Origin and distribution model of thin dolomite reservoirs in the lower sub-member of Mao 2 Member of Middle Permian Maokou Formation in Wusheng-Tongnan area, Sichuan Basin, SW China

Abstract

This paper discusses the characteristics and formation mechanism of thin dolomite reservoirs in the lower submember of the second member of the Permian Maokou Formation (lower Mao 2 Member) in the Wusheng—Tongnan area of the Sichuan Basin, SW China, through comprehensive analysis of geological, geophysical and geochemical data. The reservoir rocks of the lower Mao 2 Member are dominated by porphyritic vuggy dolomite and calcareous dolomite or dolomitic limestone, which have typical karst characteristics of early diagenetic stage. The dolomites at the edge of the karst system and in the fillings have dissolved estuaries, and the dolomite breccia has micrite envelope and rim cement at the edge, indicating that dolomitization is earlier than the early diagenetic karstification. The shoal facies laminated dolomite is primarily formed by the seepage reflux dolomitization of moderate-salinity seawater. The key factors of reservoir formation are the bioclastic shoal deposition superimposed with seepgae reflux dolomitization and the karstification of early diagenetic stage, which are locally reformed by fractures and hydrothermal processes. The development of dolomite vuggy reservoir is closely related to the upward-shallowing sequence, and mainly occurs in the late highstand of the fourth-order cycle. Moreover, the size of dolomite is closely related to formation thickness, and it is concentrated in the formation thickness conversion area, followed by the thinner area. According to the understanding of insufficient accommodation space in the geomorphic highland and the migration of granular shoal to geomorphic lowland in the late highstand of the third-order cycle, it is proposed that the large-scale shoal-controlled dolomite reservoirs are distributed along structural highs and slopes, and the reservoir-forming model with shoal, dolomitization and karstification jointly controlled by the microgeomorphy and sea-level fluctuation in the sedimentary period is established. On this basis, the paleogeomorphology in the lower Mao 2 Member is restored using well-seismic data, and the reservoir distribution is predicted. The prediction results have been verified by the latest results of exploration wells and tests, which provide an important reference for the prediction of thin dolomite reservoirs under similar geological setting.