Sedimentology and geochemistry of Eocene source rocks in the east China sea: Controls on hydrocarbon generation and source rock preservation

Abstract

As an important source rock for oil and gas generation, coal measure source rock has attracted much attention due to its high hydrocarbon generation potential. Therefore, we conducted a comprehensive evaluation of the source rocks based on organic geochemical and petrological data to clarify the sedimentary, geochemical, and hydrocarbon generation characteristics of the coal measure source rocks of the Pinghu Formation in the Xihu Depression. The source rocks were deposited in a shallow freshwater sedimentary environment with weak reduction-oxidation conditions. The organic matter is mainly derived from terrestrial higher plants, with less contribution from phytoplankton and bacteria. Compared to carbonaceous mudstone and mudstone, coal was deposited in an environment with stronger oxidation conditions, and stronger bacterial transformation. Within the study area, sag A receives relatively high contributions from phytoplankton and high liptinite content, whereas sag C receives the largest contribution from terrestrial higher plants. Sag A is dominated by source rocks with a relatively high oil generation potential and oil reservoirs, whereas other regions have limited oil generation capacity and are characterized by gas reservoirs and condensate gas reservoirs. The E2p2 member, with more terrestrial organic matter input, has a higher TOC content and hydrocarbon generation potential than the E2p1 and E2p3 members. Thick organic-rich mudstone is developed in the E2p2 member due to a high accommodation space, abundant terrestrial organic matter, corresponding to a high relative base-level. The vast intertidal setting is conducive to the development of peatlands, developing widely distributed carbonaceous mudstone and coal. The study interval is characterized by a coal measure source rock development model of ‘strong supply - rapid burial - facies-controlled’ in the Xihu Depression.