2. Institute of Geological Exploration and Development of CNPC Chuanqing Drilling Engineering, Chengdu 610051, China;
3. Research Institute of Petroleum Exploration and Development, PetroChina Southwest Oil & Gas Field Company, Chengdu 610000, China;
4. BGP Inc., China National Petroleum Corporation, Chengdu 610213, China
With the oldest reservoir (Sinian Dengying Formation) and oldest source rocks (Lower Cambrian Jiulaodong Formation) in China, the gas reservoir of the Sinian Dengying Formation in the Weiyuan Structure (Weiyuan Gas Field) is also one of the oldest geological age gas reservoirs in the world (Fig. 1). The gas reservoir was discovered in 1964 and was later placed into production and development to promote natural gas production during China's 3rd Five Year Plan period with an average annual growth of 11%, which makes the reservoir a significant contribution to the development of the modern gas industry in the early developing stages of the Peopleʼs Republic of China. At the time of this technical discovery, the guiding theory of exploration was surface anticlinal oil and gas exploration theory, which was combined with oil and gas seepage to prospect the surface anticline structure. Since the discovery of two paleouplifts in the Sichuan Basin, the Caledonian paleouplift and the Indosinian paleouplift, the paleouplift-control theory has gradually developed (Huang, 2009; Wang and Zhao, 2006; Li et al., 2001; Xu and Xiong, 1999; Luo et al., 1998; Song, 1996). Although explorations have found a number of fields, the reserves are comparatively small.
After nearly 40 years of exploration, the Gaoshiti-Moxi Structure from the Gaoshi1 Well of the Sinian Dengying Formation in 2011 and Moxi8 Well of the Cambrian Longwangmiao Formation in 2012 finally yielded industrial gas flows. These wells are another important exploration discovery in the Lower Paleozoic of the Sichuan Basin since the success of the Sinian Dengying Formation gas reservoir in the Weiyuan Structure in 1964. The exploration breakthrough of the Lower Paleozoic in the Sichuan Basin has attracted the attention of many scholars (Wu et al., 2016; Li et al., 2015; Du et al., 2014; Xu et al., 2014; Zhong et al., 2014; Zhou et al., 2014; Liu et al., 2013). Based on studying the taphrogenesis and basin-mountain system in the Sichuan Basin (Liu et al., 2012a), the Early Cambrian "Mianyang-Changning" intracratonic sag (Zhong et al., 2014; Liu et al., 2013) was discovered and the tectonic-controlled theory of marine facies hydrocarbon distribution in the Sichuan Basin was presented, which is a joint control theory of an intracratonic sag-paleo-uplift-basin mountain system (Liu et al., 2018, 2015a). Both the paleo-uplift- controlled theory and the current joint-controlled theory show that the Weiyuan Structure may play an important role in the process of Paleozoic oil and gas migration and accumulation in the Sichuan Basin.
The Weiyuan Gas Field has been investigated for a long time (Dai, 2003; Huang and Chen, 1993; Xu et al., 1989). The Weiyuan Gas Field belongs to an anticline-type bottom water block gas reservoir (Wen, 1986). Although the Weiyuan Structure is a giant dome anticline, the abundance of the Dengying Formation is relatively low with 25.4% abundance in plane and 27.3% abundance in height. The integrated abundance of the gas reservoir is only 26.4%. The gas bearing area of the Dengying reservoir is 216 km2 with a maximum height of 244 m and an average height of 84.35 m. The elevation of the original gas-water interface is -2 434 m, and the original formation pressure is 29.5 MPa. The maximum buried depth of the pay horizon in the Dengying Formation is 2 800 m with a gas reverse coefficient of 1.89×108 m3/km2, which means it is a large gas field with low abundance. However, the related research on low abundance is limited. Previously, it was thought that low abundance might be caused by inadequate gas accumulation due to the late formation time of the anticline (Zhai, 1989). In recent years, it has been thought that the low abundance may be associated with dispersion due to surface oil and gas seepage, but the evidence is insufficient (Wei et al., 2008). The problem of abundance has plagued oil and gas exploration of deep marine carbonate in the Sichuan Basin for a long time.
