Detailed Core Studies and Facies Characterization of Tight Cambro-Ordovician and Devonian Reservoirs (Tinrhert Nord Block, Illizi Basin, Algeria).

Abstract

AbstractThe Tinrhert Nord Block is located in the Illizi/Berkine-Ghadames Basin where in the central-eastern part of Algeria, near the border with Libya. The sedimentary infill of these basins is composed of thick Palaeozoic sedimentary sequences which constitute a world-class petroleum system, confirmed and producing in Algeria, Tunisia and Libya. Previously drilled wells by Sonatrach in the block had shown a considerable prospectivity for the Block, and further studies were undertaken for well planning and appraisal of the Ordovician, Silurian, and Devonian reservoirs. The target of this study was to better understand the characteristics of the reservoir and quality of the productive series in the block. A comprehensive core study was therefore run for 15 wells with a total of 647 meters of sediments in the Tinrhert Nord block (Figure 3). The study showed that 7 clastic facies for the Palaeozoic sequences can be distinguished. The Ordovician facies subdivided into 3 main facies that were deposited in Glaciomarine, Tidal-Delta and Upper shoreface-shoreline environment and is composed of diamictites, titillates and fine-grained tight reservoir facies. Silurian and Devonian sequences are composed of marine shales that are confirmed as constituting the major source rock in the basin (Silurian hot shale and Frasnian hot shales), and additional fluvial-deltaic coarse-medium-fine grained sandstone facies. SEM (Scanning Electron Microscope), routine core analysis, and fracture studies were performed on selected core intervals in order to perform a more detailed reservoir characterisation. Routine core analysis show that the Ordovician reservoir has a limited porosity up to 4-8% due to the overall texture of the sediments and shows also a low permeability (<1Md). Effective porosity of the Upper Silurian F6 reservoir is up to 15-20% and Devonian F2 reservoir is most likely up to 10%. Core fractures show drilling-induced vertical fractures, and natural hairline type healed fractures, and an additional natural open joint system. The open hole DST well tests that have shown moderate flow rates with can be related to the presence of a high porosity and main permeability system which is composed of interconnected small scale open fractures and larger scale sub-seismic fracture corridors. These features permit to produce also from the reservoir layers that generally show low porosity/permeability facies.These studies confirm that the Silurian F6 reservoirs are the best reservoirs in the basin and Devonian fluvial-deltaic sequences (C and F2)are secondary best reservoir in the basin. The Ordovician reservoirs are tighter with limited porosity and permeability but can be produced by stimulation of the natural fracture system.