Structure characteristics and major controlling factors of platform margin microbial reef reservoirs: A case study of Xiaoerbulak Formation, Lower Cambrian, Aksu area, Tarim Basin, NW China

Abstract

The structure characteristics and major controlling factors of platform margin microbial reef reservoirs in the Lower Cambrian Xiaoerbulak Formation in the Aksu area, Tarim Basin were analyzed based on 5 outcrop sections, 162 thin sections, 12 SEM samples, 52 sets of porosity and permeability data. Macroscopically, small-scale microbial reefs form the platform margin. A single microbial reef has several microbial reef progradation complexes, including reef front, fore reef, reef crest, and back reef, but microscopically, they have different kinds of microstructures. The reservoir spaces in the reefs can be divided into microbial structure reservoir space, including fenestral, frame, moldic, and oversized dissolution pores, and non-microbial structure reservoir space, including microcracks and stylolites. The statistical results of porosity and permeability show that in the Yutixi Section, porosity and permeability of the microbial reef reservoirs are generally below 5% and 1.0×10−3 μm2 respectively, characteristics of extremely low porosity and permeability reservoirs; while the reef reservoirs in the Sugaitebulake Section have a wide porosity range between 3% and 10%, and permeability range between 0.1×10−3 μm2 and 50×10−3 μm2 and strong heterogeneity, are low-medium porosity, low-medium permeability reservoirs. This demonstrates different microbial reefs have big differences in physical properties, and even the reservoir in the same reef has obvious heterogeneity. Paleotopography controls the formation of microbial reefs and sedimentation controls the facies distribution and the primary porosity development. Dissolution, controlled by the two former factors, finally decides evolution of reservoir pores. Key words: microbial reef, thrombolites, laminites, reservoirs, rimmed platform, Tarim Basin, Lower Cambrian, Xiaoerblak Formation