Hydrocarbon prospectivity of Nigeria's inland basins:From the viewpoinit of organic geochemistry and organic petrology

摘要

The inland basins of Nigeria comprise the Anambra basin,the lower,middle,and upper Benue trough,the southeastern sector of the Chad basin,the Mid-Niger (Bida) basin,and the Sokoto basin.Organic geochemical and organic petorologic studies indicate that coal beds constitute major potential source rocks in the whole of the Benue trough (Anambra basin inclusive).The generation and production of liquid and gaseous hydrocarbons from coal beds is presently indisputable worldwide.In the Anambra basin,the coal bedsin the Mamu Formation (Campanian-Maastrichtian) have total organic carbon (TOC) contents of as much as 60.8 wt.%,mean hydrogen index (HF) of 364 mg HC/g TOC,vitrinite refletance (R_o) of 0.54-0.56%,and T_max 430-433 deg C.Biomarker data indicate a dominance of high-molecular-weight n-alkanes,very high pristance/phytane ratios,pronounced odd-over-even predominance (OEP0,a preponderance of C_29 regular steranes,and relatively high contents of C_28.In the middle Benue trough,the coal beds of the Turonia-Coniacian Awgu Formation have TOC contents of as much as 79.1 wt.%,R_o of 0.83-1.07%,and mean HI of 281 mg HC/g TOC,unimodal distributions of both low-and high-molecular-weight n-alkanes with no obvious OEP,a predominance of C_29 steranes,and relatively high contents of C_27 and C_28.Coal beds from the Coniacian-lower Santonian Lamja Formation in the upper Benue trough yielded TOC contents of as much as 50.7 wt.%,with HI of 184 mg HC/g TOR,R_o of 0.70-0.73%,low-and high-molecular-weight n-alkane dominance with an unpronounced OEP,high pristance/phytane ratios,and very high contents of C_20 regular steranes.