Journal of Earth Science, 2016 online ISSN X Printed in China DOI: /s x

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1 Journal of Earth Science, 2016 online ISSN X Printed in China DOI: /s x Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution in Sinian Dengying Formation- Cambrian Longwangmiao Formation, Gaoshiti-Moxi Structure, Sichuan Basin Juan Wu 1, 2, Shugen Liu* 1, Guozhi Wang 1, Yihua Zhao 3, Wei Sun 1, Jinming Song 1, Yanhong Tian 1 1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu , China 2. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan , China 3. Chuanzhong Division of Southwest Oil& Gasfield Company, PetroChina, Suining , China ABSTRACT: Sichuan Basin is a typical superimposed basin, which experienced multi-phase tectonic movements, meanwhile Sinian-Cambrian underwent complex hydrocarbon accumulation processes, causing exploration difficulties in the past 50 years. Based on the microscopic evidence of fluid inclusions, combined with basin-modelling, this paper determines stages and time of hydrocarbon accumulation, reconstructs evolution of formation pressure and dynamic processes of hydrocarbon accumulation in Sinian Dengying Formation-Cambrian Longwangmiao Formation of Gaoshiti-Moxi structure. Three stages of inclusions are detected, including a stage of yellow-yellowgreen fluorescent oil inclusions, a stage of blue fluorescent oil-gas inclusions and a stage of non-fluorescent gas inclusions, reflecting the study area has experienced a series of complex hydrocarbon accumulation processes, such as formation of paleo-oil reservoirs, cracking of crude oil, formation of paleo-gas reservoirs and adjustment to present gas reservoirs, which occurred during Ma, Ma and Ma respectively. During the period of crude oil cracking, Dengying Formation-Longwangmiao Formation showed weak overpressure to overpressure characteristics, then after adjustment of paleo-gas reservoirs to present gas reservoirs, the pressure in Dengying Formation changed into overpressure but finally reduced to normal pressure system. However, due to excellent preservation conditions, the overpressure strength in Longwangmiao Formation only slightly decreased and was still kept to this day. KEY WORDS: Sichuan Basin, Gaoshiti-Moxi structure, Sinian-Cambrian, hydrocarbon accumulation, formation pressure, fluid inclusion. 0 INTRODUCTION Oil and gas exploration of Sinian-Cambrian in Sichuan Basin began in the nineteen fifties, it has been the major explorative horizon of lower assemblage. Since the discovery of Weiyuan gas field in 1964, the exploration had been at a standstill for a long time. After years of arduous exploration, a giant gas field was discovered in Sinian-Cambrian of Gaoshiti-Moxi region in 2013, which once again confirmed that the layer has tremendous potential (Zou et al., 2014; Du et al., 2014). As a typical superimposed basin, Sichuan Basin experienced multi-phase tectonic movements, while *Corresponding author: lsg@cdut.edu.cn China University of Geosciences and Springer-Verlag Berlin Heidelberg 2016 Sinian-Cambrian underwent complex hydrocarbon accumulation processes with early forming time, multi-stage hydrocarbon generation and high formation pressure, causing exploration difficulties in the past 50 years (Liu et al., 2009; Liu et al., 2012). Therefore, illustrating the hydrocarbon accumulation process of Sinian Dengying Formation-Cambrian Longwangmiao Formation in Gaoshiti-Moxi area is of great importance both to hydrocarbon accumulation theory of lower assemblage and exploration of new areas. During the past decades, the theory of hydrocarbon accumulation characteristics and process of Sinian-Cambrian in Central Sichuan Basin has developed in a variety of aspects. Although many researchers have conducted in-depth studies on fluid properties, fluid filling features and hydrocarbon accumulation mechanism (Wei et al., 2014; Zheng et al., 2014; Wu et al., 2014; Yuan et al., 2014; Wang et al., 2014; Liu et al., 2015), they have not systematically expounded the dynamic process of hydrocarbon accumulation. According to the Wu, J., Liu, S. G., Wang, G. Z., et al., Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution in Sinian Dengying Formation-Cambrian Longwangmiao Formation, Gaoshiti-Moxi Structure, Sichuan Basin. Journal of Earth Science.doi: /s x.

