Presenter
Pankaj Khanna
Rice University
Abstract
Pankaj Khanna1, Andre Droxler1, Michael Pyrcz2 Daniel Lehrmann3, Paul M. (Mitch) Harris1
1. Department of Earth Science, Rice University, Houston, TX, United States
2. Petroleum and Geosystems Engineering, University of Texas at Austin, TX, United States
3. Department of Geosciences, Trinity University, San Antonio, TX, United States
The discovery of hydrocarbon reservoirs in pre-salt microbial accumulations offshore Brazil and Angola, in addition to a significant microbial component in some of the world’s largest carbonate reservoirs in the Pri-Caspian Basin, has renewed interest in microbial deposits. Spectacular outcrops of Upper Cambrian microbial reefs in Mason County, Texas, offer unique opportunities to assess varying scales of their spatial variation and potentially serve as subsurface analogs to improve reservoir correlation and modeling. These outcrops are available on private Ranches, along the Llano and James rivers and Mill creek.
The microbial buildups lie within the Point Peak Member of Wilberns Formation, and are divided by Plectrotrophia zone (Plectrotrophia bridgei and species of Billingsella) into two separate units. Plectrotrophia zone is found in most of the outcrops across 2500 Km2 of the platform and hence represents a good marker bed. This study focuses on 25 Km2 outcrops exposed along Llano River, James River, and Mill Creek which expose the complete thickness of the Point Peak Member. The outcrops represent vertical stacking of proximal microbial facies (Lower Point Peak) below distal microbial facies (Upper Point Peak). Field studies, in addition of drone imagery, are used to analyze the lower Point Peak unit (bioherms and biostromes (<1m thick, associated with heterolithic and skeletal/ooid grainstone beds), and upper microbial build up unit (up to 14 m thick, associated with skeletal and ooid grainstone beds intercalated with mixed siliciclastic carbonate silt beds). The Upper microbial unit display a three phase growth model, evolving from an initial ‘colonization’ phase on a transgressive lag, through a ‘vertical aggradation and lateral expansion’ phase, and ultimately into a ‘capping’ phase. The ultimate demise of the microbial buildups in the upper Point Peak unit is interpreted by an increase of water turbidity caused by an enhanced fine siliciclastic flux.
A world class pavement outcrop, 600 x 200 m in surface area along the banks of the James River, displays a horizontal section through Phase 1 of the upper Point Peak Microbial buildup unit. A total of four scales, small (few dm), medium (few m), large (few tens of m), and complex (few hundreds of m) microbial buildups have been identified on the James River Pavement. Nine attributes – length, width, area, perimeter, anisotropy, circularity, eccentricity, and compactness are studied for small, medium and large microbial buildup scales. The major results of the statistical analyses point out that, as the size of the mapped polygon feature increases, the feature shape becomes more complex, less circular, and orients towards NE-SW.
