DETAILED CORE FACIES DESCRIPTION OF AN UPPER CAMBRIAN MICROBIAL REEF COMPLEX (JAMES RIVER, MASON COUNTY, TEXAS)

The discovery of hydrocarbon reservoirs in pre-salt microbial accumulations offshore Brazil and Angola has enhanced interest in microbial deposits and analogs used to better understand reservoir stratigraphy, facies, and heterogeneity. Focus on structures resulting from the earliest forms of life on Earth has further increased as NASA scientists search for analogous microbial structures on other planets. For the past two years, a Microbial Research Consortium, representing a collaboration between Rice and Trinity Universities with funding and interaction from Chevron, ConocoPhillips, Shell, and Statoil, has focused on a series of newly available outcrops in southern Mason County, Texas.

Outcrops reveal a 10-15 m-thick Upper Cambrian microbial reef unit covering as much as 25 km². In particular along a James River bend, the microbial reef unit includes extensive pavement exposures adjacent to cliff faces, offering a 3-D detailed view of a microbial reef complex, covering 0.2 km² and reaching 15 m at its thickest central part. Three successive microbial growth phases, repeatedly observed through the reef complex, are: 1) an initial ‘colonizing’ phase microbialite over transgressive lags of flat rip-up clasts, 2) a ‘vertical aggrading/lateral expanding’ phase, distinctly interacting with large accumulations of inter-reef grainstones in the complex center, and 3) a well-defined ‘capping’ phase, during which inter-reef grainstones have vanished. The juxtaposition of bioherms with inter-reef high-energy grainstones along the James River closely resembles the environmental conditions of modern subtidal microbialites at several localities between the Exumas Cays, Great Bahama Bank.

More than 150 cores, either 7.5 or 15 cm in diameter and up to 50 cm long, were collected along a series of lateral and vertical transects through the different bioherm growth phases as well as the inter-reef grainstones. These cores superbly illustrate the depositional textures produced within different microbial growth phases. Detailed facies analyses from the separate split cores, placed on ultra-high resolution orthophotographs, offer unique opportunities to assess spatial variation of the buildups at varying scales and can potentially provide more robust analogs to improve reservoir characterization and modeling.