Conversely, due to the signal-to-noise ratio of seismic data, land gas chimney research has its limitations and lacks of effective methods to discriminate a gas chimney. Research ideas and methods need to be innovated urgently. Based on the analysis of geological conditions in the Weiyuan Structure, combined with the related research examples of oil and gas migration and gas chimneys based on the surface or subsurface data and the geological-geophysical-geochemical comprehensive research methods, it is possible to provide a reasonable explanation for the low abundance of the gas reservoir in the Weiyuan Structure. The complex geological problems of abundances in the Weiyuan Structure can be an example of studying late reconstruction of deep oil and gas reservoirs in the superimposed basins of western China.1 GEOLOGICAL SETTING
The Weiyuan Gas Field is located among Weiyuan, Zizhong and Rong counties, Sichuan Province, approximately 30 km southeast from Zigong City (Fig. 1). The gas field was found through the Weiji Well, which was drilled on the Caojiaba surface structural high from May 1956 and stopped in the Jiulaodong Formation (2 438.65 m) on April 2, 1958 because of limited drilling conditions. On March 28, 1964, as drilling continued deeper, lost circulation occurred in the interval of 2 852.0–2 859.39 m, and then the DST observed industrial gas flow, which marked the discovery of the Sinian Dengying gas reservoir. By the end of 2001, the proven original geological reserves were 408.61×108 m3, recoverable reserves were 147.82×108 m3, gas field production was 145.94×108 m3 cumulatively and its remaining reserves were 1.88×108 m3, the resources were about to be depleted. In 2005, the Cambrian Xixiangchi Formation and Yuxiansi Formation gas reservoirs were found in a new round of rolling exploration, while they were small scale reservoirs.1.1 Stratigraphy
The strata drilled in the Weiyuan Structure are from the Late-Triassic Xiangxi Group to Pre-Sinian granite (Fig. 2 below the blue line). Wells including the Wei15, Wei28 and Wei117 were drilled into the Pre-Sinian fine-coarse grain granite. In the Wei28 Well, the granite still produces a small amount of natural gas. The Sinian Dengying Formation, with an age of 600±20–700 Ma, is the main productive layer in the Weiyuan Structure. The age of the dolomite from the Dengying Formation is 700–800 Ma in its lower value and its geological era is Late-Sinian.
The Dengying Formation is mainly composed of dolomite, and the implicit-algal dolomite is the main reservoir. The dissolution pores, structural fracture-caves and strata-deformation fractures are well developed. The cumulative thickness of the effective reservoir is 90 m, characterized by thin mono-layers (usually 1–2 m), numerous reservoir intervals and thick compacted rock between the reservoir intervals. The mean value of the effective reservoir porosity is 3.15% from core analysis and 4.4% from logging data.
The Dengying Formation has four gas pays. From the bottom to the top, the four gas pays are (1) the interval of fracture- cavities on the bottom, located in the upper submember of the 3rd member of Dengying Formation, with a thickness of 6 to 10 m. It is an important gas-bearing reservoir in the lateral. (2) The interval of fracture-cavities and pores in the middle pay, which is located in the middle of the 1st submember of the 4th member of the Dengying Formation, with a thickness of 8 to 10 m, porosity of 4.5%, wide lateral distribution and good construction of fracture- cavities and pores. (3) The interval of fracture-cavities and pores in the upper pay, which is located in the 2nd submember of the 4th member of Dengying Formation, that has a thickness of 5–7 m and porosity of 3.73%, and its lateral distribution is stable. (4) The interval of fractures at the top section pay is located in the top of the 4th member of the Dengying Formation, and it has 0–4 m thickness and poor lateral continuity. Due to cuttings of faults and fractures, the intervals are interconnected vertically. Therefore, the whole gas reservoir is a unified hydrodynamic system.
The dark grey, black grey shale of the Lower-Cambrian Jiulaodong Formation is overlain on the Dengying gas reservoir, which is also the stable source rock of the Dengying gas reservoir with a thickness of more than 230 m in the Weiyuan area. According to a rock mechanics analysis, the saturated extrusion intensity of the shale is 42.4–46.6 MPa and its shear strength is 5.2–7.6 MPa, which classifies it as a good cap rock with strong plasticity. The Middle and Upper Cambrian are mainly composed of dolomite and sandstone, interbedded with sandstone and shale. The gas production of 2.2×104 m3/d was found through drilling, while it does not reach an industrial scale. The Ordovician is also mainly composed of dolomite with biological limestone, shale and sandstone interbedded. Derived from the uplift and denudation of the Caledonian Movement, lacunas of the Silurian (only 0–140 m thick in part of the Weiyuan Structure), Devonian and Carboniferous are similar to most areas of the Sichuan Basin in the Weiyuan Structure. The Caledonian has caused angle unconformity contact between the Permian and Ordovician. The Lower-Permian is mainly composed of dark grey-black limestone, which contains a rich organism and is approximately 30 m thick. The Permian is the main regional gas-bearing strata in the Sichuan Basin, but it is only a secondary gas reservoir with micro gas production in the Weiyuan Gas Field. The Lower-Triassic Feixianguan Formation is made up of purple shale with limestone interbedded. The Lower-Triassic Jialing- jiang Formation and Middle-Triassic Leikoupo Formation are mainly composed of dolomite and limestone with gypsum interbedded, which are regional cap rocks in the whole basin. However, with part of the Triassic outcropped in the Weiyuan Structure, the seal effect may be limited.1.2 Structure
