Spatiotemporal Sedimentary Facies Variations in the Lower Permian Whitehill Formation, Ecca Group, Karoo Basin
The Lower Permian Whitehill Formation in the Karoo Basin is a potential shale gas unit in South Africa. Recharacterizing this heterogeneous formation and explaining the spatiotemporal variations in its geometry, texture, bedding features, composition, and
- PDF / 1,240,059 Bytes
- 10 Pages / 595.276 x 790.866 pts Page_size
- 37 Downloads / 196 Views
Spatiotemporal Sedimentary Facies Variations in the Lower Permian Whitehill Formation, Ecca Group, Karoo Basin Kenneth Chukwuma and Emese M. Bordy
Abstract
The Lower Permian Whitehill Formation in the Karoo Basin is a potential shale gas unit in South Africa. Recharacterizing this heterogeneous formation and explaining the spatiotemporal variations in its geometry, texture, bedding features, composition, and distribution of organic carbon content is necessary, because gas recovery can be strongly influenced by these characteristics of the host rock. Here, we report on these rock property variations and their sedimentological controls in light of recent advances in shale sedimentology by combining field descriptions, petrographic observations and geochemical proxies. We distinguish five sedimentary facies (F1–F5) that suggest changes in the Early Permian depositional conditions from overall low energy in F1 and F2, allowing pelagic snow aggregates to cover the basin floor, to higher energy in F3, F4, and F5, bringing terrestrial detritus via hyperpycnal and diluted mud flows, which possibly originated from summer melting of mountain glaciers in the Cargonian Highlands flanking the northern margin of the Karoo Basin. Keywords
Karoo basin
10.1
Shale gas
Introduction
The Lower Permian Whitehill Formation (WHF) has attracted attention from geologists for over a century (e.g., Rogers and Du Toit 1909; Schwarz 1912; CunninghamCraig 1914) due to the oil shale potential, abundant but low diversity fossil biota and conspicuous white-weathering characteristics of this black, carbonaceous shale unit. Currently, renewed interest in the WHF is due to its perceived shale gas potential (e.g., Decker and Marot 2012; Geel et al. 2013, 2015; Smithard et al. 2015). However, despite a large number of publications on its lithology (e.g., Cole and McLachlan 1991; Visser 1992), sedimentology (e.g., Visser K. Chukwuma E.M. Bordy (&) Department of Geological Sciences, University of Cape Town, Cape Town, 7701, South Africa e-mail: [email protected] K. Chukwuma e-mail: [email protected]
Permian
Mud depositional setting
Sedimentary facies
1992), paleontology (e.g., Oelofsen 1981, 1987) and hydrocarbon potential (e.g., Rowsell and de Swardt 1976; Cole and McLachlan 1991; Geel et al. 2013; Smithard et al. 2015), the WHF remains poorly understood. For instance, while reference is often made to this black, thinly laminated, carbonaceous formation as a typical anoxic facies, no consensus exists on the palaeo-water depths (e.g., Oelofsen 1981, p. 21, 143; Cole and McLachlan 1991, p. 381) or salinity levels (e.g., Faure and Cole 1999 vs. Scheffler et al. 2006). Furthermore, the widely accepted anoxic facies model requires refinement to explain why the WHF contains more siltstone and very fine-grained sandstone than ‘black shale’ in the northeastern part of the Karoo Basin (e.g., Cole and McLachlan 1991; Werner 2006). Its regional geometry (e.g., thinning from SW to NE) and style of mud sedimentation (e.g., Visser 1992; We
Data Loading...
