川东北地区碳酸盐岩层系孔隙型与裂缝型气藏成藏差异性

Differences in reservoir-forming between porous and fractured gas pools in carbonates,the northeastern Sichuan Basin

川东北地区上二叠统—下三叠统发育孔隙与裂缝两种储层类型的气藏,这两类气藏在储层沥青含量和天然气化学组成上具有明显的差异。利用地质、地球化学和盆地模拟等方法,分析了这两种类型气藏的天然气来源、输导体系与油气充注历史,及其对油气富集的影响。孔隙型气藏的油气输导体系由断层与孔隙型储集体构成,形成时间早,与烃源岩的生油气高峰期匹配,形成了古油气藏;在随后的深埋过程中,古油藏裂解成气,与此同时烃类气体与孔隙流体中的硫酸盐发生硫酸盐热化学还原反应,生成了H2S与CO2。裂缝型气藏的油气输导体系主要由断层与储层裂缝构成,形成时间晚于烃源岩生油气高峰期,与烃源岩生气晚期匹配,储层只聚集了干酪根晚期高温裂解气。烃源岩后期埋深与构造演化是造成两类气藏差异性的根本原因。

There are two types of gas pools in the Upper Permian-Lower Triassic in northwestern Sichuan Basin,namely porous and fractured gas pools.They show obvious differences in bitumen content and chemical compositions of gas.Geological,geochemical and basin modeling methods are used to analyze their gas sources,carrier systems and hydrocarbon charging history as well as their influences on hydrocarbon accumulation.The carrier system of porous-type gas pools is composed of faults and porous reservoirs and its formation is in good timing with peak hydrocarbon generation of source rocks,leading to the formation of paleo-oil/gas pools.During subsequent deep burial,the paleo-oil pools became gas pools due to pyrolysis.In the same time,H2S and CO2 were generated because of thermochemical sulfate reduction between hydrocarbon gases and sulfates in pore fluids.In contrast,the carrier system of fractured-type gas pools consists mainly of faults and fractures and their formation is later than the peak hydrocarbon generation but is good timing with the late stage of hydrocarbon generation.Therefore,only pyrolysis gas from kerogen accumulate in the reservoirs.Later deep burial of source rocks and tectonic evolution are primary causes for the differences between these two types of gas pools.