Page 134 - The Petroleum System From Source to Trap

128 Morse

DELTAIC RESERVOIRS crevasse splays, levees, marshes, and abandoned distrib­
utary fill deposits between channels. The main reservoir
Deltas occur where river-borne siliciclastic sediments facies deposited in the lower delta plain are bay-fill
meet a standing body of water and form delta plains, deposits that form between the lobes of the active
mouth bars, interdistributary bays, distributary channels, channel system when distributary channels are breached
tidal ridges, beaches, crevasse splays, and prodelta muds during flooding. Bay-fill deposits form coarsening­
(Coleman and Prior, 1982). These deposits are shaped by upward lobes of clean sand with radially bifurcating
tidal, river, and wave forces to create a particular channels (Coleman and Prior, 1982). In a large delta such
arrangement or architecture of reservoir sands (Figure as the Mississippi, bay-fill deposits can extend for 20 km
6.4). In all deltas, the finest material is dispersed seaward and form a reservoir quality sand body that is 5-7 m
and the coarsest material is deposited at the mouth of the thick for its full length and width. The bay-fills are
delta. Deltas are divided into an upper delta plain, a commonly composed of millimeter- to centimeter-thick
lower delta plain, and a subaqueous delta plain. mud and sand laminae at their bases, becoming progres­
The upper delta plain lies upstream from any tidal sively more sandy upward, and finally capped by a
influence and is dominated by migrating, braided, or marsh deposit. Bioturbation is common, especially in the
meandering channel deposits, such as point bars, mid­ more slowly deposited muddy strata. Another reservoir
channel bars, and nonchannel deposits. Nonchannel facies, distributary channel sands, form the thickest
deposits include levee and overbank silts and muds, sands but are not as widespread as the bay-fill deposits.
marsh peats, lake muds, and crevasse splays that might These channel sands generally cut into the underlying
fill the lake with fine sand. The thickest sand bodies are mouth bar of the subaqueous delta plain.
the channel sands that fine upward but lack widespread The subaqueous delta plain extends from the
continuity. In short, the upper delta plain deposits are outermost shoreline seaward to the last area actively
comparable to the upstream alluvial valley. However, receiving fluvial sediment, where water depths may be
because of their location on the protruding bulge of a 50-300 m . The most distal and largest area of the
delta, they are considered to be part of it. subaqueous delta plain receives mud and silt, forming
The lower delta plain extends from the last landward the prodelta. The main reservoir and coarsest sands of a
area that is affected by tides to the outermost shoreline. It delta are deposited at the mouth of the river channel and
contains distributary channel sands, and deposits such as form the distributary mouth bar. When river deposition

2

Beach · barner
shorelrne

3

120
. ��::j; l.COj :: :;;_?
HIGH- DESTRUCTIVE­ k -�i'
TIDE-INFLUENCED
t�i.;�
160
. ·----�
t-:i::O'l
180 �
Figure 6.4. Deltaic reservoir strata electric logs (1 and 2) and lithologic log (3) consisting of distributary mouth bar and delta
plain sands (after Coleman and Prior, 98 1 2). The geometry of these sands is highly influenced by (A) wave, (B and C) river, or
(D) tidal forces at the shoreline (from Fisher et al., 96 1 9� Lithology patterns: short dashes, marine silt and clay; long dashes
and combs, marsh or paludial deposits; small dots or white, channel, beach, or mouth bar sand.