2 Juan Wu, Shugen Liu, Guozhi Wang, Yihua Zhao, Wei Sun, Jinming Song, Yanhong Tian petroleum geologic features of Sichuan Basin, establishing relation between fluid activity and hydrocarbon accumulation is an important way to recognize the hydrocarbon accumulation process of Sinian-Cambrian. Based on the microscopic evidence of fluid inclusions, combined with basin-modelling, this paper determines stages and time of hydrocarbon accumulation, reconstructs evolution of formation pressure and dynamic processes of hydrocarbon accumulation in Sinian Dengying Formation-Cambrian Longwangmiao Formation of Gaoshiti-Moxi structure. 1 GEOLOGICAL SETTINGS Central Sichuan paleo-uplift, located in the middle of Sichuan Basin, is a large inheritance ancient uplift, which comprises a plurality of structures such as Gaoshiti, Moxi, Ziyang and Weiyuan. Gaoshiti-Moxi structure lies at the eastern end of the paleo-uplift, between Anyue, Suining, Tongnan geographically (Fig. 1). Central Sichuan paleo-uplift experienced multi-phase tectonic movements (Zou et al., 2014; Yuan et al., 2014). In the Late Sinian Epoch, the paleo-uplift had already preliminarily prototype when the central Sichuan region was tectonic uplifting. In the Early Cambrian Epoch, structural feature of the paleo-uplift strengthened and the core was on the west of Chengdu Longmen Mountain. At the end of the Silurian Period, Caledonian movement took place so that the paleo-uplift stereotyped and Silurian in the uplift was eroded thoroughly. During Hercynian to early Yanshanian, the paleo-uplift evolved continuously and buried constantly. From Late Yanshanian to Himalayan, under the influence of rapid uplifting of Weiyuan structure, the western part of the paleo-uplift experienced intense tectonic deformation, whereas the eastern part deformed weakly. According to the apatite fission track analysis, the uplifting and erosion thickness of Weiyuan anticline was generally greater than 4000 m since the Late Cretaceous Epoch. Similarly, samples taken from Moxi area indicate that the uplifting and erosion thickness there was m. The maximum eroded amplitude of Weiyuan area was 2850 m higher than that of Gaoshiti-Moxi area (Liu et al., 2014; Liu et al., 2015), and therefore the natural gas storage conditions of eastern part was superior to western part, making Gaoshiti-Moxi-Longnvsi area become the most favorable region for gas migration and accumulation. Figure 1. Location map of study area showing structures and wells where samples were taken (modified from Jin et al., 2014). 2 MATERIAL AND METHODS Fluid inclusion samples used in the experiments were taken from drilling core in Cambrian Longwangmiao Formation, Sinian Deng4 Member and Deng2 Member of Gaoshiti-Moxi structure, which are composed of microcrystalline to fine crystalline dolarenite with dissolved pores and vugs. Multi-stage fluid filling in holes is visible both in hand specimens and under microscope, i.e. grain dolomite, bitumen, euhedral quartz or dolomite filled from the edge to the center successively (Fig. 2). An enormous amount of research effort have gone into generations of dolomite in Cambrian and Sinian (Wang et al., 2014; Yuan et al., 2014; Liu et al., 2014), so it will not be discussed again in this paper. Based on the mineral filling sequence in pores of reservoirs, we shall carry out systematic observation and test on fluid inclusions, which were caught in host dolomite matrix, early dolomite, late dolomite or quartz. A petrographic microscope attached to an ultra-violet (UV) fluorescence system (Tseng and Pottorf, 2002) is usually used to identify hydrocarbon inclusions, which can be divided into different types by their occurrence, fluorescence and phase. Laser Raman spectroscopy could be used to

3 Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution d slopes, then they should intersect at some point in the P-T phase diagram, which gives the trapping temperature and pressure (Chen et al., 2002; Lu et al., 2004). The steps of reconstructing isochore equation for petroleum inclusion can be concluded as follows: Obtain the composition of hydrocarbon inclusion by decrepitation or other methods, combined with the vapor liquid ratio under normal room temperature, simulation software is used to build the P-T phase diagram, and test the homogenization temperature of aqueous inclusion as the lowest trapping temperature of hydrocarbon inclusion to estimate the minimum trapping pressure of hydrocarbon inclusion, which approximates the formation pressure when the hydrocarbon inclusion was captured. Methane inclusions can be also used for pressure simulation, especially in rich natural gas basins. Choose some high purity and single component methane inclusions to test their homogenization temperatures, then estimate their density according to the formula or thermodynamic parameter list. At the same time, homogenization temperatures of the symbiotic aqueous inclusions are measured and the peak value is got as the trapping temperature of methane inclusions, so that the trapping pressure of methane inclusions under this temperature can be calculated in accordance with the thermodynamic equation (Li et al., 2013; Wang et