Abstract: Stratigraphic Trap Potential in the Early Permian Unayzah Formation of Central Saudi Arabia

EVANS, DANIEL S., AND BASSAM H. BAHABRI

A recent oil discovery in central Saudi Arabia confirms significant stratigraphic trap potential in the Early Permian Unayzah Formation. Located about 175 km. south of Riyadh, the Usaylah-1 well targeted the updip pinch-out of the Unayzah clastic section on the east flank of the Hawtah structural trend.

Following the initial discovery of the Hawtah Field in 1989, twelve oil fields and one gas field have been discovered on anticlinal closures along the Hawtah and Nuayyim structural trends. Although structural closure is the dominant trapping mechanism in most of these fields, stratigraphic-trap potential had been postulated because the mapped vertical closure in many of the fields is not sufficient to account for the hydrocarbon column observed. Using 2-D seismic data, the recognition of the updip pinch-out of the upper Unayzah clastic section led to the acquistion of 3-D data to refine mapping in the area. Horizon slices and relative amplitude maps were constructed to show the areal distribution and updip limit of the primary objective Unayzah sandstone.

Isochron time maps are proving effective in predicting similar stratigraphic trap potential along the Hawtah and Nuayyim structural trends. Upper Unayzah sandstones are present in the paleo-structural lows (isochron thicks) and are absent on the paleo-highs (isochron thins) due to non-deposition and erosion. Mapping the paleo-highs and lows is an effective way to predict thicker, untruncated Unayzah section and map stratigraphic-trap potential along these trends.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria