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Mancini, E.A., Llinas, J.C., Parcell, W.C., Aurell, M., Badenas, B., Leinfelder, R.R., and Benson, D.J., 2004, Upper Jurassic thrombolite reservoir play, northeastern Gulf of Mexico, Am. Assoc. of Pet. Geologists Bull, v. 88, pg. 1575-1602.
Abstract
In the northeastern Gulf of Mexico, Upper Jurassic Smackover inner ramp (shallow water) thrombolite buildups developed on paleotopographic features (Paleozoic basement paleohighs or Jurassic salt anticlines and ridges). Thrombolites dominated by calcimicrobes grew in the eastern part of the Mississippi Interior Salt Basin (Melvin Field example), the Manila Subbasin (Vocation Field example) and the Conecuh Subbasin (Appleton Field example). These thrombolites attained a thickness of 58 m and covered an area of up to 12 km2. Although these buildups have been exploration targets for some 30 years, new field discoveries continue to be made in this area indicating that the development of these organosedimentary deposits is not completely understood. By studying Upper Jurassic thrombolite bioherms and reefs as preserved in outcrop, the geometries, areal extents, and facies relationships of thrombolites can be better characterized, and this characterization is useful in designing an effective exploration strategy for delineating thrombolite buildups in the subsurface. Thrombolites were best developed on a hard substrate during a rise in sea level under initial zero to low background sedimentation rates in low energy paleoenvironments, and their occurrence was not restricted by water depth, salinity, light penetration, oxygen content, or nutrient supply. The keys to drilling a successful wildcat well in the thrombolite reservoir play are to 1) utilize three-dimensional seismic reflection technology to find a paleohigh and to determine whether potential thrombolite reservoir facies occur on the crest and/or flanks of the feature and are above the oil-water contact, 2) use the characteristics of thrombolite bioherms and reefs as observed in outcrop to develop a three-dimensional geologic model to reconstruct the growth of thrombolite buildups on paleohighs for improved targeting of the preferred dendroidal and chaotic thrombolite reservoir facies, and 3) utilize the evaporative pumping mechanism, rather than the seepage reflux or mixing zone models, as a means for assessing potential dolomitization of the thrombolite boundstone.
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