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Reservoir Insight from World-Class Microbialite Outcrops - Cambrian of Central Texas


Spectacular outcrops of Upper Cambrian microbial reefs and related facies in Mason County, Texas, offer unique opportunities to assess varying scales of spatial facies variation and potentially serve as important subsurface reservoir analogs. The main microbial interval, covering 25 km2 in the study area where bioherms and biostrpomes beautifully crop out in cliffs and pavements (bedding plane exposures) along several river and creek valleys, has yielded important reservoir insight along several key themes.

Exploration-scale variability- These deposits formed on a broad ramp-like shelf during an overall transgression. Cliff exposures show the stratigraphic framework and positioning of the microbial interval above a prominent flooding surface, as well as the largest-scale clustering of reefs. Multiple 8-15 m-thick and 65-90 m-wide microbial reefs are linked laterally by a series of oolitic and skeletal grainstone beds alternating with thinner siltstones.

Development-scale variability- Closer examination of the cliffs reveals three growth phases to the reefs, evolving from an initial ‘colonization’ phase, into a ‘vertical aggradation and lateral expansion’ phase, and ultimately into a ‘capping’ phase and reef demise. Internal facies and relations between reefs and adjacent sediments vary with the growth phases. Pavement outcrops show field-scale and interwell-scale clustering of reefs along with a spectrum of reef attributes.

Reservoir modeling input- One pavement unique for its size (600m x 200m) and quality of exposure offers key scaling insight into reef heterogeneity across a range of scales (large from 15 to 40 m in length, medium 1.5 to 15 m, and smallest 10 to 80 cm) and quantification of reef attibutes such as size, shape, orientation, spacing, and rind vs core proportions. As the size of a microbial head/reef increases the shapes becomes more elongated in one direction, growing parallel to a dominant environmental condition (likely currents).

Facies definition- Extensive coring of the outcrops (n=230) shows details of rock textures for various depositional environments, including stromatolitic reef core, thrombolytic reef rind (outer margin), and inter-reef deposits.In general, thrombolitic fabrics retain minimal porosity and permeability and have been mostly unaffected by diagenesis compared to the relatively high porosity and permeability of stromatolitic fabrics, which have been heavily dolomitized.