Determining Potential Stratigraphic Traps Using the Outcrop-Subsurface Link: Example From the Triassic Sudair and Jilh Formations, Sultanate of Oman
M. Obermaier1 and T. Aigner2
Outcrop analog studies can be powerful tools for reservoir prediction, especially in combination with cuttings and core analysis on time-equivalent subsurface deposits. The Triassic Sudair and Jilh Formations in north Oman illustrate an up to 800 meters thick carbonate succession which was deposited on an extensive carbonate ramp at the western Tethys Ocean. In order to map out the lateral distribution of reservoir/seal units and detect potential stratigraphic traps, 8 outcrop sections in the Oman Mountains and 14 subsurface wells of the Sudair/Jilh in north Oman were systematically investigated and interpreted in terms of sequence stratigraphy. In the subsurface of NW Oman the almost completely dolomitized Sudair/Jilh sequence features proximal rooted marsh facies, muddy backshoal deposits with m-thick shale units and occasionally sabkha/salina anhydrites. Some 180 km towards the northeast, where time-equivalent sediments crop out in the Oman Mountains, the Triassic succession becomes increasingly grainy and consists of abundant shoal-associated oolitic/peloidal grainstones. There, close to the platform margin evaporites are absent and shale deposits are thin resulting in a lower sealing potential of Triassic deposits and an increase in potential reservoir facies. A hiatus during the Late Triassic/Early Jurassic caused the truncation of underlying Permian/Triassic sediments. The resulting erosional unconformity which cuts into older strata towards the southeast of Oman may form potential stratigraphic traps in the Triassic of Oman. The lack of Triassic subsurface core data made facies interpretations in the subsurface challenging. However, the systematic and semi-quantitative analysis of ditch cuttings allowed to determine the lithology and carbonate texture in uncored subsurface intervals so that high-resolution facies logs could be produced for the entire Triassic in the subsurface of north Oman. Subsequent sequence-stratigraphic correlations between the outcrop and subsurface highlight facies changes on a regional scale and the lateral distribution of potential reservoir and seal units. A regional 3D static facies model (265 x 160 km) which integrates subsurface and outcrop data marks areas in north Oman where grain-rich carbonates are truncated by a Late Triassic/Early Jurassic unconformity. In combination with sealing units in the overlying Jurassic Mafraq Formation potential stratigraphic traps could be mapped out in the Triassic of north Oman.
AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain