Sedimentary texture differences between lithofacies types and their control on the physical property in fan-delta conglomerate reservoirs: A case study from the Triassic Baikouquan Formation in Mahu Sag, Junggar Basin, Northwestern China

Abstract

Conglomerate reservoir developed in fan-delta environment is characterized by the diverse lithofacies types and strong heterogeneity. Lithofacies type is the main primary factor that controls the reservoir physical properties. However, quantitative sedimentary texture parameters of lithofacies types are scarcely studied, and their influence on the reservoir physical properties are still poorly understood. In this paper, lithofacies type, sedimentary texture and pore type, porosity and permeability of the Baikouquan Formation in Mahu Sag were studied to reveal the control of sedimentary texture parameters on the reservoir physical properties.The results showed that (1) lithofacies types in Baikouquan Formation can be classified into Gcm1, Gcm2, Gcm3, Gcm4, Gc, Sm1, Sm2, MSm, SSr, SSp and Mh; (2) different sorting coefficient and matrix content are present in Gcm, Gc, Sm1, Sm2 and MSm. These two texture parameters are highest in Sm2 and MSm, below Gcm and Gc, and the lowest in Sm1; (3) intra-gravel microfracture, gravel-edge microfracture and intra-matrix microfracture develop in conglomerates, which can improve their permeability; (4) porosity in Gcm, Gc, Sm1, Sm2 and MSm and permeability in Sm1, Sm2 and MSm have negative correlations with their sorting coefficient and matrix content. However, permeability in Gcm and Gc have poor correlations with these two texture parameters due to microfractures; (5) Gc and Sm1 have relatively high porosity and permeability due to their good sorting, low matrix content and consequently strong dissolution. By contrast, Gcm, Sm2 and MSm have relatively low porosity and permeability due to their bad sorting, high matrix content and consequently weak dissolution. These results indicate that Gc and Sm1 deposited in subaqueous distributary channels and lobes should be the first target in hydrocarbon exploration of fan-delta conglomerate reservoir.