Early Diagenesis In Carbonate Sediments

Extensive lacustrine carbonate-rich shale intervals including upper fourth member (Es4s shale) and lower third member (Es3x shale) of the Paleogene Shahejie Formation have been deposited in the Boxing Sag, Dongying depression, Bohai Bay Basin, East China. These two representative shale members are considered to be good exploration prospects in the study area. In order to clarify the diagenetic process and exploration significance of sparry calcite in lacustrine calcareous shales, we take calcareous shale in Es4s to Es3x members in Boxing Sag as the research object, By means of thin section identification method, EDS line scan, Scanning Electron Microscopy, Fluid Inclusion Analysis, carbon and oxygen isotope and other methods, sparry calcite is classified, and then the genesis and diagenetic fluid of various types of sparry calcite are studied, so as to clarify the formation mechanism of sparry calcite, and finally discuss the influence of sparry calcite on reservoir. Finally, the influence of sparry calcite on the development of reservoir pores was discussed. The results show that the types of sparry calcite in the study area include granular calcite, porphyritic calcite and fibrous calcite, which are mainly caused by biological oxidation and thermal evolution of organic matter. Biogenic calcite has the characteristics of rich Mg and δ¹³C, and the organic matter makes the crystal more enriched with Fe and deficient δ¹³C. The formation of three kinds of sparry calcite has different effects on pore development, which is probably beneficial to improve reservoir physical properties.