Print this pageAAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Modern Lessons for the Interpretation of Ancient Sabkhas: Examples from the Holocene of Qatar
(1) EMQI, ExxonMobil, Doha, Qatar.
(2) ExxonMobil Upstream Ventures, Houston, TX.
(3) ExxonMobil Exploration Compary, Houston, TX.
Holocene coastal and sabkha deposits of Qatar illustrate depositional and diagenetic trends that aid in interpretation of ancient, evaporate-carbonate reservoir sequences. Recent data from offshore and on land provide new insights into evaporate distribution, facies and stacking patterns of sediments deposited during the Holocene sea level rise. Comparison of coastal deposits from different regions of Qatar formed during the Holocene sea level rise reveals new insights into characterization of ancient rocks
Coastal Holocene sediments form a predictable profile from offshore to onshore, varying with sediment supply, circulation, and bathymetry. Windward-facing coasts are characterized by narrow, coarse grained facies belts dominated initially by fringing coral reefs, followed by formation of mobile sand belts and islands with algal flats, mangroves, and sheets of aeolian sands. Oblique and protected coasts are characterized by finer grain sizes, mixed carbonate and quartz sands formed in a mosaic of subtidal, beach intertidal and aeolian settings. The leeward coasts are marked by quartz sands and extremely high rates of coastline progradation.
Most coastlines are marked by low relief, with the result that high-frequency oscillations in sea level are responsible for major offsets in facies tracts. Age dating reveals that inland sabkhas are relicts of a high stand in sea level ~4000-6000 years ago. These areas are presently eroding. Extensive pedogenic modification of original marine sediments (by burrowing, infiltration, micrite precipitation) creates characteristic textures. Groundwater modification includes extensive precipitation of CaSO4 (nearly all gypsum), minor halite, micrite, and dolomite. Gypsum precipitation near the water table may reach 20-40% of sediment volume and extend over square kilometers.
Modern Qatar sabkhas are characterized by facies offsets at cycle breaks, laterally extensive erosional surfaces and associated gypsum precipitation. Documentation of these features aids in recognition of ancient sabkhas. Neither the sedimentary structures nor the biota are distinctive. Recognition of a sabkha relies on understanding styles of diagenesis that modify sediment texture and interparticle porosity. This process approach helps aid in sabkha recognition and to explain styles of diagenesis that control reservoir properties.