Characterization of a fractured basement reservoir using high-resolution 3D seismic and logging datasets: A case study of the Sab'atayn Basin, Yemen.

The Sab'atayn Basin is one of the most prolific Mesozoic hydrocarbon basins located in central Yemen. It has many oil producing fields including the Habban Field with oil occurrences in fractured basement rocks. A comprehensive seismic analysis of fractured basement reservoirs was performed to identify the structural pattern and mechanism of hydrocarbon entrapment and reservoir characteristics. A 3D post-stack time migration seismic cube and logging data of 20 wells were used and several 2D seismic sections were constructed and interpreted. Depth structure maps were generated for the basement reservoir and overlying formations. The top of the basement reservoir is dissected by a set of NW-SE step-like normal faults (Najd Fault System) and to a lesser extent, by secondary NNE-SSW oriented faults (Hadramauwt System). The Najd Fault System is dominant and dissects the reservoir in the middle of the field into two prospective uplifts. The northern and northeastern areas constitute the deep-seated downthrown side of the reservoir. Hydrocarbon emplacement is through the fault juxtaposition of the fractured basement against the organic shale source rock of the overlying Madbi Formation. Hydrocarbons are hosted in basement horsts formed by fault-controlled blocks and overlain by the regional seal of the Sab'atayn Formation. The basement reservoir rock is mainly composed of granite, quartz-feldspar, weathered silica, and mica minerals. Fractures were identified from the outcrops, cores, image logs, and the petrophysical analysis. Hydrocarbon saturation was observed in the upper and middle parts of the reservoir, more specifically in front of the highly fractured sections. The fracture porosity was less than 5% and the dead oil had an API gravity of 40° with no H2S or CO2. In conclusion, the structural highs of the Habban Field are of interest because most oil producing wells are drilled into them. We recommend extending the drilling and development activities in these uplifts.