Impacts of Basement Lithology on Reservoir of Lacustrine Carbonate and Clastic Mixed-Deposition in Member l of Shahejie Formation, Bohai Sea Area

Lacustrine carbonate and clastic mixed-deposition is developed in member 1 of Shahehjie Formation of Bohai Sea area, Bohai Bay basin. Exploration practice indicates that lacustrine mixed-deposition at deep burial depths is higher in production capacity, with proved reserves of more than 3×10⁸ m³, since it can retain higher physical properties. In this paper, it proposes that volcanic rock basement is one of the important factors of mixed-deposition scale development, based on the geological analyses, combined with thin section, isotope, and elemental analyses, and physical simulation experiment. Volcanic rocks in the basement consist of the following three types:the Mesozoic volcanic rock basement, Paleogene volcanic rock basement, and sedimentary bodies with provenance from the volcanic rocks. Simulation experiments show that the dark minerals in the volcanic rocks in the hydrolysis process can lead to enriched metal ions in surrounding waters, and as a result, microbes arise and microbial ecological community develops, which provides conditions to enrichment of bioclastics. The discovery of dolomization filamentous cyanobacteria and globular dolomite affirms the existence of penecontemporaneous biological dolomization in mixed-deposition. Early dolomization is advantageous to the reservoir property and the late dolomization, facilitating the nucleation for later dolomization. Volcanic rock lithic fragment (gravel) internal mould pores and dissolution pores account for 10%-30% of reservoir space, with significant contribution to reservoir property. Dark mineral dissolution simulation experiments confirm that volcanic hydrolysis solution can provide Mg²⁺ ions, which facilitates the burial dolomization.