Identifying intergranular pore types by distinguishing between cementation and dissolution of dotted calcite: A case study of the Xinhe Formation sandstones in the Yabrai Basin, China

Abstract

There is still no consensus among researchers about whether intergranular pores in oil-bearing sandstone are primary pores or secondary pores. Distinguishing between cementation and dissolution of dotted calcite can effectively identify intergranular pore types. In this paper, taking the diagenetic environment evolution and diagenesis sequence as the thread, we carefully observed the casting slices from the Xinhe Formation sandstones in the Yabrai Basin and finely dissected the microphenomenon by focusing on the calcite cements in its relation between the origins of substance and dissolution fluids and the occurrence mode. The genesis of the dotted calcite was clarified thus, and then the intergranular pore types and reservoir space types in the sandstones were identified. The study results show that the dotted calcite in the intergranular pore is the dissolution residue of the disseminated calcite formed in the early diagenetic stage, and the dissolution type is the consistent dissolution. The dissolution fluid, organic acid fluid formed during the middle diagenetic stage, caused the secondary pores. Therefore, it is concluded that the reservoir space in the Xinhe Formation sandstones in the Yabrai Basin consists of the secondary intergranular pores and the secondary intragranular pores such as dissolution pores of cements of feldspar, lithoclast, lithoclast.

Keywords

• dotted calcite
• cementation and dissolution
• intergranular pore
• xinhe formation sandstones
• yabrai basin