Lithofacies Combination Characteristics and Distribution Law of Fine-grained Sedimentary Rocks in the Ek2 Member of Cangdong Sag, Bohai Bay Basin

The thick-bedded fine-grained sedimentary rock2) of the second member of the Kongdian Formation is a typical lacustrine source rock, and has been considered as the main target of shale oil exploration in recent years. Fine-grained sedimentary rocks have strong heterogeneity and poor plane continuity, which is the basis of shale oil and gas accumulation. Therefore, it is of great significance to strengthen the research on lithology, lithofacies and lithofacies combination of fine-grained sedimentary rocks.

Based on the observation and description of cores from Well G108-8, Well GD12, and Well GD14, combined with analysis of thin sections and X-ray diffraction data, lithofacies of the Ek2 fine-grained sedimentary rocks can be divided into six types, including laminated felsic fine-grained sedimentary facies according to mineral compositions and sedimentary structures, massive felsic fine-grained sedimentary rocks, laminated mixed fine-grained sedimentary rocks, massive mixed fine-grained sedimentary rocks, laminated carbonate fine-grained rocks, and massive carbonate fine-grained sedimentary rocks. Due to the vertical frequent alternation of lithofacies, lithofacies assemblages are proposed to describe the distribution law of the Ek2 fine-grained sedimentary rocks, which can be named felsic lithofacies assemblages, felsic bedded with carbonate lithofacies assemblages, mixed lithofacies assemblages, carbonate bedded with felsic lithofacies assemblages, and carbonate lithofacies assemblages. The mineral compositions, sedimentary structures, and containing lithofacies types show significant differential among these lithofacies assemblages, which can be consisted with the evolution of sedimentary environment.

The method of combining the acoustic travel time (AC) log and density log is established to identify lithofacies assemblages in the study area, which can be used to study the distribution law. Lithofacies assemblages transformed from mixed to felsic bedded with carbonate in the lower part of the Ek₂¹, while from felsic to carbonate in the upper part of the Ek₂¹. From the margin to center of the lacustrine basin, lithofacies assemblages have two trends during the sedimentary period of Ek₂¹. Lithofacies assemblages present felsic, felsic bedded with carbonate, and mixed from the margin to center with the humid climate and strong detrital input. Furthermore, lithofacies assemblages present mixed, carbonate bedded with felsic, and carbonate from the margin to center with the arid climate and weak detrital input.