Meandering Channel Alluvial Systems

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Diagnostic Evidence Meandering Channel

Seismic Meandering Channel

In the subsurface, channels generally create abrupt changes in lithology. Their seismic “visibility” should, therefore, be pronounced. At the same time, where the sharp erosional base and sides of the average channel make for good velocity contrast, the upper part of the average channel grades into flood-plain deposits and thus will not generate high-quality reflections. As a result, the typical lens shape of most channels should be only relatively clear on high resolution seismic lines, as shown in Figure 1.

Contents

Diagnostic Evidence Meandering Channel Seismic Meandering Channel Wireline Logs Meandering Channel Cores Meandering Channel

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(Seismic expression of a river-cut channel. Note the abrupt termination of flat-lying reflections against the channel flanks and the change in seismic character between these reflections and those within the channel. Note also the steeper slope of the right flank of the channel, possibly indicating that this was the cut bank.) Diagnosing such a channel, however, does not identify the environment it was deposited in. Other information is required to determine whether the specific facies is a delta estuary, submarine fan, or meandering stream complex.

Since meandering, like braiding, is a lateral feature, a horizontal time slice through a block of 3-D seismic data may reveal the map view of an ancient stream. (Figure 2, Horizontal time slice showing the meandering channel geometry and oxbow lakes of an alluvial flood plain.) This is a convincing example of the advantages possible in doing this type of seismic surveying.

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However, choosing the correct time interval of a section that will show such detail is neither simple nor accidental. An abundance of both geologic and seismic data must already exist concerning the formation to be examined. Furthermore, the velocity spectrum of shale overlaps that of high porosity sand and, in Figure 2 for example, it cannot easily be surmised what comprises the channel fill.

Wireline Logs Meandering Channel

Figure 3 (Well log showing two upward-fining point-bar sand bodies) gives a suite of logs showing the variations and relationships in alluvial flood-plain sediments.

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Two upward-fining point-bar sequences are in evidence. Both are surrounded by overbank flood-plain shales. Note how the gamma ray (SP) curve shows the abrupt change from shale to sand at the base of each channel, as well as the fining-upward, bellshaped curve as point-bar sand grades into flood-plain shale at the top of each channel sequence.

The dipmeter log for such a section will be a bit complex, but will show three main depositional surfaces:

structural slope (green motif)
major accretion slopes (red motif)
cross-bedding dips (blue motif)

In relatively undeformed areas, the green (“shale”) motif is the lowest in dip amount and indicates the regional or structural slope over which the broadest depositional processes occur. The upward decreasing red motif represents, in this case, the larger-scale accretionary layers, which show flatter and flatter dips as the channel becomes filled in. These surfaces, as shown, dip in toward the channel axis, usually at right angles, and consequently point towards the thickest portion of the individual bar. They represent major successive surfaces of the point bar itself. (Figure 4, Idealized dip log pattern showing the progressively lower slope amount (red motif) characteristic of filled-in channels. Tadpoles shown correspond to dips of major accretion surfaces, in this case, those of the point bar.)

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Cross-bedding dips within these accretionary layers will not be systematically or consistently read by the dipmeter tool when it is set on a broad step distance and correlation interval. If this interval is made smaller, however, crossbed sets become discernible as a third, upwardly-increasing dip trend known as the blue motif. (Figure 5, Idealized dip log showing both the filled-in red motif at left and the upward-increasing blue motif, which indicates individual crossbed sets.

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Note that the blue pattern at right depends upon a narrow dip correlation interval-usually less than ten feet-so that both toeset and foreset dips can be recorded by the logging tool.) Crossbeds are a current direction indicator and will thus trend usually at right angles to the accretion slope, which, as mentioned, dips into the channel. An azimuth frequency diagram for the dip log of a channel should ideally, show a bimodel pattern, indicating both the lateral point bar accretion and the cross-bedding within it. The latter will show a higher frequency and reveal the principal downstream direction — but only at a particular point. Due to the meandering nature of channels, current direction will change as much 180° to 270° between locations. Therefore, only the most limited prediction for reservoir alignment can be made on the basis of dipmeter data from individual wells.

In general, log data can be used to construct isopach maps which, when combined with paleocurrent information, are an often-used base for geologists in predicting field size and trend, as well as the possible locations of upstream or downstream channels with similar reservoir properties. Various units may be contoured. In addition to the reservoir sand itself, the underlying unit into which the channel cuts is also a good choice for delineating reservoir geometry and extent. It is sometimes useful as a first step to outline the general location of the valley or plain into which the channel once cut. Isopaching the channel sand interval is very often one of the best ways to articulate the original river course on maps. Overall, the geologist should keep in mind that channels create sudden changes in subsurface formations. Such changes should be detectable through a variety of techniques.

Cores Meandering Channel

Core sampling of channels should show the range and sequence of sedimentary types and structures mentioned. However, such sequences are often truncated by overlying channels and the entire suite may not be seen.

If sufficient sampling is done, the geologist will be able to roughly predict reservoir continuity. One rule of thumb is based on the general 1:1 sand-to-shale ratio of meandering alluvial facies that reflects near-equal development of flood-plain and point-bar subfacies. This rule is shown schematically in Figure 6.

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(Two overall schemes depicting potential vertical continuity of alluvial channel sand deposits. When the sand/shale ratio for an alluvial deposit is consistently low-less than 1-, it may be assumed that flood-plain sediments dominate and point-bar sand bodies are relatively isolated. Conversely, when this ratio exceeds unity and sands are coarse, the probability is higher that channel sands are abundant and interconnected.) When shale (i.e., the flood plain) composes more than 50% of the vertical section, channels are more likely to be unconnected; reservoirs, therefore, should be smaller and more numerous. When sand (i.e., the point bar) dominates, channels have a higher chance of being stacked and interconnected. In addition, the coarser the sand becomes in this latter case, the greater the probability for extensive cut and fill, and therefore vertical intercommunication.