Gas Source And Accumulation Characteristics Of The Leikoupo Formation In The Western Sichuan Depression

At present,Zhongba gas field,Xinchang gas field,Pengzhou gas field and some other gas reservoirs or structures have found in Leikoupo Formation in Western Sichuan Depression where reveal great potential for hydrocarbon exploration.This paper is guided by the theory of petroleum geology,reservoir geology,structural geology,petroleum geochemistry and modern petroleum geology theories from domestic and international.By using the method of combining gas sampling with observations in the field and laboratory analysis indoor,macroscopic and microscopic investigation,in the light of analysis test data and site actual geological data,the gas source and the geochemical characteristics of the source rocks of gas reservoir in Leikoupo Formation are discussed in detail.Furthermore,find out the main controlling factors of the gas accumulation on the basis of dissecting typical gas reservoir,and the following conclusion can be drawn:(1)The main lithology of the reservoir in Leikoupo is granular dolomite,intergranular dissolved pore and intercrystalline dissolved pore is the main reservoir space.Influenced by sedimentary facies zones,diagenesis,weather denudation and tectonic movement the high quality reservoir is mainly distributed in the T2l3 and T2l4.(2)Gases from the target formation are generally characterized by source-mixed gas.For example,the natural gas in the third member of the Middle Triassic Leikoupo Formation in Zhongba gas field is sourced mainly from oil-type gas formed in underlying permian formation and mixed with part of coal-type gas formed in the overlying Xujiahe Formation.The geochemical characteristics of the gases from T2I4 reservoir in Xinchang gas field and well of G003-5 are obviously different from the T2l3 reservoir in Zhongba gas field,indicated that it probably comes from source rocks of Leikoupo Formation.On the other hand the gas from the well of PZ1 is similar to T2l3 reservoir in Zhongba gas field show that they have the same source but not rules out the possibility of partly derived from the source-rocks of Leikoupo Formation.(3)The correlation of gas-source rock indicated that there are three sets of source rock related to hydrocarbon accumulation of Leikoupo Formation:upper Triassic Xujiahe Formation,carbonate source rock of Leikoupo Formation and Permian.Among them,the source rock of Leikoupo Formation can be capable of generating hydrocarbon only in some area,be regarded as a subordinate gas source in Western Sichuan Depression in general.Upper Triassic Xujiahe Formation and Permian are two sets of high-quality source rocks which are abundant in organic materials and have the strong capacity of hydrocarbon generation it's the main gas-source rocks of Leikoupo Formation.(4)Fault and the plane of unconformity is the key factors of the hydrocarbon accumulation of gas reservoir in Leikoupo Formation.The hydrocarbon generated in the source rock of upper Triassic Xujiahe Formation and Permian can migrate and accumulate in the high quality reservoirs of Leikoupo Formation through the fault and the plane of unconformity.In summary,the favorably association of time and space of the major periods for hydrocarbon generation and expelling,fault development and trap forming are the key factors to the gas reservoir forming.