Page 135 - The Petroleum System From Source to Trap
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6. Siliciclastic Reservoir Rocks 129
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Figure 6.5. Characteristics of growth or slump features. (A) Section perpendicular to shore illustrating rollover along a listric
fault. (B) Thicker sands occur on the downthrown side. (C) Idealized sand isolith map on a growth fault. (D) Idealized electric
log signature comparing regular footwall sand thickness (1) with the thickened sand on the downthrown side (2) (after
Reading, 1978; Coleman and Prior, 1982).
accumulates faster than waves or tidal currents can move muds compact. Then, a third delta lobe may form and
it laterally, the delta builds seaward forming a river overlap the first. Frazier (1967) outlined 16 lobes of the
dominated lobate delta. If waves move the sand back modern Mississippi delta, many of which overlap to
against the beach to form prograding barrier beaches or some extent. Lobe switching such as this permits a river
strand plains, the delta is considered to be wave dominated delta to stack successive mouth bars on top of
dominated. Where strong tides shift sand to produce one another to create a reservoir that can hold billions of
tidal current sand ridges along numerous distributary barrels of oil, such as the Safania field in Saudi Arabia
channels, the delta is considered to be tide dominated (Ayers et al., 1982).
(Figure 6.4). Prodelta instability creates additional reservoir oppor
The distributary mouth bar of a large river, such as the tunities. Slumps, debris flows, and block slides are
Mississippi River, will form a sand body up to 40 km or common on most modem prodeltas of the world (Bouma
more wide and up to 40-50 m thick that gradually et al., 1982; Cook et al., 1982). These mass movements
increases in reservoir quality upward. The distal part of bring great quantities of shallow water deposits, such as
the distributary mouth bar forms a transition from the distributary mouth bar sands, to deeper water on the
prodelta muds and silts to the mouth bar sands and shelf. Narrow gullies 50-800 m wide or chutes exceeding
consists of interbedded silt, mud, and fine sand. The 8-10 km in length may extend down the surface of the
distributary mouth bar consists of laminated, thin <1 ° slope of the prodelta and terminate in lobe-shaped
bedded, cross-bedded, and massive sand of excellent mounds of transported material (Coleman and Prior,
reservoir potential. Rivers commonly abandon one delta 1982). Growth faults commonly form in this prodelta
lobe through channel avulsion and build another nearby. environment. These arcuate faults decrease in dip angle
Within a short time, the initial mouth bar and lower delta downward causing bed rotation and formation of
plain sink below sea level as the underlying prodelta rollover anticlines through time (Figure 6.5). Movement
