Geometry Of Reservoir-Type Sandbodies In The Holocene Rio Grande Delta And Comparison With Ancient Reservoir Analogs
Authors: Pryor Wayne A. , Fulton Ken 
Published: December 2007

Sandstone reservoirs are the result of long and frequently complex histories of geologic evolution. The combined processes of deposition, burial, compaction, diagenesis and structural deformation yield final reservoir bodies of widely varying characteristics that are difficult to predict. In unravelling the evolution of sandstone reservoirs and predicting reservoir trends on a rational basis it is necessary to have knowledge of their initial depositional characteristics. Especially important are the details of the internal geometries of sandstone bodies, the continuity of individual sand packets making up the bodies, their internal bedding forms and arrangements, and the relationships between grain textures and permeability-porosity variations. permeability-porosity variations. The Rio Grande Delta complex has been intensively studied in the subsurface with a dense drilling pattern, using wireline logs and continuous cores. Three pattern, using wireline logs and continuous cores. Three different types of sand bodies within this delta distributary complex are presented as models for reservoir inhomogeneities:distributary mouth-bar sand bodies,meander point-bar sand bodies anddelta slope sand bodies.

The Rio Grande Delta distributary complex sand bodies are composed of bundles of sand in a silt-clay matrix with relatively low lateral continuity indices. Continuity indices increase upward in the deltaic sequence. Permeabilities and porosities of these Holocene sands are relatively high, ranging from 200 millidarcies to 7 darcys and from 5 percent to 45 percent. percent. Examples of producing reservoir sandstones from the Gulf Coast, western interior and the Illinois Basin compare well with the Holocene model, illustrating the predictive reliability of reservoir models.

Introduction

Sand bodies associated with deltaic deposits are some of the most important and prolific reservoirs in clastic sedimentary basins. They occur in association with organic rich mudrocks, which not only serve as source beds but are also excellent reservoir seals. These deltaic sand bodies are of several origins with varying geometries - linear fluvial channel bodies, linear distributary bar fingers, destructive-phase sheets, and linear littoral bodies, each of these has slightly differing reservoir characteristics. Many investigations have delineated their gross geometries, distributions and relationships. However, the details of internal geometries, heterogeneities and vertical-lateral continuities, so important to reservoir continuity prediction, are lacking. This is largely the result of reconstructing these sand bodies from widely spaced data points. Forty acre spacing is normally used, so that the distance between points is often larger than the individual sand unit components that make up the bodies. Therefore, the details cannot be mapped - the trees do not stand out in the forest.

Sands do not fall out of the sky as uniform deposits, but are most often deposited as bed form or traction deposits in the form of ripples and sand waves with definite permeability boundaries, both externally and internally (Figure 1). This type of deposition results in stacked bedding units and laminae that are cross-cutting packets of cross-beds and ripple laminae. These bedding packets range in size from a few centimeters to tens of meters thick, but average less than 1 meter. Each packet is composed of inclined laminae a few centimeters thick that increase in permeability from the top to the bottom. Permeability and porosity also vary significantly between adjacent laminae (Pryor, 1973). Important discontinuities occur between laminae and bedding packets. They are most often clay laminae, clay beds and sands with interstitial clays. The clay discontinuities being the result of shifting depositional processes. The overall permeability of reservoir units is strongly influenced by these thin discontinuities and the ultimate through-flow capability or effective permeability will be largely determined by the lower permeabilities of the bounding discontinuities. Hence, lateral and vertical connectivity of reservoir units are determined by the presence and abundance of the low permeability units - and their distribution is determined by depositional processes.