Ahr, Wayne M¹, Dylan Morgan²
(1) Texas A&M University, College Station, TX
(2) Chevron-Texaco, New Orleans, LA

ABSTRACT: Microbial Buildups as Hydrocarbon Reservoirs

Cambrian carbonate buildups in Texas were known as algal bioherms until researchers reclassified them as calcimicrobial buildups. About the same time, around 1970, Mississippian mudmounds were thought to have developed by passive sedimentation of carbonate mud that accumulated with the aid of baffling organisms like crinoids and bryozoans. Now the abundant peloidal, clotted, and laminated fabrics in the buildups are considered as evidence of microbial origin. The absence of depositional fabrics indicative of current-related detrital sedimentation, along with abundant geopetal, constructed vug, and clotted fabrics in the mounds indicate that they actively grew in place. Jurassic microbial buildups in Alabama exhibit a variety of growth fabrics similar to those in Cambrian and Mississippian microbialites, and though no reservoirs are known in the Cambrian mounds, there are many productive reservoirs in Jurassic and Mississippian microbialites. Diagenetically altered Jurassic (Smackover) microbial buildups (aka reefs) at Vocation and Appleton fields are the most porous and permeable rocks in the area owing to dissolution and dolomitization of growth fabrics that transformed the microbialites, particularly thrombolites, into high-quality reservoir rocks. Diagenesis and fractures are also critical to reservoir quality in Mississippian microbialites. Diagenetically enlarged Stromatactis vugs provide much of the background porosity in Texas and North Dakota Mississippian mounds. Permeability is enhanced somewhat by dissolution but greatly by fractures. Reservoir quality therefore correlates with the abundance of through-going fractures that intersect both Stromatactis vugs and other fracture sets.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004