Difference between revisions of "Braided fluvial reservoirs"
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| + | [[file:M91FG181.JPG|thumb|300px|{{figure number|1}}Satellite photo of a braided river in Tibet. The braided river belt is about 7 km (4 mi) wide. Courtesy of the [http://www.earthobservatory.nasa.gov NASA Web site]. The lower figure was modified from Cant.<ref name=Cant_1982>Cant, D. J., 1982, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0100/0115.htm Fluvial facies models], in P. A. Scholle and D. R. Spearing, eds., [http://store.aapg.org/detail.aspx?id=627 Sandstone depositional environments]: AAPG Memoir 31, p. 115–137.</ref>]] | ||
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| + | ==Braided fluvial systems== | ||
| + | Sandy braided river systems show an intricate geometry of small bars, sand flats, and vegetated islands ([[:file:M91FG181.JPG|Figure 1]]). The river flows across and between the various bars, splitting and joining continuously in a braided pattern.<ref name=Walkerandcant_1984>Walker, R. G., and D. J. Cant, 1984, Sandy fluvial systems, in R. G. Walker, ed., Facies models: Toronto, Canada, Geological Association of Canada, p. 71–89.</ref> Repeated avulsion generates a complex of amalgamated channel segments. | ||
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| + | ==Braided fluvial sediments make good reservoirs== | ||
| + | Braided river reservoirs typically make excellent, very productive reservoirs. The net to gross can be much higher than in meander-belt reservoirs, and there is normally much less in the way of interbedded shales. Braided river deposits can occur on a continental scale with individual systems that are very thick and laterally very extensive. Gibling<ref name=Gibling_2006>Gibling, M. R. 2006, Width and thickness of fluvial channel bodies and valley fills in the geological record: A literature compilation and classification: Journal of Sedimentary Research, v. 76, p. 731–770.</ref> quoted widths in excess of 40 km (25 mi) and thicknesses up to 1200 m (3937 ft) for the very large braided river systems (Table 1). These form in response to periods of active tectonism, rapid subsidence, and a large volume of coarse sediment influx. These conditions are typical for foreland basins where braided fluvial sediments are commonly found. | ||
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| + | {| class = "wikitable" | ||
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| + | |+ {{table number|1}}Width and thickness relationships of fluvial sediments in various settings.<sup>1</sup> | ||
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| + | ! Depositional environment || Thickness || Width || Width/thickness ratio | ||
| + | |- | ||
| + | | Braided and low sinuousity rivers || 1-1200 m (3-3937 ft); most < 60 m (197 ft); common range 5-60 m (16-197 ft) || 50 m-1300+ km (164 ft-808+ mi); many > 1 km (0.62 mi); common range 0.5-10 km (0.3-6 mi) || 15-15,000+; some > 1000; common range 50-1000 | ||
| + | |- | ||
| + | | Meandering rivers || 1-38 m (3-125 ft); common range 4-20 m (13-65 ft) || 30 m-15 km (98 ft-9 mi); most < 3 km (1.8 mi); common range 0.3-3 km (0.1-1.8 mi) || 7-940; most < 250; many < 100; common range 30-250 | ||
| + | |- | ||
| + | | Delta distributaries || 1-35 m (3-115 ft); most < 20 m (65 ft); common range 3-20 m (10-65 ft) || 3 m-1 km (10 ft-0.6 mi); most < 500 m (1640 ft); common range 10-300 m (33-984 ft) || 2-245; most < 50; many < 15; common range 5-30 | ||
| + | |- | ||
| + | | Channels in eolian settings || 1-19 m (3-62 ft) || 2.5-1500 m (8.2-4921 ft); most < 150 m (492 ft) || 1-90; most < 15 | ||
| + | |- | ||
| + | | Valley fills on bedrock unconformities || 12-1400 m (39-4593 ft); most < 500 m (1640 ft) || 75 m-52 km (246 ft-32 mi); most < 10 km (6 mi) || 2-870; highly variable; mainly 2-100 | ||
| + | |- | ||
| + | | Valley fills within alluvial and marine strata || 2-210 m (6-689 ft); most < 60 m (197 ft) || 0.1-105 km (0.06-65 mi); common range 0.2-25 km (0.1-15 mi) || 4.6-3640; highly variable; common range 10-1000; many from 100 to 1000 | ||
| + | |- | ||
