Characterization and 3D reservoir modelling of fluvial sandstones of the Williams Fork Formation, Rulison Field, Piceance Basin, Colorado, USA

Abstract

This study describes the stratigraphic characteristics and distribution of fluvial deposits of the Upper Cretaceous Williams Fork Formation in a portion of Rulison Field and addresses 3D geologic modelling of reservoir sand bodies and their associated connectivity. Fluvial deposits include isolated and stacked point-bar deposits, crevasse splays and overbank (floodplain) mudrock. Within the Williams Fork Formation, the distribution and connectivity of fluvial sandstones significantly impact reservoir productivity and ultimate recovery. The reservoir sandstones are primarily fluvial point-bar deposits interbedded with shales and coals. Because of the lenticular geometry and limited lateral extent of the reservoir sandstones (common apparent widths of ∼500-1000 ft; ∼150-300 m), relatively high well densities (e.g. 10 acre (660 ft; 200 m) spacing) are often required to deplete the reservoir. Heterogeneity of these fluvial deposits includes larger scale stratigraphic variability associated with vertical stacking patterns and structural heterogeneities associated with faults that exhibit lateral and reverse offsets. The discontinuous character of the fluvial sandstones and lack of distinct marker beds in the middle and upper parts of the Williams Fork Formation make correlation between wells tenuous, even at a 10 acre well spacing. Some intervals of thicker and amalgamated sandstones within the middle and upper Williams Fork Formation can be correlated across greater distances. To aid correlation and for 3D reservoir modelling, vertical lithology proportion curves were used to estimate stratigraphic trends and define the stratigraphic zonation within the reservoir interval. Object-based and indicator-based modelling methods have been applied to the same data and results from the models were compared. Results from the 3D modelling indicate that sandstone connectivity increases with net-to-gross ratio and, at lower net-to-gross ratios (<30%), differences exist in the cumulative volume of connected sandstone bodies between the indicator-and object-based lithology models. Therefore, the types of lithology-modelling methods used for lower net-to-gross ratio reservoir intervals are important.

The life cycle of crevasse splays as a key mechanism in the aggradation of alluvial ridges and river avulsion

“…Comprising sand and silt, their contribution to connected sand volumes (which may constitute hydrocarbon reservoirs) has been acknowledged in previous studies (e.g. Jordan and Pryor, ; Pranter et al ., ; McKie, ), albeit to a limited extent. The heterolithic fill of crevasse‐splay channels connects individual splays to their coeval trunk channel, which constitutes the main hydrocarbon reservoir.…”

“…Platt and Keller, ; Mjøs et al ., ; Hornung and Aigner, ; Anderson, ; Fisher et al ., ; Hampton and Horton, ; Jones and Hajek, ; Nichols and Fisher, ; Ghazi and Mountney, ; Pranter et al ., ; Ford and Pyles, ; Gulliford et al ., ; Sahoo et al ., ; Van Toorenenburg et al ., ; Burns et al ., ) or in the subsurface (e.g. Pranter et al ., ; McKie, ; Keeton et al ., ). Yuill et al .…”

On the origin of crevasse-splay amalgamation in the Huesca fluvial fan (Ebro Basin, Spain): Implications for connectivity in low net-to-gross fluvial deposits

“…Crevasse splays are connected to channel-lag and heterolithic channel-fill deposits, which typically constitute the lithologies with best reservoir quality (e.g., Fielding and Crane, 1987;Pranter et al, 2008). Channel-lag deposits in the crevasse-splay channels connect as 'wings' to their coeval feederchannel fill (Friend et al, 1986;Hirst, 1991;Mjøs et al, 1993;Nichols and Fisher, 2007;Fisher et al, 2007) (Fig.…”

“…Floodplain deposits in low net-to-gross fluvial stratigraphy contain fine-grained reservoir bodies (e.g., Donselaar et al, 2011;McKie, 2011a;Ford and Pyles, 2014). These sub-seismic-resolution deposits are difficult to distinguish on well logs (e.g., Passey et al, 2004;Bridge, 2006) and are thought to represent a relatively small proportion of overall sandstone volumes with a reservoir quality that is generally lower than that of coarser-grained fluvial facies (e.g., Pranter et al, 2008). As a result, intervals of floodplain deposits have been discarded as 'waste zones' in conventional reservoir development (Donselaar et al, 2011) and published research on their accumulation and reservoir architecture is limited (Bridge, 2006).…”

“…Fisher et al (2007), Hampton and Horton (2007), Jones and Hajek (2007), Nichols and Fisher (2007), and Gulliford et al (2014) have provided qualitative characterisations of ancient fluvial floodplain deposits exposed in outcrop and proposed and/or applied conceptual models for their stratigraphic evolution. Jordan and Pryor (1992), Pranter et al (2008Pranter et al ( , 2009, McKie (2011a), and Ford and Pyles (2014) have presented quantitative reservoir-architecture studies of heterogeneous fluvial intervals based on outcrop and subsurface data. These authors have acknowledged the contribution of crevasse-splay sheet sands to connected reservoir volumes, but focussed on channel and bar deposits which constitute the highest-quality reservoir sandstones.…”