The Weiyuan Structure is located in the southeast slope of the Leshan-Longnüsi paleouplift and the west side of the Early Cambrian "Mianyang-Changning" intracratonic sag in the Sichuan Basin, which is a large dome anticline with an axis of NEE and characteristic of compresso-shear. The Weiyuan Structure is formed by lateral extrusion during the Himalayan period, but the specific geological age of formalization still remains controversial (Liu, 2001; Gan and Liang, 1988; Bao et al., 1985). The evolution and hydrocarbon accumulation of the Weiyuan Structure have undergone tectonic superposition by multiphase tectogenesis (i.e., extension, compression and uplifting) (Liu et al., 2008a; Liu, 2001; Song, 1996) (Fig. 3). Currently, due to the effect of intensive compression, the Weiyuan Structure is the strong transformation area in the Sichuan Basin, controlled by basement structures. The Weiyuan Structure is the largest anticline structure in the Sichuan Basin. The long axis of the Xiangxi Group (T3x) top structure on the surface is 92 km and the short axis is 30.8 km with a closed area of 1 751 km2 and a closure of 1 080 m. The long axis of the Sinian top boundary structure is 53 km and the short axis is 26 km with a closed area of 850 km2 and closure of 895 m. There are two structure highs in the top of the Dengying Formation: the main structural high is close to the Wei3 Well, with an altitude of -2 234.08 m and the west structural high is close to the Wei108 Well with an elevation of -2 277.9 m. The two flanks of the Weiyuan Structure are asymmetrical. The south flank is steep (9°33'–11°00') and the north flank is gentle (3°30'–5°30'). There are four faults near the top of the Sinian structure. The fault displacements are less than 60 m, which causes weak damage to the gas reservoir. The evolutionary process of the Weiyuan Structure has determined its distinct pattern of gas migration and accumulation.1.3 Main Structural Trap Features of the Sinian Dengying Formation in the Sichuan Basin
In summary, the Himalayan tectonic activity mainly includes the breakup of large structural traps and the migration of a structural high with uplifting and erosion. The Weiyuan area is characterized by large magnitude uplifting with strong tectonic deformation and trap reworking, while the central Sichuan Basin is characterized by small magnitude uplifting with weak tectonic deformation and trap reworking. The evolution history can be divided into two phases: (1) the compressional folding phase from 100 to 20 Ma, which was dominated by strong compression and weak uplifting and erosion. During this phase, the Weiyuan, Gaoshiti and Moxi structures were preliminarily formed and the early overpressure paleo-gas reservoirs created by oil cracking were adjusted into late overpressure paleo-gas reservoirs (Liu et al., 2014); and (2) the rapid uplifting and erosion phase since 20 Ma, which is mainly dominated by uplifting and erosion. Because the fastest and largest amount of erosion occurs at the top of the Weiyuan Structure, this phase has led to a further adjustment or damage to the late overpressure paleo-gas reservoirs, which formed the present normal pressure gas reservoirs.
Currently, the subsurface structures discovered in the Sinian Dengying Formation in the Sichuan Basin are mainly concentrated in and around the Leshan-Longnüsi paleouplift and west of the Huaying Mountain, and include more than twenty small structural traps, such as the Weiyuan, Gaoshiti, Moxi, Ziyang, Jinshi, Anpingdian, Longnüsi, Guangʼan, Zhougongshan, Hanwangchang, Laolongba, Ziliujing, Panlongchang, Dawoding, and Taigongtang (Fig. 4) structural traps.
The Jinshi Structure trending NWW is to the west of the Weiyuan Structure with a long axis of 8.2 km and a minor axis of 4.1 km. Its closure height is approximately 90 m with a closure area of 30.38 km2. For the Jinye1 Well drilled by SINOPEC, an exploration breakthrough recently occurred in the Dengying Formation with a tested gas production of 4.73×104 m3/d.
The Guangʼan Structure is located in the Guangʼan County, where the Dengying Formation has a structural high with a buried depth of approximately 6 020 m and an elevation of -5 300 m. The structure's closure height is 250 m with a spill point elevation of -5 550 m and a closure area of 210 km2. While this structure is still currently under exploration, a gas logging anomaly has been discovered in the 4th member of the Dengying Formation.
The Longnüsi buried structure is located in Wusheng County. The drilling of the Dengying Formation can be dated back to the Nüji Well in 1971. The Sinian structure has a structural top with a buried depth of approximately 5 280 m and an elevation of approximately -4 930 m. Its closure height is 80 m with a closure area of 269 km2 and a spill point elevation of -5 010 m. In the Nüji Well, the 4th member of the Sinian Dengying Formation (5 197.92–5 239.92 m) had a tested water production of 11.8 m3/d and gas production of 1.85×104 m3/d. However, this well is still under exploration and its abundance has not been identified yet.
The Gaoshiti-Moxi buried structure is located in the east of Anyue County. The structural top of the Denying Formation has a buried depth of approximately 4 900 m and an elevation of approximately -4 640 m. Its closure height is approximately 360 m with closure area of approximately 3 474 km2, while its spill point is located in the west with an elevation of approximately -4 940 m and abundance greater than 100%.
