Sedimentary Environment and Lithofacies of Fine-Grained Hybrid Sedimentary in Dongying Sag: A Case of Fine-Grained Sedimentary System of the Es₄

In order to predict the favorable lithofacies of shale fine-grained migmatite in the continental fault depressed lake basin and guide the optimization of shale oil and gas favorable targets, the sedimentary environment restoration and fine lithofacies characterization of shale fine-grained migmatite in the upper part of Es₄ Formation in Dongying sag are systematically carried out by using the data of core, thin section, whole rock diffraction, element and paleontology, etc., revealing the sedimentary environment and its combination and division to the lithofacies. The results show that the depositional period of shale fine-grained migmatites in the upper part of Es₄ Formation in Dongying depression is generally characterized by the transformation of climate from semi humid to humid, from bottom to top, it shows the process of increasing injection amount of clastic material source, increasing water depth, decreasing salinity and decreasing reducibility; the orderly and complex sedimentary environment controls the complexity of sedimentary fabric of shale fine-grained mixed rock to a certain extent, and then controls the diversity, combination and distribution regularity of fine-grained mixed rock facies. Based on the 4-terminal element division scheme of rock composition, sedimentary structure, calcareous structure and organic matter abundance, the shale fine-grained migmatites in the upper Es₄ of Dongying depression are divided into 20 types, and the complex fine-grained migmatites are divided into different lithofacies. Under the condition of semi humid with less material source, the shallow lake strong reducing salt water environment mainly developed gypsum salt, organic laminated gypsum mudstone combined with organic laminated argillaceous limestone (dolomite) facies, while the semi deep lake strong reducing salt water environment mainly developed organic laminated microcrystalline argillaceous limestone combined with organic laminated lime mudstone frequent interbedded lithofacies. In the strong reducing and brackish water environment of the semi deep lake, there are frequent interbedded lithofacies combined with organic rich horizontal micrite and organic rich laminated limestone mudstone, while in the reducing and brackish water environment of the deep lake, there are mainly organic bedded argillaceous limestone with organic rich bedded calcareous mudstone lithofacies combination. Under the condition of moist and multi-source, the deep lake strong reducing brackish water environment mainly develops the frequent interbedded lithofacies combined with organic rich layered argillaceous limestone and organic rich layered calcareous mudstone, while the deep lake reducing brackish water environment mainly develops the organic rich layered calcareous mudstone mingled with organic rich layered argillaceous limestone lithofacies association.