Genetic mechanism and permeability evaluation of low contrast oil reservoirs in M Oilfield of Wushi Sag

Abstract

The sandstone body of the main oil layer of M oilfield in Wushi Sag is large, and the lithology is mainly gravelly medium-coarse sandstone and sandy conglomerate, followed by fine sandstone.Due to the influence of special sedimentation and diagenesis, the shallow buried oil layer shows the characteristics of high resistivity, while the resistivities of some middle and deep buried oil layers are very close with that of the water layers, which brings great difficulties to the well logging based identification of fluid properties and quantitative evaluation of permeability.Based on the experimental data of NMR, mercury injection and cast thin sections, this paper analyzed the difference in reservoir characteristics between high resistivity reservoirs and low contrast reservoirs from the microscopic view.Then, based on the pore types (intergranular pores, mixed pores and mold pores), the resevoirs were diveded into three types, and the reservoir type classification standard were established. At last, the reservoir type-based permeability estimation models were proposed.The results show that the high irreducible water saturation caused by the complexity of pore structure is the main reason of the low contrast reservoirs. The accuracy of the estimated permeability is significantly improved after reservoir classification, which lays a solid foundation for the formulation and implementation of the oilfield development plan and post drilling evaluation.

Keywords

• low contrast reservoir
• genetic mechanism
• permeability
• pore type
• wushi sag1