Development and controlling factors of the carbonate cementation in the beach-bar reservoirs in the upper part of the fourth member of the Shahejie Formation on the southern slope of the Dongying depression

The sandstone-mudstone interface is considered as the active zone of water-rock interaction in interbedded sandstone-mudstone reservoirs. The littoral-shallow lake beach-bar deposits occur on a wide range of scales in the upper part of the fourth member of the Shahejie Formation on the southern slope of the Dongying depression, in which the carbonate cementation is highly intense, and results in the formation of the tight carbonate cement layers at the sandstone-mudstone interface. The present paper deals systematically with the development and controlling factors of the carbonate cementation on the basis of well logs, cast sections, physical properties and clay minerals. The carbonates have the features of multiphase cementation. The cement contents are higher at the sandstone-mudstone interface showing the basement-type cementation, and decrease gradually toward the central parts of the sandstones, showing the porosity-type cementation. The development of the cement layers is controlled by the fluids with high carbonate contents in the littoral-shallow lake mudstones formed in the semi-arid alkaline water environments, organic and inorganic components precipitated from the transformation of clay minerals and organic matter evolution, accommodation spaces for the carbonate cementation, transmission capacity of the matter between sandstones and mudstones, sandstone thickness, and lithofacies associations. The pressure resistance effects of early carbonate cementation on the reservoirs and abnormal pressure systems formed by dense layers may provide favourable conditions for the development of later dissolution pores, and filling and preservation of hydrocarbons in the reservoirs.