ABSTRACT: Incorporating Depositional and Preservational Process Systems in Reservoir Characterization: Examples from the Upper Cretaceous Ferron Sandstone, Utah, USA
GARDNER, MICHAEL H., Bureau of Economic Geology, University of Texas at Austin, Austin, TX
A reservoir compartment is a rock body that controls fluid flow at the spacing of producing and/or injecting wells. Compartment geometry and bounding surfaces, in turn, are architectural elements that define variations in the external and internal structure of a reservoir compartment. Whereas these attributes quantify styles of reservoir heterogeneity, the dimensions, proportions, and arrangement of these features within the same reservoir type, change as a function of their stratigraphic position. This occurs because the architectural elements that define a reservoir compartment reflect not only intrinsic depositional processes but also extrinsic preservational processes related to temporal and spatial changes in accommodation space, the space available for sediment to accumulate, an base level, the energy of sediment flux.Consequently, quantifying architectural elements
solely on the basis of depositional attributes without incorporating preservational trends combines statistically different populations. However, integrating both depositional and preservational process systems in reservoir characterization reduces to the lowest common denominator the factors that govern stratal architecture and control fluid flow in a reservoir, and provides the portability required to transfer outcrop relationships to subsurface reservoirs.
The Ferron Sandstone comprises seven genetic sequences that each record an episode of regression and transgression under conditions of rising relative sea-level. Each genetic sequence contains a complete spectrum of marine-shelf, shallow marine, and coastal-plain facies tracts that are bounded by time-significant flooding surfaces or other time-equivalent surfaces. These genetic sequences form a hierarchical stacking pattern of seaward-stepping, vertically stacked, and landward-stepping shifts in the position of successive facies tracts. Contrasting the stratigraphic architecture in different genetic sequences reveals systematic and predictable changes in: (1) degree of facies offset, (2) cycle symmetry and thickness, (3) facies complexity, (4) lithology ratios, (5) sediment volumes, nd (6) bed-form diversity.
In the shallow marine facies tract of seaward-stepping genetic sequences, reservoir compartments are discrete subdelta lobes characterized vertically by an upward-coarsening sandstone succession and laterally by a gently inclined, depositional-dip-oriented sandstone wedge. Compartments are typically 30-40 m in height and 1-3 km in length. Internally, amalgamated distributary mouth-bar deposits contain numerous, inclined bounding surfaces that are hundreds of meters in length along depositional dip and hundreds of meters in width along depositional strike. Reservoir compartments are replaced laterally on a km-scale spacing by mud-rich interdistributary-bay facies. By contrast, in landward-stepping genetic sequences the shallow marine facies tract contains strike-oriented compartments 2 -30 m thick and less than 1 km in length. Internally, the absence of distributary-mouth bars and interdistributary-bay facies reflects basinal wave and storm processes that produce horizontal bounding surfaces that vertically partition compartments into multiple flow units.
In distributary channel belts of seaward-stepping facies tracts, a reservoir compartment is represented by a discrete, erosive-based compound barform or macroform. Macroform diversity is low, dominated by cut-and-fill types that display low bed-form diversity and are highly interconnected. Reservoir compartments are typically 2-5 m in height and tens of meters in width. The principle bounding surfaces are basal channel lags, tens of meters in width, and numerous, shorter length reactivation surfaces. By contrast, landward-stepping channel belts contain a diverse assemblage of moderately interconnected, low-sinuosity, high-sinuosity, and abandonment-fill macroforms that display a high bed-form diversity, with vertical changes in bed-form types producing vertical partitioning. Reservoir compartments are typically 3-10 m in height and hundreds of meters in width. Basal channel lags are hundreds of m in width, with subordinate bounding surfaces of equal length represented by depositional surfaces associated with lateral and downstream channel migration.
There are systematic and predictable variations in shallow marine and distributary channel-belt reservoir compartments that reflect changes in the stratigraphic position of associated facies tracts in time and space. The most heterogeneous facies tracts (seaward-stepping shallow marine and landward-stepping coastal-plain facies tracts) record the position of the greatest accommodation space and lowest sediment flux, which produces a reservoir compartment where time is more completely represented by strata. By contrast, the most homogeneous facies tracts record the position of lowest accommodation space, greatest amalgamation and canabalization, and highest sediment flux, which produces a reservoir compartment where time is more completely represented by stratal surfaces.
AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)