Page 51 - The Petroleum System From Source to Trap

2. Petroleum System Logic as an Exploration Tool 43

described initial and final values for the percentage of Their comments regarding the presence of hydrocarbons
gas expulsion at either the top and bottom or the top, in this part of the COST-2 well are as follows:
middle, and bottom of the coal-bearing source rock layer.
The nonlinear relationship between log vitrinite During the drilling of this well, good gas shows consisting of
reflectance and percentage gas expulsion required the methane with lesser amounts of ethane, propane, and butane
were encountered between 12,190 and 14,460 ft. An examination
use of three values to obtain a reasonable approximation of the geophysical logs shows neutron-density gas anomalies
for the average of the whole layer when the percentage for much of the sandstone and interbedded coal between 12,880
gas expulsion at the bottom of the layer exceeded 20%. and 13,942 ft. No tests were run to evaluate the degree of gas
An appropriate average was calculated for the differ­ saturation in the sandstone beds with neutron-density gas
ences between the final and initial percentage gas anomalies. These anomalies probably represent partial satura­
expulsion values at each location. The resultant average tion of the sandstone with gas generated in adjacent coal beds
expulsion efficiency times 1.1 x 1012 SCF/mi2 gave the and unable to migrate further because of low permeability. This
volume of gas expelled from the pre-red source rock per sequence might conceivably be productive elsewhere in the
square mile during the basin fill deposition at the given basin.
location. We have presented a detailed discussion of these pre­
As previously stated, the pre-red source rock model red gas shows because their occurrence provides strong
described here is applicable to the western and south­ support for the migration loss model presented next.
western flanks of the Stuart subbasin. However, to illus­
trate the relative importance of the pre-red and basin fill
source rocks, we have calculated a hypothetical total
Stuart subbasin pre-red gas charge assuming that the Migration Loss Model
model applied over the whole gas-expelling area. For this
hypothetical case, the total volume of gas expelled from Since we estimated that over 98% of the gas available
the pre-red source rock during the basin fill deposition to the evaluated prospects originated in the pre-red
period is 9 x 1012 SCF. For comparison, the estimate for sequence, our major concern is to estimate the loss
the basin fill source rock was 0.12 x 1Q12 SCF for the total during migration of gas generated in the pre-red
Stuart subbasin. Therefore, where the pre-red source rock sequence. This gas must migrate through the fluvial
is present, it will yield approximately 75 times as much sandstones of the pre-red sequence, along the red uncon­
gas per unit area as the basin fill source rock. formity, and along the basement surface until a good
quality sandstone reservoir onlapping a basement high is
reached. There are losses due to the trapping of gas along
each of these parts of the migration path. The abandoned
Gas Shows in the Pre-Red Sequence oxbow sandstones and sinusoidal channel sandstones
Evidence for gas expulsion from the pre-red coals comprising much of the fluvial sandstone system create
during the basin fill period was provided by a strong gas many stratigraphic traps. The migration paths along the
show in the COST -2 well in a sandstone at about 12,200 ft red unconformity and basement surface are visualized as
and by a series of weak to moderate gas shows in sand­ a series of stratigraphic traps connected by a patchy layer
stones between 12,800 and 14,200 ft. The strong gas show of permeable sand deposited at the unconformity or on
was interpreted to come from a 4-ft-thick sandstone at the economic basement.
12,175-12,179 ft. In this show, the hydrocarbon content of A reliable estimate of the volume of gas trapped along
the mud gas reached a maximum value of about 30%. the basement surface below the reservoir section and
All of the coal beds in the pre-red section gave mud gas along the red unconformity is not possible. However, per
shows. These shows interfered with the recognition of unit area, the amounts of gas trapped in these settings is
mud gas shows originating in the sandstones between probably far less than the amount trapped in the thick
12,900 and 14,200 ft. pre-red fluvial sandstone sequence. Furthermore, the gas
A careful review of the mud log along with the show evidence provides a basis for estimating the
wireline logs showed that the mud log gas content volume trapped in the pre-red fluvial sandstones.
increased during the drilling of ten sandstone intervals Because of the jump in vitrinite reflectance in the
between 12,800 and 14,200 ft. The presence of gas in six COST-2 well at the red unconformity, the boundary for
of these sand intervals was confirmed by a lower the onset of gas expulsion was placed downdip from this
porosity reading on the neutron log than on the density well. Therefore, the gas observed in the COST-2 well had
log. To make this interpretation, we used a grain density migrated updip. This means that some of the sandstones
of 2.71 g/cm3. Core data in two different pre-red sand­ in the well, such as abandoned river channel sandstones,
stones had given grain density values of 2.68-2.78 did not have access to gas. However, in the area where
g/cm3. Therefore, the use of a grain density of 2.71 gas was expelled from the coals, virtually every
g/cm3 may have underestimated the number of gas sandstone body is charged with gas.
shows in the pre-red sandstones. In addition, some In the interval between 12,800 and 14,200 ft in the
instances of gas in shaly sandstones may have been COST -2 well, gas shows were observed on the mud log
missed because of the elevated porosity reading on the in 10 out of a possible 20 sandstone reservoirs. These
neutron log. shows are gas in stratigraphic traps whose sandstone
Turner et al. (1983) made a similar interpretation reservoir is connected to the downdip mature source
regarding the presence of gas in the pre-red section. rock. It seems unlikely that the small amount of gas