QUALITATIVE AND QUANTITATIVE ANALYSIS OF UPPER CAMBRIAN MICROBIAL REEFS, MASON, TEXAS

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 the Shepard and Zesch private Ranches, along the Llano and James rivers and Mill creek.

A drone survey was conducted in February 2013 to collect aerial photographs over ten outcrops (three pavements and seven cliffs). The photographs were used to build 3D Digital Terrain Models (DTM) using Agisoft software 1.0 version. The workflow followed six steps: first importing photographs in Agisoft 1.0, second aligning them according to the location at which the photographs were taken, third building 3D sparse point cloud. Fourth step is to improve the accuracy of the location of 3D point cloud model where ground control points (GCP’s) are used (Before the drone survey, markers were placed in the field and location data was collected using DGPS (Differential GPS) and Trimble Total Station unit). Fifth step is to build dense point cloud model and the last step is to build mesh.

Through these 3D models and field studies, three growth phases are observed in the microbial buildups (10-15 m high and tens of meters in width), evolving from an initial ‘colonization’ phase, then into a ‘vertical aggradation and lateral expansion’ phase, and ultimately into a ‘capping’ phase. 3D analyses of the buildups colonization phase on a plan view outcrop in the floor of the James River (600m x 200m pavement exposure) offers unique opportunities in scaling the growth at three quantifiable scales: large (few tens of meters), medium (few meters), and small (few decimeters). Different scales were mapped, and their length, width, orientation, and spacing were catalogued. In general the shape of buildups at all scales is elliptical to circular 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.