Log-derived petrophysical characteristics and oil potentiality of the Upper Qishn Clastic Member, Masila Basin, Yemen

The Lower Cretaceous Qishn Formation at Masila Basin, Yemen, contains substantial amounts of proven crude oil. It is divided into a lower shaly silisiclastic member and an upper carbonate member. In the present work, detailed formation evaluation of the Upper Qishn Clastic Member was carried out using the digitized well logging data from 12 wells, via computer-assisted analysis, to accomplish its reservoir characteristics and oil potentiality. Accordingly, the true resistivity (R (t)), porosity (total phi (t) and effective phi (e)), shale content (V (sh)), permeability (K), water saturation (S (w)), hydrocarbon saturation (S (h)) (movable S (hm) and residual S (hr)) and lithologic composition in each well were estimated, analyzed, and illustrated versus depth on litho-saturation cross-plots. Likewise, the areal distribution of such parameters alongside the member in the investigated area and their relations to geological setting were defined from the constructed isoparametric contour maps. Based on the obtained results, the components of Upper Qishn Clastic Member of Masila Basin are mainly shaly sandstone and minor calcareous sandstone with considerable amount of shale. Regionally, the Member was deposited on an inner neritic to shallow-marine platform setting. It is interpreted as a good-quality reservoir rock which has been established from high total porosity (15-22 %), good effective porosity (8-13 %), and variable shale volume (24-31 %). A number of high hydrocarbon saturation zones (exceeding 55 %) are detected through intervals having S (w) < 60 % and S (hm) > 20 % which have been considered as economic oil producers. The storage and pay capacities of the reservoir intervals have been resolved. The obtained results were confirmed from the available data of core analyses and production tests. The Upper Qishn Clastic Member reveals promising reservoir characteristics which should be taken into consideration during future development of the oilfields in the area.