Hydrocarbon Accumulation Mechanism of Sinian Reservoir in Anpingdian-Gaoshiti Structure, Middle Sichuan Basin

Basin formation, hydrocarbon generation and hydrocarbon accumulation are three main elements in the study of petroleum geology, and as a direct objective of exploration and development, hydrocarbon reservoir is the most important.The research on the formation process, space-time distribution of hydrocarbon reservoirs and geological controlling factors is the core issue in petroleum geology theory.Since1990s, several scholars have comprehensivized the application of geological, geophysical and sedimentary geochemistry as well as computer simulation technology to systematically study the conditions, mechanisms and interaction of hydrocarbon accumulation, and established new theories and models of hydrocarbon accumulation that have effectively guided hydrocarbon exploration.In 1990, Hunt put forward the"fluid compartment theory" (Hunt, 1990).In 1991, Magoon and Dow brought forward the"petroleum system conception" (Magoon and Dow, 1991) In 1996, Tian et al.enriched and developed the petroleum system theory by analyzing essentiality, possibilities, and means of hydrocarbon accumulation dynamic systems (Tian et al., 1996).In 2003, Jin et al.proposed the source-location based classification criteria and principles of the hydrocarbon accumulation system, and established the bursting model for hydrocarbon accumulation (Jin et al., 2003).And in 2006, 2004, Jin also proposed the"wave analysis method"to analyze the processes of"basin formationhydrocarbon generation-hydrocarbon accumulation" (Jin, 2006;Jin and Wang, 2004).In 2000, Jiang et al.considered that the three-dimensional model for coupled temperature, pressure, stress fields, fluid flow, energy transfer and transportation of chemical materials is an important area for further development of dynamics of petroleum accumulation (Jiang et al., 2000).Also, in 2002, Yang et al.considered that the accumulation processes and preservation conditions of deep petroleum reservoirs in sedimentary basins, and the mechanism and processes of rapid petroleum accumulation in active tectonic environment with overpressure became the frontier area in the studies of petroleum accumulation mechanism (Yang et al., 2002).The above studies have definitely given a great impetus to our research.

The oil-gas exploration of the Sinian–Lower Paleozoic in Sichuan basin has lasted for more than 40years.More than 40 exploratory wells have been drilled.Up to now, the Weiyuan natural gas field and the Ziyang gas pool, as well as Longnüsi and Gaoshiti-Anpingdian gas structures have been discovered (Fig. 1) (For the fourth member of Dengying Formation of Sinian, there is natural gas in the Anping1 well, and the test output was 0.248×10⁴ m³/d, the production in the same position of Gaoke 1 well was0.7×10⁴ m³/d).Although much of the Sinian–Lower Paleozoic is still left unexplored and the outcome of the previous exploration seems insignificant, it does contribute to the progress of the related petroleum geology theory.Judging from the natural gas fields discovered as well as the good short-term outcome and accumulation conditions of hydrocarbon in the Sichuan basin, Sinian–Lower Paleozoic reservoir has good exploration prospect.At present, it has no formed a big scene for hydrocarbon exploration, the unknown accumulation mechanism of hydrocarbon is one of the causes.Therefore, focusing on the hydrocarbon accumulation mechanism of the lower assemblage in the Sichuan basin has important theoretica value and practical significance.By utilizing fluid inclusion geochemistry, model of hydrocarbon generation and expulsion from source rock and balance section, this paper discusses the accumulation mechanism of hydrocarbon of Sinian in Anpingdian-Gaoshit structure, from the angle of oil and gas accumulation geology and structure evolution.The main purpose is to further understand the accumulation mechanism of the carbonate rocks of Sinian, and provide scientific basis for further hydrocarbon exploration at deeper layer of Sichuan basin.The main selected exploratory wells in the Anpingdian-Gaoshiti structure are Anping1 and Gaoke 1 wells.

Figure  1.  Sketch map showing seismic reflection structure on the top of Sinian boundary in Leshan-Longnüsi paleo-uplift in Sichuan basin.

