The Triassic Argilo Gréseux-Inferieur (TAG-I) of Algeria: High-resolution Chemostratigraphy as a Correlation Tool in Proximal to Distal Fluvial Facies

John Martin¹, Tim Pearce¹, Ken Ratcliffe², David Lawton³, Andrew Hughes⁴, and Fadila Bessa⁵. (1) Chemostrat Limited, Chemostrat Limited, Llanfyllin Ent Park, Llanfyllin, SY22 5DD, United Kingdom, phone: 01144 (0)1691 648868, johmartin@chemostrat.co.uk, (2) Chemostrat Ltd, Unit 4 Llanfyllin Enterprise Park, Llanfyllin, SY22 5DD, United Kingdom, (3) BHP Billiton Petroleum Ltd, Neathouse Place, London, SW1V 1LH, United Kingdom, (4) Burlington Resources, Canary Wharf, One Canada Square, London, E14 5AA, United Kingdom, (5) Division of Petroleum Engineering and Development, Sonatrach/PED, 8 Chemin du Reservoir, Hydra, Algiers, Algeria

Triassic sandstones of the Berkine Basin in Algeria (the TAG-I) form a prolific oil reservoir that is part of a SW-NE trending fluvio-lacustrine depositional system, extending eastwards from Algeria through southern Tunisia (Kirchaou Formation) into western Libya (Ras Hamia Formation). On both a regional and reservoir scale, the TAG-I is typified by marked lateral facies variations, which combined with poor biostratigraphic control and weak seismic definition, make stratigraphic correlation problematic.

In order to better understand the regional and intra-field stratigraphy, high-resolution chemostratigraphy has been carried out on approximately 2000 core samples, from over 40 wells. The wells in the study are oriented SW-NE along the fluvial axis, from the MLSE field in the SW to the ROD field in the NE. From SW to NE, along the fluvial axis, the sequences change from sandstone-rich braided fluvial facies (e.g. MLSE, MLN and MLC fields) to more shale-prone, low - high sinuosity fluvial facies (e.g. ROD, BSF and SFNE fields). High-resolution chemostratigraphy allows regional correlation, even though there is marked lateral facies variation. The chemostratigraphic correlation framework is based on inorganic geochemical variations attributed to temporal changes in paleoclimate and sediment provenance. On a local, intra-field scale, stratigraphic studies of reservoir sections, supported by high-resolution chemostratigraphy, enable the distribution of hydrocarbons to be mapped with more confidence. Additionally, when formation pressure and well test data are incorporated, barriers and baffles to production can be mapped, allowing the optimum strategy for field development to be formulated

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