al., 2014). U ne di te research different phase components of fluid inclusions (Dubessy et al., 2001; Chen et al., 2005; Ni et al., 2006). Homogenization temperatures of the coeval hydrocarbon and aqueous inclusions are measured under a precision heating and freezing stage. Based on the homogenization temperatures, combined with burial-geothermal history, the capture time of inclusions can be determined, thus indirectly reflecting the reservoir forming time (Chen, 2007; Chen et al., 2013). The stratigraphic erosion thickness used in the basin modelling is calculated by the fission track analysis results of the samples taken from Moxi area (Liu et al., 2015). Paleo-pressure simulation is a tedious but very important task in the study of pressure in sedimentary basins (Chen et al., 2006; Li et al., 2010; Liu et al., 2010). Petroleum inclusions and aqueous inclusions trapped simultaneously should have the same trapping temperature and pressure, i.e. the geological paleo-temperature and paleo-pressure when the inclusions were trapped. In recent years, PVTsim, VTflinc, PIT and other software is widely used to estimate the trapping condition of hydrocarbon inclusion (Aplin et al., 2000; Liu et al., 2003; Wang et al., 2008; Ping et al., 2013). A fluid inclusion can be considered as a closed system, the P-T conditions within an inclusion are constrained to fall on an isochore, of which the equations for hydrocarbon petroleum and aqueous inclusions have different Figure 2. Fluid filling sequence of Dengying-Longwangmiao reservoirs in Gaoshiti-Moxi structure. (a) Core photo of Deng 4 Member from Well GS1; (b) photo of thin section of Longwangmiao Formation from Well MX202, RESULTS 3.1 Characteristics of Fluid Inclusion Microscopic features Fluid inclusions in Dengying Formation and Longwangmiao Formation of Gaoshiti-Moxi structure are 2-14μm in long dimension. The gas-liquid ratio is between 4%-20%, with variable but typically angular shapes. Hydrocarbon inclusions and non-hydrocarbon ones can be effectively distinguished by their petrographic and fluorescence characteristics. In all samples, five types of hydrocarbon inclusions were determined in observation, i.e. pure oil inclusions, pure gas inclusions, oil-gas inclusions, oil (gas)-aqueous inclusions and asphalt inclusions. According to their occurrence, fluorescence and phase, the hydrocarbon inclusions can be divided into three types. Type I inclusions, mostly found in very finely crystalline dolomite in matrix of Longwangmiao Formation (rarely in Dengying Formation), are dominantly liquid-phase, with yellow to yellowgreen fluorescence (Figs. 3a, 3b). Type Ⅱ inclusions, found in early medium-coarsely crystalline dolomite in pores, consist of blue fluorescent oil and gas, which has slightly higher maturity than the yellowgreen fluorescent oil (Figs. 3c, 3d). Type Ⅲ inclusions have been observed mainly in late dolomite or quartz in pores, some also distribute in matrix or earlier dolomite, these are pure gas inclusions without fluorescence (Figs. 3e, 3f). According to the diagenetic phases of inclusion host minerals, type I inclusions were trapped earlier than type Ⅱ, and type Ⅲ inclusions represent the latest capture, when crude oil in reservoirs extensively cracked.

4 ISSN X te d Journal of Earth Science, 2016 online Printed in China DOI: /s x Composition Characteristics According to the composition determined by laser Raman, the Raman peak positions for most hydrocarbon inclusions in late quartz are between 2905 cm-1 and 2915 cm-1, indicating that they should mainly consist of CH4 U ne di Figure 3. Different types of hydrocarbon inclusions in the testing samples from Dengying-Longwangmiao reservoirs in Gaoshiti-Moxi structure. (a), (c), (e), (f) Images taken under transmitted light, (b) and (d) images taken under fluorescent light. (a) and (b) Dark brown oil inclusions with yellowgreen fluorescence hosted by very finely crystalline dolomite in Longwangmiao Formation, Well MX23, m; (c) and (d) tan oil-gas inclusions with blue fluorescence hosted by coarsely crystalline dolomite in Longwangmiao Formation, Well MX23, m; (e) gray gas inclusions hosted by very finely crystalline dolomite in Longwangmiao Formation, Well GS10, m; (f) gray gas inclusions hosted by quartz in Deng 4 Member, Well MX21, m. (Dubessy et al., 2001; Chen et al., 2005; Lu et al., 2007; Liu et al., 2009; Song et al., 2009). CH4 generally exists in pure hydrocarbon inclusions and gas-aqueous inclusions, small amounts of CO2, H2S, SO2 or slight other constituents are also detected in individual inclusions (Fig. 4). Figure 4. Raman spectra of fluid inclusions in quartz of Dengying-Longwangmiao reservoirs in Gaoshiti-Moxi structure. (a) Gas-liquid inclusion in Deng 4 reservoir from Well GS1; (b) pure liquid inclusion in Deng 4 reservoir from Well GS1; (c) gas-liquid inclusion in Longwangmiao reservoir from Well MX12; (d) pure liquid inclusion in Longwangmiao reservoir from Well MX17. Wu, J., Liu, S. G., Wang, G. Z., et al., Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution in Sinian Dengying Formation-Cambrian Longwangmiao Formation, Gaoshiti-Moxi Structure, Sichuan Basin. Journal of Earth Science.doi: /s x.

