6.3 SEDIMENTARY MODELS
271
as in the Cretaceous Chalk of the North Sea, from the crest of a rising salt dome. Fan
lobes are normally absent. A continuous apron of detritus is more common. The shelf
from which the detritus is derived is normally a scoured hardground (see Section 9.2.5.2)
covered by a veneer of skeletal or oolitic sands. As the sands are carried by currents over
the platform rim they may be mixed with biolithite, or contemporaneously cemented
limestone boulders dislodged from the shelf edge by storms or earthquakes. Mullins
and Cook (1986) recognize two allodapic carbonate models: slope aprons, with detritus
sloping at angles of < 4 ~ that rise from the basin floor to the shelf rim, and base of slope
aprons, with slopes of 4-15 ~ that rise from the basin floor to the base of the shelf rim
(Fig. 6.72).
6.3.2.9.3 Economic aspects of deep-sea sands
Deep-sea sands are of particular interest as petroleum reservoirs. This is because distal
deep-sea sands may interfinger with organic-rich basinal muds that may serve as petroleum source beds. Petroleum may migrate from the source shales into the interlaminated distal fan sands, and migrate up-dip toward the basin margin, moving into progressively thicker, coarser (and thus more permeable) sands. Reservoir continuity may
diminish as reservoir quality increases, however, because the basin plain sheet sands
pass up-dip into channelized units.
Petroleum can be trapped in deep-sea sand reservoirs in many ways, some structural, some stratigraphic. Conventional anticlinal entrapment is illustrated by the Long
Beach-Wilmington field of California, and the Forties field of the North Sea. In the former the anticline results from compression, in the latter from drape and compaction
over a deep seated horst (see Mayuga, 1970, and Walmsley, 1975, respectively). Closure
may be caused by diapirs, either mud lumps, such as those mentioned earlier from the
Beaufort Sea (see Section 6.3.2.5.4), or salt domes, like the Cod field of the North Sea
(Kessler et al., 1980).
In all of the examples just cited the reservoirs are principally laterally extensive basin
plain deposits of interbedded outer fan turbidites and shales. Reservoir heterogeneities
due to channels are relatively minor. Petroleum entrapment in deep-sea sands along basin margins is somewhat different, and is sometimes stratigraphic. Closure may be provided by the preserved paleotopography of a submarine fan, as in the Frigg field of the
North Sea (Heritier et al., 1981). Bizarre stratigraphic traps may occur associated with
the submarine channels cut into the slope. Sometimes channel sands may serve as a
reservoir. There are fields, however, in which turbidite reservoirs are sealed up-dip by
a clay-plugged channel (e.g., the Rosedale field of California, described by Martin,
1963). Figure 6.73 illustrates the diversity of petroleum entrapment in terrigenous
deep-sea sands.
Allodapic carbonates also serve as petroleum reservoirs. As discussed earlier (see
Section 2.2.3) reservoir quality in limestones is often unrelated to facies-controlled primary porosity, but corresponds to diagenetically induced secondary porosity. One of the
most spectacular allodapic limestone reservoirs is the Tamabra Formation of Mexico
(Guzman, 1967; Enos, 1977). This consists of grainstones, packstones, and carbonate debris flows that were shed off the Golden Lane atoll q a carbonate platform made of the
Cretaceous E1Abra limestones. A ring of fields has been discovered along the platform
271
as in the Cretaceous Chalk of the North Sea, from the crest of a rising salt dome. Fan
lobes are normally absent. A continuous apron of detritus is more common. The shelf
from which the detritus is derived is normally a scoured hardground (see Section 9.2.5.2)
covered by a veneer of skeletal or oolitic sands. As the sands are carried by currents over
the platform rim they may be mixed with biolithite, or contemporaneously cemented
limestone boulders dislodged from the shelf edge by storms or earthquakes. Mullins
and Cook (1986) recognize two allodapic carbonate models: slope aprons, with detritus
sloping at angles of < 4 ~ that rise from the basin floor to the shelf rim, and base of slope
aprons, with slopes of 4-15 ~ that rise from the basin floor to the base of the shelf rim
(Fig. 6.72).
6.3.2.9.3 Economic aspects of deep-sea sands
Deep-sea sands are of particular interest as petroleum reservoirs. This is because distal
deep-sea sands may interfinger with organic-rich basinal muds that may serve as petroleum source beds. Petroleum may migrate from the source shales into the interlaminated distal fan sands, and migrate up-dip toward the basin margin, moving into progressively thicker, coarser (and thus more permeable) sands. Reservoir continuity may
diminish as reservoir quality increases, however, because the basin plain sheet sands
pass up-dip into channelized units.
Petroleum can be trapped in deep-sea sand reservoirs in many ways, some structural, some stratigraphic. Conventional anticlinal entrapment is illustrated by the Long
Beach-Wilmington field of California, and the Forties field of the North Sea. In the former the anticline results from compression, in the latter from drape and compaction
over a deep seated horst (see Mayuga, 1970, and Walmsley, 1975, respectively). Closure
may be caused by diapirs, either mud lumps, such as those mentioned earlier from the
Beaufort Sea (see Section 6.3.2.5.4), or salt domes, like the Cod field of the North Sea
(Kessler et al., 1980).
In all of the examples just cited the reservoirs are principally laterally extensive basin
plain deposits of interbedded outer fan turbidites and shales. Reservoir heterogeneities
due to channels are relatively minor. Petroleum entrapment in deep-sea sands along basin margins is somewhat different, and is sometimes stratigraphic. Closure may be provided by the preserved paleotopography of a submarine fan, as in the Frigg field of the
North Sea (Heritier et al., 1981). Bizarre stratigraphic traps may occur associated with
the submarine channels cut into the slope. Sometimes channel sands may serve as a
reservoir. There are fields, however, in which turbidite reservoirs are sealed up-dip by
a clay-plugged channel (e.g., the Rosedale field of California, described by Martin,
1963). Figure 6.73 illustrates the diversity of petroleum entrapment in terrigenous
deep-sea sands.
Allodapic carbonates also serve as petroleum reservoirs. As discussed earlier (see
Section 2.2.3) reservoir quality in limestones is often unrelated to facies-controlled primary porosity, but corresponds to diagenetically induced secondary porosity. One of the
most spectacular allodapic limestone reservoirs is the Tamabra Formation of Mexico
(Guzman, 1967; Enos, 1977). This consists of grainstones, packstones, and carbonate debris flows that were shed off the Golden Lane atoll q a carbonate platform made of the
Cretaceous E1Abra limestones. A ring of fields has been discovered along the platform