Investigating deep geological reservoirs using seismic reflection and well logs, Tawila oil field, Yemen: Implications for structural setting and reservoir properties

Seismic reflection data were used to evaluate the deep geological structures in Tawila oil field, Masila rift basin, Yemen. A combined set of 2D seismic profiles and geophysical well logs were used to produce structural mapping at the top of the clastic Qishn reservoir and the several overlying and underlying horizons. We have investigated the distribution of reservoir fluids in relation to the identified structures. The seismic horizons with their intersecting faults have been determined to be very important in the hydrocarbon distribution. Two key fault systems have been identified to control the hydrocarbon entrapment in the study area: major, deep, E-W normal faulting that affected the northern area, which caused extensive structural deformation, and a much shallower ENE-WSW fault system to south. The shale type and distribution and the basic reservoir properties, were enhanced through analysing the logging data using the standard clastic models. The reservoir is composed of three clastic compartments (S1, S2 and S3) which are separated by thin shale beds. No hydrocarbons were found in the S1 top unit which had considerable shale volume of a dispersed shale type. Considerable to good hydrocarbon content up to 57% and 61%, associated with a structural uplift, was identified in front of the S2 and S3 units, respectively. A dispersed/laminated shale model was assigned for these two units with a laminated style priority. The hydrocarbon entrapment in the study area was caused by the sealing characteristics of the overlying layers as well as the juxtaposition of the reservoir against other seal rocks. The distribution maps show an increase in hydrocarbon saturation toward the west, north and northeast which coincided with areas of low seismic-derived properties of velocity, acoustic impedance and negative reflection coefficient. An elongated structural horst of good hydrocarbon potentiality was identified in the upthrow of the major fault. This prospective structural high is a key to explore hydrocarbons in nearby areas and other basins of similar geologic characteristics.