5 Journal of Earth Science, 2016 online ISSN X Printed in China DOI: /s x Homogenization temperatures Homogenization temperatures of aqueous inclusions in study area mainly range from 77.2 to 210.8, which can be divided into three stages, respectively coexistence with pure oil inclusions, oil-gas inclusions, pure gas inclusions (Table 1, Fig. 5). The first stage aqueous inclusions in Longwangmiao Formation have a low temperature range of , with a peak temperature value of The second stage have a higher temperature range of , with a peak temperature value of The third stage have the highest temperature range of , but with the same peak temperature as the second stage (Table 1, Fig. 5a) Similarly, the homogenization temperatures of three stage inclusions in Dengying Formation respectively range of , and Each peak temperature of Dengying Formation is 10 higher than that of Longwangmiao Formation (Table 1, Fig. 5b). Table 1 Stages and characteristics of fluid inclusions in samples from Dengying-Longwangmiao reservoirs in Gaoshiti-Moxi structure. (a) Samples from Longwangmiao reservoirs; (b) samples from Dengying reservoirs. Formation Well Depth (m) Host Mineral Longwangmiao Wu, J., Liu, S. G., Wang, G. Z., et al., Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution in Sinian Dengying Formation-Cambrian Longwangmiao Formation, Gaoshiti-Moxi Structure, Sichuan Basin. Journal of Earth Science.doi: /s x. Size (μm) gas-liquid ratio (%) Homogenization Temperatures ( ) GS coarsely crystalline dolomite Ⅱ GS finely crystalline dolomite Ⅰ finely crystalline dolomite Ⅱ finely crystalline dolomite Ⅲ GSH finely crystalline dolomite Ⅲ MX coarsely crystalline dolomite Ⅱ MX very finely crystalline dolomite Ⅱ MX MX Deng 4 MX very finely crystalline dolomite Ⅰ coarsely crystalline dolomite Ⅱ very finely crystalline dolomite Ⅲ very finely crystalline dolomite Ⅰ coarsely crystalline dolomite Ⅱ very finely crystalline dolomite Ⅰ coarsely crystalline dolomite Ⅱ Stage quartz Ⅲ Figure 5. Distribution of homogenization temperatures of coeval hydrocarbon and aqueous inclusions in Dengying-Longwangmiao reservoirs in Gaoshiti-Moxi structure. 3.2 Stages and Time of Hydrocarbon Accumulation The 45 fluid inclusions in the samples from m to m in Well GS11 can be divided into three stages, including a stage of yellow fluorescent oil inclusions, a stage of blue fluorescent oil-gas inclusions and a stage of non-fluorescent gas inclusions, of which the homogenization temperatures respectively range of , , Integrated with the burial-geothermal history, the reservoir-forming time is about Ma, Ma, Ma, respectively (Fig. 6). According to test result of all fluid inclusion samples, Sinian Dengying Formation-Cambrian Longwangmiao Formation of Gaoshiti-Moxi structure has experienced three stages of hydrocarbon accumulation (Fig. 6). In the first stage, yellow fluorescent oil inclusions with low homogenization temperatures were captured, which suggests that the process

6 Juan Wu, Shugen Liu, Guozhi Wang, Yihua Zhao, Wei Sun, Jinming Song, Yanhong Tian of the first stage accumulation was mainly connected with the initial primary oil generation and paleo-oil reservoirs formation during the mature period of source rock from the Middle Triassic Epoch to the Early Jurassic Epoch ( Ma). In the second stage, oil-gas inclusions with blue fluorescence and higher homogenization temperatures were trapped in the dolomite, indicating that the process of the second stage accumulation was mainly related to the generation of light oil and wet gas, and furthermore, part of the crude oil cracked into gas during the Jurassic Period ( Ma). Gas inclusions with the highest homogenization temperatures were caught in the third stage, when the source rock had reached an over-mature stage since the Late Jurassic Epoch (168-0 Ma). During this time, crude oil in the paleo-oil reservoirs widely cracked into gas to form the paleo-gas reservoirs and reconstructed in late tectonic movements. Figure 6. Forming time of Dengying-Longwangmiao reservoirs in Gaoshiti-Moxi structure 3.3 Evolution of Formation Pressure Features of present pressure Sinian-Cambrian located in Sichuan Basin was deeply buried and experienced a process of paleo-oil reservoirs cracking into paleo-gas reservoirs under high temperature. Many scholars have calculated the volume of gas from oil cracking, the results show that 1 ton oil can generate m3 gas maximum (Baker,1990), thus forming a great amount of abnormal high pressure (overpressure) (Isaksen, 2004; Tian