Geel Msc
Creators: name: Geel, Claire
Publisher: name: figshare
Year: 2021
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences

description: This study involves a comprehensive and detailed lithological, sedimentological, structural and geochemical description of the lower Ecca Group in the Eastern Cape, South Africa. The Ecca Group hosts 245-279 million year old black shale, which has recently been the focus of interest of petroleum companies worldwide. The shale was deposited under anoxic conditions in a setting that formed as a consequence of retro-arc foreland basin development related to the Cape Orogeny. This sedimentary/tectonic environment provided the conditions for deeply buried black shales to reach maturity levels for development in the gas window.The investigation site is called the Greystone Area, situated north of Wolwefontein en route to Jansenville. The area has outcrops of the Dwyka, the Ecca and the lower Beaufort Groups. The outcrops were mapped extensively and the data was used in conjunction with GIS software to produce a detailed geological map (~28km × 23.7km). North-south geological cross sections were drawn to give an indication of bed thicknesses, lateral facies variation and formation depths. Using the field work data, two boreholes were accurately sited on the northern limb of a shallow westerly plunging syncline. The first borehole reached 100m and the second was drilled to 295m depth (100m percussion and 195m core). The second borehole was drilled 200m south of the first, to penetrate the shale formations at a greater depth and to avoid surface weathering.Fresh core from the upper Dwyka Group, the Prince Albert Formation, the Whitehill Formation, Collingham Formation and part of the Ripon Formation were successfully extracted and a detailed stratigraphic log was drawn. The core was sampled during extraction and the samples were sent to the Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences, for geochemical analyses.At GFZ the following geochemical and geophysical procedures were performed: X-Ray diffraction (XRD) and X-Ray Fluorescence (XRF), open system pyrolysis and thermovaporization, TOC and Rock Eval, thin section and polished section microscopy, scanning electron microscopy (SEM), stable 13C and 15N isotope analyses and mercury intrusion porosimetry.The black shales of the Whitehill Formation are high in organic carbon and have an average TOC value of 4.5 wt%, whereas the TOC of the Prince Albert and Collingham Formations is below 1 wt%. High Tmax (maximum temperature) values, low HI (hydrogen index) values and bitumen reflectance measurements (~ 4% BRo), characterise these sediments as overmature. Data from XRF analysis, Rock Eval, and stable isotopes (δ 13C and δ 15N), suggest that the Prince Albert, Whitehill and Collingham Formations were deposited under anoxic conditions and have a mixed marine and terrestrial source of organic matter.Quartz, illite, muscovite and chlorite predominate in terms of mineralogical composition with some plagioclase and large amounts of accessory pyrite (especially in the Whitehill Formation). The Collingham Formation has the highest quartz content resulting in high brittleness factor. Average meso-and macro-porosity of the black shales of the Whitehill Formation (0.83%) and SEM images confirm that these sediments are tightly packed. Only dolomite occurring within the shales has a notable porosity of 2.90%, and SEM images of dolomite reveal 1.5μm wide pores.The black shales of the Whitehill Formation have characteristics suggesting that it may potentially be a successful gas producing resource, but the residual/exploitable gas is highly dependant on influences of the Cape Fold Belt and the intrusion of the Karoo dolerite suite.<br>