Abstract
Although several works have proved the existence of oil and gas in the Campanian to Maastrichtian strata of the Anambra Basin, it is still considered a frontier basin due to the challenges associated with interpretation of stratigraphy and structure arising from non-availability of subsurface data. This has led to poor understanding of the petroleum system and hindered exploration activity within the basin. This research paper aims to discuss the signatures of essential petroleum system elements, based on the study of high-quality outcrop examples. Detailed field studies reveal the presence of source, reservoir and seal rocks, and traps exposed within the four lithostratigraphic units outcropping in the basin, across the southeastern part of Nigeria. The shales of Enugu Formation, offer good source rocks. The shales, coals, coaly shales and sandstones with repeated occurrence of shale–silt–sand heteroliths of the Mamu Formation provide possible source and reservoir rocks. Thick extensive sandstones with clay laminae of the Ajali Formation offer potential reservoir that are partly compartmentalized. The shales with interstratified sandstone of the Nsukka Formation provide potential cap and overburden rocks. The presence of anastomosing fracture bands, extensional joints and faults, small-scale growth faults, and rollover anticlinal structures are good evidence of structural and stratigraphic configuration that could allow for possible migration, accumulation and entrapment of hydrocarbon. These recognized signatures of the key elements of a petroleum system and oil seeps encountered in the basin are an indication of the presence of an active petroleum system.
Highlights
Several works have been carried out within the Anambra Basin, these works were focused on basinal extent, tectono-stratigraphic evolution, sedimentary fill, lithostratigraphy age, lithological characteristics, sequence stratigraphic framework and depositional environments (Burke et al 1971; Benkhelil 1982; Nwajide and Reijers 1996; Nwajide 2006, 2013; Dim et al 2017)
The lower section of this outcrop is characterized by thick, deep greyto-black shale with siltstone and sandstone interstratification (Fig. 4a, b)
This study has shown the evidence of key petroleum system elements such as source, reservoir, traps and seals on outcropping lithostratigraphic units of the Anambra Basin
Summary
Several works have been carried out within the Anambra Basin, these works were focused on basinal extent, tectono-stratigraphic evolution, sedimentary fill, lithostratigraphy age, lithological characteristics, sequence stratigraphic framework and depositional environments (Burke et al 1971; Benkhelil 1982; Nwajide and Reijers 1996; Nwajide 2006, 2013; Dim et al 2017). As part of an ongoing review of the petroleum prospectivity of the Anambra Basin, this study utilizes surface data obtained from various outcrop locations in furthering our understanding of the key elements (encompasses source, reservoir, and seal or cap rocks) of the petroleum system and proffering better exploration guide within the Anambra Basin of Nigeria. These key elements of petroleum systems, which were evaluated by the interpretation of surface data forms the basis for assessing the hydrocarbon prospectivity of most sedimentary basins around the world. Outcrops of the Campanian to Maastrichtian rocks of the Anambra Basin studied includes the 9th Mile, Milliken–Onyeama Hills, Enugu–Onitsha
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