The Triassic Argilo Greseux-Inferieur (TAG-I), which is one of the principal hydrocarbon reservoirs in the Berkine Basin (Algeria), comprises continental fluvio-lacustrine deposits that form a 25 to 80m thick reservoir section. Poor biostratigraphic control, lateral facies variations and sandstone units of sub seismic thickness, makes reservoir correlation and therefore reservoir management problematic.
Chemostratigraphy, which involves the characterisation and correlation of strata using elemental geochemistry has been carried out on the TAG-I in 15 wells of Burlington Resources operated Block 405a, together with 25 wells from nearby blocks. Conventional cores provide the basis for high-resolution chemostratigraphy, enabling multiple samples to be analysed from each sedimentary unit. This allows correlation of beds as little as 1m thick between closely spaced wells, as well as construction of a more regional correlation framework.
The regional framework recognises a three-fold division of the TAG-I over a wide geographic area. Changing palaeoclimate and a major change in sediment provenance control the geochemical variations that define the framework. This framework provides the basis for the high-resolution chemostratigraphic reservoir characterisation within individual fields. When integrated with sedimentology, high-resolution chemostratigraphy carried out on the Triassic accumulations of the MLN Field allow an almost bed for bed correlation that highlights lateral continuity of reservoir sandstones and claystone barriers. Predictability of this analytical tool also allowed correlations to be made even where lateral facies variations were marked.
Chemostratigraphy is now routinely conducted on development wells prior to formation testing. This allows the well test engineer to formulate correct testing procedures to obtain maximum information on sandstone unit correlation, horizontal and vertical connectivity and thus ultimately connected hydrocarbon volumes.