In summary, from the Guangʼan to Longnüsi to Moxi to Gaoshiti to Weiyuan structures, the straight tectonic distances are from 45 to 48 to 28 to 105 km with a total length of 226 km. The buried depth of the Dengying Formation structural top gradually uplifts from -5 300 to -4 930 to -4 640 to -2 400 m. From the Guangʼan to the Weiyuan structures, the drop height of the structural top is 2 900 m. The spill point of structural traps gradually uplifts from -5 550 to -5010 to -4 940 to -3 200 m with a drop height of 2 250 m. Meanwhile, the abundance of gas reservoirs decreases from 100% to 26.4% from the Gaoshiti to the Weiyuan Structure.2 CHARACTERISTICS AND EFFECTS OF A GAS CHIMNEY IN THE WEIYUAN STRUCTURE
The "Gas chimney" (also called a "seal bypass system") refers to the fluid (e.g., plastic fluid, and gas) migration from the bottom to the top. It is a geological structure that caused changes in the regional geological conditions. The seismic data show abnormal characteristics. The gas chimney can be an important channel of fluids, especially for hydrate and nature gas. There are three patterns of seal bypass systems (gas chimneys), fault bypass, intrusive bypass and pipe bypass (Cartwright et al., 2007) (Fig. 5). The "gas chimney effect" is a geological phenomenon that is associated with gas chimney structures. For oil and gas geology, it intuitively reflects the initial period of oil and gas exploration that explores anticline oil and gas reservoirs using models of the surface oil and gas seepage. Due to gas migration, a gas chimney can affect regional characteristics of the gas component, pressure coefficient and geochemistry from the bottom to the top formation. An analysis of the effect of a gas chimney can be used as effective supplement evidence to seismic data to confirm the existence of a gas chimney.2.1 Features of a Gas Chimney in Marine and Land Formations 2.1.1 Gas chimney features in an ocean
A gas chimney was initially found in an ocean. Studies on gas chimneys have concentrated on fields of off-shore exploration to perform detailed analysis (Nourollah et al., 2010; Løseth et al., 2009). The North Sea Basin, in Norway, is a representative region (Hustoft et al., 2009). With intensive diapirism and fracture development in this region, a gas chimney has been formed by the leakage of oil and gas. The anomaly's conical characteristics in seismic data reflected a salse-controlled gas chimney. The fluids can migrate vertically along the faults and fractures to the seabed and spread to the sea surface (Heggland, 1997). Based on structural quantitation, the gas chimney in the North Sea Basin can be divided into three classifications (Hansen et al., 2005). In China, studies of gas chimneys mainly concentrated on the Beibu Gulf and the South China Sea, especially the Yinggehai Basin. Gas chimneys represented as mud diapirs are widely found in this region. Because the mud diapir is considered to be the source rock, the vertical migration and diffusion of hydrocarbon have formed an oil and gas migration system. Therefore, a gas chimney also represents the relationship between hydrocarbon migration and accumulation (Sun et al., 2012; He et al., 2010, 1994a, b ).2.1.2 Gas chimney features on land
The Quaternary biogenic gas reservoirs of the Sebei and Tainan gas fields in the Sanhu depression, Qaidam Basin have basement faults that connect the deep (Neogene) gas source rock. Meanwhile, the Quaternary mudstone cap-rock with high salinity water provides a better sealing capacity. The weak extensional tectonic belt provided space and a pathway for the gas chimney. Therefore, the gas chimney connects the deep source rock to the Quaternary reservoir in the Sanhu area and plays an important role in the Quaternary gas reservoir (Liang et al ., 2006). The recognition and analysis of the Cretaceous gas chimney in the Beier depression, Daqing-Hailaer Basin discovered that a connection between the oil source and reservoir was serried micro-fractures and minor faults with T-type and S-type gas bypass systems (Yang R Z et al., 2013).2.2 Mechanism Differences between Marine and Land Chimneys
First, a marine gas chimney is generally formed in the Cenozoic, Paleogene, Neogene and Quaternary formations with low compaction, low density and high water saturation. The formation is easily influenced by the interaction between an external force and deformation. Second, most gas chimney structures are mud diapirs. With high temperature and pressure, columnar fluids (gas) broke through the cap-rock and vertically migrated into overlying formations of the Paleogene, Neogene and Quaternary formations. Even with the high signal-to-noise ratio of offshore seismic data, there is a large difference in petrophysical parameters between the gas chimney and surrounding rocks. With a high wave impedance interface and seismic reflection energy, the tube anomaly characteristic is easy to identify (Figs. 6a, 6b).
There is no obvious change of lithology physical parameters between the land gas chimney and its surrounding rocks. The land gas chimneys are mostly generated in the Mesozoic and Paleozoic formations with intensive compaction, high density and consolidation diagenesis. The strata are not easy to be influenced by interaction between external force and deformation. Compared with the offshore seismic data, the onshore seismic data has a low signal-to-noise ratio. Therefore, the tubular characteristics of the land gas chimney are difficult to identify (Figs. 6c, 6d).
In terms of seismic data acquired in a complex mountainous seismic exploration such as in the Weiyuan Structure, due to the influence of surface seismic geological conditions, the suspected "pipe" anomaly in the seismic section is easy to be interpreted as a gas chimney phenomenon. However, these could be the "footprints" from seismic acquisition and processing. The current gas chimney studies are quite different from that of the Weiyuan Structure because of differences between marine and land chimneys, age of strata exposed in the surface, and difficulty of data acquisition. The recognition of a gas chimney in the Weiyuan Structure has to be based on a comprehensive analysis of seismic data and geological features affected by a gas chimney to improve the recognition reliability of a gas chimney.2.3 Gas Chimney Structure and Characteristics of Its Effects on the Weiyuan Structure
Gas seepage in practical work is the most direct evidence for the determination of a gas chimney, but the gas seepage is only easy to be found and identified at regional rivers. According to a field geological study in the Weiyuan area, a large amount of gas seepage situations are found north of Laochang Town, near the river at the Weiyuan structural core where the Lower Triassic Jialingjiang Formation is outcropped. Therefore, centered on the outcrop of the Jialingjiang Formation area in the Weiyuan Structure and combined with discovery positions of gas seepage, careful analysis and comparison of seismic interpretation and drilling data have been performed to explore the existing evidence of a gas chimney and its effects on the Weiyuan Structure.2.3.1 Seismic features of a gas chimney
Through an elaborate interpretation of 34 seismic lines acquired in 2005 and 3D seismic data acquired in the southeast of the structural core in 2010, combined with a fault analysis, the deep seismic anomaly characteristics are found near the Weiyuan structural core, especially near the Jialingjiang Formation outcropped area (Fig. 7). To determine the differences in seismic data and eliminate interference, 2D and 3D surveys of the Lower- Triassic Jialingjiang Formation outcropped area are compared.
