三塘湖盆地牛圈湖油田低孔特低渗储集层特征及形成机理

三塘湖盆地牛圈湖油田西山窑组储集层埋深浅（小于2 000 m），储集层类型却属于低孔特低渗型。为揭示其储集层特征及形成机理，通过岩心观察、铸体薄片鉴定、常规高压压汞实验和电镜扫描等技术，结合区域构造资料分析，进行了储集层特征及低孔特低渗形成机理研究。结果表明:西山窑组储集层以长石岩屑砂岩为主，储集空间类型有原生粒间孔、溶蚀孔及裂缝。沉积、成岩和构造三重因素共同作用，造成了储集层低孔特低渗；西山窑组早期因为含煤地层的酸性环境导致碳酸盐胶结弱，颗粒间缺少支撑而发生较强烈的快速压实，同时沉积作用又从粒径大小和填隙物含量上控制着储集层原始物性特征，成岩过程中发生的压实作用致使减孔率达到18%，酸性环境还导致高岭石的大量形成、石英次生加大边的发育及晚期的含铁质碳酸盐胶结的发育，胶结减孔率为5%左右；构造应力形成的裂缝为后期溶蚀作用的发生提供了一定通道，但岩石已在早期压实作用下基本固结致密，且煤层厚度较薄、酸性流体含量不足，难以在小体积粒间大规模流动，溶解强度不大，导致对渗透率贡献很大的粗喉道较少，形成了低孔特低渗储集层。 

Characteristics and formation mechanism of low porosity and extra-low permeability reservoirs in the Niujuanhu oil field,Santanghu Basin

The Xishanyao reservoir in the Niujuanhu oil field of Santanghu Basin, though shallow (usually less than 2 000 m), is a low porosity and ultra-low permeability reservoir.The reservoir characteristics and the causes for low porosity and ultra-low permeability were studied by using core and cast thin section observations, conventional high pressure mercury injection experiments, SEM, and combined with regional tectonic data. The Xishanyao Formation was mainly composed of feldspar lithic sandstones. The reservoir includes intergranular pores, dissolution pores and fractures. Sedimentation, diagenesis and tectonic effects resulted in low porosity and ultra-low permeability. During the early deposition period of Xishanyao Formation, carbonate cementation was weak due to the acidic environment of coal-bearing formations.A strong and rapid compaction occurred due to the lack of support between particles. At the same time, sedimentation also controlled the original physical properties of the reservoir from the size of particle and the content of capping agent. The compaction effect during diagenesis led to a porosity reduction of 18%, and the acidic environment also led to a large amount of kaolinite formation, quartz secondary growth and the development of late iron-rich carbonate cementation with a cementing porosity rate of about 5%. The cracks formed by tectonic stress provided channels for late stage dissolution. However, rocks have become consolidated due to early compaction. What's more, the coal seam is thin, and the acidic fluid is not enough to flow in large scale through small particles, leading to poor dissolution strength. As a result, coarse throats which have a great contribution to permeability generated little, and a reservoir with low porosity and ultra-low permeability was formed.