Page 131 - The Petroleum System From Source to Trap

6. Siliciclastic Reservoir Rocks 125

meander form stratigraphic traps that can contain up to able and unique to this environment. They are formed by
50 million bbl of oil reserves. grainflow (avalanching), grain fall (settling directly from
Provided there is an adequate regional seal to confine the air after saltating off the crest of a dune), or by
migration, alluvial sands that are closely associated or migration or translation of wind ripples on the dune
interbedded with lacustrine source rocks can provide surface (Hunter, 1977; Kocurek and Dott, 1981). Most
natural carrier beds for lateral drainage of expelled conspicuous in eolian strata are large (up to 40 m thick)
hydrocarbons (Demaison and Huizinga, 1991; Chapter 4, cross-bed sets formed by successive deposits on the lee
this volume). Migration of hydrocarbons in the fluvial side of sand dunes. Sets of dune cross strata may be
Cretaceous Sarir Sandstone in Libya for at least 80 km separated by nearly horizontal interdunal or toeset strata.
was required to charge the 1.5 billion bbl (recoverable) The cross-bed sets and interdune strata aggrade verti­
Messla field (Clifford et al., 1980). A 40-km migration in cally by bed form climbing at a low angle as long as there
the fluvial Muribeca Sandstone was required in the giant is sufficient sand and strength of prevailing wind.
Carmopolis field of Argentina (Mello et al., Chapter 31, Interdunes form flat areas between dunes in which
this volume). Oil recoveries in the predominantly sand sheets, intermittent streams, salt pans, or ponds
braided reservoirs of the large or giant fields noted in may form. In dry interdunes, a simple, nearly horizontal
Table 6.1 range from 25 to 50%. One of the most prolific erosional boundary surface is formed at the junction of
areas for oil and gas fields with fluvial reservoirs occurs the highly dipping, wind-eroded top of the underlying
in North Africa where the Cretaceous Nubian Sandstone dune and the low-dipping toeset strata of the succeeding
extends from Egypt to Algeria. Other foreland basins dune (Kocurek, 1981). In wet interdunes, poorly sorted
around the world have comparable deposits of fluvial sand, fine silt, clay, evaporite minerals, or limestone may
sediment. be deposited. Interdune bedding may be disrupted by
animals, roots, or salt.
The clean, well-sorted sand in a dune deposit com­
EOLIAN RESERVOIRS monly forms an excellent reservoir and regional carrier
bed. The extensive blanket sands of the Pennsylvanian
Eolian reservoirs are formed by wind-blown sand Tensleep Sandstone in Wyoming, for example, are
deposited along sea coasts or in vast desert areas, called thought to have carried oil up to 400 km across most of
sand sens or ergs (McKee, 1979). Deposition of extremely Wyoming from a pod of Permian Phosphoria Formation
well sorted, predominantly fine- to medium-sized sand source rock in eastern Idaho (Sheldon, 1967). Porosity
occurs in dunes and sand sheets. Less well sorted sand and permeability are the greatest in the steeply dipping
and other lithologies are found in interdunal areas. dune strata, whereas interdune strata have poor
Adjacent to these eolian environments are other conti­ reservoir characteristics. The cross-bedded sands have
nental environments, such as alluvial fans, streams, up to one order of magnitude higher permeability locally
lakes, sabkhas, or nearshore marine environments in the horizontal direction parallel to the strike of the
(Ahlbrandt and Fryberger, 1982). An overall model of a cross bed than in the horizontal direction in which the
complete eolian sequence is illustrated in Figure 6.2. The cross beds are dipping or in the vertical direction
initial marginal dune deposits are overlain by central (Lindquist, 1988). Preserved paleotopographic or
dune field or erg deposits that migrated downwind and, erosional relief can form stratigraphic traps in eolian
in turn, are capped by back-erg sands that were formations (Fryberger, 1984; Eschner and Kocurek, 1988).
deposited as the sand supply was cut off and the dune Fields listed in Table 6.2 illustrate many of the
field growth stopped. prominent eolian reservoirs of the world. The fields
Individual strata in a dune or sand sheet are identifi- generally occur in the same 15°-40° north and south

Table 6.2 (continued)
Reserves (80E) Gas Migration
Recoverable In place or Oil Trap Stylec Distance Depth Porosity Permeability
(x10 9) (x1 0 9 ) Produced Typeb (km) (m) (%) (md) References
1
10.9 G 81 v 914-1981 Stauble & Milius, 9 70;
Halbouty, 1 9 80
0.6 G 81 v 285G-2877 14 30-80 Gage, 1980
0.07 0 WrA v 1G-30 1 0 -1200 Chapter 31
0.9 0 A-St Lt 300 Koelmel, 1 9 86;
Fryberger, 1 9 79
0.1 G St v Mancini et al., 1 9 88;
Pense, 1 9 88
1
0.58 1 . 43 0 A Lt 1 5 0 Carmalt & St. John, 9 86�
0.91 O, G Th v 2918 1 4 23 Lamb, 1 9 80