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HYDROCARBONSOURCEROCK CONDITIONS

The thickness of black shale of Qiongzhusi Formation in the Cambrian ranges from 90 to 120 m.The organic matter is over mature at present.The actual mean value of measured Ro of Anping 1 well is 2.86%H/C is 0.2–1.40;the range of kerogen carbon isotope generally is from-27‰to-31‰; the organic matter is nonfixiform and belongs to type-I kerogen (Zhang, 1998);the mean value of organic carbon is 2.18%, which indicates that the organic matter is mainly from the low organism; the hydrocarbon generation potential is great, and the shale is the most important hydrocarbon source rock.

Moreover, according to Song (1996), the mean value of TOC in Sinian Dengying Formation algae dolomite is 0.13%.He explained that the Dengying Formation algae dolomite cannot reach the lower limit of abundance for hydrocarbon source rock.The TOC reflects the anthraxolite abundance, which is the secondary bitumen characteristic of reservoir rock, other than the organic matter abundance characteristic of source rock.Qiu (1994), and Qiu et al. (1994) thought that the later anthraxolite backfill may cause the organic carbon of rock to be 4 to 10 times higher than the average organic matter abundance of carbonate rocks.Therefore, in Dengying Formation algae dolomite, the high content of organic carbon is essentially caused by the oil migration pollution, and the Dengying Formation algae dolomite cannot be an effective hydrocarbon source rock.The generalized analysis indicates that the gas source of the gas pool in this area mainly comes from Cambrian Qiongzhusi Formation mudstone overlying Dengying Formation.For example, at the Anping 1 well, Cambrian Qiongzhusi Formation mudstone is good oil source rock, for its thickness is 92.4 m, and the remaining organic carbon content is 1.95%.The organic matter is type-I kerogen.These indicate that the organic matter abundance of source rock in Anping 1 well is higher in Lower Cambrian, and organic matter type is better.

TRAP CONDITIONS

The landform of the Middle Sichuan is a monocline with regional gentle fold.The deep declination is in opposite direction against the center and shallow fold (before and after Permian).Deep-layer structure pattern is a large-scale anomalous arched structure belt, northwest of which is high and southeast of which is low (Tong, 1992).Anpingdian-Gaoshiti, located between Anyue and Suining-Tongnan, is a part of Anyue-Moxi structural belt.Macroscopically, the Sinian top structure within the Anpingdian-Gaoshiti area is a giant nosing uplift, west of which is high and east of which is low.The local structures distribute at the axis and two wings of the nosing uplift, showing the west-east direction.However, the Anpingdian, Gaoshiti, Moxi, etc.are at the axis of the giant nosing uplift.There are two rows of local structures in the area, namely, Anpingdian-Moxi and Gaoshiti-Tongnan (Fig. 1).Among them, the scale of Gaoshiti is the largest, with trap area being 251 km², closure 200 m (Table 1), the main high spot 5 km to north of Gaoshiti, and the high spot elevation-4 600 m.Simultaneously, the west side of Gaoshiti has a subaltern high spot, with its north being opposite to the high saddle of Anpingdian, and its south gently transiting to Shiyangchang.The Anpingdian structure is the second largest, with trap area being 100 km², closure 150 m, and the high spot elevation 100 m lower than that of Gaoshiti.Its south, a high saddle compared with Gaoshiti constructure, extends gently to the Anpingdian along the north.It is a brachyanticline in which the south is steep and the north is gentle.There also exist several other buried structures, but with small scale and low uplift scope.

Table  1.  The structural element in Anpingdian-Gaoshiti structure

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The analysis of the geology, seismic survey, and drilling data in the Middle Sichuan basin and neighboring areas implied that although this area had undergone several tectonic movements, the movements were mainly elevating movements (Guo et al., 1994) and even the latest Himalayan movement was not intense in this area.As a result, the structure of this area was gentle, and the breakage did not grow.Song (1996) revealed that faults did not develop in the area, but there are several small underlying reversed faults, the throws of which are 30–80 m long.However, based on the analysis of the present constructional map, it has been discovered that the west side has Longnüsi succession break which starts from the Sinian.According to the structural evolution history, these faults should be more active in the Caledonian and late Hercynian should be their main active stage, which positively contributed to the transformation of the reservoir.However, in every epoch that followed Hercynian, the faults seemed to be inactive in Anpingdian-Gaoshiti area.Moreover, in the IndoChinese epoch, faults should be relatively active in Longnüsi and Moxi areas, the significant evidence of which was that Nüshen 5 and Moxi 1 wells produced gas in Cambrian.Therefore, in the Indo-Chinese epoch and later, Longnüsi and Moxi were still the areas where faults were relatively active.Hydrocarbon may migrate through the factures to the excellent Cambrian reservoir, and accumulate under the appropriate trap conditions (Xu, 1999).

