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本文通过对川西坳陷气源岩的划分,提出孝泉构造天然气藏以上三叠统须4段底部为界,划分出以须2气藏为主的下部成藏系统和以中侏罗统、上侏罗统为主的上部成藏系统,且认为除上侏罗统及以上气藏外,主要为近源气藏.天然气运移到储层,只要储层稍有倾斜,天然气就会靠重力分异向上运移,若有一定的封盖条件就会增压;岩石致密化不仅缩小了储层的孔隙体积,而且增大运移阻力,因而形成异常高压,当储层压力大于地层破裂压力时,则形成微裂缝.微裂缝的形成期也是溶蚀孔的形成时期.笔者认为须2气藏、中侏罗统气藏在形成过程中主要受基底隆起和同生隆起的控制,燕山末期岩石的致密化使其定型或基本定型.须2气藏是生储盖组合优良的近源气藏,虽储层孔隙度较小,但因微裂缝和溶蚀孔发育仍具有形成大型气田的基本条件.新场气田X851井获得稳产高产(151×104m3/d)天然气,就是这一认识的佐证.

Basic Property of Forming Gas Reservoirs in Xiaoquan Structure in West Sichuan Depression

??Through the division of the gas source rocks in West Sichuan Depression,it is proposed that by taking the bottom of T₃x⁴ as the limitation,the natural gas reservoirs in Xiaoquan structure may be divided into two petroleum systems,i.e. the lower petroleum system which takes the T₃x² gas reservoir as the center and the upper petroleum system which takes the Middle Jurassic-Upper Jurassic as the center.Except the Upper Jurassic and its overlying gas reservoirs,the others are mainly the near-source gas reservoirs,which provides a full basis for determining early gas reservoir formation.According to the mechanisms of uplifting,pressure-increasing and fracture-forming,it is considered that after natural gas's having entered into reservoir bed owing to hydrocarbon-generating and pressure-increasing,natural gas will migrate upward as long as a little tilt occurs and formation pressure will incerase if a certain of seal condition happens.In late stage,because of rock compactification,not only the pore volume of reservoir is diminished but also the formation of abnormally high pressure is guaranteed because of increasing the resistance of gas migration in reservoir.When reservoir pressure is larger than formation fracturing pressure,microfissures occurrs.And the period of forming microfissures was that of forming solution pores.According to the mechanism of reservoir formation by uplifting and in combination with the change law of Xiaoquan structure in space during the periods of hydrocarbon generation,reservoir formation and reservoir compactification in the history of forming gas reservoirs,it is thought that the T₃x² gas reservoir and Middle Jurassic gas reservoir were mainly controlled by basement uplift and syngenetic uplift in the process of forming gas reservoirs and the rock compactification made the gas reservoirs finalized or basically finalized in the end of Yanshan epoch.Although the reformation of the early structural configuration was intensified in Himalayan movement,it was very difficult to change the control of palaeostructure over gas trap because of an essential loss of pores in reservoir.In this paper,it is clearly proposed that the T₃x² gas reservoir is a near-source gas reservoir with a fine source-reservoir-seal assemblage;and although reservoir porosity is small,it possesses the fundamental condition of forming large-sized gas field owing to the growth of microfissures and solution pores.It is the proof of such an ideal that the natural gas with stable and high production (open-flow capacity of 151×10⁴m³/d) was found in well ??-851 in Xinchang gas field.