Source Rock Potential of Selected Shale Samples from Orange Basin, South Africa
A set of shale samples from the Orange Basin of South Africa were evaluated for their source rock potential. Orange Basin sediments comprise of passive margin deposits overlying rift ones; Rifting initiated in response to the break-up of the African and South American continental plates and resulted in the development of a series of half grabens and grabens that are approximately parallel to the present sea-floor spreading axis (Paton et al., 2008). Fully marine conditions which yielded Orange Basin’s highest quality source rocks developed during the Barremian - Aptian in response to the initiation of the main drift phase of the South Atlantic Opening which has been dated to be Neocomian. The Barremian - Aptian black shales sequence is overlain by more than 5500 m (18,045 ft) of postrift sediments that are dominantly shales or claystones.
The 20 shale samples that were evaluated had been sampled from a close to 1400 m thick sediments sequence ranging from 1910 m to 3321 m penetrated in a borehole that was drilled through Lower to Upper Cretaceous units. These samples exhibited Total Organic Carbon content values ranging from 1.06% to 2.17% (mean - 1.56), S2 values varying from 0.08 to 2.27 (mean - 0.93) and Hydrogen Index (HI) values varying from 7 to 128 (mean - 60). S2/S3 values ranged from 0.03 to 3.49 (mean - 1.15) thereby indicating presence of poor to fair hydrocarbon generative potential gas prone Type III to Type IV Kerogen. The streak of fair potential hydrocarbon generative source rock was sampled at 2225 m and yielded TOC%, S2, HI, S2/S3 and Tmax values of 1.77, 2.27, 128, 3 and 440 respectively; further sampling of this streak of fair potential source rock is worthwhile to establish how thick it is and also determine its Kerogen type from petrographic maceral analysis. The maturity of the 20 samples analyzed as indicated by Tmax (0C) values ranged from immature to the oil window: 428 - 446 (mean - 436). Gas chromatographic data of the rock extracts suggest both marine OM and terrestrial OM input.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009