The influence of seismic data quality on abnormal responses of a gas chimney can be obtained by comparing 2D and 3D seismic data. An example is line 05WY16 of the 2D work area where a suspected gas chimney is found (Fig. 8a). Considering the accuracy and shooting conditions of 2D seismic data, we have compared it with the latest overlapped 3D data to improve reliability of abnormal features. The comparison indicates that the interface between the top of Dengying Formation and the overlying bottom of the Lower Permian have an obvious and continuous reflection in the 3D area, while the interbed behaviour is obviously a messy reflection with subtle pull-down events but clear reflection on both sides. The characteristics of the gas chimney are very obvious (Fig. 8b).
For the 3D work area, the PSTM (pre-stack time migration) of Trace 155 shows that there are obvious structure deformations and event anomalies in the overlying formations of the Dengying Formation with good event continuity from strong to weak and lasting with strong reflection features. In the seismic section of the adjacent Trace 257 line, there are visible vertical zonal distributions of an event drop-down phenomenon over the structure high of the top of Dengying Formation, which indicates low interval velocity. These distributions may be the gas filling or bottom-up gas chamney effects. Based on the analysis of seismic data with different dimensions, we think that the vertical zonal distributions are probably caused by the gas chimney (Fig. 9).
As an important channel of oil and gas migration, fracture- fault is one of the main gas chimney models. Four faults are developed at the top of the Dengying Formation in the Weiyuan Structure (Fig. 9). One is located in the interior of the Dengying gas reservoir (No. 3), and the other three faults are located outside of the Dengying gas reservoir (Nos. 1, 2, 4).
Seismic anomaly characteristics show that: (1) there are large seismic anomaly areas in the surface Triassic outcropped area, especially in the Jialingjiang Formation and Leikoupo Formation outcropped area, and (2) the distribution of anomaly characteristics is centered on the Dengying gas reservoir and spreads outside of the boundary.2.3.2 Pressure coefficient
The pressure coefficient can directly reflect the geological environment of gas reservoirs. Because the Weiyuan Structure is the current structural high of the Dengying Formation in the Sichuan Basin, the Dengying gas has migrated and accumulated in the Weiyuan area over a large scale and long distance. With a pressure coefficient of approximately 1.0, the pressure system of the Dengying Formation is relatively stable (Liu et al., 2015b). From the bottom to the top, the pressure coefficient of each layer in the Weiyuan area is similar. For example, the average coefficient is 1.05 in the Sinian Formation, 1.049 in the Cambrian Jiulaodong Formation, 1.0 in the Yusiansi Formation, 1.003 in the Qiongzhusi Formation and 1.02 in the Middle Permian. The pressure coefficient analysis has shown that the Weiyuan area has a normal-weak low pressure environment with a bottom-up interconnected pressure system, in which the gas in the Dengying Formation has a significant vertical transportation under the influence of gas chimney effects.
In comparison with the Weiyuan Structure, the pressure coefficient is 1.06 in the Sinian Dengying Formation of the Gaoshiti-Moxi Structure. However, the pressure coefficient in the Lower Cambrian Canglangpu Formation, upper Longwangmiao Formation and Middle Permian are between 1.587– 1.696 and reach up to 1.9 in the Canglangpu Formation. Due to the overpressure in the overlying gas bearing formations of the Dengying Formation, the coefficients of the Gaoshiti-Moxi Structure indicate a good vertical preservation condition of the Dengying Formation, but the gas can migrate in the lateral direction (Fig. 10).2.3.3 Isotope geochemistry
The 36Ar content is a constant value that indicates the existence of argon, while 40Ar is formed by radioactive decay of 40K. 40Ar is mainly from the decay product of a potassium mineral in sedimentary rocks, secondary from the bedrock and mantle. Therefore, the older a source rock stratum in a geological era is, the higher the 40Ar from radioactive decay in the gas is, which leads to a larger 40Ar/36Ar ratio, which is called the 40Ar accumulation effect (Dai, 2003).
An analysis of the isotope geochemical data in the Weiyuan Structure has shown that the 40Ar/36Ar ratios of the Dengying Formation in three wells are 4 440, 7 232, and 9 255, respectively, with an average ratio of 7 009. The argon isotope value in the Cambrian is 7 681 in the Wei26 Well, which is slightly below the Sinian Dengying Formation. Another 2 wells have shown that the 40Ar/36Ar value in the overlying Permian gas reservoir are still as high as 2 855 and 5 222 with an average of 4 038 (Table 1); these values have large differences from other Permian gas reservoirs in the Sichuan Basin (distribution range of 340–1 622) (Xu et al., 1979). 3He/4He is stable from the bottom to the top. The isotope geochemical characteristics of the Cambrian–Permian gas reservoir in the Weiyuan Structure indicate gas channelling in the Dengying Formation and the development characteristics of a gas chimney.
