Mike Izuchukwu Akaegbobi¹, Herwig H. Ganz²

(1) Department of Geology, University of Ibadan, Ibadan, Nigeria, N/A, Nigeria
(2) Shell Exploration and Production Ltd., London, UK, London, England

ABSTRACT: Thermal evolution of alkylated sulphur-containing aromatics in the Post-Santonian Carbonaceous Shale source rock sequences, Anambra basin, Nigeria

A set of twenty shale and coal samples selected from the Campano-Maastrichtian Nkporo Formation were subjected to a quantitative assessment of extractable sulfur aromatics by capillary gas chromatography. Relative concen-trations of various benzo-thiophenes(BTs), dibenzo-thiophenes (DBTs) and se-lected alkylphenanthrene and alkyldibenzothiophene isomers were determined for a group of selected sample set. The samples are moderately rich in organic matter and features Pr/Ph ratio (>3.0) and Ph/n-C18 ratio (1.5) reflecting pre-dominance of terrestrially derived organic facies. Bulk geochemical data from Rock-Eval analysis revealed poor quality organic matter with hydrogen Index (HI) values ranging from 50-114 mg HC/gTOC. Vitrinite reflectance (%Ro) values ranging from 0.51 to 0.82% are quite in agreement with the depth trend classified the Nkporo shale as mature. The samples generally yielded relatively high concentrations of C11+ aromatic hydrocarbon fractions. DBTs appeared to be present in all the samples whereas the BTs are observed only in bituminous shale and coal sequences, which are recognized as source bed for the patches of oil/gas pooled within the Anambra basin. Investigation of quantitative distribution of C1-C3 BTs and DBTs in nine samples with reflectance values varying between 0.51 to 0.82% revealed a sig-nificant generation of C2-BTs relative to DBTs at 0.73% Ro. The concentration of C2-BTs varies between 0.20 and 1.55 mg/gTOC exceeding C2-DBT by one order of magnitude in some samples. The onset of significant generation of DBTs was evident from concentrations in the range of 0.60-6.5 mg/gTOC. A considerably lower relative abundance of BTs in the bituminous shales at the deeper section as compared to the coal series in the updip may be attributed to selective expulsion and/or primary migration. The threshold of the BT generation curve was at-tained at a vitrinite reflectance of 0.65% as shown by C2-BT concentrations in the range 0.7-1.55 mg/gTOC.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado