Geophysical Investigations of a Subsurface Structural Anomaly, Gullebi Field, Ghadames Basin, Libya

Gullebi Field lies in the southwestern part of Concession NC7A in the central Ghadames Basin. The field is characterized by a northeast-southwest reverse fault named the Gullebi Fault. The fault extension is about 45 km and is the conduit for the main hydrocarbon accumulation of the Devonian reservoirs. Based on available seismic data and other geophysical methods that were used in the past for hydrocarbon exploration in the field, there was evidence of a structural complexity—a radial rim faulting system with a deep depression surrounding a central dome. This domal structure has an average circular depression ranging from 152 to 305 m deep. The total width of the anomaly is about 13 km in diameter, while the radius of the dome is about 3 km. To investigate the structural complexities of this anomaly, Arabian Gulf Oil Company (AGOCO) conducted a new airborne gravity and magnetic survey over the entire area of the NC7A concession addition to the existing seismic data within the concession. Based on the results of all geophysical methods used for this investigation, two hypotheses were established to identify the origin of this anomaly. The first cause could be the possibility of an igneous or volcanic intrusion that originated from the basement. The second theory assumed the anomaly was generated by possible meteorite impact that created synclinal rims around the domal structure. In this paper we present the geophysical assessment of seismic, gravity, and magnetic analyses and mapping conducted in the area to enable the geological interpretations to be more feasible.