Abstract:

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.

Key words: carbonate reservoir, high permeability zone, bioturbation, dissolution, intergranular pore, Cretaceous Khasib, Iraq