Analysis of dolomite reservoir characteristics of Lower Permian Maokou Formation and dolomite origins in Chuanzhong Area

Abstract

Dolomite of Lower Permian Maokou Formation has a strong heterogeneous formation in-plane and vertical in Chuanzhong Area. Its formation mechanism is not yet fully understood. Thus, this study focuses on the Maokou Formation core and conducts thin section analysis, isotope analysis, cathodoluminescence, and data analysis of fluid inclusion using a combination of geological and chemical analysis methods to summarize the characteristics of dolomite reservoir layer of Maokou Formation and its formation mechanism. The Maokou Formation is believed to be the result of a deformity of dolostones, medium–fine grained dolomite, and a few gray dolomites, with the exception of gypsum layer. In the reservoir space of dolostones, pore type occupies the first layer, and dissolved seam and particle-dissolved pores are in the second layer. The dolomite value of δ 13 C was distributed in the range of 1.78‰–3.95‰(PDB), and the value of δ 18 O was distributed in the range of −7.39‰ to −3.8‰(PDB) with above 120 salinity. Thus, the dolomite reservoir was formed in a normal marine environment. Inclusions show that dolomite in the study area have a multi-phase fluid filling and a homogenized temperature of the maximum value of above 160 °C, which is not within the value that the normal temperature gradient can reach. Thus, buried dolomitization, seepage flow, Penesyndiagenesis, and Sabha genetic model are ruled out from the study area of Maokou Formation. The fault development that connected the basal layer with the stratum of Maokou Formation was distributed in the G3 well area, which provides a channel for the upper channeling of the basal hydrothermal fluid. In the Chuanzhong Area, the dolomite of the Lower Permian Maokou Formation is considered to be mainly of tectonic-hydrothermal origin.