Shiyou shiyan dizhi (Sep 2023)
Characteristics, controlling factors and exploration prospects of microbial dolomite reservoirs in the second member of Dengying Formation, Penglai-Zhongjiang area of central Sichuan Basin
Abstract
The formation and evolution of dolomite reservoirs in the Sinian Dengying Formation in the Sichuan Basin have attracted much attention recently. Due to their economic significance for hosting natural gas resources, determining the key factors that govern the formation and evolution of these dolomite reservoirs are important for hydrocarbon exploration and development. Based on the drilling and seismic data in the Penglai-Zhongjiang area of the Sichuan Basin, the main controlling factors for the development of high-quality reservoirs in the second member of Dengying Formation are investigated by integrating core and thin section observation and geochemical analysis. Lithologies of the reservoirs in the second member of Dengying Formation are dominantly microbial dolomites, dolo-grainstones, and brecciaed dolomites. Moreover, reservoir spaces in these dolomites are mostly dissolution pores, residual framework pores, and breccia pores. The porosity ranges from 2.0% to 8.0% with an average of 4.39%, and the average permeability is 0.53×10-3 μm2, and the reservoir thickness is 170-320 m. Synsedimentary faulting had caused the differentiation of depositional paleo-geomorphology and led to the formation of submarine high barriers, which provided favorable conditions for the development of microbial mound (reef) and shoal complexes. Furthermore, fault activities could have broken the weakly consolidated carbonate sediments into breccias, thus resulting in the brecciaed dolomite reservoir. Penecontemporaneous dissolution is the key to the development of high-quality reservoirs in the second member of the Dengying Formation, which is consistent with their occurrence in the upper part of the shallowing-upward cycles. Differentiation in microfacies leads to distinct diagenetic pathways and porosity evolution of these microbial-dominant carbonate sediments. Overall, dolomite cementation during shallow burial had controlled thedegree of pore preservation. It is predicted that three fault-controlled, platform-margin mound regions in the study area, i.e., Penglai-Zhongjiang, Yanting-Mianyang, and Cangxi-Guangyuan, with areas of 1 600 km2, 1 870 km2 and 2 280 km2, respectively, had been developed in the second member of Dengying Formation in the north slope of the central Sichuan paleo-uplift, showing multi-stage and multi-zone characteristics. Microbial mounds in the platform margin of the second member of Dengying Formation in the Yanting-Mianyang and Cangxi-Guangyuan areas, with three sets of high-quality source rocks, may form a variety of favorable source-reservoir combinations. Additionally, the accumulation conditions may have been superior. The estimated natural gas resources in the study area, over one trillion square meters, makes it a favorable area for ultra-deep carbonate exploration and is expected to be a breakthrough site for the next one trillion square gas field in the Sichuan Basin.
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