镇泾地区延长组八段低渗岩性油藏形成过程动态分析

Hydrocarbon accumulation process of Chang-8 low-permeability lithological reservoirs in Zhenjing area,Ordos Basin

为了更好地揭示鄂尔多斯盆地镇泾地区延长组八段(长8段)低渗砂岩岩性油藏的成藏特征和成藏主控因素,在该区埋藏史、热史、生烃史和成岩作用史研究的基础上,开展长8段岩性油藏动态解剖,确定储层孔隙度、圈闭形成期次、烃源岩地层压力及石油充注动力窗口等成藏要素的演化特征,综合分析油藏的动态形成过程。结果表明:成藏期早白垩世中期长8段储层孔隙度为20%,成藏后储层逐渐致密化到现今12%,具有先成藏后致密的特点;长8段岩性圈闭形成于晚侏罗世,持续发育至今,圈闭形成早、发育时间长;成藏期长7段烃源岩内部地层压力达22 MPa,早白垩世末增加到35 MPa,压力系数达到1.5,至新近纪逐渐演化为正常压力;石油充注动力大,充注动力与阻力的压力差达15 MPa,充注动力窗口从早白垩世持续到新近纪末期,持续时间长。各成藏要素和成藏作用过程在时间和空间上的有效配置决定长8段低渗岩性油藏的形成,其中成藏期砂岩物性和充注动力是成藏主控因素。

In order to better understand the characteristics and main controlling factors of hydrocarbon accumulation in low-permeability lithological reservoirs of Member 8 of Yanchang Formation（Chang-8）in Zhenjing area,Ordos Basin,we performed dynamic analysis of the Chang-8 lithological reservoirs based on study of burial history,thermal history,hydrocarbon generation history and diagenetic process.The evolution features of various hydrocarbon accumulation factors were clarified such as porosity,timing of traps,formation pressure of source rocks and the window of hydrocarbon charging kinetics.Finally,a comprehensive analysis of the dynamic process of hydrocarbon accumulation was performed.The results indicate that porosity of Chang-8 reservoir was of 20% during hydrocarbon accumulation in the middle Early Cretaceous and lowered progressively to current 12% due to reservoir tightening of the later evolution stage.The lithological traps of the Chang-8 were formed at the Late Jurassic and kept on developing till now,thus they are characterized by early formation and long process of development.The formation pressure of Chang-7 source rocks was 22 MPa during the period of hydrocarbon accumulation.It increased up to 35 MPa with a pressure coefficient of 1.5 at the end of Early Cretaceous and later lower progressively to normal pressure in the Neogene.The charging pressure of oil was 15 MPa higher than the pressure of resistance,and the window of hydrocarbon accumulation lasted for a long time from the Early Cretaceous to the end of Neogene.The formation of the low-permeability lithological reservoirs in the Chang-8 is controlled by the favorable temporal and spatial matching of between the factors and process of hydrocarbon accumulation.Physical properties of sandstone and charging kinetics are the main factors controlling hydrocarbon accumulation.