Changes

Deep-water marine reservoirs have been increasingly found since the 1970s, particularly as a result of an increase in offshore drilling activity. Many of these are Tertiary in age, although large reservoirs of Jurassic and Cretaceous age have also been found, particularly in the North Sea.

Deep-water marine reservoirs have been increasingly found since the 1970s, particularly as a result of an increase in offshore drilling activity. Many of these are Tertiary in age, although large reservoirs of Jurassic and Cretaceous age have also been found, particularly in the North Sea.

The term deep water has been used in two different ways. It applies in a geological context to deep-water systems that have been transported by gravity flow processes in a marine setting.<ref name=WeimerandSlatt>Weimer, P., and R. M. Slatt, 2004, Petroleum systems of deep-water settings: SEG/EAGE (Society of Exploration Geophysicists/European Association of Geoscientists and Engineers) Distinguished Instructor Series 7, 465 p.</ref> Deep water is also defined as present-day sea depths in excess of 500 m (1640 ft) deep.

The term deep water has been used in two different ways. It applies in a geological context to deep-water systems that have been transported by [[gravity]] flow processes in a marine setting.<ref name=WeimerandSlatt>Weimer, P., and R. M. Slatt, 2004, Petroleum systems of deep-water settings: SEG/EAGE (Society of Exploration Geophysicists/European Association of Geoscientists and Engineers) Distinguished Instructor Series 7, 465 p.</ref> Deep water is also defined as present-day sea depths in excess of 500 m (1640 ft) deep.

Since 1984 there has been an intensive effort in exploring for reservoirs located in present-day deep water with numerous prolific discoveries.<ref>Pettingill, H. S., and P. Weimer, 2001, Global deep-water exploration: Past, present and future frontiers, in R. H. Fillon, N. C. Rosen, P. Weimer, A. Lowrie, H. W. Pettingill, R. L. Phair, H. H. Roberts, and B. Van Hoorn, eds., Petroleum systems of deep-water basins: Global and Gulf of Mexico experience: GCS-SEPM Foundation, p. 1–22.</ref> Deep-water exploration in the Gulf of Mexico, Brazil, and west Africa is targeting and finding a large number of hydrocarbon pools in deep-water marine-sand systems. Only about 20% of these reservoirs had been developed to 2004.<ref name=WeimerandSlatt />

Since 1984 there has been an intensive effort in exploring for reservoirs located in present-day deep water with numerous prolific discoveries.<ref>Pettingill, H. S., and P. Weimer, 2001, Global deep-water exploration: Past, present and future frontiers, in R. H. Fillon, N. C. Rosen, P. Weimer, A. Lowrie, H. W. Pettingill, R. L. Phair, H. H. Roberts, and B. Van Hoorn, eds., Petroleum systems of deep-water basins: Global and Gulf of Mexico experience: GCS-SEPM Foundation, p. 1–22.</ref> Deep-water exploration in the [[Gulf of Mexico]], Brazil, and west Africa is targeting and finding a large number of hydrocarbon pools in deep-water marine-sand systems. Only about 20% of these reservoirs had been developed to 2004.<ref name=WeimerandSlatt />

==Deep-water marine reservoirs can be prolific reservoirs==

==Deep-water marine reservoirs can be prolific reservoirs==

| Widespread amalgamation of channel-fill sandstones in channelized systems || Creates laterally and vertically connected high-volume reservoirs ||  

| Widespread amalgamation of channel-fill sandstones in channelized systems || Creates laterally and vertically connected high-volume reservoirs ||  

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| Shale drapes or late-stage channel-fill shales common in channel-fill sandstones || || Reduces vertical and lateral connectivity between individual channel-fill sandstones

| Shale drapes or late-stage channel-fill shales common in channel-fill sandstones || || Reduces vertical and [[lateral]] connectivity between individual channel-fill sandstones

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| Preferential water ingress along channel axes || || Banked oil may form along channel margins

| Preferential water ingress along channel axes || || Banked oil may form along channel margins

| Levee sediments in channel-levee complexes are thin bedded but can show reservoir connectivity across a large area || Levee sediments can be a production target in their own right

| Levee sediments in channel-levee complexes are thin bedded but can show reservoir connectivity across a large area || Levee sediments can be a production target in their own right

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| Laterally extensive mudstones commonly form permeability barriers to vertical flow || Encourages edge-water drive and can suppress early water production || Creates hydraulic units; water overrun is common

| Laterally extensive [[mudstones]] commonly form permeability barriers to vertical flow || Encourages edge-water drive and can suppress early water production || Creates hydraulic units; water overrun is common

