Diagenesis of carbonate rocks in the lower part of 1st Member of Eogene Shahejie Formation in Huanghua depression

Abstract：There is a set of lacustrine facies carbonate rocks 20-60m in thickness in the lower part of 1st Member of Eogene Shahejie Formation in the Huanghua depression. There exist rich oil and gas resources in biological limestone, oolitic (dolomite) limestone and micrite dolomite, suggesting good exploration prospects. Through macro- and micro- observation of thin sections and rocks from more than 10 wells in combination with geochemical characteristics, the authors made a detailed investigation into diagenetic types and characteristics and examined the relationship between the diagenesis and the evolution of reservoir space. The results show that the diagenetic evolutionary process of carbonate rocks in the lower part of 1st Member of Shahejie Formation was very complex and characterized by a variety of diagenetic types, and that the reservoir quality was obviously affected by the diagenetic process. The compaction and pressure solution action and the multi-stage calcite and dolomite cementation resulted in drastic reduction of intergranular holes, biological coelomopores and framework holes. The mixed water dolomitization and the syngenetic-penecontemporaneous and burial dissolution constitute the keys to the formation of high quality reservoir, with the formation of massive intercrystal pores, intercrystal dissolution pores, intragranular dissolution pores, moldic pores, biological coelomopores, intergranular dissolution pores and corroded fissures, which have now become the main oil and gas accumulation space.