| + | | 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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| + | Some very big oil fields are known from braided river reservoirs, including the Prudhoe Bay field in Alaska and several giant oil fields in the Sirte basin of Libya. | ||
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| + | ==Lateral continuity== | ||
| + | Lateral continuity is typically excellent in braided fluvial reservoirs. The net to gross of these systems is normally very high (>80%), and, as such, these types of reservoirs are usually well connected laterally. In detail, they can be internally complex with intervals of upward-decreasing permeability profiles, but the lack of organized stratification or laterally continuous shales results in braided fluvial reservoirs showing effectively layer-cake geometry and acting as a single integrated reservoir at the larger scale.<ref name=Gallowayandhobday_1996>Galloway, W. E., and D. K. Hobday, 1996, Terrigenous clastic depositional systems: Applications to petroleum, coal, and uranium exploration: New York, Springer-Verlag, 489 p.</ref> | ||
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| + | Braided river systems normally comprise medium to coarse-grained sands and gravels, and the rock properties can be excellent. Oil recovery factors can be very high in braided river reservoirs, commonly more than 50%.<ref name=Martin_1993>Martin, J. H., 1993, A review of braided fluvial hydrocarbon reservoirs: The petroleum engineer's perspective, in J. L. Best and C. S. Bristow, eds., Braided rivers: Geological Society Special Publication 75, p. 333–367.</ref> Laterally extensive braided river reservoirs tend to be in communication with strong aquifers. | ||
| + | |||
| + | ==Barriers to vertical flow== | ||
| + | Not much mud is preserved in braided river systems under normal conditions. What mud there is, collects as bar tops or represents fragments of floodplain silts and muds. The shales tend not to be very extensive and are randomly distributed. Formation tester data in braided river reservoirs typically show good vertical pressure communication with only localized evidence for flow baffling.<ref name=Martin_1993 /> | ||
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| + | In some braided river sediments, shales are laterally more extensive and may be correlatable.<ref name=Geehanetal_1986> Davies et al., 1993). Where floodplain shales cover a large area, they can act as permeability barriers bounding hydraulic units. Widespread lacustrine shale beds act as barriers within the Jurassic braided river reservoir of the Jackson field of Australia (Hamilton et al., 1998) and in the Tertiary Merecure unit B interval of the Budare field in Venezuela (Hamilton et al., 2002). | ||
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Revision as of 22:24, 10 August 2015
THIS PAGE IS UNDER CONSTRUCTION
| Oil Field Production Geology | |
| |
| Series | Memoirs |
|---|---|
| Part | The Production Geologist and the Reservoir |
| Chapter | Braided fluvial reservoirs |
| Author | Mike Shepherd |
| Link | Web page |
| PDF file (requires access) | |
| Store | AAPG Store |
Braided fluvial systems
Sandy braided river systems show an intricate geometry of small bars, sand flats, and vegetated islands (Figure 1). The river flows across and between the various bars, splitting and joining continuously in a braided pattern.[2] Repeated avulsion generates a complex of amalgamated channel segments.
Braided fluvial sediments make good reservoirs
Braided river reservoirs typically make excellent, very productive reservoirs. The net to gross can be much higher than in meander-belt reservoirs, and there is normally much less in the way of interbedded shales. Braided river deposits can occur on a continental scale with individual systems that are very thick and laterally very extensive. Gibling[3] quoted widths in excess of 40 km (25 mi) and thicknesses up to 1200 m (3937 ft) for the very large braided river systems (Table 1). These form in response to periods of active tectonism, rapid subsidence, and a large volume of coarse sediment influx. These conditions are typical for foreland basins where braided fluvial sediments are commonly found.
