Characteristics and dominating factors of lamellar fine-grained sedimentary rocks: A case study of the upper Es_4 member-lower Es_3 member, Dongying Sag, Bohai Bay Basin
A case study was made in the Dongying Sag to meet the needs of shale oil zone evaluation.The characteristics and dominating factors of lamellar fine-grained sedimentary rocks from the upper Es_4 member to the lower Es_3 member in the Dongying Sag were studied based on core and cast thin section observations combined with drilling and logging data. By comparing the lamellar fine-grained sedimentary rocks in ancient lakes with those in recent lakes, five types of laminae were identified, including silty, clay, carbonate, organic-rich and mixed ones. The origin and sedimentary environment of each type of laminae was analyzed. The study considered both macroscopic and microscopic features of laminae and the factors of sedimentary source, transport process and depositional environment, and made a conclusion that tectonic movement, climate change and source determined laminar characteristics as follows. (1) From the upper Es_4 member to the lower Es_3 member, the distribution zone of lamellar fine-grained sedimentary rocks migrates from east to west, which is compatible with the evolution tendency of the tectonic subsidence center. Different laminar types developed in different tectonic units. The organic-rich laminae are mainly distributed in the deep depression and the southern slope. The silty laminae occur in the central anticline and the southern slope. The carbonate laminae are distributed in the inner slope. The mixed laminae are found in the central anticline and the southern slope. The clay laminae are distributed widely in all the above mentioned tectonic units. (2) An arid climate led to high lake salinity and carbonate laminae were deposited. So the carbonate laminae are relevant to an arid climate. The organic-rich laminae tend to be deposited in damp climates, which are well-developed in lacustrine transgressive system tracts from the upper Es_4 member to the lower Es_3 member. (3) Provenance has a significant control on limestone laminae distribution. A development mode of lamellar fine-grained sedimentary rocks was proposed.