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Hierarchical,
Self
-Affine Fluvial Sand Body Shapes from Ancient and Modern Settings*
By
Paul A. Dunn, J.C. Van Wagoner, and M. Deffenbaugh
Search and Discovery Article #40083 (2003)
ExxonMobil Upstream Research Company, Houston, Texas
*Adapted from "extended abstract" of poster session presentation at AAPG Annual Meeting, May 14, 2003, Salt Lake City, Utah.
NOTE: This is the third of six presentations on the general subject of the shapes of siliciclastic sedimentary bodies presented by this group of ExxonMobil researchers under the leadership of John C. Van Wagoner. Click to view a list of all these articles.
Hydrocarbon production rates and recovery efficiencies depend on the spatial distribution of reservoir properties at scales ranging from the pore network seen in a core plug to the distribution and connectivity of bodies resolvable at seismic scales. Analog studies of modern environments and outcrops help span the range of length scales, but there is always uncertainty related to the applicability of a given analog to a subsurface data set. If there are common stratigraphic elements or bodies found in a wide range of depositional environments and over a wide range of dimensions, then reservoir models could be built of such objects without reliance on a specific analog. This paper describes the preliminary results of an on-going effort to test the hypothesis that most coarse-grained fluvial strata are composed of bodies that have strong commonalties in shape, internal structure and property distributions over a range of depositional styles and dimensions. In conjunction with work reported in other abstracts (see Van Wagoner et al., Hoyal et al., and Beaubouef et al., this volume), we hypothesize that sedimentary bodies in fluvial systems share a common shape and evolutionary pathway with bodies in other environments of deposition such as deltas, submarine fans, crevasse splays, and washover fans, among others.
We postulate that similarity in sedimentary body shape over this spectrum of environments of deposition, including fluvial strata, is a function of similar global dynamics of flow and particle deposition. We believe that these are the dynamics of nonequilibrium thermodynamics of open systems that form structures (beds, bars, lobes, fans) to dissipate excess kinetic and potential energy (see Van Wagoner et al., this volume).
Stratigraphic studies of ancient (Jurassic, Salt Wash Mbr., Morrison Fm. Green River, Utah) and modern (Red River, Randlett, Oklahoma) fluvial bar complexes delineated multiple orders of nested sand bodies which are statistically similar in shape and have consistent internal grain size and sedimentary structure distributions. Sand body shape is defined as the 3D surface bounding all genetically related sedimentary particles. In practice, the boundary is determined by using a ''cut off'' criterion based on thickness, grain size, or other characteristic. Where thickness information is not available, planform (mapview) outline can be used as a measure of shape. Shape planforms from the Salt Wash and modern Red River were analyzed as part of a larger effort to characterize sandstone body shape in many depositional environments.
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uStatistical shape similarities uSpatial variation in grain size uFluvial sandbar complex development
uStatistical shape similarities uSpatial variation in grain size uFluvial sandbar complex development
uStatistical shape similarities uSpatial variation in grain size uFluvial sandbar complex development
uStatistical shape similarities uSpatial variation in grain size uFluvial sandbar complex development
uStatistical shape similarities uSpatial variation in grain size uFluvial sandbar complex development
uStatistical shape similarities uSpatial variation in grain size uFluvial sandbar complex development
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Length-area plots
of depositional fluvial planforms yielded slope exponents near two, indicating
space filling (non-fractal) behavior. However, box counting of perimeters of 34
fluvial bar planforms from the Red River using high resolution aerial photos
taken during a low flow stage of the Red River generated statistically
Field testing of a
new approach to analyzing siliciclastic deposits as hierarchical structures
built of similar elements required data sets from a variety of depositional
environments where three-dimensional sand body morphology and internal
properties could be assessed. For fluvial deposits, data types included
topographic surveys, aerial photos, and ground penetrating radar (GPR) surveys
for morphology and geometry assessment as well as vertical and point
observations of grain size, sorting, sedimentary structures, and paleocurrent
indicators. All surveying was carried out using a real time kinematic global
positioning system (RTK-GPS) which allowed all observations to be located to
within k2cm. A sandbar complex in the Red River valley near Taylor Oklahoma had
the following features which made it attractive for study: (1) composite nature
of the bar complex permits
A unique outcrop
of the fluvial Salt Wash member of the Jurassic Morrison formation 15 miles
south of
Green River,
Utah was selected for study by virtue of its extensive The composite nature of the Salt Wash bar complex can be seen in the aerial photo where elements ranging in size from crossbed sets, bars and bar complex appear as light areas against the dark, vegetated background (Figures 4 and 5). The terminology used to describe the elements of the complex is arbitrary in the sense that there is a continuous spectrum of body sizes. Allowing for a gentle regional structural dip to the NE, bar complex topography shows the overall down flow expanding, digitate morphology of the bar complex (Figure 4). The exhumed depositional topography shows bar and bar complex apices are narrow highs that have steep upstream (western) margins. These steep exhumed slopes are interpreted to result from filled scours in the proximal bar as well as positive depositional relief of the bar crest. The medial bar forms a downstream expanding platform with low relief ''grooves'' oriented parallel to paleoflow. Distal