Summary
Quantitative models are needed to predict interactions between rock properties and drive mechanisms in geologically complex reservoirs. Analog studies using outcrop data provide insights for modeling, understanding, and predicting the behavior of oil and gas reservoirs. Stratigraphic cornerpoint grids preserve the geometries and facies distributions of outcrop data sets. Flow simulations of two outcrop exposures of sandstone-rich fluvial-deltaic tongues within the Cretaceous Age Ferron sandstone (Utah) revealed differences in fractional flow, recovery efficiency, and deliverability that can be related to stratigraphic setting. Compared with homogeneous models, models based on the landward-stepping tongue exposed at the Picture Flats locality had more tortuous flow paths and lower gas recovery efficiency. In the seaward-stepping tongue exposed at the Interstate 70 location, the displacement was layer like. Gas deliverability at the Interstate 70 locality varied with the well location; it was highest when the well penetrated high-permeability shallow-marine sediments and lowest when flow was restricted by a shale-lined valley-fill succession.
Introduction
Emerging technologies continue to improve reservoir modeling methods. Measurements such as borehole imaging and three dimensional (3D) seismic provide data at high density and resolution, and geostatistical methods enable construction of large, heterogeneous models.1 Cornerpoint grids with non-neighbor connections can represent complex geometries for reservoir simulation.2,3 However, we often lack the data and methods necessary to build detailed reservoir models at scales of interest. Outcrop studies provide data and insights to build models. The Ferron sandstone outcrop study combines regional stratigraphic relationships with a detailed reservoir-to interwell-scale view of layering, facies distribution, permeability, and flow behavior.4–8 The data sets discussed in this article contain hundreds of sandstone and shale layers and thousands of sedimentologic and petrophysical measurements. Layers are laterally discontinuous, nonrectangular, and nonhorizontal. Thin shales intermittently separate sandstone layers. Rock properties depend on facies, and sandstone layers may comprise more than one facies. These layer and lithofacies geometries are difficult to model using a Cartesian grid.
Reservoir simulation models were built from layer, shale, and facies diagrams. Vertical measured sections recorded grain size, permeability, sedimentary structures, and facies. The diagrams were edited, sorted, and discretized to create stratigraphic cornerpoint grids that conform to observed layer geometry. These non-Cartesian grids used void blocks and non-neighbor connections extensively. Hierarchical layer ordering preserved stratigraphic grouping throughout the modeling process. The flow behavior of these models was predicted using a reservoir simulator.3
Geologic Setting.
The Ferron sandstone is a lithostratigraphically defined member of the Mancos Shale Formation exposed in east-central Utah.9 The Ferron fluvial-deltaic system was deposited during a widespread regression of the Western Interior Seaway as thrust-belt sediments were shed eastward and accumulated along the margin of a rapidly evolving foreland basin during Late Cretaceous (Turonian) time.10,11 The Ferron sandstone is composed of two distinct clastic wedges:11,12 an early wedge derived from the northwest (the Clawson and Washboard sandstones) and a later wedge derived from the southwest (the Ferron clastic wedge).
Marine shales divide the Ferron clastic wedge into five sandstone-rich tongues, each comprising a delta-front sandstone body overlaid by a coal (Fig. 1).13 The tongues are as much as 100 ft thick and extend basinward 3 to 30 mi. Early tongues (numbers 1 to 3) step seaward, whereas later tongues (numbers 4 and 5) stack vertically or step landward. Each tongue contains many upward-coarsening and upward-shoaling shallow-marine successions4,5 or parasequences.14 Individual parasequences are 15 to 45 ft thick and extend basinward 1/2 to 5 mi. Within each tongue a nonconformity, which is marked by an incised fluvial system and an abrupt basinward shift in facies, separates underlying progradational-to-aggradational parasequences from overlying aggradational-to-backstepping parasequences.6
Patterns of Stratal Architecture.
The spatial arrangement of facies within tongues is related to the stratigraphic position within the Ferron clastic wedge (Fig. 1).4,5 In seaward-stepping tongues, sandstone is preserved mainly within the shallow-marine facies tract. The shallow-marine deposits are broadly lenticular parasequences separated by thin marine mudstones. Individual parasequences (as much as 30 ft thick and 3 mi long) stack progradationally to aggradationally, forming composite delta-front bodies (as much as 100 ft thick and 30 mi long in the dip direction). These delta-front sandstone bodies are locally incised and replaced by homogeneous ribbon-like sandstone bodies that are as much as 80 ft thick and 1/2 mi wide. The crosscutting sandstone body comprises many channel-form sandstone bodies that are as much as 25 ft thick and approximately 30 to 700 ft wide. The channel-form bodies are similar in scale and structure to channel stories defined by Allen;15 they are interpreted to be deposits of fluvial channels and bars. The channel-form bodies are thin relative to the crosscutting body, and they do not interfinger with adjacent shallow-marine strata. Thus, the channel-form bodies are interpreted to be deposits of a fluvial system that aggraded within an incised valley.7 Although the volume of valley-fill sandstone is small compared with the volume of the shallow-marine sandstone, valley fills may connect or segregate reservoir units. This is the setting of the Interstate 70 locality (Ferron sandstone cycle 2, Fig. 1).
