Geochemical evaluation of source rocks within the upper Ecca, main Karoo

Abstract

Early works completed by Bouma, Johnson, Catuneanu, Wickens and others have paved the way in gaining a better understanding of the Karoo complex, from a sedimentology perspective. Research in this area is becoming more readily available as the oil industry seeks to gain more knowledge in this area for exploration purposes. Understanding this area can help aid in offshore development that is not as readily accessible as the onshore sediments. Using new modern technologies as well as old geochemical techniques we can expand our knowledge on hydrocarbons within this onshore environment. The Karoo in South Africa is an area in which petroleum studies are being reviewed with particular interest, especially within the Tanqua and Laingsburg subbasins of the Karoo. The particular formations of interest comprises of the Laingsburg, Fort Brown, Kookfontein and Vischkuil of the Upper Ecca group, located within the Laingsburg subbasin as well as the Kookfontein formation found within the Tanqua subbasin. The present work dealt with interpreting and, distinguishing the different types of source rock lithologies, to identify the source rock zones and organic matter within the Laingsburg, Fort Brown, Kookfontein and Vischkuil formations. This was achieved by utilising different geochemical analyses (such as rock eval pyrolysis and TOC) and organic chemistry (XRD) to assess the maturity, mineralogy and the quality of the source rocks. Forty samples were sent for TOC analyses. Rock samples were taken at various localities in the Fort Brown, Vischkuil and Laingsburg Formations, within the Laingsburg subbasin, whereas, Kookfontein core samples were obtained from the Tanqua subbasin. Overall, ten samples were selected from the 40 samples that had undergone TOC analysis. Using XRD, the core and rock outcrop samples showed clay minerals predominantly of illite, montmorillonite and chlorite. The TOe and rock-eval S 1 and S2 values from the formations within this study were anomalously low. The HI vs. TOe contained Type 1 and Type 2 kerogen plots, but this did not correspond with the HI vs. 01 index, because the samples were heavily oxidised.