Examples Reservoir Meandering Channel
The vast, shallow interior seaway of the Later Cretaceous divided the North American continent essentially in half, and encouraged the development of numerous meandering river systems. These fed into the great embayment from both the east, where tectonic activity was mild and the west, where aggressive mountain-building was underway. On land, this major period of marine transgression brought with it a more humid, temperate climate and a rise in base level. While thick sequences of organic-rich marine shale accumulated in the axis of this seaway, fluvial complexes, whose sands would later serve as updip traps for petroleum, fed into its margins. Continued tectonism divided much of the western part of this seaway into variously-sized intermontane basins (Denver-Julesberg basin, Laramie basin, Powder River basin, Big Horn basin, Wind River basin), which individually generated substantial amounts of hydrocarbon reserves. Due to the rising uplands to the west, many of the river systems were characterized by higher slopes and thus straighter, less sinuous channels than some modern-day classic rivers such as the Mississippi.
For example, Figure 1 (Generalized stream channel pattern in the Lloydminster, Alberta area) shows the configuration of channels in the Lloydminster field area, on the Alberta-Saskatchewan border.
Abundant well data has indicated the pattern shown. The sands themselves are discontinuous, with individual bodies being less than 30 m thick and 10 km long. The vein-like pattern displays several generations of stacked channels and should not be mistaken as indicating braided stream flow. Figure 2 and Figure 3 (Maps delineating small field area in Lloydminster,
Part a shows an isopach map of several channels from the producing area and part b is a structural contour map of the same area. Note the small anticlinal arch in the westernmost channel sand body) reveals the actual dendritic plan view. A cross section through one of the sand bodies is given in Figure 4.
(Cross section of fluvial channel sands in the mannville group, Lloydminster area) and shows the lenticular geometry of the fluvial sandstones (compare also Figure 5, Isopach map and cross section of the Cretaceous Denkman and Little Creek field, Mississippi.
Note the scale of the meander loops and the lenticular nature of the sand body.) Note how the changes in the SP log particularly have been used to trace the lateral and vertical extent of this channel and its flanking sands.
In this particular case, the main channel sandstones are draped over a basement high that has created anticlinal trapping. Note the structural closure in Figure 2 and Figure 3.
As a second example, the Coyote Creek field in the eastern Powder River basin (Wyoming) provides an excellent example of a point-bar sand reservoir.(Figure 6,
Isopach map of Coyote Creek field, Wyoming-eastern Powder River basin and Figure 7, Log cross section of point-bar stratigraphic trap in the Cretaceous Fall River Sandstone, also from Coyote Creek field, Wyoming.
Note the bell-shaped gamma ray log curves through the point bar and the subdued profile through the oxbow fill.)
Isopaching reveals the orientation and large size of the bar, as well as the shale-filled abandoned (oxbow) channel that forms its hem. Several low areas, called swales, exist on the point bar itself, and are filled with siltier, low-porosity material. These limit lateral fluid flow. As in the above example, SP logs provide a relatively reliable indication of the location and extent of the sandbody. Well A-21 shows a well-developed, bell-shaped curve in the point bar. How does the curve in this well compare with that shown for Well B-2? The reservoir sands themselves are rather fine-grained, well-sorted, with porosities between 15% and 24% and permeabilities of up to 300 md. Gross pay is about 40 ft, and total reserves have been calculated at 20 million bbl of oil.
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Assessment
1. Although meander channel systems generally possess good porosity and permeability their importance as major petroleum reserves are limited because:
A .thin discontinuous shoestring geometries.
B .lack of proximity to marine source rocks.
C .continuity between channel systems is poor.
D .A, B and C
E .A and C
2. The Coyote Creek Field in the Powder River Basin represents a stratigraphic trap in a pointbar deposit.
A .True
B .False
3. Alluvial flood plains cut by a single meander channel occur in regions characterized by relatively low gradients, fine- to medium-grained sediment, and continuous discharge.
A .True
B .False
4. In a meandering stream, the erosional edge is called:
A .oxbow cutoff
B .cutbank
C .point bar
D .channel lag
5. Shoestring sand bodies are a result of meander stream deposition.
A .True
B .False
6. The azimuth of the dips in a point bar sequence generally point:
A .downstream
B .at right angles to downstream
C .upstream
7. The most common fossils found in meandering stream deposits are:
A .gastopods
B .vertbrate bones
C .plant debris
D .spores and pollens
8. The most common associated facies to the meandering stream deposits are:
A .deltaic and submarine fan
B .braided stream and alluvial fan
C .deltaic and braided stream
