断裂调整型致密砂岩气藏成藏过程模拟与成藏模式

Accumulation process simulation and an accumulation model of a fault-adjusted tight sandstone gas reservoir

鄂尔多斯盆地边缘致密气勘探起步较晚,但突破明显,以东北缘临兴地区上古生界致密气为典型代表。但相较于盆地内部而言,盆地边缘地质条件复杂,受断裂调整改造的致密气藏成藏过程和成藏机理尚待系统揭示。基于对临兴地区致密气层分布特征剖析、储层流体包裹体特征分析以及盆地埋藏—热演化史模拟等,恢复了盆地边缘断裂调整型致密砂岩气的成藏过程并阐明了成藏模式。研究区致密气层在源内、近源及远源3套成藏组合中表现出差异分布特征:源内成藏组合气层个数最少,近源成藏组合气层数量居中,远源成藏组合气层个数最多;近源气层厚度最小,源内气层厚度居中,远源气层厚度最大。流体包裹体主要赋存于石英颗粒的微裂隙中,少量赋存于方解石胶结物和石英次生加大边中。流体包裹体均一温度分布范围很广,为81.7~180℃,这可能与紫金山岩浆侵入对烃源岩热历史产生影响有关。研究区天然气充注成藏分两个阶段,在成藏早期由于断裂和裂缝不发育,天然气以源内充注为主、近源充注为辅;到成藏中后期,伴随紫金山岩浆侵入产生大量断层和裂缝,天然气优先沿断层和裂缝充注到近源和远源储层中,而对源内储层的贡献相对较少。总体而言,断裂调整型致密气藏成藏过程受断层和裂缝控制作用明显,断层起到“气泵”的作用,因此,在盆地边缘致密气勘探中,要高度重视断层和裂缝的研究。

An apparent breakthrough has been made in tight gas exploration in the edge of Ordos Basin,although tight gas exploration started late.The typical example of tight gas at the edge of the basin is the Upper Paleozoic tight gas in the Linxing area on the northeast edge of Ordos Basin.However,compared with the interior of the basin,the geological conditions at the edge of the basin are complex,and the reservoir-forming processes and the mechanism of tight gas reservoir adjustment and transformation by faults needs to be systematically revealed.Based on the analysis of tight gas reservoir distribution characteristics,reservoir fluid inclusion characteristics,and basin burial-thermal evolution history simulation in the Linxing area,the reservoir-forming process of fault-adjusted tight sandstone gas at the edge of the basin is now understood,and the reservoir-forming model is clarified.The tight gas reservoirs show differential distribution characteristics in three sets of reservoir-forming combinations.The number of in-source reservoir-forming combinations is the least,the number of near-source reservoir-forming combinations is in the middle,and the number of far-source reservoir-forming combinations is the most.The thickness of nearsource gas reservoirs is the smallest,the thickness of inner source gas reservoirs is the middle,and the thickness of far source gas reservoirs is the largest.Fluid inclusions mainly occur in microcracks in quartz grains,and a small amount occurs in calcite cement and quartz overgrowths.The homogenization temperature of fluid inclusions ranges from 81.7℃to 180℃,showing the influence of the Zijinshan magma intrusion on the thermal evolution of source rocks.There are two stages of natural gas charging and accumulation in the study area.In the early stage of gas accumulation,faults and fractures were not developed.The natural gas was mainly charged into the in-source reservoirs,supplemented by the near-source reservoirs.In the middle and late stages of hydrocarbon accumulation,a large number of faults and fractures were generated with the intrusion of the Zijinshan magma.Natural gas was preferentially charged into the near-source and far-source reservoirs along the faults and fractures caused by the intrusion of the magma,and the contribution to the in-source reservoirs was relatively tiny.Generally speaking,the tight gas charging at the basin edge is obviously controlled by faults and fractures,with faults playing the“gas pump.”To explore fault-adjusted tight sandstone gas reservoirs,we should attach great importance to studying faults and fractures.