四川须家河组层序地层格架与天然气成藏的关系

须家河组天然气成藏受层序地层格架的控制。Ⅲ-1层序先后沉积了包括碎屑岩构成的低位体系域,泥岩、砂质泥岩构成的水进体系域和砂岩、砂质泥岩、煤层构成的高位体系域,显示了由川中向川西前积充填的特征；Ⅲ-2层序则先沉积了由湖泊及进积三角洲沉积构成的低位体系域,后又沉积了主要由泥质岩构成的水进体系域,形成由周缘向盆地内部充填的沉积格局；Ⅲ-3层序在沉积了由巨厚盆底扇构成的低位体系域后,继之形成了由湖泊、退积三角洲沉积体系构成的水进体系域,后又充填陆源碎屑为主的高位体系域,亦具由周缘向盆地内部前积充填的沉积特色。水进体系域因其泥岩厚度大,有机碳丰富为主要烃源岩及区域盖层,低位体系域和高位体系域由于砂岩孔渗性好是为储集层。川中、川西北以Ⅲ-1层序低位体系域为主,次为Ⅲ-2层序低位体系域,川东、川南以Ⅲ-3层序高位体系域占优势。显示天然气成藏分布由须家河组二段低位体系域占优势转变为须家河组五段上部高位体系域为主,有利储集相带向东迁移的趋势。

RELATION BETWEEN SEQUENCE STRATIGRAPHIC-FRAMEWORK AND GAS POOL-FORMING OF XUJIAHE FORMATION,SICHUAN

The formation of the gas pools in Xujiahe Formation was controlled by its sequence stratigraphic framework.Ⅲ-1 sequence is constituted by lowstand system tract formed of clastic rocks,transgressive system tract formed of sandstone,arenaceous rock and highstand system tract formed of arenaceous rock and coal beds;Ⅲ-2 sequence contains respectively lowstand system tract formed of lacustrine facies and transgressive delta sediments,transgressive system tract formed of argillaceous rocks;Ⅲ-3 sequence is composed of lowstand system tract with huge thickness of fan deposits in the internal part of the basin,the transgressive system tract with lacustrine and regressive delta sediments,the highstand system tract formed of terrigenous debris.Both Ⅲ-2 and Ⅲ-3 sequences show the framework of progradation from periphery to the interior part.It is suggested that the transgressive system tract contains thick mudstone and abundant organic matter,and it should be the main source rock and regional caprock;the lowstand and highstand tracts contain sandstones with favourable porosity and permeability,and should be considered as reservoir rocks.