Detailed Core Facies Description of an Upper Cambrian Microbial Reef Complex (James River, Mason County, Texas)

Abstract

The discovery of hydrocarbon reservoirs in pre-salt microbial accumulations, offshore Brazil and Angola, has heightened interest in microbial deposits and, in particular, analogs that can be used to better understand reservoir stratigraphy, facies, and potential heterogeneity. For the past two years, a Microbial Research Consortium, representing collaboration between Rice and Trinity Universities with funding and interaction from Chevron, ConocoPhillips, Shell, and Statoil, has focused on a series of newly accessible and spectacular outcrops in southern Mason County, Texas. Outcrops reveal a 10–20 m-thick Upper Cambrian microbial reef unit covering as much as 25 km². In particular along a bend of the James River, the microbial reef unit crops out including extensive pavement exposures adjacent to spectacular cliffs thereby providing a unique 3-D detailed view of a microbial reef complex, covering 0.2 km² in surface area and reaching 15 m at its thickest central part. Three successive growth phases, repeatedly observed on the outcrops of the reef complex, are: 1) an initial 'colonization' phase of microbialites over transgressive flat pebble conglomerates, 2) a 'vertical aggrading/lateral expanding' microbial phase, interacting in particular in the central part of the complex, with the accumulation of large volume of grainstones in the inter-reef areas, and 3) a well-defined 'capping' phase, during which the inter-reef grainstones have vanished. The juxtaposition of individual bioherms along the James River with inter-reef high energy grainstones closely resembles the environmental conditions of modern subtidal microbialites at several localities in the Exumas Islands portion of Great Bahama Bank. More than 80 cores, either 7.5 or 15 cm in diameter and up to 50 cm-long, were collected using a hand held drill along a series of transects laterally across and vertical through the different bioherm growth phases. In addition, the inter-reef vertical and lateral grainstone variations were also sampled with cores. Depositional textures produced within the different phases of the microbial growth are superbly illustrated in the cores. Detailed facies analyses from the different split cores, placed on ultra-high resolution orthophotographs, offer unique opportunities to assess spatial variations of the buildups at varying scales and can potentially provide more robust analogs to improve reservoir characterization and modeling.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015