Page 47 - The Petroleum System From Source to Trap

2. Petroleum System Logic as an Exploration Tool 39

PROSPECT SO. TETON YELLOW PUP CASCADE CHUGACH CHUGACH
OC - Y - 0414 0407 0398 0430 0425
S
o

2
1-
u.
�
3636'
:I: 4
1-
a.
w 4951'
c EXPLANATION
6 6093'
Em RESERV ROCK
� SEAL ROCK 6913'
• SUSTONE
8 7869'
ECONO
0 BASE - Position of Blue Seismic Event
D NO DA T A

Figure 2.21. Generalized stratigraphy of Stuart subbasin tests showing distribution of seal intervals and
sanck:ontaining intervals.

tionships. The prospect names given by Exxon and Elf 2.16, 2.22, and 2.23). To reach commercial reservoirs on
(Desautels, 1988) were used for the four prospects they the prospects, the gas must migrate along the
drilled. For the other two prospects, Cold Duck and West sediment-basement contact. The gas expelled from the
Cold Duck, we have used the names given by Shell. As pre-red coals must first migrate through the fluvial
shown in Figure 2.19, gas expulsion is restricted to a rela­ sandstone either to the basement contact or to the red
tively small central portion of the Stuart subbasin, unconformity. The gas that migrates to the red unconfor­
whereas all the prospects expected to derive gas from the mity must then migrate along the unconformity to the
Stuart subbasin are located outside the gas expulsion basement. There are migration losses along every part of
region. Therefore, critical facets of this evaluation include the migration path, particularly in the fluvial sandstones.
determining the migration paths of the gas, the limits of The deepest sandstone reservoirs of good quality are
the fetch areas for the various prospects, and the expected to onlap the basement and be located near the
volumes of gas retained along the migration paths. "blue horizon" (Figures 2.16 and 2.22). Figure 2.20
Figures 2.22 and 2.23 present cross-sectional views presents the structure at the blue horizon. Note the
through the center of the Stuart subbasin. As indicated in possible onlap traps on South Teton and Chugach at this
Figure 2.23, some gas is expelled from the basin fill structural level. From the COST-2 well stratigraphy and
sequence. The primary source of gas is the coal in the the structural characteristics of these two prospects,
pre-red sequence, which will be called the pre-red source onlap traps in good quality reservoir sandstones are
rock. possible over an interval about 1000 ft thick. Hydro­
The COST-2 wireline logs show the distribution of the carbon migration from one onlap trap to the next higher
coals (Figure 2.24). Each coal bed is characterized by trap would have occurred until a trap was reached
increases in resistivity and sonic travel time and a beneath a top seal that extends over the basement high.
decrease in density. The coals are interbedded with Migration above this top seal may have occurred where
shales, siltstones, and low-permeability sandstones. The the displacement created by a fault juxtaposes the
coal-bearing section was deposited under fluvial and sandstone reservoir below the seal against the sandstone
paludal conditions. Under these conditions, each reservoir immediately above the seal. This concept of
sandstone is expected to have a relatively narrow width hydrocarbon migration across a fault plane when two
and to follow a sinusoidal path providing many oppor­ reservoirs are juxtaposed was advanced by Allan (1989)
tunities for stratigraphic trapping. and has been used by Shell from about 1970.
All of the prospects are on basement highs adjacent to The processes of generation, migration, and accumu­
the Stuart subbasin. A thick seal rock rests on the red lation of hydrocarbons in the Stuart subbasin area lead to
unconformity and onlaps the basement highs (Figures the use of a single petroleum system for evaluation of the