伊拉克A油田白垩系孔隙型生屑灰岩高渗透层成因及分布特征

伊拉克A油田白垩系孔隙型碳酸盐岩高渗透储层具有分布普遍、类型复杂、成因多样等特征。岩石类型主要为泥晶生屑砂屑灰岩、亮晶生屑砂屑灰岩及泥晶生屑灰岩;生物扰动加剧了碳酸盐岩储层非均质性,导致高渗透储层在全油田广泛分布。通过岩心观察、常规薄片和铸体薄片的观察和测量,孔隙度、渗透率和毛管压力的测定,综合区域地质研究成果,发现高渗层的形成受层序演化、次级相对海平面变化、沉积环境、硬底形成、生物扰动及(准)同生期暴露淋滤等复杂多因素共同控制。高渗透储层形成模式表现为Khasib组沉积时期为晚高位域期,在Kh2-1-2L段上部沉积过程中,发生首次次级相对海平面下降,形成沉积间断。沉积间断期间硬底和区域规模性的生物扰动同时发育,在Kh2-1-2L段硬底尚未被胶结的相对疏松的基质中掘穴,形成厚几十厘米、迂曲状互相贯通的生物扰动通道。沉积间断结束后,在晚期高位域层序背景下,次级相对海平面上升,松散生屑砂屑充填扰动通道。由于沉积物堆积速率较快,限制了胶结作用的发生,使扰动通道内充填物能够发育连通性好的粒间孔,而后次级相对海平面再次下降,发生(准)同生期暴露淋滤。由于扰动部位连通性好,成为溶蚀流体优势通道,导致沿扰动部位发育溶孔及扩溶缝,形成了“粒间孔+溶孔+溶缝”高渗透网络。A油田高渗透储层物性表现出发育强烈生物扰动的部位具有明显高渗特征,其孔喉较粗,排驱压力低,孔喉配位数高、连通性好。高渗层全区稳定发育,由西至东厚度逐渐增大,高渗层的分布对油田单井产量、含水上升和油田采出程度影响较大。

High permeability zone of Cretaceous porous carbonate reservoir of A Field,Iraq: Genesis and distribution characteristics

The high permeability zone of Cretaceous porous carbonate reservoir with complex reservoir type and origin developed extensively cross the A Field, Iraq. The rock types in the high permeability zone are mainly argillaceous clastic limestone, clastic limestone and micrite bioclastic limestone, whilst bioturbation increases reservoir heterogeneity causing it to expand. Here, we carried out core and thin section analyses along with porosity, permeability, capillary pressure and probe permeability measurements to investigate the characteristics and genesis of the high permeability zone. Combined with regional geology, we conclude that the formation of high permeability zone is controlled by stratigraphic evolution, sea level change, sedimentary environment, bioturbation, etc. The high permeability zone is thought to originate in the lower Khasib Formation, where in the Kh2-1-2 layer during deposition sedimentary discontinuity is formed as sea level declines, meanwhile bioturbation leads to formation of burrow network tens of centimeters thick. As deposition ends, sea level rises and bioclastic grainstones fill the pores, without cementation, to form finely connected pores. As sea level declines again, the reservoir is exposed, and dissolved pores are generated. Thus a high permeability zone made of intergranular pores, dissolved pores and dissolved seams is formed. In the high permeability zone of the A Field, locations with physical properties consistent with strong bioturbation—such as relatively thick pore throat, high pore-throat coordination number, and good connectivity—have obvious high permeability characteristics. The high permeability zone, developing stably across the oil field with its thickness gradually increasing from west to east, greatly influences the development and production of the oil field.