Dual sea-level-climatic controls on the stratigraphic distribution of total organic carbon content and macerals in the Permian black shales of southwest Gondwana

Abstract

The Whitehill Formation (WHF) is part of the organic-rich black shales deposited in the shallow seas that occurred within the continental interior of southwest Gondwana during the Lower Permian and has been identified as the main potential shale gas reservoir in South Africa. Here, high-resolution total organic carbon (TOC) content, organic petrographic composition, and multiple geochemical studies conducted on samples of the WHF in peak oil to gas maturity (vitrinite reflectance, Ro, range between 0.98 and 1.26%) were used to evaluate the influence of relative sea-level and climatic fluctuations on paleoproductivity, bottom-water redox condition, and clastic supply, and their combined control on the stratigraphic distribution of TOC content and macerals. Organic petrographic observations and geochemical data allowed recognition of three stratigraphic intervals (ORI-1 to ORI-3) that show marked differences in TOC content and maceral type, as well as mineralogical composition and submillimeter-to centimeter-scale sedimentary textures. TOC content increases from 0.93% at the base to 16.75% at the top. Data further revealed that the lower interval (ORI-1) is dominated by organic macerals biologically produced in the water column, including algal Tasmanites cysts, spores, and radiolaria, and depleted in land-derived macerals (inertinite, vitrinite). The mid (ORI-2) and upper ORI-3) intervals are shown to be composed of both production-derived components including bacterial biomass and aquatic plants (algae and radiolaria), as well as land-derived macerals, which are intermixed with clay-sized mineral particles, forming interconnected algal/bacterial mats. Trace and major element data indicate that these stratigraphic variations and heterogeneity of both TOC content and maceral reflect the variability of paleoproductivity, redox conditions, and clastic supply, which in turn were influenced by fluctuations of relative sea level and climate. Specifically, the accumulation of ORI-1 occurred during sea-level highstand and cool-climate, resulting in relatively higher paleoproductivity, reduced bottom-water oxygenation, and low clastic supply. However, pervasive degradation of organic matter at the seabed occasioned by low clastic detritus resulted in lower TOC content. Moreover, reduced supply of clastic detritus and severe organic matter degradation diminished the sizes of telalginite and the concentration of macerals into interconnected networks. On the contrary, intermittent decreases of relative sea-level (shallowing) and warmer conditions were detected in the mid and upper intervals, and the associated buildup of clastic sediment enhanced organic matter burial. Together these conditions resulted in higher TOC and better preservation of labile macerals. To conclude, the scale of stratigraphic heterogeneity of both TOC content and maceral type reported here may be present in many other black shale successions. Therefore, this type of study should constitute a fundamental step in the characterization of shales for their unconventional oil/gas reservoir potential.