STRUCTURAL EVOLUTION

Since Sinian, the Middle Sichuan area has been on the stable basement of uplift.Although it has undergone several tectonic movements, the movements are primarily elevating movements.Thus, the formation and the development of the ancient and modern structures in Sichuan show strong successivity.The structure framework fell into a pattern after the Caledonian event at Late Silurian, and developed greatly after Indo-Sinian movement at Late Triassic, and formed the present structure framework after the stageⅢof Himalayan movement (Wang, 2002).The development of structure is mainly divided into two stages in the area, namely, Caledonian event at the end of Silurian and Indo-Sinian movement at the end of Late Triassic, which directly dominated the basic framework of present structure.

At Late Silurian, before Caledonian event, the Gaoshiti tectonic belt was a low amplitude uplift (Fig. 2), and Gaoke 1 well was located in the slope of the uplift.At Late Permian, the Gaoshiti tectonic belt uplifted slightly, with its culmination point displaced to the east, and Gaoke 1well was still located at the slope At Late Triassic, the Gaoshiti tectonic belt continued to uplift slightly, with its culmination point displaced towards Gaoke 1 well which was still located at the slope.At Late Jurassic, the closed area of the Gaoshiti tectonic belt expanded slightly.But according to the section map, the Gaoshiti tectonic belt is in the slope of Longnüsi uplift belt of Leshan, with obviously high altitude.The present tectonic form is similar to that at Late Jurassic, and the Gaoshiti tectonic belt is still a low amplitude uplift in depression, with Gaoke 1 well in the slope of the low amplitude uplift.

Figure  2.  The structural evolution profile of the study area, Middle Sichuan basin.

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Similarly, at Late Silurian before the Caledonian event, the Anpingdian tectonic zone was a small culmination point, but with low uplift amplitude.At Late Permian, the culmination point of the Anpingdian tectonic belt was more obvious, and Anping 1 well was located in the slope of anticlinal bowing.At Late Triassic, the Anpingdian anticlinal bowing amplitude was obvious, but Anping 1 well was still located in the slope of anticlinal bowing.At Late Jurassic, the Anpingdian anticlinal bowing was more obvious than that at Late Triassic, and only the position of culmination point displaced slightly.Still, Anping 1 well was located in the slope of anticlinal bowing.Thus, the present tectonic form differs little from that at Late Jurassic.

The generalized analysis indicates that since the local structure formed and developed successively on the stable basement of the uplift, the present structure framework is mainly dominated by Caledonian and Indo-Sinian paleo-structure.In fact, the tectonism in Himalayan only further complicated the details of the structure.The local structure belongs to the gentle structure.The influence of tectonism in this area is extensive, but the amplitude of uplift is low.Therefore the closed area is generally small.The formation of the structure was much earlier than the generation, so the traps formed earlier than the peak of oil generation, which is favorable for hydrocarbon accumulation and preservation.

MECHANISM OF HYDROCARBON ACCU-MULATIONOFANPINGDIAN-GAOSHITI STRUCTURE IN SINIAN

Hydrocarbon-Generation Evolution of Source Rock

The paleogeothermal history of Anping 1 well and the generation history of Qiongzhusi Formation source rock in Cambrian indicate that the source rock, whose burial depth was approximately 2 000 m and the Ro value of which was bigger than 0.5% (Fig. 6), entered into the oil threshold at Late Ordovician–end Silurian, which was the first hydrocarbon generation stage.In terms of the Dengying Formation reservoir of Sinian, it was the first oil charge stage.Since the strata uplifted after the Caledonian event, the formation of Silurian suffered the denudation and the generation stagnated.At the Early Permian, as a result of the overlying formation deposition, the burial depth of the source rock of the Cambrian continued to increase.At the Middle–Late Permian, the Ro value was 0.7%and the source rock began to generate oil massively.At the Early–Middle Triassic, the Ro value increased to 1.0%, which is the peak of oil generation.The second generation stage was from the Middle Triassic to the Early Jurassic.It was the second charge stage of the Dengying Formation reservoir of Simian.From the Early to Middle Jurassic, the Ro value of the Cambrian source rock increased from 1.3%to 2.0%, and it was the generation stage of moisture gas.In this stage, the Sinian reservoir was mainly charged by the moisture gas.At the end of Middle Jurassic, the Ro value reached 2.0%, and massive dry gas started to generate.Owing to the secondary generation stage of the Cambrian source rock, the Sinian reservoir also presented multi-stage charging.The burial depth of the top of the Simian reservoir was about 4 500 m at the early Middle Jurassic.The temperature was 160 and the℃oil in the reservoir started to crack to gas (Schenk et al., 1997) and bitumen was generated at the same time.At the late Middle Jurassic, the burial depth of the topof the Simian reservoir was 6 200 m.The ground temperature was 215℃, and the amount of gas generated from oil cracking reached the maximum.

Characteristics of Fluid Inclusion and Accumulation History of the Sinian Gas Pool

England et al.(1995, 1987), England and Mackenzie (1989), Rueckheim and England (1989) have described the mechanism of the secondary migration and charge and utilized the information of fluid inclusion, such as ingredient, phase, temperature, etc., to demarcate the charge and accumulation history The stages of accumulation are always divided by using homogeneous temperature of fluid inclusion on the basis of restoring the geohistory and thermal history of the sedimentary basin.The homogeneous temperature usually refers to the temperature of saline water fluid inclusion that contains hydrocarbon, other than the homogeneous temperature of organic fluid inclusion.The homogeneous temperature of organic fluid inclusion is often lower than that of saline water fluid inclusion at the same stage (Lu, 2000;Lu et al., 1990).The fluid inclusion displayed the essential characteristics of the fluid that existed in diagenetic and mineralization process, thus it can provide a series of original information related to the formation of mineral, such as the temperature, the ingredient, the salinity, etc..However, the organic fluid inclusion is the hydrocarbon captured and wrapped by mineral at each evolution stage of the organic matter and during the migration and accumulation process of hydrocarbon.Therefore, it recorded the evolution and migration history of hydrocarbon.The homogeneous temperature and characteristics of the fluid inclusion obtained from the fillings in the pore and fissure of the Simian of Anping 1 well are listed in Table 2.

Table  2.  The analysis results of fluid inclusion of Anping 1 well from Dengying Formation

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(1) The fluid inclusion in the fillings of quartz crystal, whose homogeneous temperature can be measured, is mainly saline water and contains hydrocarbon fluid inclusion; the gas/liquid ratio is 10–25, and the size is 2–8µm.The homogeneous temperature of saline water fluid inclusion ranges from 120 to 250℃, and mainly concentrates on 190–220℃and240–250℃, indicating that the formation and growth of the quartz displayed multistage features.It is noteworthy that the quartz filled at the first stage, whose homogeneous temperature of saline water fluid inclusion is 130–70℃, is approximately formed at the end of the Triassic, when it was the peak of oil production.Thus the quartz can capture oil and gas.

(2) The data on dolomite temperature are few, and the fluid inclusions are mainly the saline water inclusions.The homogeneous temperature is 170–220℃, and the fluid inclusion whose temperature is lower or that contains hydrocarbon has not been found.However, according to Table 3, the authors can discover that the mineral that includes the single-stage oi fluid inclusion or the oil/water fluid inclusion is nearly the dolomite.The two types of the fluid inclusions indicate that it formed in the generation stage of immature oil.We suppose that the homogeneous temperature is 80–110℃, and according to the burial-therma evolution, the two types of the fluid inclusions formed at the Early–Middle Triassic.It seems that the dolomite growth lasted for a considerably long time, and the dolomite filled in the initial stage formed earlier than the quartz filled in the first stage which grows along the wall of fracture.

