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Quantitative 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, United Kingdom.

Outcrops of the Barremian-Aptian Qishn Formation in the Huqf region are laterally continuous over at least 1 km, and minimally deformed, providing geometric information on sedimentary systems and system tracts 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.

The 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 in a sequence stratigraphic framework. 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 were studied.

The stratigraphic architecture shows a general layer-cake pattern. Two types of grainstone bodies were recognized based on their different geometries and positions in the depositional profile. The high-energy subtidal shoal facies present a high degree of vertical and lateral heterogeneity. One-meter thick by tens of meters wide cross-stratified rudstones pass laterally into tabular floatstones and rudstones. Tidal flats beds are laterally irregular, as they are occasionally cut by 10 meters wide 30 cm deep storm scours infilled with carbonate mud.

Quantitative data on fracture distribution reveal that lithology has acted as a major control on fracture density. Mudstone and wackestones present widely spaced sets of non-stratabound fractures whereas grainstones are saturated with stratabound fractures of a single set only.

The 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 Center funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California