Controls on the Organic Matter Accumulation of the Marine- Continental Transitional Shanxi Formation Shale in the Southeastern Ordos Basin

The marine-continental transitional shale with favorable geological conditions for shale gas accumulation and broad resource prospects is widely distributed in China, which is also well developed in the Ordos Basin. The reservoir characteristic and gas-bearing properties of the transitional Shanxi Formation shale have been studied in previous studies. However, the factors influencing the organic matter (OM) accumulation have not been well studied, which restricts shale gas exploration and development of the Shanxi Formation in the Ordos Basin. According to analyses of organic geochemistry, mineral compositions, and major and trace elements, this paper has studied the paleoenvironment characteristic and its influence on the OM accumulation of the Shanxi Formation shale. The results indicate that the OM is characterized by the high mature stage and type III kerogen while the total organic content (TOC) of Member 1 is higher than that of Member 2. From Member 1 to Member 2, the paleowater depth gradually decreases, along with a gradually relative cold and dry climate, decreasing the terrestrial influx intensity and paleoproductivity and increasing the oxygen content in the water column. For Member 1, the OM accumulation is mainly controlled by the terrestrial influx intensity and paleowater depth, and other paleoenvironment factors have an unobvious contribution to the OM accumulation. For Member 2, the OM accumulation is commonly controlled by the weak terrestrial influx, low paleoproductivity, and oxic water column, resulting in the low TOC of Member 2. This study reveals the paleoenvironment characteristic and controls on the OM accumulation of the Shanxi Formation shale, which is beneficial to the reservoir selection of the Shanxi Formation in the southeastern Ordos Basin and the understanding of the OM accumulation in other transitional shales.