Exploration history of the Ara intrasalt carbonate stringers in the South Oman Salt Basin

Abstract

ABSTRACT The Ara intrasalt carbonate ‘stringer’ play is one of oldest petroleum occurrences known in the world (terminal Neoproterozoic to Early Cambrian age), and it constitutes one of the most complex and unconventional deep oil and gas plays in Oman. The reservoirs are commonly over-pressured, and consist of porous dolomitic carbonates that are encased in salt at depths of 3–5 km. The mostly shallow-water carbonates pose a challenge; both in terms of understanding the origin and spatial distribution of the various lithofacies, and in building predictive reservoir models. In addition, early phases of salt movement influenced carbonate sedimentation and dolomitization, the sedimentation of reservoir and source-rock facies and the structural development that later governed oil migration. While the thick halite sequences provide the seal for the intra-formational trapping of hydrocarbons, the geometry of these thick salt pillows (combined with deep, present-day burial of these reservoirs) affects seismic resolution. The intrasalt stringers were previously regarded as a self-charging hydrocarbon system, containing carbonate source rocks in close proximity or even within the dolomite reservoirs. More recent documentation of a presalt charge, in some of these stringers, adds a new level of complexity to this petroleum system. Carbonate intrasalt stringer exploration in the South Oman Salt Basin (SOSB) started with the unexpected discovery of moveable oil in Nasir-1 in 1976. This launched the first phase of stringer exploration that focused on the Birba and Dhahaban areas. Despite the addition of significant reserves during this campaign, the stringer play proved to be complex. Limited knowledge of the depositional systems and diagenetic history of the stringers made the predicton of reservoir quality difficult and the understanding of production behaviors next to impossible. Difficulty in delivering expected reserves forced the play to become dormant in 1986. The second phase of stringer exploration started in 1988 after a review of deep exploration opportunities that highlighted the play potential outside the proven Birba and Dhahaban areas. All wells drilled during this phase failed to discover commercial hydrocarbon accumulations, thus forcing the play to become dormant for a second time. This short-lived campaign, however, led to the Al Noor Athel discovery, a silicious tight reservoir also encased in the Ara salt. This discovery launched an Athel exploration campaign that lasted until 1997. The Athel campaign did not result in any further commercial discoveries either, however it revived the interest in the Ara stringers with the discovery of oil in Harweel Deep-1 in 1997. Continued success in the Harweel area has maintained interest in stringer exploration to this day and has led to the fast-track development of some Harweel stringer discoveries, which are now contributing significantly to PDO’s oil production. Since 2001, the play has been tested outside the Harweel fairway with limited success. A substantial part of the prospect portfolio remains untested mostly residing outside the Harweel area. Increasing the chance of exploration success will require significant improvements in seismic imaging, better prediction of reservoir occurrence, improved produceability from discovered reservoirs, and a better understanding of hydrocarbon charge history.