| Depositional environment | Thickness | Width | Width/thickness ratio |
|---|---|---|---|
| Braided and low sinuousity rivers | 1-1200 m (3-3937 ft); most < 60 m (197 ft); common range 5-60 m (16-197 ft) | 50 m-1300+ km (164 ft-808+ mi); many > 1 km (0.62 mi); common range 0.5-10 km (0.3-6 mi) | 15-15,000+; some > 1000; common range 50-1000 |
| Meandering rivers | 1-38 m (3-125 ft); common range 4-20 m (13-65 ft) | 30 m-15 km (98 ft-9 mi); most < 3 km (1.8 mi); common range 0.3-3 km (0.1-1.8 mi) | 7-940; most < 250; many < 100; common range 30-250 |
| Delta distributaries | 1-35 m (3-115 ft); most < 20 m (65 ft); common range 3-20 m (10-65 ft) | 3 m-1 km (10 ft-0.6 mi); most < 500 m (1640 ft); common range 10-300 m (33-984 ft) | 2-245; most < 50; many < 15; common range 5-30 |
| Channels in eolian settings | 1-19 m (3-62 ft) | 2.5-1500 m (8.2-4921 ft); most < 150 m (492 ft) | 1-90; most < 15 |
| Valley fills on bedrock unconformities | 12-1400 m (39-4593 ft); most < 500 m (1640 ft) | 75 m-52 km (246 ft-32 mi); most < 10 km (6 mi) | 2-870; highly variable; mainly 2-100 |
| Valley fills within alluvial and marine strata | 2-210 m (6-689 ft); most < 60 m (197 ft) | 0.1-105 km (0.06-65 mi); common range 0.2-25 km (0.1-15 mi) | 4.6-3640; highly variable; common range 10-1000; many from 100 to 1000 |
| 1From Gibling[3], Journal of Sedimentary Research. Reprinted with permission from, and © by, the SEPM (Society for Sedimentary Geologists). | |||
Some very big oil fields are known from braided river reservoirs, including the Prudhoe Bay field in Alaska and several giant oil fields in the Sirte basin of Libya.
Lateral continuity
Lateral continuity is typically excellent in braided fluvial reservoirs. The net to gross of these systems is normally very high (>80%), and, as such, these types of reservoirs are usually well connected laterally. In detail, they can be internally complex with intervals of upward-decreasing permeability profiles, but the lack of organized stratification or laterally continuous shales results in braided fluvial reservoirs showing effectively layer-cake geometry and acting as a single integrated reservoir at the larger scale.[4]
Braided river systems normally comprise medium to coarse-grained sands and gravels, and the rock properties can be excellent. Oil recovery factors can be very high in braided river reservoirs, commonly more than 50%.[5] Laterally extensive braided river reservoirs tend to be in communication with strong aquifers.
Barriers to vertical flow
Not much mud is preserved in braided river systems under normal conditions. What mud there is, collects as bar tops or represents fragments of floodplain silts and muds. The shales tend not to be very extensive and are randomly distributed. Formation tester data in braided river reservoirs typically show good vertical pressure communication with only localized evidence for flow baffling.[5]
In some braided river sediments, shales are laterally more extensive and may be correlatable.<ref name=Geehanetal_1986> Davies et al., 1993). Where floodplain shales cover a large area, they can act as permeability barriers bounding hydraulic units. Widespread lacustrine shale beds act as barriers within the Jurassic braided river reservoir of the Jackson field of Australia (Hamilton et al., 1998) and in the Tertiary Merecure unit B interval of the Budare field in Venezuela (Hamilton et al., 2002).
See also
- Eolian reservoirs
- Meandering fluvial reservoirs
- Deltaic reservoirs
- Deep-water marine reservoirs
- Carbonate reservoirs
References
- ↑ Cant, D. J., 1982, Fluvial facies models, in P. A. Scholle and D. R. Spearing, eds., Sandstone depositional environments: AAPG Memoir 31, p. 115–137.
- ↑ Walker, R. G., and D. J. Cant, 1984, Sandy fluvial systems, in R. G. Walker, ed., Facies models: Toronto, Canada, Geological Association of Canada, p. 71–89.
- ↑ 3.0 3.1 Gibling, M. R. 2006, Width and thickness of fluvial channel bodies and valley fills in the geological record: A literature compilation and classification: Journal of Sedimentary Research, v. 76, p. 731–770.
- ↑ Galloway, W. E., and D. K. Hobday, 1996, Terrigenous clastic depositional systems: Applications to petroleum, coal, and uranium exploration: New York, Springer-Verlag, 489 p.
- ↑ 5.0 5.1 Martin, J. H., 1993, A review of braided fluvial hydrocarbon reservoirs: The petroleum engineer's perspective, in J. L. Best and C. S. Bristow, eds., Braided rivers: Geological Society Special Publication 75, p. 333–367.
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