Page 139 - The Petroleum System From Source to Trap

6. Siliciclastic Reservoir Rocks 133

by only the largest turbidity flows. Because fans The most prolific sandstone reservoirs of the world
prograde into the basal plain as they receive more and are deltaic distributary mouth bar and channel sands.
more sediment, an overall coarsening and thickening of Thick, laterally extensive reservoir sands can be
potential reservoir strata occurs upward in the deposited by one lobe of a delta. Because of delta lobe
subaqueous fan sequence (Figure 6.7 . With lobe switching and subsidence of inactive lobes by
)
switching, multiple coarsening and thickening-upward compaction of prodelta silts and clays, multiple lobes can
packages are likely to be stacked in the suprafan areas be vertically stacked, which can greatly increase the
(Normark, 1978). Thus, reservoir quality sands are likely amount of reservoir sand. Tertiary deltaic sediments
to form in the upper and middle fan channels and as underlying present-day major deltas of the world
turbidite sands deposited on the middle fan lobes. Berg account for significant petroleum reserves.
( 1 9 82) and Mitchum ( 1 9 88 ) have described the The second most productive sandstone reservoirs are
prograding sequences of fans in seismic sections. from shallow marine sediments. Stacking of reservoir
Proximal subaqueous fan deposits contain sizeable facies due to fluctuations in sea level adds significantly to
reserves of hydrocarbons. The middle and outer fan their reservoir volume. Subaqueous fans are a relatively
deposits, athough not reservoirs, are interbedded with underexplored reservoir type that holds great promise
very fine grained pelagic shales that are commonly rich because of the amount of sand that is present, the
in organic matter. These shales act as both a source for potential for stacking fan lobes, and the likely proximity
hydrocarbons and as a seal. Because of the lateral conti­ to deep basinal source rocks.
nuity of middle and lower fan turbidite sandstones, these Although nonmarine reservoirs have not produced as
strata serve as natural carrier beds to bring hydrocarbons many giant hydrocarbon accumulations as deltaic or
to the main reservoir sands in the middle and upper fan. shallow marine sands, they do offer significant targets.
Stratigraphic traps are common in subaqueous fans and Many lacustrine-related fields in China have yet to be
may be detected with sequence stratigraphy. Examples carefully documented and published outside China.
include the Jurassic Brae Formation of the Miller field Eolian sands must always be considered because of their
(McClure and Brown, 1 9 92 ) and the Eocene Alba high quality and thick reservoirs. Fluvial sands, particu­
Formation of the Alba field (Mattingly and Bretthauer, larly in north Africa, have great potential when they can
1992 ), both occurring in the North Sea area. The Wilm­ be tied to a source rock and a regional seal
ington field in the Los Angeles basin, with about 2.5
billion bbl of recoverable oil (Mayuga, 1970), is one of the
largest subaqueous fan reservoirs. Table 6.6 identifies
prominent fields with fan reservoirs in the United Acknowledgments Preparation of this paper was encour­
Kingdom, Brazil, and the United States. The fans form in aged by W a llace G. Dow o f DGSI and Leslie B. Magoon o f the
rapidly subsiding basins produced by wrench faulting or USGS. Their subsequent reviews and that of Peter van de
rifting with deep water adjacent to a shelf. Submarine Kamp significantly improved its presentation. Linda Rausch
canyons commonly also form at the shelf edge in associa­ helped to prepare this chapter.
tion with major rivers entering the sea. Because of the
instability of the prodelta slope, turbidity flows, mass
flows, and slope creep sediments are often funneled to References Cited
the canyons and hence to subaqueous fans building on
the basal plain. The huge Green Canyon Block discov­
eries in the Gulf of Mexico off the Mississippi delta are Ahlbrandt, T. S., and S. G. Fryberger, 1982, Eolian deposits, in
interpreted as having subaqueous fan sandstone reser­ P. A. Scholle and D. Spearing, eds., Sandstone depositional
voirs (Brannon et al., 1993 ) . environments: AAPG Memoir 31, p. 11-48.
Albright, W. A., W. L. Turner, and K. R. Williamson, 1980,
Ninian field, UK sector, North Sea, in M. T. Halbouty, ed.,
CONCLUSIONS Giant oil and gas fields of the decade: 1968-1978: AAPG
Memoir 30, p. 173-193.
Depositional environments of siliciclastic sandstone Ayers, M. G., M. Bilal, R. W. Jones, L. W. Slentz, M. Tartir, and
reservoirs were reviewed here to characterize the distrib­ A. 0. Wilson, 1982, Hydrocarbon habitat in main
ution of reservoir facies. The main reservoir facies in producing areas, Saudi Arabia: AAPG Bulletin, v. 66, n. 1,
nonmarine sandstones include point bar or braid bar p. 1-9.
sands in fluvial environments; dune sands in eolian Bacoccoli, G., R. G. Morales, and 0. A. J. Campos, 1980, the
settings; and shoreface, mouth bar, fluvial channel, and Namorado oil field: a major oil discovery in the Campos
basin, Brazil, in M. T. Halbouty, ed., Giant oil and gas
subaqueous fans in lacustrine settings. In deltas, the main fields of the decade: 1968-1978: AAPG Memoir 30, p.
reservoirs are distributary mouth bars and channel 328-338.
sands. Primary shallow marine reservoirs include barrier Balducchi, A., and G. Pommier, 1970, Cambrian oil field of
beach and shoreface sands and offshore bars. Petroleum Hassi Messaoud, Algeria, in M. T. Halbouty, ed., Geology
production in subaqueous fans is mainly from upper and of giant petroleum fields: AAPG Memoir 14, p. 477-488.
middle fan channel and proximal turbidite sands. Barlow, J. A., and J. D. Haun, 1970, Regional stratigraphy of
Examples of prominent oil and gas fields with reservoirs Frontier Formation and relation to Salt Creek field,
in each type of depositional environment are given in Wyoming, in M. T. Halbouty, ed., Geology of giant
petroleum fields: AAPG Memoir 14, p. 147-157.
Tables 6.1-6.6.