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6. Siliciclastic Reservoir Rocks 123
1 9 85). Migration of petroleum from a pod of active Braided rivers tend to have coarser sediment load,
source rock to the trap requires a conduit that may higher gradients, more flashy or variable flow, more
involve vertical migration, such as along fractures or easily eroded banks, and higher width-depth ratios than
faults, or lateral migration within a reservoir quality meandering rivers (Walker, 1979). Braided river deposits
carrier bed. The dynamics of migration are examined by may form alluvial fans (Nilsen, 1982), cover a broad
England (Chapter 12, this volume). Rates of migration in alluvial plain, or be confined to a restricted valley.
sandstone carriers are in the range of 1-1000 km/m.y., Braided river deposits (Figure 6.1 B) are formed by aggra
two orders of magnitude greater than for limestones. dation of mid-channel bars and sinuous-crested,
Lateral migration requires both a continuous carrier bed subaqueous megaripples producing planar and trough
and seal (Demaison and Huizinga, 1991; Chapter 4, this cross-bedded strata; overbank silts and clay deposits are
volume). Favorable areas for long distance migration of rarely preserved. River depth determines the thickness of
up to several hundred kilometers occur in foreland an ideal fining-upward braided river sequence (Figure
basins and intracratonic sags where blanket sands of 6.1A), which begins with a basal lag, followed by trough
transgressive, eolian, or fluvial origins are sealed by cross beds, planar cross beds, and ending with a thin
evaporites or shales. bedded sandy cap. Complete sequences are commonly
This chapter reviews siliciclastic reservoir rocks found abbreviated by truncation from successive braid
in hundreds of giant and supergiant oil and gas fields by channels, resulting in highly laterally connected sands.
depositional environment (Halbouty, 1970, 1980b, 1992). Meandering river deposits (Figure 6.1C) are formed
The nonmarine reservoir rocks were deposited in fluvial, by the lateral downstream accretion of point bars, filling
eolian, and lacustrine environments, whereas the marine in of channels cut off at the narrow neck of a meander
reservoirs were deposited in deltaic, shallow marine, and loop with silt and clay, and the vertical accretion of fine
deep marine environments. The packaging of deposi sand and mud on the flood plain during high water. The
tional facies, the distribution of principal reservoir rocks ideal point bar vertical sequence (Figure 6.10) consists of
in the depositional setting, the lateral continuity or archi a basal lag, trough cross-bedded sands of set amplitude
tecture of the facies, and important world examples are and grain size, decreasing upward to rippled, fine
compiled here from the literature into tables by deposi grained sand, and finally laminated or bioturbated silt
tional environment. and clay of the flood plain. Preserved mud to sand ratios
are high.
Porosity and permeability of fluvial reservoirs (Table
FLUVIAL RESERVOIRS 6.1) are functions of the rock matrix, lithologic hetero
geneity, compaction, and cementation. The best primary
Reservoir rocks deposited as fluvial or river deposits porosity is found in channel sands. Flood plain strata are
are formed by braided, multi-channeled rivers or by poor to nonreservoir quality sediments. Fluvial sands
single-channeled, meandering rivers, or a combination of tend to be more mineralogically and texturally immature
these "end members" (Figure 6.1). Miall (1978), Ethridge than other sands because they occur close to the
and Flores (1981), Cant (1982), and Collinson and Lewin sediment source. Alluvial fans may mark the beginning
(1983) provide comprehensive guides to fluvial deposits.
of a fluvial system; with decreasing gradients, next are
Table 6.1 (continued)
Reserves (BOE) Gas Migration
Recoverable In place or Oil Trap Stylec Distance Depth Porosity Permeability
(x1 09) (x1 09) Produced Typeb (km) (m) (%) (md) References
5.8 1 1 . 7 0 A-Up Lt, V 2 1 0 0 Magloire, 1 9 70;
Hamouda, 1 9 80-A
8.0 2 1 . 2 0 BI-A 2590 Sanford, 1 9 70;
Halbouty et al., 1 9 70
1 . 5 3.0 0 St-UpBI Lt 80 2644 1 7 500 Clifford et al., 9 80
1
9.0 25 0 D-Up 3350 2-1 2 Balducchi & Pommier, 9 70
1
0.8 3.2 0 A 3950 Roberts, 1 9 70
0 Bl v 3350 1 7 236 Lelek et al., 1 9 92
0.77 3.1 0 BI-Up Lt 2526 23 380 Jorde and Diesen, 1 9 92
0.57 0 WrA 2560 1 7 80 Halbouty, 1 9 80;
Chapter 22
0.25 0 BI-Up V, Lt 690-2400 Ghignone & Andrade, 1 9 80
1 . 2 0 D 40 Chapter 31
0 5-26 1 0 -150 Chapter 32
