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6 DEPOSITIONAL SYSTEMS
A notable example of a marine-dominated delta is provided by the Brent Group
(Middle Jurassic) of the northern North Sea. This is an important petroleum reservoir
(Morton et al., 1992). The Brent delta prograded northward down the axis of the Viking
graben, debouching its sediments into the opening ocean. It provides a classic example
of Walther's law (see Section 6.4.1). The delta slope deposits of the Rannoch Formation
are overlain by the delta-front sands of the Etive Formation, which are overlain in turn
by the delta plain coal-bearing Ness Formation. Rising sea level reworked the top of the
delta to deposit the shallow marine sands of the Tarbert Formation (Fig. 6.38).
The Brent delta, just described, graded seaward into open marine muds. Others,
such as the Pennsylvanian deltas of the Appalachian plateau, and those of the Yoredale
series of northern England, prograded across carbonate platforms (e.g., Ferm, 1970;
Moore, 1959). There is, however, yet another important type of delta that generates turbidite sands at its foot. The development of slumps and slides is known from modern
deltas, and there is evidence that these transport sand by turbidity currents onto the basin floor. This has been described from the Mississippi, Fraser, and Niger deltas (Shepard, 1963, pp. 494 and 500; Burke, 1972). Great depth of both water and sediment make
it hard to study modern delta-front turbidites. Many ancient examples have been described however. Notable case histories have been documented from the Carboniferous
of England (Walker, 1966; De Raaf et al., 1964), from the Ordovician rocks of the Appalachians (Horowitz, 1966), from the Coaledo Formation of Oregon (Dott, 1966), and
from the Tertiary-Recent wedge of the Niger delta (Fig. 6.39).
To conclude, at its simplest the delta process generates upward-coarsening lobes of
sediment that grade from marine muds, upward and shoreward, into diverse nonmarine
sands, muds, and often coals. This simple model may be modified by marine destructive
influences. Furthermore, if the delta slope was sufficiently unstable to slide and slump,
then redeposited turbidite sands may be present at the delta foot.
Recognition of these diverse deltaic models is critical to the effective exploitation of
hydrocarbons from ancient deltas. Sand reservoirs in fluvial-dominated deltas are radiating shoe-strings on the delta platform. Marine-dominated deltas tend to have arcuate
motifs of shoal sands. Additional reservoir sands may be present in the submarine canyons and fans of high-slope deltas (see Section 6.3.2.9.1).
6.3.2.5.4 Economic aspects of deltaic deposits
Ancient deltaic deposits are extremely important economically. They host most of the
world's coal, and many major petroleum provinces. Environments of coal formation are
not discussed now. They are dealt with in Section 9.3.3. Deltas make excellent petroleum
provinces because they fulfil all the conditions necessary for petroleum source bed formation, petroleum generation, and entrapment (Selley, 1977).
The deltaic process is a way of depositing lobes of sand (potential reservoirs) into envelopes of organic-rich marine muds (potential source beds). Deltaic environments deposit many potential stratigraphic traps, including mouth bars, barrier bars, and channels. Rapid deposition often leads to overpressuring. This may generate diapiric traps
and roll-over anticlines. Deltas need a basin, or at least some subsidence, before they
may form. Subsidence implies crustal stretching and thus increased heat flow. This expedites the maturation of source beds. No wonder then that ancient deltas are major
6 DEPOSITIONAL SYSTEMS
A notable example of a marine-dominated delta is provided by the Brent Group
(Middle Jurassic) of the northern North Sea. This is an important petroleum reservoir
(Morton et al., 1992). The Brent delta prograded northward down the axis of the Viking
graben, debouching its sediments into the opening ocean. It provides a classic example
of Walther's law (see Section 6.4.1). The delta slope deposits of the Rannoch Formation
are overlain by the delta-front sands of the Etive Formation, which are overlain in turn
by the delta plain coal-bearing Ness Formation. Rising sea level reworked the top of the
delta to deposit the shallow marine sands of the Tarbert Formation (Fig. 6.38).
The Brent delta, just described, graded seaward into open marine muds. Others,
such as the Pennsylvanian deltas of the Appalachian plateau, and those of the Yoredale
series of northern England, prograded across carbonate platforms (e.g., Ferm, 1970;
Moore, 1959). There is, however, yet another important type of delta that generates turbidite sands at its foot. The development of slumps and slides is known from modern
deltas, and there is evidence that these transport sand by turbidity currents onto the basin floor. This has been described from the Mississippi, Fraser, and Niger deltas (Shepard, 1963, pp. 494 and 500; Burke, 1972). Great depth of both water and sediment make
it hard to study modern delta-front turbidites. Many ancient examples have been described however. Notable case histories have been documented from the Carboniferous
of England (Walker, 1966; De Raaf et al., 1964), from the Ordovician rocks of the Appalachians (Horowitz, 1966), from the Coaledo Formation of Oregon (Dott, 1966), and
from the Tertiary-Recent wedge of the Niger delta (Fig. 6.39).
To conclude, at its simplest the delta process generates upward-coarsening lobes of
sediment that grade from marine muds, upward and shoreward, into diverse nonmarine
sands, muds, and often coals. This simple model may be modified by marine destructive
influences. Furthermore, if the delta slope was sufficiently unstable to slide and slump,
then redeposited turbidite sands may be present at the delta foot.
Recognition of these diverse deltaic models is critical to the effective exploitation of
hydrocarbons from ancient deltas. Sand reservoirs in fluvial-dominated deltas are radiating shoe-strings on the delta platform. Marine-dominated deltas tend to have arcuate
motifs of shoal sands. Additional reservoir sands may be present in the submarine canyons and fans of high-slope deltas (see Section 6.3.2.9.1).
6.3.2.5.4 Economic aspects of deltaic deposits
Ancient deltaic deposits are extremely important economically. They host most of the
world's coal, and many major petroleum provinces. Environments of coal formation are
not discussed now. They are dealt with in Section 9.3.3. Deltas make excellent petroleum
provinces because they fulfil all the conditions necessary for petroleum source bed formation, petroleum generation, and entrapment (Selley, 1977).
The deltaic process is a way of depositing lobes of sand (potential reservoirs) into envelopes of organic-rich marine muds (potential source beds). Deltaic environments deposit many potential stratigraphic traps, including mouth bars, barrier bars, and channels. Rapid deposition often leads to overpressuring. This may generate diapiric traps
and roll-over anticlines. Deltas need a basin, or at least some subsidence, before they
may form. Subsidence implies crustal stretching and thus increased heat flow. This expedites the maturation of source beds. No wonder then that ancient deltas are major