Meandering Stream Reservoirs

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Introduction Braided stream deposits, described in a previous article in this series, andmeandering stream deposits commonly are excellent reservoirs. Meanderinghigh-sinuousity channels are found on flat alluvial plains with slopes lessthan 1 1/2 degrees [0.026 rad]. These rivers have wide ranges of dischargesfrom low-water flow to flood stage. Two main processes are responsible fordevelopment of sand bodies. These are point-bar deposits left by channelmigration, and oxbow-lake deposits left in loops of the river course abandonedwhen the stream cuts a new course during flooding. Extremely high floods spillover the banks and deposit sheets of very fine sand, silt, and clay onto theflood plain. Point-Bar and Oxbow Deposits Point-Bar and Oxbow Deposits The formation of apoint-bar deposit is illustrated in Fig. 1 Most of the sediments are carried inand deposited during a flood stage. The fastest bottom currents are in thedeepest water, which is usually on the outer banks of the loops. The coarsestsand is deposited in the deep, highest-energy part of the channel and finersand is deposited in the shallower water. As shown in Fig. 1. the depositionalslope across a channel is 10 to 20 degrees [0.17 to 0.34 rad]. During theflood, the outer banks arc undercut by the rapid currents and the channelsmigrate laterally. During falling-flood and low-river stages, the banks cave inand fine silts and clays are then draped over the depositional surfaces. Thenext flood stages will lay down new layers of sand as the point bar growslaterally. If the river course becomes too sinuous as the loops grow, the riverwill cut a new course during a flood. Abandoned loops form oxbow lakes that arefilled by silts, and clay during later floods, forming effective barriers tolateral flow. During geologic time, the stream may change its course severaltimes, leaving vertical sequences of point-bar and oxbow deposits within ameander belt 1 to 20 miles [1.6 to 32 km] or more wide. Individual point-barsand bodies are sinuous and elongated along the course of the ancient streambed. The clay-drape beds, which occur mainly in the upper half of the unit, usually have very limited lateral extent-a few tens to hundreds of feet. Hence, they reduce the effective vertical permeability. Similarly, clay plugs left inthe ancient channel beds usually lie above some gravel or sand, so they are notcomplete barriers to lateral flow. The quality of the sand in a point bar isbest a the base and decreases toward the top. Log responses and grain sizesshown in Fig. 2 are typical examples. The conceptual geologic model for apoint-bar sand body is shown in Fig. 3. The unit is about 60 ft [18 m] thick, the depth of the river, and 2 miles [3 km] wide, reflecting a possible lateralgrowth during, a depositional sequence. The possible lateral growth during, adepositional sequence. The idealized permeability profile shows sands andgravels have more than 3,000 md at the base and less than 10 md at the top. Theclay drapes shown at the top are small, isolated barriers that reduce but donot prevent lateral and vertical continuity within sands left in a meander loopgrowing outward. However, lateral continuity may be poor between sandsdeposited in adjacent loops because the loops grow in opposite directions andmay leave clays and silts in the region in between, as shown in Fig. 3. Development of point-bar reservoirs requires special planning and studies. While sand quality and continuity within planning and studies. While sandquality and continuity within an individual point bar deposited in a givensequence may be excellent, continuity between units deposited at differenttimes may be limited. Oxbow lakes and abandoned channels filled with silt andclay can form effective barriers to lateral flow. Similarly, shale beds formedabove a point-bar unit while that area of the meander belt was temporarilyabandoned by the river course may be preserved when the river re-enters thatarea. Thus, careful studies of geologic data, cores, and logs are required tocorrelate and to map the individual point bars and internal geometry of thereservoir in locating, and completing, the wells to drain each potentiallyseparate sand body. Later studies of reservoir performance often reveal theneed for infill wells or additional perforations to improve recoveries. JPT P. 1501