Abstract
TRACT: The Cretaceous sediments in the Anambra Basin (SE Nigeria) consist of a cyclic succession of coals, carbonaceous shales, silty shales and siltstones interpreted as deltaic deposits. The objective of this study is to compare the hydrocarbon generation potential of organic matter from shale sediments along Isugwuato-Okigwe axis in the Anambra Basin, Nigeria. Data obtained indicates the presence of Type III kerogen with Tmax values are between 424 and 441ºC indicating that the shales are thermally immature to marginally mature with respect to petroleum generation. Hydrogen Index (HI) values range from 14 to 388.9mgHC/gTOC while S1 + S2 yields values ranging from 0.2 to 1.0mgHC/g rock, suggesting that the shale have gas generating potential. The TOC values rangesfrom 1.3 to 3.0%, an indication of a good source rock of terrestrially derived organic matter. The high oxygen index (OI) (16.3 mgCO2g-1TOC), TS (1.35) and TOC/TS (1.5) suggest deposition in a shallow marine environment. Based on the kerogen type, shales from the studied area will equally generate oil and gas if its organic matter attained sufficient thermal temperature.
 Keywords: Shale, kerogen type, maturity, oil generation.
Highlights
The Cretaceous sediments in the Anambra Basin (SE Nigeria) consist of a cyclic succession of coals, carbonaceous shales, silty shales and siltstones interpreted as deltaic deposits
Total organic carbon in a source rock comprises three basic components: (1) organic carbon in a retained hydrocarbons as received in the laboratory; (2) organic carbon that can be converted to hydrocarbons, called convertible carbon (Jarvie, 1991) or reactive or labile carbon (Cooles et al, 1986) and (3) a carbonaceous organic residue that will not yield hydrocarbons because of insufficient hydrogen commonly referred to as inert carbon (Cooles et al, 1986; Jarvie, 1991)
Adequate amount of organic matter is a necessary prerequisite for sediment to generate oil or gas (Cornford, 1986, Uzoegbu and Ikwuagwu, 2016b).High total organic carbon (TOC) contents of >>1.95 wt% and hydrogen index (HI) between 14 and 388.9 mg HC/g TOC characterise the shale beds of the study locality (Table 1)
Summary
The Cretaceous sediments in the Anambra Basin (SE Nigeria) consist of a cyclic succession of coals, carbonaceous shales, silty shales and siltstones interpreted as deltaic deposits. The objective of this study is to compare the hydrocarbon generation potential of organic matter from shale sediments along Isugwuato-Okigwe axis in the Anambra Basin, Nigeria. Based on the kerogen type, shales from the studied area will generate oil and gas if its organic matter attained sufficient thermal temperature. The authors consider to estimate source rocks characteristics in this basin by using well established geochemical techniques such as total organic carbon (TOC), hydrogen index, Tmax and biomarker validation. The objectives of this study were to characterize a shale sample from the basin using modern techniques of petroleum geochemistry in order to: (i) assess in detail the quality of its organic matter; (ii) evaluate its thermal evolution, and (iii) highlight its potential as a source.
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