et al., 2008). The existing drilling results reveal that a considerable portion of the overpressure generated in the process of the paleo-oil reservoirs cracking into gas reservoirs has been preserved. Measured pressure data indicates that the present pressure coefficients in Longwangmiao Formation of Gaoshiti-Moxi area vary between 1.46 and 1.70, with average 1.64, belonging to overpressure range (Fig. 7). However, Deng 4 Member and Deng 2 Member, with pressure coefficients of under 1.20 and 1.11 on average, which are normal in pressure. According to the distribution characteristics of present pressure, the shallow buried Longwangmiao Formation is an overpressure system, while the deep buried Dengying Formation is a normal pressure system, which is independent of the other one. Figure 7. Longitudinal distribution of present pressure in the study area

7 Journal of Earth Science, 2016 online ISSN X Printed in China DOI: /s x Simulation of paleo-pressure Based on fluid inclusion analysis, using PVT simulation and methane inclusion thermodynamic simulation methods, we have calculated the pressure of Dengying Formation and Longwangmiao Formation during the process of paleo-oil reservoirs cracking into paleo-gas reservoirs and the time after paleo-gas reservoirs formed. Based on fluid inclusion analysis, using the VTflinc software, the second stage of inclusions with complete data were chosen to estimate the trapping pressure during the time when paleo-oil reservoirs cracked into paleo-gas reservoirs. Take the inclusion samples from Longwangmiao Formation and Deng 4 Member for example, a inclusion with homogenization temperature of in Longwangmiao Formation was simulated to get the trapping pressure of 55.1 MPa, combined with the burial-geothermal history, we estimate the ancient buried depth is 4200 m and the corresponding time is 177 Ma. Therefore, the pressure coefficient at this time was 1.34, which is the ratio of trapping pressure to hydrostatic pressure (Fig. 8, Table 2). In the same way, the inclusion with homogenization temperature of in Deng 4 Member was captured at 163 Ma, when the pressure coefficient was Results of PVT simulation indicate that during the early stage of paleo-oil reservoirs to the paleo-gas reservoirs ( Ma), the pressure coefficients of Longwangmiao Formation and Dengying Formation were , which belonged to weak overpressure-overpressure system. On the basis of microscopic observation, fluid inclusions in late quartz and dolomite that filled the dissolved holes in Dengying Formation and Longwangmiao Formation are mainly rich in the methane. Samples collected from m to m in Deng 4 Member of Well GS1 have methane inclusions in quartz with homogenization temperatures between -87 and -84.7, so the methane densities are calculated to be , which belong to high-density methane. The homogenization temperatures of aqueous inclusions associated with methane inclusions range from 190 to 210, with a peak value of 200, combined with the burial-geothermal history, the buried depth is estimated to be 4200 m and the trapping time is 177 Ma. It is calculated that the trapping pressure of methane inclusions varies from 79.4 MPa to MPa and the corresponding pressure coefficients are , which can be taken as weak overpressure-overpressure (Fig. 9, Table 2). The inclusions in the quartz samples from m to m in Deng 4 Member of Well GS1 are calculated to be captured at 23 Ma when the pressure coefficients were , which is abnormal high pressure, indicating that Deng 4 Member was an overpressure system before 20 Ma. However, the pressure relief has took place during rapid uplifting and erosion phase since 20 Ma, resulting a normal pressure system in Deng 4 Member presently. Different from it, inclusions in the samples taken from m to m in Longwangmiao Formation of Well MX17 was at 90 Ma. The Longwangmiao Formation had obvious overpressure features before the paleo-gas reservoir adjustment. According to the thermodynamic simulation results of methane inclusions, in the early adjustment process of paleo-gas reservoirs, Dengying Formation was a weak overpressure system while Longwangmiao Formation was overpressure environment. Figure 8. PVT simulation of paleo-pressure in Deng 4 Member and Longwangmiao Formation in Well MX21 Wu, J., Liu, S. G., Wang, G. Z., et al., Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution in Sinian Dengying Formation-Cambrian Longwangmiao Formation, Gaoshiti-Moxi Structure, Sichuan Basin. Journal of Earth Science.doi: /s x.

