Distribution and mechanisms of overpressure generation and deflation in the late Neoproterozoic to early Cambrian South Oman Salt Basin


Corresponding author:Peter A. Kukla, Geological Institute, Wuellnerstrasse 2, RWTH Aachen University, D-52056 Aachen, Germany.
Email: kukla@geol.rwth-aachen.de. Tel: +49 241 8095720. Fax: +49 241 8092151.


Late Neoproterozoic to early Cambrian intra-salt Ara reservoirs of the South Oman Salt Basin represents 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 characterized by high initial production rates because of hard overpressures. A database of more than 30 wells has been utilized to understand the distribution and generation of overpressures in intra-salt reservoirs that can be separated by up to 350 m of salt. A temporal relationship of increasingly overpressured 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−1, 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. Structural and geometric information indicates 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. A proposed pressure deflation mechanism can be related to the complex interplay of salt tectonics and fast deposition of early Cambrian to Ordovician age clastics.