Compared with the Weiyuan Structure, the 40Ar/36Ar of the Gaoshiti-Moxi area in the Sinian Dengying Formation is higher and ranges from 1 132 to 9 559 with an average of 4 696. The 40Ar/36Ar in the Middle-Cambrian Longwangmiao Formation decays faster and has a ratio of only 1 024 to 1 388, which is on-average 1 229 lower than that in the Sinian Formation. The average value of 3He/4He decreases rapidly from 7.443×10-8 in the Dengying Formation to 2.781×10-8 in the Middle-Cambrian Longwangmiao Formation (Wei et al., 2014). The isotopic feature indicates that the bottom of the Dengying Formation and the Middle-Cambrian Longwangmiao Formation are disconnected, relatively independent gas systems. On the other hand, the similar isotope values of the Dengying Formation indicate that the gas system of the Dengying Formation between the Weiyuan and the Gaoshiti-Moxi area is united.2.3.4 Non-hydrocarbon gas composition
Based on the non-hydrocarbon gas composition of the Sinian Dengying Formation, Middle-Cambrian Yuxiansi Formation, Qiongzhusi Formation and Permian gas reservoirs in the Weiyuan Structure, we found that the gas composition characteristics of the overlying gas reservoirs are very similar with the Sinian gas reservoir.
The Dengying gas reservoir is dominated by organic oil-cracking gas in the Sichuan Basin, which also features high contents of carbon dioxide, nitrogen, helium, argon and other non-hydrocarbon gases (Fig. 11). According to the drilling data: (1) vertically, there are high contents of carbon dioxide in all the gas reservoirs. In addition to the Weihan101 Well, the carbon dioxide content ranges from 4.63% to 8.39% with an average of 5.16%. For the Upper-Permian, the component gradually reduces from 4.529% to 2%. (2) Nitrogen (N2) content is generally higher than 6%–8% and up to 15.46% in the Wei117 Well. The nitrogen content is from 5.69% (Weihan6 Well) to 7.364% (Weihan1) in the Middle-Cambrian Yuxiansi Formation, 6%–7% of the Upper- Cambrian Xixiangchi Formation and reaches 3.2% at the top of the Upper-Permian. In conclusion, the nitrogen content gradually reduced from the deep Dengying Formation to the Permian, but it is still far higher than that in the counterparts in the Gaoshiti-Moxi Structure (ranging of 0.5%–1%). (3) Helium (He) content also gradually decreased from the Sinian to Permian Formations and is generally greater than 0.2% and up to 0.342% in the Wei34 Well. The helium content is 0.154%–0.217% in the Yuxiansi Formation (Є2y), 0.15%–0.19% in the Xixiangchi Formation (Є3x), and an average of 0.14% at the top Permian Maokou Formation, which shows a decreasing trend. The helium content in the Weiyuan Structure is much higher than that in the counterparts in adjacent areas of the Gaoshiti-Moxi Structure. The helium content only averaged 0.06% in the Dengying Formation and 0.02% in the Longwangmiao Formation (Є2l). (4) There are abnormal high argon (Ar) content values of each gas bearing formation in the Weiyuan Structure. From the bottom to the top, all of them are on the order of 10-2 percent, and some are up to 10-1 percent, which are commercial values, in the Cambrian in a few wells. The argon content is consistent with the increasing tendency of nitrogen and helium from the bottom to the top. At the top of the Permian gas reservoir, the argon content is as high as 0.02%, far higher than that in the adjacent regions (0–0.05% in the Middle of Sichuan and the highest value of approximately 0.15% in the east of Sichuan). (5) Like the other gas fields, such as the Ziliujing and the Huanglongchang gas fields, the gas composition of the Permian gas reservoir contains propane, butane and other heavy hydrocarbons. However, there are no heavy hydrocarbons in the Permian gas reservoir of the Weiyuan, which is similar with the characteristic of the Sinian Dengying Formation (Zhao, 1988).