8 Juan Wu, Shugen Liu, Guozhi Wang, Yihua Zhao, Wei Sun, Jinming Song, Yanhong Tian Figure 9. Thermodynamic simulation of paleo-pressure in Deng 4 Member and Longwangmiao Formation in Well GS1 Table 2 Data of paleo-pressure simulation of Dengying-Longwangmiao Formation in Gaoshit-Moxi structure Formation Well Depth(m) Stage Longwangmiao Deng 4 Homogenization temperature ( ) Buried Depth (m) Trapping age (Ma) Trapping pressure (Mpa) Paleo-pressure coefficient GS Ⅱ MX Ⅱ MX Ⅱ MX Ⅲ MX Ⅱ GS Ⅲ GS Ⅲ DISCUSSION Controlled by the high geothermal background and multi-phased tectonic activities of Sichuan Basin, Sinian Dengying Formation-Cambrian Longwangmiao Formation in Gaoshiti-Moxi structure experienced a series of complex hydrocarbon accumulation processes, including formation of paleo-oil reservoirs, cracking of crude oil, formation of paleo-gas reservoirs and adjustment to present gas reservoirs (Fig. 10). 4.1 Formation of Paleo-Oil Reservoirs The Cambrian Qiongzhusi source rock entered oil window(ro>0.5%) and produced a small amount of oil in the Silurian Period. In the end of the Silurian Period, subject to uplifting in the Caledonian tectonic movement, hydrocarbon generation broke off for a period of time, but restarted with strata burying again in the Permian period. During the Triassic Period, source rock entered the middle-mature stage (0.7%<Ro<1.3%), as the buried depth increased rapidly and plenty of crude oil was generated till the Early Jurassic. With the crude oil migrating into the reservoirs, a stage of oil inclusions were captured in microcrystalline dolomite of Dengying Formation and Longwangmiao Formation. These inclusions mainly consisted of low molecular weight hydrocarbon with yellowgreen fluorescence. At the same time, reservoir pressure increased, contributing to a weak overpressure system in Longwangmiao Formation with pressure coefficient of about Cracking of Crude Oil During the Jurassic Period, the source rock entered into high-mature stage (1.3%<Ro<2.0%), generating light oil and wet gas. Due to high temperature, the kerogen and crude oil thermally cracked into gas so that liquid hydrocarbon sharply reduced and the remaining oil was less of resin and asphaltene. At the same time, a stage of gas-liquid two-phase hydrocarbon inclusions with blue fluorescence were captured in Dengying and Longwangmiao reservoirs. Substantial gas generation caused reservoir pressure greatly increasing. In early cracking period, the pressure coefficients of Longwangmiao Formation had reached more than 1.46, which belonged to overpressure, while the value of Dengying Formation was around 1.30, representing weak overpressure system.

9 Journal of Earth Science, 2016 online ISSN X Printed in China DOI: /s x Figure 10. Reservoir forming periods and accumulation process of Dengying-Longwangmiao Formation in Gaoshiti-Moxi Structure 4.3 Formation of Paleo-Gas Reservoirs and Adjustment to Present gas reservoirs Up to the Late Jurassic Epoch, the source rock had already become into over-mature (Ro>2.0%), thus the kerogen and crude oil substantially cracked into dry gas and bitumen till the Late Cretaceous Epoch (Fig. 2). In this period, a stage of non-fluorescent gas inclusions mainly consisted of methanewere captured in the Dengying and Longwangmiao reservoirs. The reservoir pressure increased rapidly with the formation of thermal-craking gas. According to the results of inclusion pressure simulation, the pressure coefficients of Dengying and Longwangmiao reservoirs had reached more than 2.0 before the tectonic uplifting in the Late Cretaceous Epoch, showing strong overpressure features. When Himalaya tectonic movement started, hydrocarbon generation broke off as a result of the tectonic uplifting and the temperature decreasing. In Early Himalayan stage, with formation of Ziyang structure, the high position of Gaoshiti-Moxi structure moved from Moxi area to Gaoshiti area. Additionally, gas in Dengying reservoirs migrated laterally from Gaoshiti area to Ziyang area, resulting in decrease of the pressure coefficient (1.46). Subsequently, gas in Guang'an-longnvsi area of the eastern Central Sichuan paleo-uplift migrated along the unconformity surface on the Wu, J., Liu, S. G., Wang, G. Z., et al., Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution in Sinian Dengying Formation-Cambrian Longwangmiao Formation, Gaoshiti-Moxi Structure, Sichuan Basin. Journal of Earth Science.doi: /s x.

