Pressure Generation and Deflation Mechanisms in Deeply Buried Intra-Salt Reservoirs of the Late Neoproterozoic to Early Cambrian South Oman Salt Basin

Kukla, Peter A.¹; Reuning, Lars¹; Becker, Stephan¹; Urai, Janos L.¹; Schoenherr, Johannes²
¹Energy & Mineral Resources, RWTH Aachen University, Aachen, Germany.
²ExxonMobil Production Deutschland GmbH (EMPG), Hannover, Germany.

Late Neoproterozoic to early Cambrian intra-salt Ara reservoirs of the South Oman Salt Basin represent a unique self-charging petroleum play with respect to hydrocarbon and overpressure generation and dissipation. Reservoir bodies (termed "stringers") are isolated in salt and frequently contain low permeable dolomites that are characterised by high initial production rates due to hard overpressures.

A database of more than 30 wells has been utilised to understand the distribution and generation of overpressures in intra-salt reservoirs that can be separated by up to 350 metres of salt. A temporal relationship of increasingly overpressured and compartmentalized reservoirs within stratigraphically younger units is observed, and two distinctly independent trends emerge from the Oman dataset; one hydrostatic to slightly above hydrostatic and one overpressured from 17 to 22 kPa/m, almost at lithostatic pressures.

Structural, petrophysical and seismic data analysis suggests that overpressure generation is driven by fast burial of the stringers in salt, with a significant contribution by thermal fluid effects and kerogen conversion. Pyrobitumen confirms local contribution by a high-temperature hydrothermal event. Structural and geometric information indicate that present day hydrostatic stringers have been overpressured in their earlier geologic evolution. Evidence for these initial overpressures in currently hydrostatic reservoirs is provided by hydrocarbon-veined cores from halite overlying the reservoirs. Pressure deflation responsible for presently hydropressured reservoirs is conceivable by structural configurations to adjacent clastic minibasins or by further isolation and fluid injection into surrounding rock salt once minimum principle stress levels have been reached to dilate the salt. This is witnessed by black, hydrocarbon-stained cores of Ara salt directly above and below some of the stringer reservoirs. Pressure generation and deflation mechanisms are controlled by salt tectonic, microstructural (grain boundary network) and thermo-kinetic (burial and kerogen conversion) constraints and parameters. The processes revealed in this study are considered significant for other evaporite basins.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012