Abstract
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 CO 3 2 − {\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 CO 3 2 − {\text{CO}}_{3}^{2-} formed by organic acid splitting decomposition in late diagenesis and CO 3 2 − {\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.
Highlights
Carbonate cements are important type of cement in sandstone diagenesis demonstrating multistage precipitation, complex and diverse genesis, and wide distribution and can be generated in different geochemical environments [1,2,3]
The morphological and geochemical characteristics of carbonate cements in the sandstone of Shahejie Formation in Dongying Sag were studied by thin-section casting, cathodoluminescence, backscattering, electron probe analysis, and isotope analysis
The carbonate cements in the early stage are mainly calcite, which can be divided into two types based on their production: the first type is mainly micritic high magnesium calcite, which usually has a good degree of automorphism, and is mostly in the form of micritic equal thickness rim, which is semi-basal-basal cementation and occurs on the surface of a small amount of clastic rock particles (Figure 3a)
Summary
Carbonate cements are important type of cement in sandstone diagenesis demonstrating multistage precipitation, complex and diverse genesis, and wide distribution and can be generated in different geochemical environments [1,2,3]. Due to differences in material sources and precipitation mechanisms, different oxygen isotope values during the precipitation of diverse carbonate cements indicate the presence of different fluid–rock interaction systems and strengths [9]. The formation temperature of such carbonate cements is generally low [5,10]; carbonate cements in the middle and late stages have relatively high inclusion temperatures, indicating that they precipitate at elevated temperatures during diagenesis [5,9]. Material sources involved in redeposition-diagenesis mainly determine carbonate cement formation, such as Ca2+, Mg2+, Fe2+, This work is licensed under the Creative Commons Attribution 4.0
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