99/00155 Depositional environments and oil potential of Jurassic/Cretaceous source rocks within the Seychelles microcontinent

Abstract

Organic-rich shale and claystone are present within the Lower Cretaceous Abu Gabra Formation of the Muglad Basin, representing excellent oil source rocks. The source rock potential of Abu Gabra organic-rich whole rock samples from two oilfields in the Muglad Basin was investigated using bulk and quantitative pyrolysis techniques. The analytical program included TOC, pyrolysis (SRA), pyrolysis GC, bulk kinetics, elemental analysis and kerogen microscopy. The results were used to characterize the kerogen composition and type of petroleum generated from these Abu Gabra organic-rich sediments to clarify the structural variability of the organic matter in the Abu Gabra source rock. The investigated Abu Gabra organic-rich sediments proved to contain excellent quality lacustrine kerogens. The samples have moderate to high hydrogen index (HI) values (267–837 mg HC/g rock), consistent with predominantly oil-prone Types I and II kerogen with a minor contribution of Type II–III kerogens. This is supported by the abundant liptinitic materials (i.e., lipid-rich structured algae, structureless (amorphous) alginite, and sporinite) and high atomic hydrogen-to-carbon (H/C) ratio. However, pyrolysis GC analysis proved that all the investigated Abu Gabra samples contain homogeneous Type I kerogen that produces mainly waxy oil.Bulk Kinetic experiments on these samples indicate that the source rock facies derived from a homogenous organic matter, spans an extremely narrow distribution of activation energy (Ea) ranges from 49 kcal/mol to 51 kcal/mol. The magnitude order of frequency factors (A) also show a narrow distribution range from 1012 to 1013/s. Based on the discrete simulated model using 3.3 °C/My heating rate, the average of predicted geological temperature is 0.73% Ro where the peak generation of 0.85% Ro, thus shows a typical characteristic of Type I Kerogen.