REINHARDT, JOACHIM W., JOACHIM E. AMTHOR, and FOLCO HOOGENDIJK
Petroleum Development Oman, Muscat, Sultanate of Oman

Abstract: Deep Oil Exploration in an Unconventional Reservoir -- The PreCambrian Intra-Salt Carbonate Play of Oman

Intra-salt carbonates of the Precambrian Ara Formation represent one of the complex deep oil exploration plays in Oman and presently constitute a significant part of Petroleum Development Oman's undrilled prospect portfolio. The total-in-place proven oil reserves within the intra-salt carbonates exceed 300 million barrels.

During the late 1970's to early 80's Petroleum Development Oman drilled several oil and gas discoveries in carbonate objectives in the Precambrian Ara Salt of the South Oman Salt Basin. The carbonate reservoirs consist of porous dolomites stratigraphically trapped in salt at 3 to 4 km depth, hence commonly over-pressured. Although potentially rewarding, with well flow rates up to 8800 bbls/day of 30° API oil, the play was found to be complex with respect to reservoir quality prediction, hydrocarbon charge, production behavior, and seismic imaging.

The play complexity is a direct result of its unconventional nature:

i) The Precambrian age of the mostly shallow-water carbonates poses a challenge both in understanding the origin and spatial distribution of the various lithofacies and in building a more predictive set of reservoir models.
ii) The vuggy and fractured dolomite reservoirs are part of several evaporite-carbonate cycles deposited in a restricted platform setting. Early phases of salt movement influenced the stages of carbonate sedimentation and dolomitization, creating both new depositional sites for reservoir and source rock facies and structural development that later governed oil migration. The thick halite sequences provide the seal for the intraformational trapping of hydrocarbons. The geometry of the thick salt pillows, however, hampered the 2D seismic imaging of the deep (>3000m) reservoir objectives.
iii) The intra-salt carbonates are interpreted as a self-charging hydrocarbon system, containing carbonate source rocks in close proximity or even within the dolomite reservoirs.

These complexities resulted in a number of dry wells. For that reason the play had attracted only little attention in the past ten years. In 1996 an integrated project team was formed to reassess the full potential of the play. Newly acquired high-quality 3D seismic data were interpreted and integrated using new seismic methods and geological concepts in order to improve understanding of this unconventional play. The new models and approaches were successfully tested with the drilling of two new oil deep discoveries.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas