Analysis Of The Main Controlling Factors Of The Middle Permian Qixia Formation In Northwest Sichuan

The dolomite reservoirs of Qixia Formation are well developed in Northwestern Sichuan Basin,and the test production of some wells was high,especially in recent years,great breakthroughs have been made in oil-gas exploration in Shuangyushi structure,which shows tremendous prospects of hydrocarbon resource.However,due to the influence of foreland orogeny,oil and gas in Qixia Formation often have the characteristics of multi-source filling,multi-stage accumulation,multi-stage reformation,which results in unclear understanding of the accumulation condition and main controlling factors of gas accumulation in Qixia Formation in this area,and that the progress of oil and gas exploration is restricted.Therefore,it is necessary to conduct in-depth analysis and research on the accumulation conditions and main controlling factors of gas accumulation,in order to determine the main controlling factors of hydrocarbon accumulation in different tectonic zone and establish the accumulation modes,so as to provide new ideas for the next direction of exploration.Under the guidance of petroleum geology,petroleum exploration geochemistry and other related disciplines,this paper has carried out the following work and obtained the following understandings.(1)On the basis of observation of cores and outcrops as well as the test of samples of source rocks in the study area,this paper clarified the distribution of five sets source rocks related to Qixia formation gas accumulation,determined the geochemical characteristics of source rocks,including the abundance,type and maturity of organic matter,and calculated their hydrocarbon-generating intensity eventually.The results of evaluation of source rocks show that;the organic matter abundance of five sets of source rocks in the study area is in a good quality rating,of which the source rocks of Qiongzhusi Formation are the highest;the organic matter mainly consists of the type of ?,?1 and ?1-?2;the maturity of all sets of source rocks is high,and now is in a high mature-over mature stage,in which mainly produce gas.(2)The origin and sources of natural gas in Qixia Formation of different tectonic zone were discussed by the use of gas composition,carbon isotope data and biomarkers.The results show that the origin and sources of natural gas in different tectonic zone are quite different.It often has the characteristics of multi-source filling.The source rocks in Cambrian Formation are one of the main source rocks in this area.(3)Through vertical and horizontal correlation of formation and hydrocarbon show in single well,the types of source-reservoir-cap assemblages of different structural belts in the study area were cleared.It is found that the main types of source-reservoir-cap assemblages in the study area are lower-source with upper-reservoir,self-generation with self-storage and upper-source with lower-reservoir assemblage.(4)Based on the analysis of fluid inclusions and the thermal evolution history of source rocks,it is found that the hydrocarbon accumulation of Qixia Formation in this area includes at least four stages,and abundant hydrocarbon which charged in the reservoirs was from middle Triassic to middle Jurassic,Qixia Formation traps in Northwest Sichuan were generally formed in the early Indosinian epoch,adjusted in Yanshanian period and shaped in Himalayan period,while the hydrocarbon filling begun at the end of Indosinian epoch Therefore,the match of spatio-temporal is good.(5)Based on the anatomy of typical hydrocarbon reservoirs,the main controlling factors of gas accumulation of different tectonic zone were analyzed,and the accumulation modes were established.The results show that the reservoir forming conditions and accumulation modes in different tectonic zone are quite different.The thrust nappe zone has the characteristics of“early reservoir formation,late adjustment,preservation controlling";the concealed fault zone has the reservoir formation mode of "multi-source filling,multi-stage migration and accumulation,gas-source-fault controlling";the depression zone has the characteristics of "near source filling,continuous hydrocarbon supply,interlaminar fracture controlling".