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Integrated 3-D Sedimentological and Structural Study of a Lower Cretaceous Analogue Outcrop, Qishn Formation, Central Oman

Sena, Claire N.*1; John, Cedric M.1; Cosgrove, John W.1
(1) Earth Science and Engineering, Imperial College London, London, United Kingdom.

Outcrops of the Barremian-Aptian Qishn Formation in the Huqf region are laterally continuous over at least 1 km, and minimally deformed, allowing a high-resolution interpretation of the sedimentary record at the inter-well scale. These carbonates could be excellent outcrop analogues for the Upper Kharaib and Lower Shu’aiba oil reservoirs in the Interior Oman basins.

This study focuses on the facies geometries and fracture patterns of shallow-water carbonates in a shallow burial context. It presents outcrop-based quantitative data of facies and fractures distribution from a 1000 m long and 500 m wide butte. A bed-by-bed sampling of six 30 meters-thick outcrop sections provides material for geochemical and petrographic analysis. Fractures were measured on a 1500 m window and whenever possible, on bedding planes, and their type, orientation, spacing was studied.

The paleo-environments recorded range from supratidal to deep subtidal above the storm-wave base. The sediments were deposited on a low angle epeiric platform and transported by waves and currents mainly during storms. The subtidal carbonate sand bars are associated with rudists biostromes, backshoal and foreshoal deposits that pinch out laterally, giving considerable lateral and vertical heterogeneity to the reservoir rock.

Three genetic types of discontinuities surfaces are differentiated: omission, erosion and composite surfaces. They are laterally irregular and can be associated with storm scours infilled with carbonate mud. Twelve of these surfaces are lateral extensive and record high-frequency sea level oscillations. Their lateral extent and morphology is important to assess reservoir compartmentalization

Lithology has acted as a major control on fracture density and length. Mudstone and wackestones present widely spaced various sets of non-stratabound fractures whereas grainstones are saturated with stratabound fractures of a single set only. Early cementation is believed to be the main controlling factor on fracture saturation. Links between primary lithology, diagenesis and fracturing helps to drive predictive rules on the distribution of fractures in carbonates.

Outcomes of this study could be used to reduce uncertainties on facies geometries and fracture distribution of storm influenced peritidal carbonate models.

This study is part of the Qatar Carbonate Carbon Storage Centre funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain