Natural Gas Accumulation And Enrichment Regularity Of The Jurassic In West Sichuan Depression

The western Sichuan depression contain very rich natural gas resources,and the exploration and development potential of oil and gas is enormous.However,at the early stages of the exploration and development of the Jurassic,we mainly focus on the severalpositive structures,because the so many difficulties.For example,the Sichuan basin has a very complicated evolutionary history;the features of multi-source,multi-reservoir,andmulti-phase;the high rate of the subsided andthe deposition in western Sichuan foreland basin;the high degree of the rock densification;the strong heterogeneity;the complicated accumulation process;very difficult to predict the “sweet spot'' and so on.So,based on well studied and understoodthe previous research results,we mainly carried out our work onthe accumulation geological background,the accumulation geological conditions,the accumulation mechanism,the main control factorof accumulation,the accumulation regulation,the comprehensive evaluation for the Jurassic gas reservoir of the west Sichuan foreland basin.The episodic compression of surroundingmountains is pulsed wave about stress which is accumulation long term-slow and release short term-instantaneous,it is the dominant factor of controlling the sequence stratigraphy and sedimentary infill pattern in the west Sichuan foreland basin.The changes of lake-level(basal level)and accommodation space are the “dual feature” mutation which are result by periodic episodic compression and relaxing.On this basis,we established the “pulsedwave”sequence structure of the “dual feature” system tract.By multi-attribute provenanceanalyzing,we defined the sedimentary characteristics of the Jurassic in the west Sichuan depression are:coexistof the long-axis and the short-axis provenance;intersection of the near-source sand and the far-source sand;coexistof the multi-depositional systems;the multilayer sand stackedin the vertical direction;continuous distributionin the lateral direction.Wedefinedthe basic characteristics of the reservoir and the distribution regularities vertically and laterally,and establishedthe diagenetic evolutionary sequencesand the porosity evolution mode for each section of the Jurassic,west Sichuan depression.We identified seven lapidofacies typesby using the quantitative ~ semi-quantitative method and determinedthe corresponding plane distributive characteristics for each section.We deepened the understanding ofthe major control factorsof the relatively high quality reservoir and proposed the three-elements(provenance,sedimentaryfacies and diagenetic facies,structure and fault)mode,furthermore,refined the geological prediction modelof thehigh quality reservoirs,and realized the fine evaluation of reservoir.We carried out the typical gas reservoir analysis of different deformation zone by applying the “source,facies,position”three-elementsreservoir-controlling theory.As a result,we ascertained the main controlling factors for the different deforming zones and established the corresponding accumulation modes,and then,summarized the rules of oil and gas enrichmentsystematically.On the basis of study the relationship among the source rockburial history,reservoir densification history,main accumulation period and thetectonic evolutionhistory,consequently,we ascertained the critical period of accumulation,confirmed the reservoir-controlling main factor of each key accumulation periodchronologically.Using the three-elements reservoir-controlling factor of each key accumulation period as the evaluation indexes,we carried outthree-elementsdynamic assessment at the trap level and established the corresponding dynamic evaluation selection process.And then,we carried out more sophisticated comprehensive screeningandevaluation,and pointed out thefavorable zones of further exploration.