Spatial and temporal variations in the geometry and composition of the Permian Whitehill Formation South Africa

Doctoral Thesis


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University of Cape Town

The Lower Permian Whitehill Formation (WHF) is an important hydrocarbon resource unit in the main Karoo Basin where it occurs in outcrops and up to 4 400 m in the subsurface and thus it is distributed over an area in excess of 260 000 km² in the southern half of South Africa. Although the formation is composed predominantly of black laminated carbonaceous shales, earlier studies detected significant spatiotemporal variability in its stratigraphy and composition, particularly the distribution of its organic carbon content across the basin. Because these stratigraphic variations and compositional heterogeneities remained largely uninvestigated, there are conflicting interpretations of not only the hydrocarbon potential of this resource unit but also the paleoenvironmental conditions that prevailed in the Karoo Basin during its accumulation. Following the recent global proliferation of unconventional hydrocarbon resources originating in organic-rich shale successions, the WHF is seriously being considered for gas shale exploitation in South Africa. Consequently, re-characterizing and explaining the spatiotemporal variations in its geometry and composition would be invaluable to the energy industry as well as the larger scientific community. With this aim, this study applied a combination of field descriptions, vintage borehole data, micro- to nano-scale petrographic observations, and multiple geochemical data so that a more critical understanding of the sedimentological controls responsible for the variability can be established. Using this integrated approach, five primary sedimentary facies (F1-F5, i.e., stratigraphic subunits in the WHF) were identified, which show specific and systematic variations in nature and content of organic carbon, stable isotopic composition (of δ¹³Corg and δ¹⁵N), C/N ratio, major and trace elemental enrichment, nature and content of iron sulfides, quartz texture, and CIA across the basin. The lower dark grey to black thinly laminated pyritic, carbonaceous fine shales (facies F1 and F2) contain up to 16.5 % TOC, δ¹³Corg of -15.57‰, δ¹⁵N of 12.49‰, C/N ratio of 1.50, average CIA of 68.11, Rb/K (x10⁻³) and Sr/Ba ratios of 6.56 and 0.67, respectively. Relative to average shale, this unit is up to 6.27 and 3.11 times richer in Mo and Fe, respectively. The organic materials in this facies comprise Tasmanites cysts, colonial algae cells, and amorphous macerals and occur in well-defined laminae (lamalginites) as well as in discrete organic domains loosely associated with mineral grains (organo-minerallic aggregates). At least 25% of the silicate in this facies is of early diagenetic origin, possibly derived from alteration of air-fall volcanic ash. Iron sulfides occur dominantly in form of framboidal aggregates of pyrite. Marcasite in form of lags cement and nodules is also reported. A binary mixture of organic matter and phosphorites with botryoidal textures is also abundant. The upper medium to light grey calcareous-siliceous silty lenticular shale (F3-F5) contain up to 2.04% TOC, δ¹³Corg of -24.71‰, δ¹⁵N of 4.93‰, C/N ratio of 17.62, average CIA of 74.33, Rb/K (x10⁻³) and Sr/Ba ratios of 3.83 and 0.36, respectively. Relative to average shale, this unit was up to 2.65 and 0.43 times richer in Mo and Fe, respectively. Their organic macerals comprise disarticulated plant remains in disseminations with few amorphous macerals. At least 85% of the quartz content is of detrital origin likely sourced from the basin margins and transported to the basin by the action of bottom-hugging currents. Few iron sulfides occur dominantly in form of octa- and euhedral pyrite grains. The data presented in this thesis suggest that the lower WHF (subunits F1 and F2) may have accumulated in a marine setting with high bioproductivity of organic carbon delivered in form of flocculated organo-minerallic aggregates (pelagic snow) onto an anoxic seabed overlain by dysoxic to oxic waters. Reduced terrigenous input, presence of phosphorites, increasing CIA and increasing δ¹³Corg values with higher TOC point to a depositional setting that resulted from an interplay of sea-level highstand and climatic warm-ups. In contrast, the upper WHF (subunits F3-F5) was deposited largely under non-marine conditions with OM in the company of great detrital debris sourced from terrestrial settings, transported into the basin mainly in form of fluid mud flows and deposited above storm wave base. The presence various body and trace fossils, as well as the variations in bioturbation styles and intensities in subunits F3-F5, indicate that colonisation of the basin by invertebrate and vertebrate organisms is related to the different stages in the oxygenation of the sediment-water interface in an increasingly oxygenated setting.