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Modeling Upper Cambrian Microbial Reefs: Bridging the Gap From Outcrops to Reservoir-Scale

Abstract

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, Texas1, offer unique opportunities to assess varying scales of their spatial variation and potentially serve as subsurface analogs to improve reservoir correlation and modeling. A drone survey was conducted over these outcrops, from 40 m altitude, and ultra-high resolution orthophotographs, with a resolution of 1 cm, were processed using Agisoft 1.0. Three growth phases are observed in the microbial bioherms (10–15 m high and tens of meters in width), evolving from an initial 'colonization' phase, through a 'vertical aggradation and lateral expansion' phase, and ultimately into a 'capping' phase. 3D analyses of the bioherm colonization phase on a plan view outcrop in the floor of the James River offers unique opportunities in scaling their growth at three quantifiable scales: large, medium, and small. Different bioherm scales were mapped, and their length, width, orientation, and spacing were catalogued. Absolute and comparative analyses were conducted within scales and between the scales showing that the statistical analysis conducted for these bioherms could be used for the entire area and are not specific to a certain part of the outcrop. In general the shape of bioherms at all scales is elliptical with large scale ranging in length from 15 to 40 m, medium scale ranging from 1.5 m to 15 m and smallest scale ranging from 10 cm to 80 cm. Trend analyses demonstrate possible bioherm orientation and size trends. Training images (TIs) were generated using a simple rule-based, object-based method with constraints such as the geometries spacing between bioherms at different scales. Additional work could explore methods to amalgamate the bioherms in a more realistic manner, but the existing TIs are considered suitable for providing reasonable spatial heterogeneity constraint when augmented by stationary region and non-stationary trend models for multi-point-statistics (MPS) modeling. In addition, these models provide an opportunity to test the impact of these architectures on flow response.