Table  3.  The analysis results of fluid inclusion of Anping 1 well from the 4th member of Dengying Formation

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(3) The measured homogeneous temperatures of the organic fluid inclusion and the saline water fluid inclusion are scattered, mainly ranging among160–170℃, 210–220℃and 250–260℃, which reflects the hydrocarbon expulsion differences in time and depth.The bitumen-bearing fluid inclusion is common since 200–220℃; it revealed that the liquid hydrocarbon has gradually cracked and entered into the dry gas stage; its geological period is from the Middle–Late Jurassic to the Early Cretaceous.

(4) There are few gaseous hydrocarbon fluid inclusions.According to the formation mechanism of fluid inclusion, the quantity of the gaseous hydrocarbon fluid inclusion can directly show the natural gas abundance.Absolute quantity of gas fluid inclusion depends on the quantity of flaws in the crystal and the gas abundance during crystallization, that is, the fullness level of natural gas is high and thus the gaseous hydrocarbon fluid inclusion quantity is large.From Tables 2 and 3, it can be seen that irrespective of whether we obtain data of the homogeneous temperature, we have not discovered the pure gaseous hydrocarbon fluid inclusion.

Moreover, in Table 3, the quantity of the organic fluid inclusion (unable to identify the ingredients of the organic matter oil, gas, or bitumen) and the saline water fluid inclusion, which includes a few hydrocarbon, accounts for only 31%of the total fluid inclusion.According to Fu (1989), in the Weiyuan natural gas field in South Sichuan, the proportion of the gaseous hydrocarbon fluid inclusion in the industrial gas formation is 60%–85%, and that in the gas showing formation is 31%, but in the non-gas bearing formation, less than 15%.This indicates that the trap, which includes Anping 1 well, has good gas source, but probably, the natural gas has not fully accumulated in the trap.

Process of Hydrocarbon Accumulation (Reformation) in Sinian Reservoir

Formation and destruction of the paleo-pool

Figure 3 is the pool formation and evolution profile of Gaoke 1 and Anping 1 wells, analyzed with the generation history of the Cambrian generally.At the end of Silurian (Figs. 2, 3a, 6), the area including Anping l or Gaoke 1 wells presents certain trap shape.The top of Sinian formation of Gaoke 1 well is very flat, and the trap size of Gaoke 1 well is larger than that of Anping 1 well.During this period, the Ro value of the source rock of Cambrian is 0.5%–0.6%, and the hydrocarbon has begun to migrate into the Sinian trap.This is the formation stage of the paleo-pool.

Figure  3.  The formation and evolution of hydrocarbon reservoir in Sinian, Middle Sichuan basin.

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Before the Permian, the paleo-pool was disintegrated and destructed because of the uplifting, denudation in Caledonian event.The biomarkers in bitumen sample of the two wells have recorded this process; they present the biodegradation in various degrees, and the oxidative degradation bitumen was mainly generated in this period (Figs. 3b, 4).

Figure  4.  The crude oil formation, damage, superimposition evolution model of Sinian hydrocarbon reser-voir, Anpingdian-Gaoshiti structural belt.

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The saturated hydrocarbon chromatograms of the bitumen samples A12 and A16 from the Dengying Formation of Gaoke 1 and Anping 1 wells are obtained.Sample A12 is the bitumen, which filled in the bug hole of grey micrite dolomite, and sample A16 is the bitumen from laminated algae dolomite.The two samples'saturated hydrocarbon chromatograms demonstrate unresolved complex mixture in various degrees.The two curves are in bimodal distribution; ∑C_21-/∑C₂₂₊are respectively 0.72 and 0.44, and both are dominated by the later peaks.It is related to the degree of thermal evolution that they suffered (Dahl et al., 1993).The samples G15 and G5 from Gaoke 1well are respectively the bitumen that filled in the bug hole of grey micrite dolomite of the Sinian and that distributing along the fracture or from the microfissure these two samples'prominent peak carbon are respectively C₁₈ and C₁₉; ∑C_21-/∑C₂₂₊are respectively 0.97and 2.13;the former presents that the latter peaks occupy predominantly, while the latter presents that the former peaks occupy predominantly.From these samples'saturated hydrocarbon chromatograms, it can be concluded that the paleo-pool of the Sinian has suffered from the biodegradation in various degrees in geohistory process (Xu et al., 2007).The biodegradation caused the losses of light hydrocarbon firstly, made the early-charged crude oil retain the heavy carbon, and because of the recharge of the post mature crude oil later, the saturated hydrocarbon chromatogram is characterized by the former peaks predominantly.However, since the sample experiencedthermal evolution again at a later stage, the chromatograms of saturated hydrocarbon present current ap-pearance.These saturated hydrocarbon chromatograms are different between the two samples from Gaoke 1 well and the two samples from Anping 1 well; the chromatogram of Anping 1 well presents bimodal distribution, but the chromatogram of Gaoke 1 well presents unimodal distribution.This is obviously related to the sufficient degree of the recharge of the crude oil in later period, and the degree of the thermal evolution that they experienced at a later stage (Hu et al., 2003).Considering the structure evolution history, the paleo-pool suffered the serious destruction in Caledonian event.Figure 4 is the evolution model of the destruction and recharge.

