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Characteristics and Origin of the Carbonate Cements in Sandstone Reservoir under Multi-episode Oil Entrapment: A Case Study from the Eocene Shahejie Formation, Chexi Sub-sag, Bohai Bay Basin, China


Carbonate cementation is an important factor that can affect the accumulation of oil in sandstone reservoir. The characteristics and origin of carbonate cements of the Eocene Shahejie sandstones in the Chexi Sub-sag were deduced based on comprehensive analysis of thin sections, cathodoluminescence, SEM, electron microprobe, XRD, stable carbon and oxygen isotopes, and fluid inclusions. Two episodes of oil charging events are identified. The first oil charged during 28-25 Ma ago, and the second within 15-0 Ma. Three phases of carbonate cements are identified according to their relationships with comprehensive analysis and oil charging events. Early carbonate cements with orange luminescence were strong dissolved. Interim ferroan carbonate cements with orange-red luminescence often filled in corrosion pores of feldspar or along the side of intergranular pores while late ferroan calcites with dark red luminescence developed in the center of the intergranular pores. Carbon origin of early carbonate cements in the sandstone reflect a mainly inorganically sourced (δ13C values: -2.88‰ - 0.34‰), in contrast, others’ carbon origins (richer in Fe, δ13C values: -6.68‰ to -12.15‰) are related to early carbonate cements dissolution and CO2 expelled from the thermal decarboxylation of organic matters in the adjacent source rocks. Carbonate cementation was limited in the sandstones with high mud contents and ductile grains, which had been strongly compacted before the first oil arrival and were not charged later. Samples with high oil saturation have low carbonate cements because the early carbonate cements are dissolved completely by organic acids and the early oil filling inhibits the formation of carbonate cements in the interim and late episode. Low oil saturation in early charge does not effectively inhibit the interim and late carbonate cementation, thus the contents of carbonate cements are higher in samples with low oil saturation. Early carbonate cementation before the first oil charging can inhibit compaction and their late dissolution improves the physical properties of the sandstone reservoir, while a large amount of interim and late carbonate cements is ubiquitous and reduce the reservoir properties. The cementation is mainly dominated by ankerite in the samples associated with high oil saturation; therefore, ankerite may be good mineralogical signatures of oil charging and migration.