uIntroduction

uFigure Captions

u3D seismic examples

uOutcrop & GPR examples

uModel

uConclusions

uAcknowledgments

uReferences

uIntroduction

uFigure Captions

u3D seismic examples

uOutcrop & GPR examples

uModel

uConclusions

uAcknowledgments

uReferences

uIntroduction

uFigure Captions

u3D seismic examples

uOutcrop & GPR examples

uModel

uConclusions

uAcknowledgments

uReferences

uIntroduction

uFigure Captions

u3D seismic examples

uOutcrop & GPR examples

uModel

uConclusions

uAcknowledgments

uReferences

Figures Captions

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3D Seismic Examples of Erosional Remnants 

3D seismic imaging of Pleistocene slope strata, DeSoto Canyon area, northeast Gulf of Mexico documents erosional remnants that formed within a single leveed slope channel complex. These erosional remnants formed as a result of successive downcutting by small individual channel threads confined within the leveed master channel (Figure 1). In contrast, 3D seismic data from the Triassic-aged Montney Formation (western Canada) provides examples of erosional remnants between adjacent channels on the basin floor. The remnants are characterized by high amplitude and, based on available borehole data, are interpreted as being largely dissected frontal splays. Flanking the remnants are channels characterized by low-amplitude seismic reflections inferred as predominantly shale filled (Posamentier et al., 2003).  

The Gulf of Mexico erosional remnants are found on the slope within a larger leveed master channel complex (Posamentier, 2003) (Figure 1). Successive deeply incised channel threads within the master channel are illustrated in Figure 1A. The first erosional remnant formed within a meander loop cut-off, between channel threads 2 and 3 (Figure 1A). The later high sinuosity channel thread 4 dissected the larger, earlier-formed meander-loop cut-off remnant, resulting in two smaller mounds. Figure 1B shows the topographic relief and the high seismic reflection amplitude of the preserved substrate, interpreted as remnants of an earlier deposited frontal splay. The larger of the two remnants pinches and swells along depositional dip, forming an elongate mound within the master channel complex. The smaller of the two remnants forms a small mound with no preferential elongation.

Erosional remnants from the Montney Formation are found between channels on the basin floor and are not confined to a master channel complex. Figure 2 shows four different channel orientations and their associated erosional remnants. The channels have incised into the underlying frontal splay at slightly differing times and orientations. Variation in the orientations of the channels resulted in the lens- and crescent-shaped erosional remnants (Figure 2). They are significantly larger than the Gulf of Mexico examples.

Outcrop and GPR Examples of Erosional Remnants 

The Dad Sandstone Member of the Lewis Shale in Wyoming exhibits erosional remnants on an outcrop and at GPR scale. These erosional remnants are similar to the Gulf of Mexico examples in that they are observed within an interpreted larger leveed master channel complex. Compositionally, the erosional remnants comprise levee/overbank deposits, and they are thought to differ in lithology from the high-amplitude Montney and Gulf of Mexico examples.  

In outcrop (Figure 3), the erosional remnants are thinly bedded siltstones with rare interbedded thin sandstones. They are found in abrupt contact with the associated channel-fill. The uppermost sandy fill onlaps the remnant, indicating a hiatus between channel incision and fill. GPR lines, obtained from behind an outcrop (where the beds dip beneath the ground surface) at a different locality, confirm a similar relationship (Figure 4). On GPR lines, the erosional remnants are topographically high mounds within the channel complex. They are acoustically opaque zones as the result of rapid attenuation of the GPR waves due to the fine-grained nature of the remnant (Young et al., 2003). In contrast, the sand-prone channel fills contain numerous reflections representing bed boundaries. The spatial extent of these erosional remnants has proven problematic due to the two-dimensionality of the outcrop and limited GPR lines. However, initial evidence indicates they are a similar, to somewhat smaller, size than the Gulf of Mexico examples.

Erosional Remnant Model 

The formation of erosional remnants within a master leveed channel complex indicates several stages of incision within a confined setting. The combination of levee construction from bypassing flow and overall incision into the substrate provides the topographic relief necessary to confine the succession of channelized flows. The confining nature of this channel fairway creates a narrow conduit (thread) within which flows are concentrated. Concentration of the flows within this conduit causes erosion and incision into the underlying and laterally adjacent strata (Beaubouef, in press). Infilling of the relief created by the original thread results in lithofacies onlapping the channel margin. Erosion and re-incision from later bypassing flows will result in a new channel thread. If this new channel thread crosses the path of an earlier-formed channel thread in two places, an erosional remnant will be formed. The lithologic composition of the erosional remnant will be the same as the strata incised into by the channel threads. Repeated incision will result in the formation of numerous erosional remnants within the larger master slope channel complex.  

Erosional remnants that form in an unconfined setting (e.g., the basin floor) are observed between neighboring crosscutting incising channels. These channels are not commonly active at the same time. Lithologically, these remnants also reflect the composition of the precursor substrate. Due to the lack of lateral constraints common in slope channels, erosional remnants on the basin floor can be significantly larger than those within slope channels.

Conclusions 

Two different types of erosional remnants have been observed: within leveed slope channel complexes and between adjacent channels on the basin floor. The remnants forming within a leveed slope channel complex appear to be smaller than those observed between adjacent channels on the basin floor. Within a leveed slope channel complex, erosional remnants are formed as a result of channel thread migration or shifting in conjunction with progressive downcutting of successive channel threads. Repeated incision results in crosscutting channel threads, forming erosional remnants between the threads. In contrast, crosscutting of channels in an unconfined environment such as a basin floor can create erosional remnants between channels. The lithology of the erosional remnant is genetically distinct from that of the channel, but a function of the strata being eroded.

The examples shown document the internal complexity of erosional remnants, the geomorphology of such features, and the overall complexity that they add to deep-water environments. Within a channel-dominated reservoir section, erosional remnants can compartmentalize sands, giving rise to tortuous fluid flow paths. They can be an important element of the deep-water channel model and should be considered in reservoir modeling and management planning.

Acknowledgments 

The authors would like to thank Anadarko Canada Corporation and Paradigm Geophysical, with special recognition going to both Randy Evans (ACC) and Paul Lepper (PG) for their patience and insight. Research was made possible through contributions from the Gene & Astrid VanDyke Scholarship and the Heston Geology Scholarship.

References 

Beaubeouf, R.T., (in press). Deep-water channel complexes of the Cerro Toro Formation, Upper Cretaceous, Southern Chile: AAPG Bulletin.

Posamentier, H.W., 2003, A linked shelf-edge delta and slope channel turbidite system: 3D seismic case study from the eastern Gulf of Mexico in Shelf Margin Deltas and Linked Down Slope Petroleum Systems: Global Significance and Future Exploration Potential: 23^rd Annual Gulf Coast Section Economic Paleontologists and Mineralogists Foundation Bob F. Perkins Research Conference, Houston, p. 115-134.

Posamentier, H.W., Chaplin, C., and James, D.P., 2003, Integrated analysis of the Triassic Montney turbidites: a case study from northern Alberta (abstract): CSPG and CSEG Annual Convention, Calgary.

Young, R.A., Slatt, R.M., and Staggs, J.G., 2003, Application of ground penetrating radar imaging to deepwater (turbidite) outcrops: Marine and Petroleum Geology, v. 20, p. 809-822.

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