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Meandering fluvial reservoirs (view source)
Revision as of 22:00, 3 August 2015
, 22:00, 3 August 2015no edit summary
| colspan="4" | <sup>1</sup>''From Gibling<ref name=Gibling_2006 />, Journal of Sedimentary Research. Reprinted with permission from, and © by, the SEPM (Society for Sedimentary Geologists).''
| colspan="4" | <sup>1</sup>''From Gibling<ref name=Gibling_2006 />, Journal of Sedimentary Research. Reprinted with permission from, and © by, the SEPM (Society for Sedimentary Geologists).''
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==Meandering fluvial macroforms==
Macroforms found in meander belts include point bars, crevasse splays, and mud-rich channel plugs within a background of floodplain muds. Coals are found in fluvial systems with high water tables. Levees sometimes border rivers but they do not appear to be a major feature preserved in the subsurface.<ref name=Gibling_2006 />
[[file:M91FG175.JPG|thumb|300px|{{figure number|4}}Point bars within background floodplain shales and crevasse splays, Ebro basin, Spain.]]
==Point bars==
The main sand-prone macroforms found in meandering river sediments are point bars ([[:file:M91FG175.JPG|Figure 4]]). These form by lateral accretion of sediment on the inside of meander bends, and they occur as discrete sand bodies with a lenticular or half-moon shape in plan view. A multitude of point bar sandstone bodies may be found studded within a meander belt.
[[file:M91FG176.JPG|thumb|300px|{{figure number|5}}Flow geology influences in meandering river sediments.]]
Jordan and Pryor<ref name=Jordanandpryor_1992>Jordan, D. W., and W. A. Pryor, 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0010/0000/1601.htm Hierarchical levels of heterogeneity in a Mississippi river meander belt and application to reservoir systems]: AAPG Bulletin, v. 76, no. 10, p. 1601–1624.</ref> made detailed measurements on sediment body dimensions along a 10-mi (16-km) stretch of the Mississippi River meander belt system in southeastern Missouri. The point bars here are 15–45 m (49–147 ft) thick, a few kilometers long (typically 3 km; 2 mi), and between 600 and 1800 m (1968 and 5905 ft) wide. The Mississippi is a continental-size river stretching the length of the United States. As a general rule, big rivers like the Mississippi will tend to deposit large point bar sand bodies; lesser rivers will tend to produce smaller point bars ([[:file:M91FG176.JPG|Figure 5a]]). For example, the typical width of individual point bars in the 35-1 sand of the Widuri field in the Java Sea is 1200-1500 m.<ref name=Carter_2003>Carter, D. C., 2003, [http://archives.datapages.com/data/bulletns/2003/06jun/0909/0909.HTM 3-D seismic geomorphology: Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea]: AAPG Bulletin, v. 87, no. 6, p. 909–934.</ref> A well located in one of the point bars has produced 3.2MM bbls of oil. By contrast the fluvial sands in the Jonah Gas field of Wyoming are estimated to have a P50 width ranging from only 60 to 210 m.<ref name=Shanley_2004>Shanley, K. W., 2004, [http://archives.datapages.com/data/specpubs/study52/CHAPTER10/CHAPTER10.HTM Fluvial reservoir description for a giant, low-permeability gas field: Jonah field, Green River Basin, Wyoming, U.S.A.], in J. W. Robinson and K. W. Shanley, eds., Jonah field: Case study of a giant tight-gas fluvial reservoir: AAPG Studies in Geology 52, p. 159–182.</ref>
Several technical papers give cross plots of fluvial sand-body widths versus the maximum bankful depth of rivers (e.g., Bridge and Mackey<ref name=Bridgeandmackey_1993>Bridge, J. S., and S. D. Mackey, 1993, A theoretical study of fluvial sandstone body dimensions, in S. S. Flint and I. D. Bryant, eds., Geological modeling of hydrocarbon reservoirs: International Association of Sedimentologists, Special Publication 15, p. 213–236.</ref>) These plots have been used to model thickness-to-width ratios for 3-D geological models.
Miall<ref name=Miall_2006 /> criticized the use of empirical relationships for fluvial geometries in too prescriptive a manner. He suggests that they should only be used as approximate guidelines for developing alternative scenarios of fluvial reservoirs for modeling purposes. Shanley,<ref name=Shanley_2004 /> characterizing the Jonah field in Wyoming, preferred to estimate a range of possible dimensions for fluvial bodies instead of using a single unique value for the width-to-thickness ratio.
Werren et al.<ref name=Werrenetal_1990 /> described a vertical profile for a point bar deposit in the Cretaceous reservoir of the Little Creek field in Mississippi. An erosional base is overlain by channel lags with intraformational shale rip-up clasts. Above this are large-scale cross-bedded sandstones, which pass upward to beds showing horizontal and small-scale ripple cross laminae, clay drapes, micaceous and carbonaceous streaks, local mud balls, and intraclasts. The overall pattern is fining upward.
