Biomarker indications as evidence for organofacies variations within the Abu Gabra formation of the Muglad basin, Sudan

摘要

The Lower Cretaceous Abu Gabra formation is the main source rock of the Muglad basin oils. It has been previously believed that this source rock consists of a single organofacies. This study reveals that at least two different organofacies units exist. A detailed biomarker study of a suite of 57 oil samples was carried out using Gas chromatographic (GC) and Gas Chromatographic/Mass Spectrometric (GCMS) data. Two main oil families ("Family-A" and "Family-B") have been identified based on this. Distinction between these was solely based on variations in Pristane/Phytane (Pr/Ph) ratios, Diahopane indices, Diasterane/Regular sterane ratios, and Regular steranes distributions. "Family-A" being basically restricted to oils from Heglig, Unity, and Bamboo fields, tend to show relatively low (Pr/Ph) ratios in the range of (1.3 to 1.9), low Diahopane indices (4.1-9.6), low diasterane/regular sterane ratio (0.34-0.60), and relatively high C_(28) Regular sterane content (24-32). In contrast, "Family-B" that is typified by the remaining oils, displays a relatively high (Pr/Ph) ratios (1.23 to 2.27), high Diahopane indices (10.8-32.0), high diasterane/regular sterane ratio (0.52-1.30), and relatively low C_(28) Regular sterane content (23-28). The facies unit that generated "Family-A" oils is more algal (high C_(28) Steranes), and was deposited in a more closed, less oxic high-stand system (low Pr/Ph, Diahopane index, and Diasterane/regular sterane) typical of early rift phase lacustrine environments. "Family-B" oil seems to have originated from a more terriginous suite (higher Pr/Ph, low C_(28) Regular Sterane), and was deposited in a more open, oxic-sub-oxic, shallow-stand system (high Diahopane index, diasterane/regular sterane), that is a late rift phase environment.