Lithofacies and reservoir characterization of a source-controlled carbonate succession in a lacustrine rift basin, the Shulu Sag of Bohai Bay Basin, East China

Abstract

The lower part of the third member of the Eocene Shahejie Formation (Es3L) is one of the most important petroleum exploration targets in the Bohai Bay Basin. Source-controlled carbonate deposits are widely distributed in the Es3L unit of the Shulu Sag and display strong vertical and horizontal variations in lithofacies, geochemical, and reservoir characteristics. Based on core data, thin-section petrography, stable δ13C isotope analyses, and organic geochemistry data, eight lithofacies with distinctly different characteristics and origins are identified. Clast-supported terrigenous carbonate rudstone and matrix-supported terrigenous carbonate rudstone were sourced from the surrounding Cambrian-Ordovician carbonate bedrocks and are widely distributed at the edge of the sag in fan delta environments. Some of the rudstones were transported from the littoral zone to profundal zone by slide and slump processes. Mixed-source rudstone was mainly deposited by sandy debris flow. Carbonate lithic sandstone and carbonate lithic siltstone were mainly formed by turbidity current deposition in a sublacustrine setting. Irregular-laminated marlstone was mainly formed by muddy turbidity currents, and massive marlstone was formed by turbid clouds in a relatively turbulent deep water environment. Regular-laminated marlstone is thought formed by seasonal laminae due to seasonal variations by suspension settling in the anoxic deep, quiet lake environment. Calcareous mudstone possibly formed by shallow lake gravity flow in a shallow lake environment. Various of pores have been identified in the Es3L rocks: pores and cracks inside gravels, seams around gravels, intergranular pores, intercrystalline pores, intragranular pores, dissolution pores, organic matter pores, fractures and fracture-related pores. Different lithofacies have different space types, porosities and permeability. Clast-supported terrigenous carbonate rudstone has the best reservoir qualities, with the highest porosity and permeability. Laminated marlstone is the second best reservoir type. These two lithofacies can be the target for tight oil exploration in the study area.