New Geochemical Evidence for the Origin of Dolomites in the Second Member of the Lower Permian Maokou Formation in the Hechuan-Tongnan Area

The reservoirs in the second member of the Maokou Formation of the Lower Permian in the Hechuan-Tongnan area are mainly dolomite reservoirs. The genetic mechanism of dolomite is still unclear, which to some extent restricts the analysis of favorable reservoir distribution rules and the prediction of favorable reservoirs. In order to clarify the diagenetic environment and evolution characteristics of dolomitization, on the basis of mineralogical analysis such as core description and rock thin section identification, inclusion homogenization temperature, element geochemical analysis and laser in-situ U-Pb dating technology were used to obtain the elemental geochemical and chronological information of different dolomitization, and analyze the diagenetic environment and genetic mechanism of dolomitization. The analysis results indicate that the second member of the Maokou Formation has developed two types of dolomite reservoirs: matrix dolomite and saddle dolomite, which are formed by two stages of fluids. Geochemical evidence shows that matrix dolomite was formed earlier than the quasi-contemporaneous shallow burial period and the matrix dolomite is mainly composed of formation fluid. Saddle dolomite was formed in the middle burial period and was formed by geothermal formation water. The intercrystalline pores formed by matrix dolomite preserve the pores of the original rock. Saddle dolomite mainly grows along the inner wall of early formed dissolution fractures, leading to filling and partial-filling of fractures, which to some extent damages the physical properties of the reservoir. However, unfilled and partial-filled residual pores and fractures are still the main reservoir space of the reservoir. The research results confirm for the first time that saddle shaped dolomite is a formation fluid in the middle stage of burial, providing a geological basis for predicting favorable dolomite reservoirs.