Hydrocarbon recharge, ancient oil and gas pool formation and adjustment

At the end of the Triassic (Figs. 3c, 6), the Ro value of source rocks of Cambrian increased from0.7%to 1.3%, this was the main oil charge stage, with a little natural gas charged simultaneously, and the measured homogeneous temperature of inclusion was mainly 110–130℃.The charging natural gas led to gas cutting, deasphalting and a little bitumen precipitation.

At the end of the Jurassic (Figs. 3d, 6), the source rock of Cambrian Ro value evolved from 1.3%to2.8%, and the authors obtained two formations of inclusion homogeneous temperatures, respectively210–220℃and 250–260℃.It is the main natural gas charging time, primarily oil cracked gas, and it is the ancient gas pool formation stage.During this period, the oil started to crack massively to gas and formed the thermal evolution bitumen.According to the calculated result, the bitumen was formed mainly in163–157 Ma, i.e.Jurassic (Wang et al., 2002).Simultaneously, owing to temperature elevation and pressure enlargement, the produced natural gas was dissolved in water, and formed the water soluble gas (determined the enclosure by physical examination to discover massive liquid state methane).

In the oil gas charge process, the unconformity between the Sinian and the Cambrian acted as the main passageway for oil and gas.The content of bitumen near unconformity is very high; and that of bitumen far away reduces obviously.The bitumen content of the Cambrian layer base of Anping 1 well is10%, and that of Sinian layer crown is approximately6%.The Cambrian Qiongzhusi Formation base of Gaoke 1 well (core section 4 955–4 986 m) has the highest bitumen content, and the average bitumen content of 6 samples is 15.5%;that of Sinian Dengying Formation crown is at the second place (core section 4 986–4 994 m), and the highest bitumen content is 30%, the smallest 2%, and average 8.54%.

The bitumen content in unconformity, with upward or downward formation, shows decreasing trend (Fig. 5).It indicates that it indeed has oil and gas charge and the oil gas accumulation history along the crust of weathering or its surface.

Figure  5.  Characteristics and distribution of Sinian–Lower Paleozoic bitumen and paleo-reservoir in Si-chuan basin.

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At the end of Cretaceous (Figs. 3e, 6), the structure was uplifted at late Yanshanian, and because of temperature reduction, original balance of gas pool was broken, the exsolution of natural gas took place massively in the water, and followed by the lost and transference of partial natural gas (integrated with tectonic analysis, possibly transfering to Weiyuan, Ziyang), this was the gas pool adjustment stage, and formed the present residual gas pool finally.

Figure  6.  The hydrocarbon accumulation event, Anpingdian-Gaoshiti structural belt in the Middle Sichuan basin.

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It is obvious that the evolution of Sinian natural gas pool in Anpingdian-Gaoshiti structure belt mainly went through the following several stages: (1) the formation stage of the paleo-oil pool at the end of Silurian; (2) the destruction stage of the paleo-oil pool at the Caledonian; (3) the re-charge stage of oil and gas when source rocks of Cambrian generate hydrocarbon at a second time at the Early Permian–Early and Middle Triassic; (4) the stage of oil cracking to gas and water soluble gas formation at the Late Triassic–Middle and Late Jurassic; (5) the stage when the present remnant oil pool developed from the episodic uplifting of the strata, the exsolution of the gas and the readjustment and reconstruction of the paleo-gas pool from the Late Cretaceous.

