Lithofacies characteristics and sedimentary environment of Es4U and Es3L:A case study of Well FY1 in Dongying sag, Bohai Bay Basin

Characteristics summarized through core and microscopic thin section observations, including the organic matter content, inorganic mineral composition and sedimentary structure, were used to categorize the lithofacies types of the fine-grained sedimentary rocks in the upper section of the fourth member of Shahejie Formation(Es4U) and the lower section of the third member of Shahejie Formation(Es3L) in Dongying sag, Bohai Bay Basin. As a result, 11 lithofacies types were identified. The vertical variation of the sedimentary environment was analyzed in terms of paleoclimate, palaeosource, paleo-water-depth, paleosalinity and paleo-oxidation-reduction, based on the content of major and trace elements as well as vertical variation of specific element ratios. The fine-grained sedimentary rocks formed in semi-deep to deep brackish-to-saline lacustrine environments with relatively humid climate and reduction-to-strong reduction. In addition, the evolution of sedimentary environment is obviously periodic, featuring increase in water depth, decrease in salinity, climate change from arid to humid, change from strong reduction to weak reduction then to reduction, and terrigenous sediment supply change from large to small then to large quantities. All the five elements of sedimentary environment collectively influence the vertical distribution and combination of lithofacies types. With the water depth increasing, the proportion of carbonates to lithofacies assemblages will decrease. Without considering the gravity flows, a combination of water depth, offshore distance and terrigenous debris supply will affect the contents of quartz and feldspar, which will in turn influence the development of siltstone and silty fine-grained sedimentary rocks. The setting featuring high salinity and strong reduction facilitates the preservation of organic matters, prone to produce organic-rich lithofacies types. Variation of paleoclimate makes other paleoenvironmental elements change correspondingly, then affecting the distribution and combination of lithofacies types. © 2019, OIL & GAS GEOLOGY Editorial Board. All right reserved.