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| Fill and spill geometries || || Potential to create bypassed oil volumes in cellar oil accumulations

| Fill and spill geometries || || Potential to create bypassed oil volumes in cellar oil accumulations

==Typical settings for deep-water marine reservoirs==

==Typical settings for deep-water marine reservoirs==

[[File:M91FG193.JPG|thumb|300px|{{figure number|1}} Depositional model for channelized turbidites and a basin-floor fan complex, Brushy Canyon, Texas. From Beaubouef<ref name=Beaubouef1999>Beaubouef, R. T., C. Rossen, F. B. Zelt, M. D. Sullivan, D. C. Mohrig, and G. D. C. Jennette, 1999, Field guide for AAPG Hedberg field Research Conference, April 15–20, 1999, Deep-water sandstones, Brushy Canyon Formation, west Texas: AAPG Continuing Education Course Note Series 40, 48 p.</ref>. Reprinted with permission from the AAPG.]]

[[File:M91FG193.JPG|thumb|300px|{{figure number|1}} Depositional model for channelized turbidites and a basin-floor fan complex, Brushy Canyon, Texas. From Beaubouef.<ref name=Beaubouef1999>Beaubouef, R. T., C. Rossen, F. B. Zelt, M. D. Sullivan, D. C. Mohrig, and G. D. C. Jennette, 1999, Field guide for AAPG Hedberg field Research Conference, April 15–20, 1999, Deep-water sandstones, Brushy Canyon Formation, west Texas: AAPG Continuing Education Course Note Series 40, 48 p.</ref>]]

The main settings include

The main settings include

==Production from channel-levee complexes==

==Production from channel-levee complexes==

[[File:M91FG195.JPG|thumb|300px|{{figure number|3}}Schematic section across a deep-water channel-levee complex based on outcrops from the Cerro Toro Formation, Upper Cretaceous, southern Chile (from Beaubouef<ref name=BB2004 />). The lower section shows a series of idealized gamma ray logs. Reprinted with permission from the AAPG.]]

[[File:M91FG195.JPG|thumb|300px|{{figure number|3}}Schematic section across a deep-water channel-levee complex based on outcrops from the Cerro Toro Formation, Upper Cretaceous, southern Chile (from Beaubouef<ref name=BB2004 />). The lower section shows a series of idealized gamma ray logs.]]

Channel-levee complexes can show highly variable continuity between the channels and levees ([[:File:M91FG195.JPG|Figure 3]]). It is common for hydrocarbons in the channel-fill sandstones to be poorly connected with the levee sediments. The channel fills may be younger than the levees themselves, and the beds in the proximal levee deposits may be discontinuous.<ref>Cronin, B. T., A. Hurst, H. Celik, and I. Turkmen, 2000, Superb exposures of a channel, levee and overbank complex in an ancient, deep-water slope environment: Sedimentary Geology, v. 132, p. 205–216.</ref><ref name=BB2004>Beaubouef, R. T., 2004, [http://archives.datapages.com/data/bulletns/2004/11nov/1471/1471.HTM Deep-water leveed channel complexes of the Cerro Toro formation, Upper Cretaceous, southern Chile]: AAPG Bulletin, v. 11, p. 1471–1500.</ref> Kneller et al.<ref>Kneller, B., M. Dykstra, and P. Thompson, 2007, Collapse of submarine channel levees, examples from outcrop and subsurface, and reservoir implications: Abstracts of the 2007 AAPG Annual Convention and Exhibition, p. 77.</ref> noted that collapse structures, including rotated blocks, slide sheets, slump folds, and thick debris flows, are common on levee margins and may contribute to poor reservoir continuity.

