Unconventional shale oil and gas systems in northern Africa: Mineralogical and geochemical analysis from Silurian Shales: Ghadames Basin Southern Tunisia

Abstract

In Northern Africa, the Lower Silurian (Tannezuft Formation) organic-rich shales charge the Paleozoic and Triassic reservoirs and have an important potential for oil and gas shale. This study is based on the analysis of cutting, core, SWC samples from several wells of Ghadames Basin located in Southern Tunisia. The reservoir assessment was based essentially on geochemical (Rock Eval, GC, GC-MS, thermal extraction…) and mineralogical (XRD, XRF, SEM, QEMSCAN analysis, wireline logs ..) techniques including: (1) source-rock distribution, thickness, organic richness, maturity level, (2) organic facies distribution, quality, kerogen transformation; (3) mineralogical assessment, clay type, quartz richness and (4) reservoir quality, porosity, permeability. The Tannezuft « Hot shales » exhibit good to excellent organic content and petroleum potential (averaging 7% and 15Kg HC/t of rock respectively). Original HI averaging 500 mg HC/g of TOC suggesting a type II organic matter and variable maturity level as indicated by the distribution of Ro from 0.8 to 1.2% and high TR ranging from 0.45 to 0.92%. The abundance of redox sensitive trace metals (V, Cr, U, Mo) indicate that the paleoproductivity was high during deposition of the Hot shales. Quartz, carbonate and clay contents are highly variable in the Hotshales averaging from 15%, 9% and 30% respectively. The Clay contents are highly rich in illite (20-40%). Three types of porosity were identified in these systems: matrix porosity, organic porosity derived from decomposition of organic matter and fracture porosity. The compilation and interpretation of the mineralogical, lithological and geochemical data indicate the Formation is highly heterogeneous and exhibit strong vertical and lateral facies variations with possibly lead to strong variances in the shale gas potential. The properties of the source rock were compared with published data from oil and gas shale indicating significant differences and suggest that the Tannezuft Formation is a promising oil and gas shale reservoir in North Africa.

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AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015