In terms of non-hydrocarbon gas composition variations, it may be an anomaly if only one single gas component varies in a certain pattern, but when multiple components varying in a similar pattern, the pattern can be considered to be a law. With only 26.4% fullness of the Dengying gas reservoir and micro gas production in the overlying strata, the following principle can be summarized. Due to the creation of a gas chimney, the upper gas reservoirs have accumulated in small-scale areas from the leakage of the Dengying Formation. With the potential migration energy decreasing in the vertical direction, only a portion of the gas has been continuously adjusted. Therefore, constant mixing, exchanges and seal bypass processes exist in the vertical direction that can be used as the main indicators of gas loss in the reservoirs. A law in which the percentage of the non-hydrocarbon gas component decreases in a vertical sequence should be explained as one of the gas chimney effects. Based on the decreasing trend of non-hydrocarbon, rare-gas contents, pressure coefficient and isotope geochemistry, the analysis reflected an obvious gas channelling phenomenon in the Dengying Formation and a gas chimney in the Weiyuan Structure.3 IMPORTANT ROLE OF THE WEIYUAN STRUCTURE ON DIFFERENTIAL ACCUMULATION OF THE DENGYING FORMATION GAS RESERVOIRS IN THE SICHUAN BASIN 3.1 Formation Mechanism and Geological Significance of a Gas Chimney in the Weiyuan Structure
Apatite samples from 22 locations in the Weiyuan and Gaoshiti-Moxi areas have been fission track tested. The results of the fission track test analysis indicated that during the Late Mesozoic to Cenozoic, the Weiyuan area experienced fast and large magnitude uplifting (Liu et al., 2008b). The uplift caused the erosion of the Lower Triassic Jialingjiang Formation and Middle Triassic Leikoupo Formation in the Weiyuan structural core, which formed an anticline with the largest surface area and the oldest outcropped strata (carbonate rocks) to the west of Huaying Mountain, Sichuan Basin. Based on the modeling results of the apatite fission track, the uplifting and erosion (thermal history) of the Weiyuan area can be divided into three types (Deng et al., 2009). (1) Subsidence-uplifting type. With a rising burial depth and temperature after the Late Cretaceous (100 Ma), the maximum buried depth occurred during the Early Cenozoic (the general buried depth is greater than the apatite fully anneal isothermal surface). After a short period of retention, the formations were uplifted rapidly and annealed by the surface temperature. (2) Retained annealing-rapid uplifting type occurred during the Late Cretaceous–Paleogene. The apatite fission track indicates that some areas remained in the partial annealing range and experienced slow uplifting and cooling. Since the Neogene (20–15 Ma), these areas have gone through rapid uplifting and cooling to the surface temperature. The uplifting and denudation thickness of the Weiyuan Structure is generally greater than 4 000 m. (3) Staged rapid uplifting. With staged rapid uplifting, cooling-slow uplifting, rapid uplifting and cooling again, this type of uplifting is retained for a short time in apatite fission track annealing period compared to the whole. Especially during the Late Paleocene–Early Eocene (60–45 Ma) and Neogene (20 Ma), rapid uplifting cooling and annealing occurred with an average denudation thickness of 2 000 m (Deng et al., 2009).
The confirmation of gas chimney effects of the non-hydrocarbons component, pressure coefficient and isotope geochemistry provide sufficient evidence of vertical gas leakage in the Dengying Formation. Intensive uplift and compression in the Himalayan has caused severe erosion in the Weiyuan area and formed the current Weiyuan Structure. The regional cap- rocks of the Jialingjing Formation and Leikoupo Formation gypsum rock in the Sichuan Basin have been denudated by substantial uplift. Therefore, seal strength and trap closure the capabilities in the Weiyuan Structure have been significantly reduced because the sedimentary cover is unable to seal the Dengying gas reservoir. With an ineffective seal capability, the slow gas leakage of the Dengying gas reservoir has formed a pipe gas chimney. Meanwhile, the uplift and denudation has developed several large-scale faults from the basement to the Permian with some faults leading up to the shallow strata. With the outcropped regional cap-rock, the faults and fractures have accelerated the communication of gas migration vertically and formed a complete accumulation system with a uniform pressure coefficient from the bottom to the top in the Weiyuan Structure. Therefore, a seal-failure is associated with a faults- fractures gas chimney in the Weiyuan Structure (Fig. 12).3.2 New Model of Differential Accumulation—Evolution from the Paleo-Gas Reservoirs of the Dengying Formation (Gas Storage Center) to Present Gas Reservoirs (Gas Preservation Center)
In 1954, the hydrocarbon differential entrapment theory regarding traps that formed from gradual elevation variation of spill points with genetic relations was first proposed (Gussow, 1954). Based on this theory, further studies presented evidence that the effects of seal strength and trap closure are fundamental to the controlling of hydrocarbon accumulation, dissipation and distribution and that traps can be divided into three types (Sales, 1997). In recent years, several researchers have presented the hydrocarbon migration, accumulation and dissipation model in uplifted areas (Ohm et al., 2008). Through the in-depth study of gas reservoirs in the Sinian Dengying Formation of the Gaoshiti- Moxi area, it is possible to show the genetic relationship among the petroleum reservoirs of the Sinian Dengying Formation in the Ziyang, Jinshi, Weiyuan, Gaoshiti, Moxi, Longnüsi and Guangʼan in the Sichuan Basin with transformation mechanisms of paleo-gas reservoirs (gas storage center) to present gas reservoirs (gas preservation center) (Liu et al., 2012b). Based on the current exploration results of the Sinian Dengying Formation in the Sichuan Basin, the traps of the Dengying Formation, such as the Guangʼan, Longnüsi, Moxi, Gaoshiti and Weiyuan, are gradually uplifting spill points during regional uplifting of the Himalayan period. The gas migration and distribution might have followed the hydrocarbon differential migration and entrapment theory presented by the researchers mentioned above. However, the process of evolution has a great effect (Liu et al., 2015b) (Fig. 13).
Based on a comprehensive analysis of all the references, differential oil and gas entrapment requires four conditions: (1) abundant oil and gas sources in the regional downdip direction, (2) a good regional migration pathway that allows long-distance migration of oil and gas, (3) a series of contiguous traps in the regional background with a spill point ascending in the updip direction, and (4) the reservoirs are filled with static formation water.