[[file:M91FG177.JPG|thumb|300px|{{figure number|6}}Upward-decreasing permeability profile in a point bar sandstone in the Peoria field, Colorado (from Chapin and Mayer<ref name=Chapinandmayer_1991 />). Reprinted with permission from, and © by, the SEPM (Society for Sedimentary Geology).]]
Fining-upward profiles are typical for point bars; the permeability declines upward with decreasing grain size ([[:file:M91FG177.JPG|Figure 6]]). The decrease in permeability commonly occurs in a step-like fashion rather than showing a gradual upward decrease.
Upward-decreasing permeability profiles are unfavorable to efficient sweep. Water will flood through the high-permeability basal part of the point bar leaving the uppermost section unswept ([[:file:M91FG176.JPG|Figure 5b]]).
[[file:M91FG178.JPG|thumb|300px|{{figure number|7}}Mud-lined lateral accretion surfaces in the upper section of a point bar sandstone, Ebro basin, Spain. The point bar grew by accretion from left to right.]]
Sweep will be retarded where the upper sections of point bars form mud sheets along lateral accretion surfaces ([[:file:M91FG176.JPG|Figure 5c]], [[:file:M91FG178.JPG|Figure 7]]). These are inclined surfaces formed by the lateral growth of the point bar as the meander loop migrates. Mud-lined lateral accretion surfaces develop by mud deposition during ponding at low river stages.<ref name=Jordanandpryor_1992 /> The inclined mud drapes form a series of shingled barriers to both vertical and horizontal flow.<ref name=Maetal_1999 /><ref name=Pranteretal_2007 />
Detailed data integration analysis has been made for oil recovery from meander belt sandstones in the Daqing field in China.<ref name=Xue_1986 /><ref name=Fuzhiguoetal_1998 /> After 30 yr of production from one of the reservoir units, the sweep efficiency is only 29.8%. It was found that although the basal intervals of the fluvial sandstones are well swept, there is much oil left behind in the upper part. Water cuts in the production wells can reach 90% with only the basal section of the fluvial sandstones contacted by the waterflood. A horizontal well was drilled as a pilot trial to determine whether this could recover the oil in the upper sections of the point bar sandstones. The well found an estimated net pay of 2–4 m (7–13 ft) but produced less than expected as a result of a combination of formation damage and poorer than predicted vertical permeability.<ref name=Fuzhiguoetal_1998 />
Meander belt sediments may be better suited as gas reservoirs than oil reservoirs. The labyrinth of numerous dead ends in these systems will not tend to trap nearly so much gas as oil. The expansion of gas on the reduction of pressure with depletion will cause much of the gas to spill out of the dead ends in fluvial reservoirs. Gas can also flow through the low-permeability connections that exist in fluvial systems, which would otherwise not allow oil to pass.
==Crevasse splays==
Crevasse splays are sandy overbank deposits, which are found intercalated with background floodplain muds. They form fan-shaped sheets, tens to hundreds of meters wide and typically 0.3–2 m (1–6.5 ft) thick. Mjos et al.<ref name=Mjosetal_1993 /> gave a width-to-thickness ratio for crevasse splays of 150–1000. The splays thin laterally toward the margins of the floodplain.<ref name=Miall_1996 /> Individual flows may amalgamate into thicker composite intervals.
Crevasse splays do not normally contain large volumes of hydrocarbons, although they may provide a target for infill wells onshore. Ambrose et al.<ref name=Ambroseetal_1991 /> noted that crevasse splays in the meander belt reservoir of the La Gloria gas field in Texas were only partially depleted or undepleted. These have limited lateral extent and pinch out less than 460 m (1509 ft) along depositional strike from the channel fill deposits. The crevasse splays are often found with much higher pressures than the main producing intervals in the field. Nevertheless, they deplete rapidly on production, indicating that they contain only small, isolated volumes of gas. However, the overall production potential is thought to be significant as these splay compartments are numerous.
==Mud plugs==
Fluvial reservoirs may be partially or totally compartmentalized by abandoned channel mud plugs.<ref name=Ambroseetal_1991 /> A meander loop can be cut off by the river breaking through into a new course during a flood. The abandoned meander channel is quickly isolated from the river flow, typically forming lakes and ponds for a period of time. The channels slowly fill up with clay, silt, and peaty organic material.
Mud plugs are crescentic in plan view and lenticular in cross section. They are narrow with widths of tens to hundreds of meters. In the Widuri field, offshore Java, mud plugs are described as 50–150 m (164–492 ft) wide and up to 5 m (16 ft) thick.<ref name=Carter_2003 /> A meander belt may contain many individual mud plugs, which can collectively make up a large volume of the total system. In a study area comprising 16 billion m<sup>3</sup> of a meander belt in the modern day Mississippi River, Jordan and Pryor<ref name=Jordanandpryor_1992 /> estimated that 11.1 billion m<sup>3</sup> of this volume comprises point bar and splay sandstones and the remaining 4.9 billion m<sup>3</sup> consists of clay plugs within numerous abandoned channels.
Clay plugs can be difficult to recognize in the subsurface. It is sometimes possible to identify them on correlation panels if the wells are closely spaced. They can also be picked out from amplitude displays on good quality seismic data.
==Connectivity in meander belts==
Determining the connectivity of the sand bodies in a meander belt system is critical to evaluating the commerciality of types of reservoirs. Individual point bars are relatively small reservoir bodies likely to contain only a few million barrels of recoverable oil at best. They may be successfully drilled onshore where wells are relatively cheap, but they are less likely to make much profit as a primary target offshore. However, if several of these sand bodies overlap with each other, then they can combine to form a larger connected sand volume.
[[file:M91FG179.JPG|thumb|300px|{{figure number|8}}Net sand isochore map of the Q reservoir in the Little Creek field in Mississippi. The reservoir comprises three connected point bar sandstones in a background of floodplain mudstones and siltstones. Just to the north is the Sweetwater field, which produces from a depositionally isolated point bar in the same meander belt system (from Werren et al., 1990). Reprinted with permission from, and © by, Springer Ltd.]]
Technical papers indicate that connectivity in meander belt sediments can be highly variable and prone to chance factors. An example of this is the Little Creek field in Mississippi.<ref name=Werrenetal_1990 /> The lower reservoir unit comprises three connected point bar sandstones ([[:file:M91FG179.JPG|Figure 8]]). The Sweetwater field immediately to the north is believed to form part of the same fluvial system and produces from a fourth point bar sand body along the same trend. Nevertheless, the Sweetwater field is isolated from the Little Creek field on the evidence of a 24-m (79-ft) higher oil-water contact. The two fields are thought to be separated by a shale plug or an area with relatively high capillary displacement pressure. A similar observation was made by Carter<ref name=Carter_2003 /> for a meander belt reservoir in the Widuri field in the Java Sea. Following the depletion of a well on the updip side of a 100-m (328-ft)-wide abandoned channel, a second well was drilled on the opposite site of the clay plug. A full oil column was found in the new well, unaffected by production from the previous well. In the Saddle Lake area of Alberta, Canada, oil and gas pools are restricted to point bars completely surrounded by clay plugs.<ref name=Edieandandrichuk_2003 />
It seems from these case examples that clay plugs can be an important element limiting horizontal connectivity in meander belt sediments ([[:file:M91FG176.JPG|Figure 5d]]). Some point bars show flow connectivity with each other, others do not. Connectivity may be effective where the clay plug does not totally separate two point bars areally. Richardson et al.<ref name=Richardsonetal_1987 /> noted that there is commonly some sand or gravel underneath clay plugs that can allow communication. Similar observations have been made by Donselaar and Overeem.<ref name=Donselaarandovereem_2008 /> Clay plugs occur at the same level of the point bar that it partially encloses. If vertical connectivity is effective between incised point bars, the clay plug obstruction can be bypassed above and below. If no effective vertical communication occurs, then clay plugs are more likely to act as lateral flow barriers ([[:file:M91FG176.JPG|Figure 5e]]).
==What outcrops of meander belt sediments indicate about connectivity==
Well-exposed outcrops of meander belt sediments can be used to get an understanding of the reservoir connectivity in three dimensions. Connectivity can result from the vertical incision of one point bar into an older underlying point bar, creating multistory sand bodies (Figure 176f). Point bars commonly connect with each other across the shallow, sandy course of the river where the tips of point bars overlap on opposite banks. Crossovers like this can create a connected system of point bars which Donselaar and Overeem (2008) describe as a string-of-beads sandstone body. Crevasse splays may also link up one point bar with another.
Nevertheless, it probably does not take much for the connectivity between the various sand bodies in a meander belt to be disrupted. The connections between the various macroforms are likely to be through apertures of limited cross-sectional area such as erosional windows, crossovers, and crevasse splay-point bar intersections. Carbonate cementation of the basal lag by circulating groundwater can create permeability barriers at the base of individual point bars. Precipitation of carbonate cement may be accentuated where calcrete fragments form part of the basal lag (Mckie and Audretsch, 2005).
Abundant mud chips caked along the base of point bars have the potential to attenuate communication. For example, Chapin and Mayer (1991) found that the vertical connectivity between stacked point bars was severely impeded by mudstone-rich lags at the base of individual point bars in the reservoir of the Peoria field in Colorado. Doyle and Sweet (1995) found that mudclast lags at the base of point bar sandstones have a patchy distribution in the Gypsy Sandstone of Northern Oklahoma. They consider them more likely to form baffles to flow instead of continuous barriers. Shanley (2004) noted that where basal lags contain abundant mudclasts, they can be mistaken for shales on the gamma-ray log. Caution should be taken where a shale-like wireline log response is seen within thick multistory fluvial sandstones.
Coals commonly act as significant flow barriers where they occur in more humid fluvial systems. The precursor peat deposits to coal occur as thick mats of flexible and intertwined plant material and these can withstand strong erosive forces to stay substantially intact.