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Petroleum Exploration Strategies in the Search for Microbial Reservoirs in the Eastern Gulf Coastal Plain of the United States

Mancini, Ernest A.1
1 Geological Sciences, University of Alabama, Tuscaloosa, AL.

In the eastern Gulf Coastal Plain, Upper Jurassic (Oxfordian) Smackover inner ramp (shallow water) microbial buildups developed on encrusted and cemented firm to hard surfaces or on paleotopographic crystalline rock features, such as Paleozoic basement paleohighs. These thrombolites attained a thickness of 58 m and are present in an area of up to 6.2 km2. Although these buildups have been exploration targets for some 30 years, new field discoveries continue to be made in this area. Microbiolites in this region were best developed on a firm to 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, temperature, light penetration, oxygen content, or nutrient supply. The key to drilling a successful wildcat well in search of microbial reservoirs associated with Paleozoic basement paleohighs is using three-dimensional seismic reflection technology to find a paleohigh and to determine whether potential microbial reservoir facies occur on the crest and/or flanks of the feature and are above the oil-water contact. The keys to drilling a successful wildcat well in search of microbial reservoirs associated with a stratigraphic trap include: thorough knowledge of the regional geology of the area, detailed subsurface mapping, and reservoir characterization based on an integration of wire-line log and core data. Porosity in these boundstone reservoirs is chiefly a result of diagenetic processes, including dissolution and dolomitization. Porosity in the most hydrocarbon productive reservoirs is characterized by a high percent of vuggy pores. An understanding of the depositional and diagenetic processes affecting the microbial facies and reservoirs is critical in the design of a sussessful exploration strategy.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009