Origin of carbonate cements in deep sandstone reservoirs and its significance for hydrocarbon indication: A case of Shahejie Formation in Dongying Sag

Abstract

Carbonate cements are primary cement types formed in deep sandstone reservoirs of Dongying Sag. We have proposed three stages of carbonate cements with different origin and material sources: carbonate cements in early stage are rim-shaped high-Mg calcite, which is the product of quasi-contemporaneous period; and calcite filled with primary pores without obvious compaction and diagenetic transformation is mudstone compaction during the drainage process. Carbonate cements in middle stage are calcite and dolomite filled with feldspar secondary dissolved pores. The rich Ca2+, Mg2+, and CO32−{\text{CO}}_{3}^{2-} in overpressure fluid enter the reservoir and mix with Ca2+ in the original formation water. Carbonate cements in late stage are ferrocalcite and ankerite that filled the dissolution pores of early- and middle-stage carbonate cements. They were products of CO32−{\text{CO}}_{3}^{2-} formed by organic acid splitting decomposition in late diagenesis and CO32−{\text{CO}}_{3}^{2-} formed by dissolution of carbonate cements in early and middle stages, combined with Mg2+, Ca2+, Fe2+ plasma in pore fluid. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for obvious positive drift in the δ 13CPDB‰ values of carbonate cements. Carbonate cements in middle stage and late stage, respectively, represent the early hydrocarbon charging of Dongying Formation and the end of Guantao Formation to the present.

Keywords

• dongying sag
• material sources
• carbonate cements
• hydrocarbon charging