«Hot Shales» Facies Heterogeneity: Highlights from the Mineralogical and Geochemical Analyses of the Lower Silurian Source Rock of a Palaezoic Basin (SW Algeria)

Abstract

Abstract In Northern Africa, the Lower Silurian organic-rich shales charge both intra-paleozoic and basal Triassic reservoirs and might have an important potential for shale gas. The shales were deposited in a confined marine environment during the Lower Silurian transgression. On the Gondwana scale, they are laterally discontinuous while their distribution and thickness are controlled by palaeo-reliefs inherited from tectonic and Ordovician glacial events (Luning et al., 2000). On the field-scale, the mineralogical and geochemical analyses of cutting samples indicate significant vertical and lateral facies variations, which contradict the apparent homogeneity of this formation. The study is based on 80 cutting samples from 5 wells of a Palaezoic basin in SW Algeria, with a maximum spacing of 13 km (Fig. 1). Each sample has been analysed in terms of its mineralogy / petrography (QEMSCAN analysis) and organic matter features (Rockeval pyrolysis). The fully automated QEMSCAN system (Quantitative Evaluation of Materials by Scanning Electron Microscopy) provided a bulk modal analysis, particle maps, a textural analysis, mineral-by-mineral chemical assay and trace mineral hunt. The cutting analysis facilitated the identification of 21 minerals, which were subsequently clustered into 5 assemblages (1) carbonate, (2) mica, Fe-oxide and siderite, (3) Clay, (4) Clay, pyrite, chlorite and feldspar and (5) remaining minor minerals. The focus of this presentation is to describe the observed heterogeneity and to discuss the parameters that control the vertical and lateral distribution of these facies (sedimentology, diagenesis...). The Lower Silurian "Hot Shales" are highly heterogeneous and exhibit strong vertical and lateral facies variations, which might explain the complex distribution of TOC values and brittleness properties. Through a combination of geochemical and mineralogical analyses from cutting samples, it will be possible to model the source rock heterogeneity, predict the distribution of TOC values and brittleness properties and ultimately evaluate the shale gas potential.