FAVORABLE AND UNFAVORABLE FACTORS OF HYDROCARBON ACCUMULATION IN SINIAN

Favorable Factors of Hydrocarbon Accumulation

The structure of Sinian, Cambrian, Ordovician was based on the basement block uplift, with successive development resulting in stereotype at Caledonian movement of Late Silurian, which met the good trap conditions; although it underwent several tectonisms which didn't change the early stage conformation greatly.

Moreover, the Cambrian source rock develops well and quite thick with great hydrocarbon generation potential and the Cambrian oil source rock reached the oil-forming fastigium in Triassic, and matched with trap well.

Unfavorable Factors of Hydrocarbon Accumulation

Although Anpingdian-Gaoshiti structural bel was in the paleohigh axial region, and local uplift and trap area were larger at Caledonian, since the Indo-Chinese epoch (unboiled oil peak), the paleohigh moved southward to Gaoshiti, and the rate of uplift reduced greatly, and so did the closed area, causing the hydrocarbon to migrate again and overflow.

① The contrast of regional data indicates that the Sinian reservoir accumulation conditions of this area are quite the same as those of the Ziyang area; the main difference is that the secondary corrosion holes and the fractures did not develop, the filling degree a later stage, compared with Ziyang area, is higher (Liu et al., 2008).The microscope observations indicated that the major part of the secondary corrosion holes and the fractures got entire-half filled with ageing bitumen or dolomite, kiesel, etc., leading to the limited space of surplus reservoir and infiltration.

② Oil and gas migrated and accumulated laterally for long time, Leshan-Longnüsi paleohigh presents the tendency of high west and low east after Caledonian, particularly that structure is more prominent after Himalayan.For example, Zi 1 well is located in the western end at axial region of the paleohigh; its Sinian burial depth is 1 400 m lower than Anping 1 well.Therefore, the oil and gas completely migrate to and accumulate in the west area, such as Ziyang paleo-trap, Anyue, Weiyuan structures, etc..

③ From current constructional map and structure evolution analysis, it is found that Anping 1 and Gaoke 1 wells are located at the structural slopes of Anpingdian and Gaoshiti, rather than the structural height, which is the most unfavorable factors for comparatively bad efficiency of exploration in the two wells.

CONCLUSIONS

(1) The Sinian gas pool is mainly sourced from mudstone source rock of overlying Cambrian Qiongzhusi Formation, with the following characteristics: large thickness, high abundance of organic matter saprople type-I kerogen, and larger hydrocarbon generation potential.

(2) The structure of Sinian, Cambrian, Ordovician was based on the basement block uplift, with successive development resulting in stereotype at Caledonian movement of Late Silurian, which meets the good trap conditions; although it underwent several tectonisms which did not change the form of early stage significantly.Moreover, the Cambrian oil source rock reached the oil-forming peak in Triassic, matching with trap well.

(3) The homogeneous temperatures recorded by the fluid inclusion in Sinian reservoir mainly range among 130–170℃, 210–220℃, 230–260℃, which indicates that the reservoir underwent at least three stages of fluid charges.At Middle, Late Jurassic–Early Cretaceous, the most important stage started from about 200–220℃, with bitumen as the most common organic inclusion, which implied that liquid hydrocarbon gradually cracked into dry gas stage.

(4) The Sinian gas pool mainly formed in natural gas exsolution and paleo-gas pool & apos; s adjustment stage.During this period, natural gas appeared to be lost, transferred, and redistributed, which led to remnant gas pool finally.

(5) Sinian gas accumulation experienced from paleo-oil pools to paleo-gas pools, and to the present adjustment and reformation. It is mainly characterized by: at the Middle Triassic or earlier, organic ma ① tter maturation, hydrocarbon generation, and expelled process led to the early multi-stage hydrocarbon accumulation, taking on a different process and multi-stage migration and accumulation; at the ② Middle Jurassic or earlier, oil cracked to gas and bitumen formation, which resulted in middle accumulation of natural gas, revealed by deep burial, high temperature and oil transferred into gas; at the Him ③ alayan, the formation of structural trap, and the uplifting and denudation led to late accumulation of hydrocarbon, which mainly features uplifting, denudation, energy field adjustment, gas exsolution from water, gas pools at later destruction or gas accumulation by redistributing.