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David R.D. Boote1, Duenchien Mou2, Robert K. Olson3
(1) Occidental Oil and Gas Corporation, Bakersfield, CA
(2) Occidental de Colombia, Bogota, Colombia
(3) Independent, Houston, TX

Abstract: SAFAH FIELD, OMAN. From Pre-discovery through Appraisal and Development -- A study of the limits in geological prediction under changing uncertainty

Safah Field is an Upper Shuaiba (Lower Aptian) carbonate reservoired accumulation located in northwestern Oman. Understanding of the field’s reservoir architecture and hydrocarbon charge history advanced considerably through pre-discovery, appraisal and development as the constraining data increased in volume and sophistication. A review of this conceptual evolution through time provides an insight into the level of predictive accuracy which can be achieved under varying degrees of uncertainty.

The Safah 1 discovery well was drilled in 1983 on the north plunging Lekhwair Arch. The area lacked any significant structural closure, but a rather loosely constrained regional analysis suggested the northeastern platform margin of the Upper Shuaiba intra platform Bab Basin crossed the Arch in the area of an enormous pre- Miocene paleoclosure. It was envisaged this closure might have been charged from the east during early Tertiary, later spilling entrapped hydrocarbons southwards during regional mid-Tertiary tilting, to be dammed against the carbonate margin porosity pinch-out.

The internal complexity of the field became increasingly apparent during subsequent appraisal. Additional drilling revealed a more heterogeneous reservoir architecture than predicted, with an early highstand platform sequence onlapped by later lowstand shales and shoal carbonates. Reservoir quality was poorer than anticipated and rather than a simple porosity pinch out, discontinuities within the lowstand carbonates proved to form the critical updip seal. The charge history was also more complex than expected, with at least two different incompletely mixed oils - a relatively early charge spilled from the northern paleoclosure and a later one from the east.

However despite these unanticipated variances, the original exploration concept remained valid on a more general fundamental level. With further refinement of the rather ambiguous seismic control, a stratigraphic accumulation of 1Bbbls OIP was successfully predicted early during the appraisal of the field.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana