 160 and 200 °C, which was the peak of massive oil cracking; in late Jurassic-early Cretaceous periods, the Dengying Formation, between 190 and 210 °C in formation paleo-temperature, entered the end of oil cracking.

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 —43.5% during Triassic Period to -55.2%o during the early stage of early Jurassic Period, then increased to —23.5% at the end of Jurassic Period. In contrast, the analysis data of carbon isotope values of methane in natural gas in the Sinian Dengying Formation is between —32.3% and —32.5% , which is similar to the carbon isotope value of later products of crude oil cracking (later Jurassic Period). This conclusion coincides with the Sinian burial history and thermal evolution history mentioned above. The Sinian crude oil in the Moxi region also reached the end of thermal cracking in late Jurassic Period. Hence, the natural gas accumulated in the Sinian System should be the products of cracking of oil in palaeo-Sinian oil reservoirs, and this period should be in late Jurassic, being the product of later oil cracking.

In summary, the development of reservoir asphalt in the Dengying Formation not only shows that hydrocarbons in Sinian palaeo-oil reservoirs mainly comes from Lower Cambrian source rocks, but also proves that the development and distribution of current reservoir asphalt are the result of the palaeo-oil reservoir cracking before the deposition of the Jurassic System, and the natural gas accumulated in the Sinian System must be the product of late palaeo-oil reservoir cracking. Gas reservoir development is generally related to large palaeo-oil reservoir development, while smaller palaeo-oil reservoirs couldn't form gas reservoirs by crude oil cracking.

3. Conclusions

1) The asphalt in the Dengying Formation in the Tongwan Palaeo-uplift of the Sichuan Basin exists in various pores, such as intergranular pores, intercrystal pores, dissolution pores, dissolution vugs and dissolution

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 Emei Taphrogeny and hydrothermal event in the middle-later Permian Period.

2) The distribution of reservoir asphalt in the Dengying Formation in the Tongwan Palaeo-uplift of the Sichuan Basin is mainly related to the palaeo-structures in the Indosinian-Yanshanian Period. The high position of the palaeo-structures in this period has higher asphalt content. Large-scale hydrocarbon accumulation in the Sinian System in the palaeo-uplift occurred in the Indosinian-Yanshanian period, and formed Leshan-Ziyang and Gaoshiti-Longniisi-Guang'an palaeo-oil reservoir development belts. Crude oil cracking in the major parts of these palaeo-oil reservoirs control the development of current natural gas reservoirs.

3) The development of reservoir asphalt in the Dengying Formation in the Sichuan Basin not only shows that hydrocarbons in palaeo-oil reservoirs in the Dengying Formation mainly come from Lower Cambrian source rocks, but also prove that the natural gas accumulated in

Fig. 7. Evolution pattern of carbon isotope in methane in gas produced by crude oil cracking in Sinian geological history.

the Sinian reservoirs should be the product of late palaeo-oil reservoir cracking.

4) The palaeo-structures of the Dengying Formation in the Sichuan Basin during the Indosinian-Yanshanian period were favorable regions for the development of palaeo-oil reservoirs. Looking for regions with large-scale hydrocarbon accumulation in the Sinian System in this period is of great significance for opening up new areas of Sinian natural gas exploration.

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