Digital characterization of thrombolite-stromatolite reef distribution in a carbonate ramp system (terminal Proterozoic, Nama Group, Namibia)

Abstract

The stratigraphic architecture of a terminal Proterozoic carbonate ramp system (ca. 550 Ma, Nama Group, Namibia) was mapped quantitatively with digital surveying technologies. The carbonate ramp consists of a shoaling-upward ramp sequence in which thrombolite-stromatolite reefs developed at several stratigraphic levels. The reefs are associated with grainstone and heterolithic facies and exhibit diverse geometries and dimensions related to the position in the sequence-stratigraphic framework. Laterally extensive reefs with a tabular geometry developed when accommodation was relatively low, whereas discontinuous oblate dome-shaped reefs developed during times when accommodation space was relatively high. Collecting sedimentological and stratigraphic data digitally in an extensive canyon system allowed a comprehensive documentation of the three-dimensional (3-D) architecture and dimensions of the reefal buildups. Both deterministic and stochastic methods were used to extend outcrop observations to construct 3-D models that honor the observed stratigraphy. In particular, the accuracy with which dimensions of reefal buildups can be measured is critically important in the statistical modeling of the dome-shaped buildups. Calculations and corrections can be applied directly to the digital data set and serve as input during model building. The final 3-D model faithfully reproduces the outcrop distribution of facies and geological objects and has a high spatial resolution, compared with petroleum industry reservoir models. The organization of the reefal buildups in the stratigraphic framework has direct implications for reservoir continuity and connectivity in analogous settings. The digital characterization and 3-D outcrop models presented in this article can be subsequently used to condition dynamic reservoir-simulation modeling of geologically similar areas.