10 Juan Wu, Shugen Liu, Guozhi Wang, Yihua Zhao, Wei Sun, Jinming Song, Yanhong Tian top of Dengying Formation and accumulated in the high position of Moxi-Gaoshiti structure, causing the pressure coefficient of Dengying Formation increase again (1.67). However, Since 20 Ma, as the uplifting intensified gradually and the strata overlying the top of Weiyuan area were eroded rapidly, natural gas of the Dengying Formation leaked off and escaped from the structural window of Lower Triassic Jialingjiang Formation in Weiyuan structure, leading to the conversion of overpressure to normal pressure in Dengying Formation. Paleo-gas reservoirs in Longwangmiao Formation also experienced a series of structural adjustments, but due to the lack of internal migration pathways (Luo et al., 2015; Ma et al., 2015), the gas did not experience large-scale lateral migration, so pressure only slightly decreased and the overpressure system was still kept to present. 5 CONCLUSIONS Three stages of fluid inclusions have been detected in reservoirs of Sinian Dengying Formation and Cambrian Longwangmiao Formation in Gaoshiti-Moxi structure, including a stage of yellow-yellowgreen fluorescent oil inclusions, a stage of blue fluorescent oil-gas inclusions and a stage of non-fluorescent gas inclusions. The homogenization temperatures of these three stage inclusions in Longwangmiao Formation range of , and respectively, while the ones of Dengying Formation range of , and , respectively. The study area has experienced a series of complex hydrocarbon accumulation processes, such as formation of paleo-oil reservoirs, cracking of crude oil, formation of paleo-gas reservoirs and adjustment to present gas reservoirs, which took place in Ma (Middle Triassic-Early Jurassic), Ma (Jurassic), Ma (Middle Jurassic to now) respectively. During the Permian Period to the Early Jurassic Epoch, large quantities of crude oil was generated and formed the weak overpressure paleo-oil reservoirs. Furthermore, a stage of oil inclusions were captured in Dengying and Longwangmiao reservoirs. After entering the Jurassic period, the source rock began to generate light oil and wet gas. Additionally, a part of the crude oil in paleo-reservoirs cracked into gas and bitumen, causing the reservoir pressure obviously increased to show weak overpressure-overpressure characteristics. At the same time, a stage of gas-liquid two-phase hydrocarbon inclusions were captured. Up to the Late Jurassic Epoch, with high temperature, the kerogen and crude oil substantially cracked into methane and bitumen till the Late Cretaceous Epoch, so that the formation pressure rapidly increased from week overpressure to strong overpressure, which was recorded by a stage of pure gas hydrocarbon inclusions. Since Himalayan movement started, Dengying reservoirs have experienced a series of adjustments to form the present normal pressure gas reservoirs. In contrast, owing to the favorable preservation condition, the pressure of Longwangmiao reservoirs only slightly decreased and the overpressure features were still kept to this day. ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (No ), the 973 Program of China (No. 2012CB214805), the China Postdoctoral Science Foundation (No. 2014M552327), and the research grant from the Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences (No. TPR ). We thank the editors and reviewers for their constructive comments. REFERENCES CITED Aplin, A. C., Larter, S. R., Bigge, M. A., et al., PVTX History of the North Sea's Judy Oilfield. Journal of Geochemical Exploration, 69-70: Barker, C., Calculated Volume and Pressure Changes During the Thermal Cracking of Oil to Gas in Reservoirs. AAPG Bulletin, 74(8): Chen, G., Li, S. H., Zhang, H. R., et al., Fluid Inclusion Analysis for Constraining the Hydrocarbon Accumulation Periods of the Permian Reservoirs in Northeast Ordos Basin. Journal of Earth Science, 24(4): Chen, H. 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11 Multi-Stage Hydrocarbon Accumulation and Formation Pressure Evolution Li, J., Zha, M., Determination of Oil Accumulation Period and Building Up of Paleopressure of Wumishan Formation in Renqiu Oilfield by Using Fluid Inclusion. Journal of China University of Petroleum, 34(4): (in Chinese with English Abstract) Liu, D. H., Dai, J. X., Xiao, X. M., et al., High Density Methane Inclusions in Puguang Gasfield: Discovery and A T-P Genetic Study. Chinese Science Bulletin, 54: Liu, D. H., Xiao, X. M., Mi, J. K., et al., Determination of Trapping Pressure and Temperature ofpetroleum Inclusions Using PVT Simulation Software-A Case Study of Lower Ordovician Carbonates From the Lunnan Low Uplift, Tarim Basin. Marine and Petroleum, 20(6): Liu, D. H., Xiao, X. M., Tian, H., et al., Identification of Natural Gas Origin Using the Characteristics of Bitumen and Fluid Inclusions. Petroleum Exploration and Development, 36(3): (in Chinese with English Abstract) Liu, S. G., Huang, W. M., Jansa, L. F., et al., Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation, East Sichuan Basin, China. Acta Geologica Sinica-English Edition, 88(5): Liu, S. G., Ma, Y. S., Cai, X. Y., et al., Characteristic and Accumulation Process of the Natural Gas From Sinian to Lower Paleozoic in Sichuan Basin, China. Journal of Chengdu University of Technology: Science and Technology Edition, 33(4): (in Chinese with English Abstract) Liu, S. G., Qin, C., Sun, W., et al, The Coupling Formation Process of Four Centers of Hydrocarbon in Sinian Dengying Formation of Sichuan Basin. Acta Petrologica Sinica, 28(3): (in Chinese with English Abstract) Liu, S. G., Sun, W., Zhao, Y. H., et al., Differential Accumulation and Distribution of Natural Gas and Their Main Controlling Factors in the Upper Sinian Dengying Fm, Sichuan Basin. Natural Gas Industry, 35(1): (in Chinese with English Abstract) Liu, W., Wang, G. Z., Liu, S. G., et al., Characteristics and Geological Significance of Fluid Inclusions in Longwangmiao Formation of Moxi Structure in Central Sichuan, China. Journal of Chengdu University of Technology: Science and Technology Edition, 41(6): (in Chinese with English Abstract) Lu, H. Z., Fan, H. R., Ni, P., Fluid Inclusion. Science Press, Beijing (in Chinese) Lu, W. J., Chou I. M., Burruss, R. C., et al., A Unified Equation for Calculating Methane Vapor Pressures in the CH4-H2O System with Measured Raman Shifts. Geochimica et Cosmochimica Acta, 71: Luo, B., Zhou, G., Luo, W. J., Discovery from Exploration of Lower Paleozoic-Sinian System in Central Sichuan Palaeo-uplift and Its Natural Gas Abundance Law. China Petroleum Exploration, 20(2): (in Chinese with English Abstract) Ma, T., Tan, X. C., Li, L., Sedimentary Characteristics and Distribution of Grain Shoals in the Lower Cambrian Longwangmiao Formation of Sichuan Basin and Its Adjacent Areas. Journal of Palaeogeography, 17(2): (in Chinese with English Abstract) Ni, P., Ding, J. Y., Rao, B., In Situ Cryogenic Raman Spectroscopic Studies on the Synthetic Fluid Inclusions in Thesystems H2O and Nacl-H2O. Chinese Science Bulletin, 51(1): Ping, H. W., Chen, H. H., Thiéry, R., Thermodynamic Modeling of Petroleum Inclusions: Composition Modeling and Prediction of the Trapping Pressure of Crude Oils. Fluid Phase Equilibria, 346: Song, S. G., Su, L., Niu Y. L., et al., CH4 Inclusions in Orogenic Harzburgite: Evidence for Reduced Slab Fluids and Implication for Redox Melting in Mantle Wedge. Geochimica et Cosmochimica Acta, 73: Tian, H., Xiao, X. M., Wilkins, R. W. T., et al., New Insights into the Volume and Pressure Changes During the Thermal Cracking of Oil to Gas in: Implications for the in-situ Accumulation of Gas Cracked from Oils. AAPG Bulletin, 92(2): Tseng, H. Y. and Pottorf, R. J., Fluid Inclusion Constraints on Petroleum PVT and Compositional History of the Greater Alwyn-South Brent Petroleum System, Northern North Sea. Marine and Petroleum Geology, 19(7): Wang, G. Z., Liu, S. G., Liu, W., et al., Process of Hydrocarbon Accumulation of Sinian Dengying Formation in Gaoshiti Structure, Central Sichuan, China. Journal of Chengdu University of Technology: Science and Technology Edition, 41(6): (in Chinese with English Abstract) Wang, J. Z., Yang, S. W., Jiang, S. B., et al., Methods for Paleopressure Reconstruction Based on Thermodynamic Simulation Technologies of Fluid Inclusion and Some Problems Needing Attention. China Petroleum Exploration, 13(1): (in Chinese with English Abstract) Wei, G. Q., Wang, D. L., Wang, X. B., et al., Characteristics of Noble Gases in the Large Gaoshiti-Moxi Gas Field in Sichuan Basin. Petroleum Exploration and Development, 41(5): (in Chinese with English Abstract) Wu, J., Liu, S. G., Zhao, Y. H., et al., Fluid Characteristics of the Upper Sinian-Lower Cambrian Petroliferous Strata in Gaoshiti-Moxi Structure of the Central Sichuan Basin. Journal of Chengdu University of Technology: Science and Technology Edition, 41(6): (in Chinese with English Abstract) Yuan, H. F., Liu, Y., Xu, F. H., et al., The Fluid Charge and Hydrocarbon Accumulation, Sinian Reservoir, Anpingdian-Gaoshiti Structure, Central Sichuan Basin. Acta Petrologica Sinica, 30(3): (in Chinese with English Abstract) Yuan, H. F., Zhao, M. X., Wang, G. Z., et al., Phases of Hydrocarbon Migration and Accumulation in Cambrian

12 Juan Wu, Shugen Liu, Guozhi Wang, Yihua Zhao, Wei Sun, Jinming Song, Yanhong Tian Longwangmiao Formation of Moxi Structure, Central Sichuan, China. Journal of Chengdu University of Technology: Science and Technology Edition, 41(6): (in Chinese with English Abstract) Zheng, P., Shi, Y. H., Zou C. Y., et al., Natural Gas Sources in the Dengying and Longwangmiao Fms in the Gaoshiti-Maoxi Area, Sichuan Basin. Natural Gas Industry, 34(3): (in Chinese with English Abstract) Zou, C. N., Du, J. H., Xu, C. C., et al., Formation, Distribution, Resource Potential and Discovery of the Sinian-Cambrian Giant Gas Field, Sichuan Basin, SW China. Petroleum Exploration and Development, 41(3): (in Chinese with English Abstract)