Influence of multistage oil emplacement on carbonate cementation in the Eocene Shahejie Formation, Dongying Sag, Bohai Bay Basin, China

Abstract

Abstract The influence of multi-stage oil emplacement on carbonate cementation was investigated on the basis of determining the sequence of oil emplacement and carbonate cementation in the clastic reservoirs of the upper part of the fourth member of the Eocene Shahejie Formation (Es4s) in the Yanxie 229 area, Dongying Sag. The sequence of multistage oil emplacement and carbonate cementation was determined by projecting the homogenization temperatures (Ths) of two-phase aqueous inclusions on a burial and thermal history plot. The ratio of the number of quartz grains with yellow-fluorescent oil inclusions therein to the total number of quartz grains (YGOI) and the ratio of the number of quartz grains with blue-fluorescent oil inclusions therein to the total number of quartz grains (BGOI) were calculated to analyze the paleo-oil saturation of each stage of oil emplacement. The YGOI and BGOI values reflect the relative level of the paleo-oil saturation of the first-stage yellow-fluorescent (FSYF) oil and the second-stage blue-fluorescent (SSBF) oil, respectively. A large number of primary pores were preserved in the oil layers, whereas only a small number of dissolution pores and micropores were developed in the oil-bearing water layers and dry layers. Calcite, dolomite, ferrocalcite, and ankerite cementation and two-stage oil emplacement occurred in the Es4s reservoirs. The FSYF oil emplacement occurred earlier than the ankerite cementation and ferrocalcite cementation but later than the calcite cementation and dolomite cementation. The SSBF oil emplacement occurred later than all phases of carbonate cementation. The dolomite cementation was not influenced by the FSYF oil emplacement because it occurred earlier than the FSYF oil emplacement. The ankerite cementation was dominantly inhibited by the FSYF oil emplacement in the oil layers and the degree of inhibition increased with the increase in paleo-oil saturation of the FSYF oil. The SSBF oil emplacement further increased the paleo-oil saturation in the oil layers, which enhanced the inhibition of the ankerite cementation and even stopped it. The ankerite cementation in the dry layers and oil-bearing water layers was slowed down or stopped due to the lack of pores and the low transportation rate of external material, which was caused by the intense ankerite cementation prior to the SSBF oil emplacement.