川西坳陷中段须家河组四段钙屑砂岩气层的成因

Origin of Calcarenaceous Sandstone Gas Formation of the 4th Member of Xujiahe Formation in the Middle Part of Western Sichuan Depression

钙屑砂岩在川西坳陷须家河组四段分布特别广,几乎每个构造、每口井都能见到。岩石大部分致密,目前只在局部井段见到孔隙发育能成为有效储集层。在大量岩石薄片鉴定及资料统计的基础上,对丰谷地区钙屑砂岩的岩石学、成岩作用、储集物性和孔隙发育特征进行了详细的研究。结果表明,丰谷地区钙屑砂岩气层单层砂体厚度大,碳酸盐岩岩屑含量高,方解石胶结物含量低,次生孔隙发育,孔隙类型主要为溶蚀粒间孔。天然气主要来自下三叠统马鞍塘组、须家河组三段、须家河组四中亚段烃源岩。本区钙屑砂岩气层与所经历成岩作用密切相关,属于成岩圈闭型气层。丰谷地区钙屑砂岩气层的形成过程对于勘探类似岩性气藏具有借鉴与指导意义。

Calcarenaceous sandstone is widely distributed in the fourth member of Xujiahe formation（ T3x^4 ） in West Sichuan Depression, and it can be seen in every structure and well. Most of the rocks are tight. Presently, well-developed pore that can be effective reservoir can be seen in local place. On the basis of thin sections identification and data statistics, the lithology, diagenesis, petrophysical properties and pore development properties for calcarenaceous sandstone in Fenggu area are analyzed in detail. It is indicated from the research that calcarenaceous sandstone layers in Fenggu are characterized by large thickness of single sand body, high content of carbonate debris, low content of calcite and well-developed secondary pores. Major pore type is dissolved intergrnular pores; and natural gas is mainly produced from source rock of lower Triassic Maantang formation, the third member of Xujiahe formation （ T3x^3 ） and the forth member of T3x^4. Gas layers of calcarenaceous sandstone are closely related to its diagenesis,which belong to diagenetic trap. The form process of calcarenaceous sandstone gas layers in Fenggu area offers guidance for exploration of similar lithologic gas reservoirs.