Geochemical, microstructural, and petrophysical characteristics of Lokpanta Shale, Anambra Basin, Nigeria

Abstract

The Lokpanta Shale of Anambra Basin, a potential unconventional hydrocarbon resource, has limited studies on its microstructure, diagenetic history and fluid storage-transport capacity. This study presents geochemical, imaging and petrophysical analyses on shallow samples (<25 m) to characterize their rock properties. The unit contains thermally immature and non-porous type II kerogen with patchy-amorphous, sub-lenticular and structured morphologies. The laminated (texturally anisotropic) microfabric is defined by its sub-lenticular organic matter, weakly aligned clays and few subparallel microfractures. Calcite dissolution, moldic-dissolution and phyllosilicate framework are the dominant pores in an isolated pore network consisting of 0.2 − 60.2 μ m large near-spherical to ellipsoidal pores and 2− 180 nm nanopores. Calcite minerals underwent diagenetic alterations, including dissolution, cementation and neomorphism. The clay minerals in the analyzed core of the Lokpanta Shale include two tracts of R3 I/S mixed clay layers (ordered interstratified illite-smectite), rectorite (partially ordered illite-smectite), illite and negligible smectite, which suggests limited smectite to illite diagenesis. The organic matter reached immature thermal maturation (Tmax of 429 − 434 °C), which is close to the oil window threshold (435 °C). The organic matter maturity status and clay profile at shallow depths (<25 m) suggest an earlier exposure to a higher but time-limited thermal stress regime (burial then uplift). Higher clay and organic matter content in the unit directly correlate with wider pore distribution, lower tortuosity, higher external specific surface area and higher dielectric constant (higher fluid storage, transport, and adsorption capacity). This study enhances source rock and/or reservoir characterization, which could be vital for exploration strategies in the Lokpanta Shale.