川中须家河组流体包裹体与天然气成藏机理

Study on Fluid Inclusion and Gas Accumulation Mechanism of Xujiahe Formation of Upper Triassic in the Central Sichuan Basin

基于本区构造演化、生烃条件和储集条件及其时空配置的综合分析,根据烃类包裹体形成时间分布的广泛性和离散性,认为须家河组油气充注和聚集成藏是一个持续的过程,主要从晚侏罗世开始持续到古近纪末。天然气成藏时期不是表现为某一或某几个时刻或时期,须家河组煤系烃源岩具有连续生烃、持续充注的特点。根据川中地区的沉积埋藏史,将须家河组的成藏过程分为早期深埋阶段和晚期抬升阶段。深埋阶段(晚侏罗世-晚白垩世)也是成岩圈闭的形成阶段和大量生烃阶段,油气呈层状蒸发式运移,形成"泛气田"(圈闭内气饱和度较低,致密砂岩介质中有一定的分散滞留气,大面积连片砂体普遍含气)。构造抬升阶段(白垩纪末-古近纪末)产生断裂和裂缝,烃源岩中的气进一步释放,分流汇聚于孔渗性好的砂体中,形成大面积的"孤立气藏"(圈闭内气饱和度较高,致密砂岩介质滞留分散气较少)。须家河组连片砂体与煤系烃源岩互层,生储盖配置得当,天然气成藏是一个持续充注的过程。大范围烃源层和储集层层状间互展布,产状平缓,油气以短距离垂向运移为主,是形成大面积低丰度大气区的主要原因。

Based on the comprehensive analyses of tectonic evolution, hydrocarbon generating conditions, reservoir conditions and the hydrocarbon inclusion distribution is temporally extensive and continuous in the central Sichuan Basin, this study suggested that hydrocarbon filling and accumulation of Xujiahe formation of Upper triassic in Sichuan Basin was a successive processes which lasted from Late Jurassic to Late Paleogene. The coal-measure source rocks generated hydrocarbon continually and the gas charged continually, so the gas accumulation stages were not limited to one or several certain periods. According to the deposit history of the central Sichuan Basin, the accumulation process of Xujlahe formation was divided into two stages, early burial stage and late tectonic uplift stage. During the burial stage（from Late Jurassic to Late Cretaceous） , which was also the forming stage of diagenetic traps and the amount hydrocarbon generation stage, the gas migrated in a form of layered vapor and formed＂indistinct gas fields＂ where the trap saturation was low, retained gas scattered in tight sandstone and large area continuous gas-bearing sand bodies distributed widely. During the tectonic uplift stage（from Late Cretaceous to Late Paleogene）, as faults and fractures were generated, the gas from the source rock discharged continuingly and gathered in sand bodys with high pore ratio and permeability and finally formed large area ＂isolated gas reservoirs＂ where the trap saturation was high and retained gas scattered in tight sandstone was less. In Xujiahe formation in central Sichuan Basin, as continuous sand bodies interbedded with coal-measure source rocks, source-reservoir-cap rock assembled in a good form, so the gas accumulation was a continuous charging process. The extensively distributed hydrocarbon containing rock layers, the gentle occurrence, and oil migration was mainly in short distance and vertical directions were proposed to be the main geological reasons of forming large gas region with low gas abundance.