ARCHITECTURE AND SCALING OF UPPER CAMBRIAN LARGE MICROBIAL REEF COMPLEXES (MASON COUNTY, CENTRAL TEXAS, LLANO UPLIFT): OUTCROP ANALOGUES FOR BRAZILIAN PRE-SALT RESERVOIRS

Academic and industrial research into lacustrine microbialites has intensified in response to recent giant oil field discoveries in pre-salt strata offshore Brazil. Several studies have reported similar grain types, textures, and porosity structure between marine microbial carbonates and those from alkaline lakes. These similarities indicate that marine microbial carbonates may also provide meaningful analogues for understanding the heterogeneity of pre-salt reservoirs. The lacustrine marine similarities and the large scale of recently accessible Upper Cambrian microbial mound and reef complexes within the Wilberns Formation in private ranches of Mason County (Llano Uplift, Central Texas,) prompt us to embark in a detailed outcrop study of the facies architecture, diagenesis, and porosity/permeability distributions of microbialites as potential analogues for pre-salt microbial carbonate reservoirs.

Our initial reconnaissance studies of the Wilberns microbial reef complexes only focused on exposures along the Llano River through kayaking. A series of spectacular newly accessible outcrops on private ranches along Mill Creek and James River were discovered in the past year, south of the Llano River. The reef complexes are spectacularly exposed at various stratigraphic levels in surface view in river bottoms, and in cross-section view in cliff walls along the Llano River, Mill Creek, and James River. Those reefal edifices were deposited near the western margin of the vast Cambrian-Ordovician Great American Carbonate Bank. Thick reef mounds developed in the Point Peak and lower San Saba members likely formed during the Wilberns Formation transgressive systems tract. In contrast, thinner, tidally oriented bioherms within the upper San Saba member indicate shallower water deposition during high stand conditions.

The large microbialite bioherms are arrayed into gigantic microbial reef complexes that span several square kilometers in surface area. The arrangement of large bioherms into reef complexes appears to mimic the arrangement of smaller thrombolite mounds into bioherms. There is a great deal of heterogeneity in the facies, porosity and permeability at the reservoir scale, and our preliminary observations suggest a complex, fractal like arrangements of microbial structures.