Mechanisms for the anisotropic enrichment of organic matter in saline lake basin: A case study of the Early Eocene Dongpu Depression, eastern China

Abstract

Organic-rich terrestrial shales, the most promising shale oil production layer, are widely distributed in major basins in China, and they are usually interbedded salt rocks. Understanding the role of salt rocks in the organic matter (OM) enrichment in shale sediments has important enlightenment for shale oil exploration. The shale of the third member of the Eocene Shahejie Formation (Es3 shales) in the Dongpu depression of the Bohai Bay Basin is dominated by clay minerals, with a non-negligible proportion of gypsum, anhydrite, and halite. They contain abundant OM with a mean total organic carbon content of 1.26 wt %. The development of salt rocks revealed the arid and transiently humid climate during the Es3 period and an evaporating environment with a shallow lake level at the Dongpu Ancient Lake Basin. OM enrichment was synthetically affected by the palaeoenvironment during the Es3 period. The paleolake with a paleosalinity as high as 23.43‰ was nutritious and promoted the prosperity of salt loving organisms causing high paleoproductivity. The Es3 shales were deposited under anoxic and dysoxic conditions in a deep lake, which provided good conditions for OM preservation. In addition, salt rocks itself are good caprocks to prevent the loss of shale oil. Under the dual guarantee of high productivity and good preservation conditions, OM is much enriched in the Es3 shales. OM enrichment was synthetically affected by the palaeoenvironment during the Es3 period. The horizontal difference of paleoclimate controlled the discrepancy of salinastones distribution, OM enrichment, paleosalinity, andpaleoproductivity in different saline surroundings of Dongpu Depression. The composite patterns of palaeoenvironmental variation affected the vertical anisotropic enrichment of OM. Seven stages were divided for the Es3 period to reflect the effects of vertical anisotropy on OM enrichment. Stages I, V, and, VII deposited thick salt under an arid climate and provided a good seal-capping and preservation condition for the OM enriched in stages II, III, IV, and VI. The shale reservoirs deposited in stages III and IV were favorable for shale oil exploration.