Significant Biologic and Hydrodynamic Control on Reservoir Oil Properties and Resultant Reservoir Geometry in the Wafra, First Eocene Reservoir, Partitioned Zone, Saudi Arabia and Kuwait

Since 1956 the Wafra First Eocene Reservoir in the Partitioned Zone has produced a significant amount of heavy oil from a dolomitized inner-ramp reservoir. Primary oil recovery is only 3-5% and the operators are currently evaluating plans for a full field steamflood project to improve oil recovery. Plans for potential thermal EOR operations require an understanding of the oil in place geometry.

While previous workers have observed a geographical asymmetry of hydrocarbon distribution within the First Eocene Reservoir, detailed saturation mapping conducted as part of this study suggests that the up-dip lateral extent and base of the hydrocarbon accumulation has a more complex geometry than previously documented. A dry hole near the structural crest defines the up-dip margin of the reservoir, while the subsea depth of the oil transition zone varies by over 500ft (152m). Well-log hydrocarbon saturations, oil geochemistry data as well as physical and chemical reservoir fluid property data were incorporated with the overall hydrodynamic setting for the First Eocene Reservoir to understand the distribution and genesis of the hydrocarbon accumulation.

Calculations of the reservoir oil/water interface angle under hydrodynamic conditions (using existing oil gravities and various horizontal water potentiometric slope estimates) yield hydrocarbon contact inclination values that approximate the observed base of oil tilt on the up-gradient, western side of the reservoir. Areal and vertical variations in oil gravity appear to play a role in the aspect of the oil/water transition. The strike of the oil/water transition plane is perpendicular to the regional water gradient and provides further evidence hydrodynamic control of reservoir geometry.

Vertical profiles of oil geochemical signatures from core extracted oil samples suggest aerobic bacteria played a significant role in altering the vertical distribution of oil gravity within the reservoir. In turn, the variation of biodegradation and resultant oil density distribution is thought to have played a significant role in the origin of the original oil in place geometry. As the Hubbert Equation (1954) predicts, the low gravity (<12 API) highly biodegraded oil along the hydrodynamic front of the reservoir forms the steepest inclination of the oil/water transition plane.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California