Characteristics and sedimentary environment of organic-rich shale in the second member of Paleogene Funing Formation, Subei Basin

Abstract

Two sets of organic-rich shales are developed in the second member of Funing Formation (E1f2) in the Subei Basin, which are E1f2shale 1 and E1f2shale 2. They are widely distributed, with rich shows and favorable shale oil conditions. Based on core and thin section observations, and combined with systematic organic geochemistry, X-ray diffraction and trace element analyses, the characteristics and sedimentary environment of the organic-rich shales were studied. The characteristics of the E1f2shale 1 and E1f2shale 2 are totally different. The E1f2shale 1 mainly consists of dark gray blocky limestones and siliceous mudstones, with slight bedding, 2.21%-3.41% organic carbon content, and type I organic matter. The E1f2shale 2 mainly consists of dark oil shales and dolomitic mudstones, calcite mudstones, muddy dolomites and muddy limestones, with laminar bedding, 2.02%-2.99% organic carbon content, and type I organic matter. The sedimentary palaeo climate of E1f2 has experienced semi-arid, reducing, brackish water, to dry, strongly reducing, salt lake, and to warm humid, oxic, freshwater in the Subei Basin. The organic-rich shales were formed in the transition formations near the maximum flooding surface, between strongly reducing, dry and hot, saltwater environments and oxic, humid, normal water environment. The E1f2shale 2 was formed in a strongly reducing, dry and hot, salt lake environment, while the E1f2shale 1 was deposited in a strongly reducing, damp to semi-arid and brackish water environment. Sedimentary environment evolution resulted in the differences of lithology, electrical properties, and quality of hydrocarbon source rocks, occurring in the muddy shale interlayer and internal organic-rich shale.

Keywords

• organic-rich shale
• sedimentary environment
• shale oil
• second member of funing formation
• paleogene
• subei basin