Channel-levee complexes can show highly variable continuity between the channels and levees ([[:File:M91FG195.JPG|Figure 3]]). It is common for hydrocarbons in the channel-fill sandstones to be poorly connected with the levee sediments. The channel fills may be younger than the levees themselves, and the beds in the proximal levee deposits may be discontinuous.<ref>Cronin, B. T., A. Hurst, H. Celik, and I. Turkmen, 2000, Superb exposures of a channel, levee and overbank complex in an ancient, deep-water slope environment: Sedimentary Geology, v. 132, p. 205–216.</ref><ref name=BB2004>Beaubouef, R. T., 2004, [http://archives.datapages.com/data/bulletns/2004/11nov/1471/1471.HTM Deep-water leveed channel complexes of the Cerro Toro formation, Upper Cretaceous, southern Chile]: AAPG Bulletin, v. 11, p. 1471–1500.</ref> Kneller et al.<ref>Kneller, B., M. Dykstra, and P. Thompson, 2007, Collapse of submarine channel levees, examples from outcrop and subsurface, and reservoir implications: Abstracts of the 2007 AAPG Annual Convention and Exhibition, p. 77.</ref> noted that collapse structures, including rotated blocks, slide sheets, slump folds, and thick debris flows, are common on levee margins and may contribute to poor reservoir continuity.

==Production from sheet complexes==

==Production from sheet complexes==

[[File:M91FG112.JPG|thumb|300px|{{figure number|4}}Formation tester data taken in wells that have been drilled postproduction provide invaluable data on how the reservoir splits up into hydraulic units showing different pressures. This example is from the Magnus field in the UK North Sea (from Morris et al).<ref>Morris, P. H., S. N. J. Payne, and D. P. J. Richards, 1999, Micropalaeontological biostratigraphy of the Magnus Sandstone Member (Kimmeridgian to early Volgian), Magnus field, UK North Sea, in R. W. Jones and M. D. Simmons, eds., Biostratigraphy in production and development geology: Geological Society (London) Special Publication 152, p. 55–73.</ref> Reprinted with permission from the Geological Society. GR = Gamma Ray; RFTtrade = Repeat Formation Tester; UKCF = Upper Kimmeridge Clay Formation.]]

[[File:M91FG112.JPG|thumb|300px|{{figure number|4}}Formation tester data taken in wells that have been drilled postproduction provide invaluable data on how the reservoir splits up into hydraulic units showing different pressures. This example is from the Magnus field in the UK North Sea (from Morris et al).<ref>Morris, P. H., S. N. J. Payne, and D. P. J. Richards, 1999, Micropalaeontological biostratigraphy of the Magnus Sandstone Member (Kimmeridgian to early Volgian), Magnus field, UK North Sea, in R. W. Jones and M. D. Simmons, eds., Biostratigraphy in production and development geology: Geological Society (London) Special Publication 152, p. 55–73.</ref> Reprinted with permission from, and &copy; by, the Geological Society. GR = Gamma Ray; RFTtrade = Repeat Formation Tester; UKCF = Upper Kimmeridge Clay Formation.]]

Sheet sandstones form excellent reservoirs. Their characteristics include simple tabular geometries, good lateral continuity, and few erosional features.<ref name=WeimerandSlatt /> A large volume of deep-water sheet sandstones can be produced by a single production well. Width-to-thickness ratios are large, more than 500:1 for sheet complexes compared to a range of 10:1 to 300:1 for channels.<ref name=WeimerandSlatt /> Vertical connectivity can be variable depending on the amount of interbedded shales or the degree of sand-on-sand amalgamation.

Sheet sandstones form excellent reservoirs. Their characteristics include simple tabular geometries, good lateral continuity, and few erosional features.<ref name=WeimerandSlatt /> A large volume of deep-water sheet sandstones can be produced by a single production well. Width-to-thickness ratios are large, more than 500:1 for sheet complexes compared to a range of 10:1 to 300:1 for channels.<ref name=WeimerandSlatt /> Vertical connectivity can be variable depending on the amount of interbedded shales or the degree of sand-on-sand amalgamation.

Cellar oil targets can also occur ([[:File:M91FG194.JPG|Figure 2d]]). Early sand input into a receiving basin tends to pond into the bathymetric lows. The seabed will eventually become smoother as the lows are filled in. Later sand flows will spill beyond the extent of the previous flows. These show a more tabular geometry and will be spread across a larger area than the underlying sediments, creating a fill and spill geometry. The ponded sand bodies can potentially hold isolated oil volumes by comparison to the more extensive later flows, which may be better swept.

Cellar oil targets can also occur ([[:File:M91FG194.JPG|Figure 2d]]). Early sand input into a receiving basin tends to pond into the bathymetric lows. The seabed will eventually become smoother as the lows are filled in. Later sand flows will spill beyond the extent of the previous flows. These show a more tabular geometry and will be spread across a larger area than the underlying sediments, creating a fill and spill geometry. The ponded sand bodies can potentially hold isolated oil volumes by comparison to the more extensive later flows, which may be better swept.

==References==

==References==

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