bar and bar complex relief is subdued, forming a broad region of relatively flat sandstone. GPR surveys of 100, 250 and 500 MHz image the internal geometries of the bar complexes, bars and crossbed sets respectively. As imaged in the 100 and 250 MHz GPR surveys, bar complexes form down flow expanding lobate bodies resting on a composite basal surface that is erosional beneath the proximal bar complex and becomes conformable or downlapping in the distal bar complex. When viewed transverse to paleoflow, the basal composite surface forms a U-shaped proximal profile that gradually broadens down flow beneath the wide, tabular distal bar complex. The surface is composite in that it is formed by lower bounding surfaces of multiple bars. Where bars overlie one another they show dominantly lateral, compensational stacking. Bars that make up the proximal bar complex are thicker and narrower than the bars in the distal complex. Intermediate resolution 250 MHz and high-resolution 500 MHz GPR data provide the best imaging of the stratal geometries at the bar scale. The basal surfaces of most bars form the same down flow expanding and shallowing U-shape profile observed in the bar complex. There is a range of vertical successions seen in the GPR sections, but the basic motif seen in most cases permits a two-fold subdivision. The lower bar is dominated by parallel to very low angle trough cross stratification which are nearly conformable to the basal surface, but commonly onlap the proximal scour surface and downlap in the distal bar region. Where exposed at the surface, the lower bar facies exhibit flat or broadly mounded topography. The upper bar is composed of trough crossbed sets that onlap and downlap on to local scours that broaden and diminish in relief in a down flow direction. Upper bar facies form rough surfaces where exposed with paleoflow parallel groves formed at the lateral boundaries of the crossbed sets. The upper bar facies may not be present over the entire bar area. Upper bar deposits are thickest in the proximal bar and taper gradually or abruptly and downlap in the medial or distal bar region. Paleoflow directions taken from cross bedding show more variation (lateral divergence away from the bar core) in the upper bar facies than in the lower bar (dominantly parallel to general paleoflow). Spatial Variations in Grain-Size
Grain-size
Spatial grain-size distributions within the Salt Wash bar complex were assessed using visual estimates of point locations on the outcrop surface. Coring or hand-sampling was not permitted by the BLM in this area and, as a result, vertical grain-size variations could not be assessed. Figure 5 shows grain-size variation with distance along two transects originating from the upstream apex of the bar complex. Note the overall decay in grain size from proximal to distal bar complex. Downstream fining of fluvial deposits is not a new observation. The matter of real interest in these two transects is the composite nature of the grain-size decay. The plot of the northern transect shows a power law fit through the coarse end-members of each bar as well as individual visual fits through the coarsest samples in each bar. Much of the scatter seen in these data for the component bar elements can be attributed to grain size variation in the most proximal areas where winnowing has produced poorly sorted lags.
Fluvial Sandbar Complex Development The geometric observations of the proximal, medial, distal bar regions, and the vertical facies succession are consistent with deposition from expanding, decelerating, flows. Bar nucleation is initiated immediately down stream of a zone of scour. As the bar builds the scour is commonly filled and the deposit begins to interact more with the flow. Bedload features (trough crossbedding) dominate the upper bar. Flow over the bar top is forced to diverge away from the crest resulting in formation of crossbed sets that diverge obliquely away from the bar axis. With progressive shoaling, grooves form on the bar top as flow begins to develop enhanced pathways to the margins of the bar complex. Finally, the complex is abandoned after avulsion moves the channel to a new location. This process of scour formation and bar growth is interpreted to form in response to jets in the flow, jet deposits, and subsequent evolution of the jet deposit into a ''leaf-like'' deposit (Hoyal et al. and Van Wagoner et al., this volume).
Initial research on modern and ancient fluvial deposits indicates that there are strong similarities in sand body shape over a wide range of length scales. However, more work is necessary to prove the volumetric significance and preservation potential of these fundamental elements and the effects of container (channel) geometry on sand body shape. The relationship between deposit morphology and internal properties also requires further assessment. At this point we make the following conclusions about the fluvial deposits studied so far: 1. There are statistical similarities among fluvial sand body shape over a range of length scales. 2. The basic building blocks of fluvial sand deposits are lobate or teardrop shaped bodies which are narrow and thick at the upstream apex and broaden and thin towards their down flow edges. These elements resemble lobes in distributive environments such as deltas and fans even though they were deposited in confined settings. 3. Scour is concentrated in the proximal zones of each element in the fluvial sand body hierarchy. 4. Grain size diminishes away from the upstream apex and away from the medial axis of all elements with more noise in the trend of downstream grain size decay in proximal areas (winnowing, lags, dominance of bedload) and at smaller length scales. 5. Spatial variation in grain size and other flow related parameters appear to be consistent within depositional complexes that issue from a single apex or ''orifice''. |
Figure 1.
Principal component 
Figure 2. Red
River field program and preliminary grain size trends (inset).
Figure 3. Red
River bar complex topography, GPR sections and examples of core facies. See
Figure 4. Salt
Wash Member fluvial bar complex field program and topography (right). 
Figure 5. Salt Wash
grain size from visual estimates of outcrop surface.