Analyzing Spatial Patterns in Modern Grainstone Accumulations to Increase Their Value as Reservoir Analogs

Author: Paul M. (Mitch) Harris

Date: Fri 07/08/2011


Given that grainstones constitute major hydrocarbon reservoirs worldwide, including portions of San Andres, Grayburg and Queen reservoirs in the Permian Basin, quantitative morphometric characterization of grainstone deposits is valuable for exploration and production optimization. Beyond revealing architectural trends that may be of use in reservoir characterization, morphometrics may also provide insight into the processes controlling grainstone distribution and sculpting their geometry.
Landsat remote sensing and bathymetric surfaces assembled into a GIS were used to interrogate the spatial patterns of sandbars within three carbonate sand bodies on Great Bahama Bank, which show a range of depositional facies patterns (Fig. 1). Rimming the southern end of Tongue of the Ocean (TOTO) is the broadest expanse of “high-energy” sands found in the Bahamas characterized by narrow sandbars separated by wide, deep channels and a lack of islands. A variation of the tidal bar motif with broader and more irregular sandbars, relatively narrow channels, and few small islands occurs at the northern end of Exuma Sound (Schooners). And sands associated with tidal channels and the numerous islands of the Exumas chain along the western edge of Exuma Sound occur primarily as flood tidal deltas.
Considered are properties such as the size, shape, complexity, distribution, orientation, and topography of the individual sandbars. Profiles and spatial analysis tools enable sandbar and channel spacing, position relative to the platform margin, connectedness, separation distances, and density to be characterized. Certain aspects of the geometry of the sandbars are found to behave in a systematic and hence predictable manner. Because the sand bodies are disparate in their overall extent and depositional settings, e.g., orientation and prevalence of islands, these findings have the potential to impart considerable insight to the characterization of grainstone systems.
Size - Unlike the well-reported behavior of reef-dominated seascapes, areas of the sandbars are not power-law distributed, as there is an underrepresentation of large geobodies compared to the power-law prediction. Instead, the sandbars robustly adhere to an exponential distribution, which like the power-law, confers a simple predictive relationship. Also, the ratio of sandbar to bar crest area falls within a narrow range, which is somewhat surprising given the differences in size and morphology between the sand bodies.
Shape - There is a surprising similarity of shape of sandbars given that the sand bodies themselves have differences, and systematic behavior is observed between sandbar shape and size with a clear trend of greater elongation with increasing area.Sandbar centerlines, which are a proxy for well-sorted grainstone bar crests, are commonly simple and the propensity for the formation of sinuous and intricate centerlines is similar between the sand bodies. Sandbars with highest surface-to-planar ratio, an efficient means with which to appraise the complexity in three-dimensional form, are typically closer to the seaward margin of the sand body and their geometry is linear and elongated.
Spacing - Mean separating distances between sandbars, which equals channel breadth, falls within a narrow threshold of only a couple of hundred meters and is more consistent than the width of the sandbars themselves. Separation distance relates predictably with respect to sandbar shape, as rounded sandbars are found within close proximity to one another and those separated by great distance have a propensity to be elongate. But the distance between sandbars seemingly has no bearing on sandbar size.
* Based on collaborative studies with Samuel J. Purkis (National Coral Reef Institute, Nova Southeastern University, Dania Beach, FL, USA) and James Ellis (Ellis GeoSpatial, Walnut Creek, CA, USA) which were published as:
Harris, P. M., Ellis, J., and Purkis, S., 2010, Delineating and Quantifying Depositional Facies Patterns of Modern Carbonate Sand Deposits on Great Bahama Bank: SEPM Short Course Notes No. 54, paper p. 1-51, appendix p. 1-31, and 2 DVDs.

Harris, P. M., Purkis, S. J., and Ellis, J., 2011, Analyzing Spatial Patterns in Modern Carbonate Sand Bodies from Great Bahama Bank: Journal of Sedimentary Research, v. 81, p. 185-206.

Category: wtgs | Sub Category: core workshop presentation