David S. McCormick1, John P. Grotzinger2
(1) Schlumberger-Doll Research, Ridgefield, CT
(2) Massachusetts Institute of Technology, Cambridge, MA
ABSTRACT: Digital Mapping of Neoproterozoic Thrombolite-Grainstone Facies Architecture in Namibia: Analog for Intrasalt Reservoirs in South Oman Salt Basin
We have used digital surveying equipment that includes real-time kinematic global positioning system (RTK GPS) receivers, total station, and reflectorless laser rangefinder to survey key geological relationships. We have also used photogrammetry to extract a digital elevation model (DEM) for the study area and draped aerial imagery on the DEM. This 3D model is the basis for studying stratigraphic geometries from cm's to km's.
We apply these digital surveying technologies to the quantitative characterization of carbonate field settings in the Neoproterozoic thrombolite-stromatolite reefs and associated grainstones of the Nama Group of central Namibia. These facies occur at several stratigraphic levels within the Omkyk Member. Reefs form laterally continuous biostromes (late HST), isolated patch reefs and isolated pinnacle reefs (TST) ranging in scale from a meter to several kilometers in width. Pinnacle reefs are up to 7 km wide and up to 350 m thick, whereas grainstone-patch reef assemblages form sheets extending for over 50 km laterally with thicknesses up to 30 m. These bioherms are believed to be excellent analogies to current exploration targets in Oman of equivalent age.
The collection of 3D digital data from outcrop enables the visualization of complex stratigraphic relationships that are not apparent from conventional 2D representations. Where such outcrop data sets have been collected with the intention of conditioning 3D reservoir models, the data must be collected in 3D so that the geometries are not misrepresented as a result of projections into 2D.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado