Paleozoic Reservoir Geology of the Zerafa and Djebel Hirane Blocks (Western Desert, Algeria): Advances in Understanding a Challenging Play

Andrea Moscariello¹, Willhelm Kolkman², Aziz Oudjida³, Andrea James⁴, Stephanie Murfitt⁵, et al.
¹and TU Delft, NL, Rijswijk, Netherlands
²Shell International E&P, Rijswijk, Netherlands
³Shell International Exploration & Production, Rijswijk, HOLLAND, Algeria
⁴Badley Ashton and Associates Ltd, Winceby,
⁵Shell International E&P,

Deep (> 3000 m) clastic, gas reservoirs in the Zerafa and Djebel Hirane blocks (Western Desert, Algeria) are relatively unexplored and still poorly known. The successful exploration and effective development of these reservoirs requires a sound understanding of sedimentology, facies types and vertical and lateral variability both at reservoir and block scale, and especially, distribution of reservoir properties. The latter are often characterized by matrix permeability lower than 1 mD with occasional occurrence of high-perm streaks (e.g. 50-80 mD). Preliminary core examination of the 4 gas-bearing reservoirs of lower Carboniferous and Lower Devonian age, indicate that they primarily consist of mid to low n:g, shallow marine clastic sequences alternated with thick packages of non-reservoir, offshore marine sequence. Overall, facies types and associations of both Carboniferous and Devonian reservoirs seem to point toward tidally-influenced paralic depositional environments. In places, seismic investigation allows the identification of paleovalleys that help to localize areas of thicker and confined gross reservoir packages probably genetically related to buried estuarine valley systems. However, lateral changes in seismic character of key reservoir horizons would suggest lateral facies variability related to changes in depositional paleo-environment mostly related to difference response to relative sea level changes and related accommodation space (e.g. low stand deltas vs. estuarine and shoreface vertical aggradation). The lateral variability in seismic-facies is consistent with the occurrence of multiple erosional or exposure surfaces (sequence boundaries) observed in cores. Preliminary observations suggest that 3D architecture, reservoir facies development is consistent with the predictive approach indicated by a sequence stratigraphic framework. This approach is being used in order to investigate and unravel the possible relationships between reservoir property distribution and sedimentary facies, in particular the potential impact of sea-level changes in early diagenetic history (chlorite vs quartz cement and related porosity preservation) of all reservoir sequences.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece