Translating Outcrop Data to Flow Models, with Applications to the Ferron Sandstone
Authors: White Christopher D. , Barton Mark D. 
Published: December 2007

Hydrocarbon recovery efficiency depends on spatial distribution of transport properties and on process physics. Property variability and process physics are sometimes highly coupled, and detailed models may be required to predict recovery accurately. The well spacing in hydrocarbon reservoirs makes constructing a detailed model difficult; analog data collected from outcrops can constrain or guide model construction. Outcrop data sets may be complex. The Ferron Sandstone outcrop data sets contain hundreds of sandstone and shale layers. Beds are discontinuous, nonrectangular. and nonhorizontal. Facies and property distributions crosscut bedding planes. Such features are difficult to model, especially using a Cartesian grid.

The method presented in this paper uses descriptive bedding diagrams prepared by a geologist. Custom algorithms edit, sort, and grid the surfaces described in the bedding diagram. The resulting cornerpoint grid with nonneighbor connections preserves bedding geometry and hierarchy and allows accurate placement of shale barriers.

Flow simulations using Ferron outcrop data sets show the importance of bed geometry, shale barriers, and property distributions. Comparative simulations of two fluvial-deltaic tongues within the Ferron Sandstone show that differences in depositional setting can alter fractional flow, sweep, and deliverability of analogous reservoirs.

Introduction

Emerging technologies have altered reservoir modeling methods. New measurement technologies such as borehole imaging and 3-D seismic provide data at greater density and resolution. Geostatistical methods enable geoscientists to construct large, detailed, and heterogeneous models. Reservoir simulation software can now incorporate irregular reservoir geometries, and the feasible size of models has increased manyfold. These technologies have been accelerated by advances in computing. Taken together, the result has been a huge increase in our ability to image and model reservoirs. However, geoscientists often lack methods, concepts and data needed to build reasonable constrained models of reservoirs at scales of interest.

Outcrop studies provide geoscientists data and insights to build models. Some outcrop studies address large-scale stratigraphic issues, others focus on smaller-scale characterization issues. The Ferron Sandstone outcrop study combines regional stratigraphic relationships with a detailed reservoir to interwell scale view of bedding, facies distribution, permeability and flow behavior.

Geological Setting. The Ferron Sandstone Member of the Mancos Shale Formation (Fig. 1) was deposited during a widespread regression of the Western Interior Seaway during Late Cretaceous (Turonian) time as thrust belt sediments were shed eastward and deposited along the margin of an evolving foreland basin.

Work by Ryer summarizes the depositional and tectonic history of the Ferron Sandstone. Briefly, the Ferron Sandstone is a fluvial-deltaic system containing a range of depositional facies from fluvial through shallow marine. The Ferron Sandstone is subdivided into a number of shallow-marine sandstone tongues partially bounded by marine mudstones (Fig. 2). The sandstone tongues are as much as 100 ft thick and extend basinward from 3 to 30 miles.

Patterns of stratal architecture. Initial sandstone tongues step strongly basinward whereas younger sandstone tongues are vertically stacked to landward stepping. The stratal architecture of identical facies tracts at different positions within the Ferron system changes as the stacking pattern of sandstone tongues changes.

Seaward-stepping tongues. In seaward-stepping sandstone tongues the shallow marine facies tract is the principal sandstone repository. Deposits consist of numerous broadly lenticular upward coarsening sandstone successions that are vertically and laterally separated by thin marine and marginal marine mudstones Fluvial strata interpreted as valley fills are preserved as lithologically homogeneous ribbonlike sandstone bodies that crosscut underlying shoreline deposits.

Landward-stepping tongues. In landward-stepping sandstone tongues these relationships are almost reversed. The shallow-marine tract is nearly completely eroded and replaced b fluvial and estuarine deposits, interpreted as valley fills. P. 233^