Lacustrine shale lithofacies and depositional environment in the paleocene second member of the funing formation, subei basin, China: Insights into shale oil development prospects

Abstract

Lacustrine fine-grained sedimentary rocks play a crucial role in the petroleum industry, particularly in the exploration of unconventional oil and gas resources. This study focuses on a petroliferous fine-grained succession deposited prior to the Paleocene–Eocene Thermal Maximum (PETM), specifically, the Paleocene second member of the Funing Formation. This formation represents the primary target for shale oil exploration in the Qintong Depression of the Subei Basin. Employing detailed sedimentological descriptions and utilizing multiple geochemical proxies, our investigation aimed to investigate lithofacies distribution and reconstruct the paleoenvironmental conditions prevalent in the Qintong Depression. Through the integration of lithological observation and well log data, the second member of the Funing Formation was categorized into three distinct system tracts: the early expanding system tract (EEST), the late expanding system tract (LEST), and the highstand system tract (HST). Within the established sequence stratigraphy framework, six lithofacies were identified, displaying comparable lateral and planar distributions. Results indicate that lithofacies exhibit greater thickness in the Deep Sag zone compared to the Slope and Fault Terrace. The EEST is characterized by laminated calcareous mudstone (LCM), laminated dolomitic mudstone (LDM), and massive argillaceous dolomite (MAD). Whereas the LEST predominantly comprises LCM and massive calcareous mudstone (MCM). These lithofacies were deposited under relatively arid climatic conditions, characterized by high salinity and strong terrigenous input, with a moderate abundance of organic matter consisting of type Ⅱ and type Ⅲ kerogen. In contrast, the HST is dominated by MCM with stable thickness, indicative of deposition under relatively humid climatic conditions with low salinity and terrigenous input. These conditions fostered an environment conducive to plankton and algae, resulting in a high abundance of organic matter composed of type Ⅰ kerogen. These new characterizations have significant implications for considering favorable lithofacies regarding the vertical orientation and target zones regarding the planarity of shale oil resources in the Qintong Depression of the Subei Basin.