The reasons that the gas of the Dengying Formation accumulated and migrated over a basin-wide long distance are the following: (1) the gas source charging the Dengying Formation is sufficient (i.e., an overfilled or over-supplied basin). The gas source rocks of the Dengying Formation include the Upper- Sinian Doushantuo Formation, the 3rd member of Dengying and the Lower-Cambrian Qiongzhusi Formation. These source rocks have an extensive distribution, are thick and have a high content of organic matter. The Qiongzhusi Formation has the best source rocks in the Sichuan Basin (Liu et al., 2013). Currently, the gas of the Dengying Formation is mostly from cracking of crude oil under high temperature (Sun et al., 2010, 2007; Liu et al., 2008b), and the distribution of the reservoir bitumen shows that in a large region, the paleo-oil reservoirs provided sufficient gas for the Dengying Formation itself (Liu et al., 2012b). (2) During the Himalayan period, from NE to SW, there are a series of structural traps that gradually ascend in spill point elevation that formed in the Guangʼan, Longnüsi, Gaoshiti-Moxi and Weiyuan areas. (3) The main migration pathway of the unconformity surface on the top of the Deng- ying Formation not only has well-developed pores, cavities and good permeability, but also stably distributes across the Sichuan Basin (Song et al., 2013).
Thus, the differential gas accumulation and distribution of the Dengying Formation has the following differences compared to the Sichuan Basin. (1) Different initial driving force: Rather than abundant gas sources in the downdip direction, the large-scale long-distance migration of the Denying Formation natural gas is driven by natural gas leak-off and escape from the Weiyuan Structure. (2) Different initial fluid in traps: before the large-scale and long-distance migration, the traps of the Dengying Formation were filled with gas rather than formation water with possible gas fullness of nearly 100%. (3) Different results of gas differential accumulation: classical differential oil and gas entrapment theory shows that the source rocks continuously generated and expelled oil and gas. As a result, there should be more and more gas in traps. In contrast, when large-scale and long-distance gas migration and differential accumulation happened in the Dengying Formation, the source rocks (the Dengying Formation and the upper Qiongzhusi Formation) had almost stopped hydrocarbon generation and expulsion, and as gas leaked off from the top of the Weiyuan Structure, the gas in the Dengying Formation continuously decreased (Liu et al., 2015b).3.3 Relationship between Gas Chimneys and Hydrocarbon Accumulation (Dengying Formation) in the Sichuan Basin
Therefore, it can be concluded that the long distance gas migration, adjustment and dissipation of the Dengying Formation still occurs, and the gas reservoirs of the Dengying Formation are still the dynamic formation in the Sichuan Basin. With uplifting and erosion, a "skylight" for gas leakage has been generated at the top of the Weiyuan Structure. Therefore, the Weiyuan Structure is the main escape pathway for the Deng- ying Formation gas in the middle and west Sichuan Basin.
The gas seepage in the Weiyuan structural core is connected with the dynamic adjustment of the pressure coefficient, fluid characteristics and reserves scale in the regional Dengying Formation (Fig. 14). However, at the highest trap of the differential accumulation in the Dengying Formation, Sichuan Basin, the Weiyuan Structure is a giant dome anticline with a large closure area and height such that the regional gas of the Dengying Formation could migrate along the Tongwan unconformity to the Weiyuan Structure. This is the most important reason why the seal strength of the Weiyuan Structure Deng- ying Formation is poor with only 26.4% abundance.4 CONCLUSION
The confirmation of a gas chimney indicates a disabled seal system and poor preservation condition in the Weiyuan Structure. This result explains why the Dengying gas reservoir only has low abundance even with the giant scale of the Weiyuan Structure.
The Jialingjiang Formation and Leikoupo Formation gypsum rocks are the most important cap-rocks of the Sichuan Basin. With substantial uplifting and intensive denudation, the seal strength of the Weiyuan Structure was significantly reduced, and a seal-invalid gas chimney was developed.
The gas differential accumulation and distribution in the Sinian Dengying Formation, Sichuan Basin is consistent with the classical differential oil and gas entrapment theory in respect to migration and accumulation force, homogeneous conducting system and unified hydrodynamic force system. However, the most significant difference from the classical theory lies in the initial source force of gas migration and accumulation, which is differential migration caused by the evolution from the paleo-gas reservoirs of the Dengying Formation (gas storage center) to the current gas reservoirs (gas preservation center) itself.
Acting as the top trap of gas differential accumulation in the Dengying Formation, Sichuan Basin, the Weiyuan Structure forms a "skylight" of the Dengying gas reservoir. In vertical gas migration, complicated geological evolution has evoked large dynamic adjustment in pressure coefficient, fluid characteristics and reserves scale. From the perspective of hydrocarbon migration and accumulation in a late stage, the latest geological significances of the Weiyuan Structure can be instructive for deep marine carbonate oil and gas explorations in the Sichuan Basin.ACKNOWLEDGMENTS
This study was financially supported by the Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, the 973 Program of China (No. 2012CB214805), the SINOPEC research project (No. P16109) and the National Science and Technology Major Project of China (No. 2017ZX05005003-007). The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0962-y.
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