Depositional Architecture, Paleoenvironmental Dynamics and Hydrocarbon Potential of the Jurassic Da'anzhai Member, Sichuan Basin: A Comprehensive Sequence Stratigraphic and Geochemical Analysis

Abstract

The Early Jurassic Da'anzhai Member of the Ziliujing Formation, located in the Sichuan Basin, represents a key stratigraphic unit for unconventional hydrocarbon exploration due to its organic-rich facies and favourable depositional conditions. This study employs an integrated approach, combining lithofacies analysis, sequence stratigraphy and high-resolution paleoenvironmental reconstruction, to investigate the factors governing organic matter accumulation and to identify prospective exploration targets. Previous studies, supported by sedimentary and geochemical evidence, classify the Da'anzhai Member as a nonmarine lacustrine deposit, which is further categorised into three facies associations: marginal, shallow and semi-deep lacustrine. These are differentiated based on a combination of sedimentary structures formed by wave- and current-generated structures, fossil assemblages and lithological variations, all of which reflect differences in depositional energy and water depth relative to the wave base. A third-order sequence stratigraphic framework was established comprising four systems tracts: Highstand Systems Tract 1 (HST1), Transgressive Systems Tract (TST), Highstand Systems Tract 2 (HST2) and Falling Stage Systems Tract (FSST). Within this framework, paleoenvironmental conditions evolved in response to changing climatic and hydrological regimes. Geochemical proxies revealed a transition from a relatively arid, shallow, and brackish lacustrine environment during HST1 to a deeper, fresher and highly productive anoxic lake during the TST, which peaked at the Maximum Flooding Surface (MFS). This was succeeded by complex highstand deposition (HST2), followed by a return to shallower, more evaporative conditions during the FSST. The entire sequence was interpreted as a terrestrial basin's response to the global Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma). This allocyclic climate-forced model suggests that the initial lacustrine transgression was driven by an intensified hydrological cycle associated with peak T-OAE warming, whereas the ensuing regression during the FSST corresponds to the global climate system's recovery phase. This climate-driven model has significant implications for hydrocarbon exploration in lacustrine shale systems. This climate-forced model has direct implications for hydrocarbon exploration. The primary geological ‘sweet spots’ were concentrated around the MFS, corresponding to peak transgressive conditions with optimal organic matter preservation. However, the most favourable engineering sweet spots were found at the TST/HST2 boundary. These zones provide an optimal balance of high organic content and improved fracability, resulting from lower concentrations of ductile calcite and higher concentrations of brittle minerals such as quartz and feldspar. Collectively, the integrated sequence-stratigraphic and palaeoenvironmental framework developed in this study not only refines understanding of the depositional history of the Da'anzhai Member but also provides a strategic basis for identifying high-potential intervals for hydrocarbon production in climatically controlled lacustrine shale systems.