Page 120 - The Petroleum System From Source to Trap

114 Peters and Cassa

indicates that the Deer Shale was an active source rock ratio is 1 . 6 and the CPI is 1 . 1 . A more definitive
sometime in pre-Tertiary time. Burial history charts oil-source rock correlation using biomarkers was not
(Figure 1 . 2, Chapter 1 ) in several places in the area completed. Based on the rock extract and oil geochem­
confirm that the source rock was active in late Paleozoic istry, the oil probably originated from the Deer Shale,
time. therefore the level of certainty for the Deer-Boar
Because the hydrocarbon shows in the wells are proof petroleum system is hypothetical.
of the petroleum system, an understanding of the distrib­ The maps and cross sections in Figures 1 . 2- . 5
1
ution of these shows is important to determine the (Chapter 1) and in Figures 5.12 and 5.13 indicate the
geographic and stratigraphic extent of the system. Most stratigraphic and geographic extent of the Deer-Boar
of the wells were drilled to basement. Where the petroleum system. Based on the burial history chart, the
basement rock, underburden rock, and source rock Deer Shale was an active source rock in late Paleozoic
intervals were penetrated, shows were not detected. time. The oil show map of the Boar Sandstone reservoir
Hydrocarbon shows were detected in the overburden rock interval indicates the oil migrated along two
rock and seal rock intervals. corridors into anticlinal and stratigraphic traps. In
Slight oil shows (streaming cut-fluorescence) were Tertiary time, a rift graben formed, isolating the oil accu­
detected near the base of the overburden rock in the mulations on the east from those on the west side of the
north-central part of the area, and gas shows were graben and the pod of inactive source rock Oil samples
detected at a depth of 350 m in the central part of the area from the shows and accumulations were used in an
(Figure 5.13A). Well site evaluation of the gas shows oil-oil correlation. Results showed that all these oils orig­
indicated substantial gas on the hot wire that was 99.7% inated from the same source rock The confidence that
methane on the GC (dry gas). Laboratory analyses of this oil originated from the Deer Shale is based on the
headspace gas from canned cuttings collected through similarity of certain geochemical parameters for both the
this interval indicate a carbon isotope value of -71.6%o, oil and rock extract even though a detailed oil-source
or biogenic gas. The conclusion is that this gas is part of a rock correlation was not completed.
separate petroleum system probably related to Carbonif­ After the source rock is demonstrated to be effective,
erous coals in the overburden rock. that is, it generated and expelled hydrocarbons, then the
Within the seal rock, two shows were detected near question remains as to how effective. Decreased TOC
the base of the unit (Figure 5.13B). The show in the north­ and pyrolysis yields of source rocks resulting from
central part of the area underlies the oil show in the over­ thermal maturation must be taken into account to assess
burden rock Both shows are interpreted to be vertically their original generative potential accurately and to make
migrating oil from the reservoir rock below. South of this volumetric estimates of petroleum generated (Dow,
show is another show in the seal rock, which is also inter­ 1977b; Schmoker, Chapter 19, this volume). For example,
preted to be oil that migrated from the reservoir rock
below. roc is little affected by maturation of rocks containing
The reservoir rock interval, the Boar Sandstone, has type N kerogen, but TOC can be reduced by 12-20 wt. %
the most abundant oil shows of any interval investigated for type ill and by as much as 50 and 70 wt. % for types II
and I, respectively (Daly and Edman, 1987). Failure to
(Figure 5.13C). Of the 32 wells that penetrated the
reservoir horizon, 7 have oil shows and 6 recovered oil account for these effects on mature or spent source rocks
when drill-stem tested. Based on the distribution of these can cause source intervals to be overlooked on geochem­
oil shows and their proximity to the pod of mature ical logs and can result in underestimates of original
source rock, there is little doubt that the oil originated source rock potential or oil generated.
from this source rock. If this is true, the pattern of oil The volume of hydrocarbons generated and accumu­
shows should give some indication as to the migration lated can be demonstrated in many ways. The case
path of the oil. Two east-west bands of shows are shown, studies in this volume use a mass balance method
one at the top and one in the middle of the map with five (Schmoker, Chapter 19), and Demaison and Huizinga
wells in between that lack shows. These bands are good (1991; Chapter 4) use the SPI. Because the case studies
circumstantial evidence that the oil migrated from the adequately explain the mass balance method, only the
mature source rock on the west through the reservoir SPI method is outlined here.
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rocks along two corridors to the east (Figure . 3, Chapter The residual SPI map is constructed from the net
1). The distribution of these oil shows and the pod of source rock thickness map and the genetic potential (51 +
mature source rock provide a basis for the geographic S2) (Table 5.4) of the source rock from Rock-Eval
extent of the petroleum system. pyrolysis (Figure 5.12E). The SPI ranges from 0.6 to 11.3.
Gas chromatography of the Deer Shale source rock The residual SPI where the source was buried deepest
extract shows that the pristane-phytane ratio is above 1.5 suggests that up to 8 SPI units were lost as the source
and the carbon preference index (CPI) is 1 . 2. Organic rock generated oil. Based on the preliminary SPI classifi­
petrography indicates that the organic matter is mostly cation (Figure 4.4), the burial of this source rock indicates
marine amorphous material with up to 35% terrigenous a large drainage area with moderate to high potential.
material. Using regional geology and SPI, this petroleum system
The API gravity of the oil ranges from 32 ° to 43 ° and can be classified as a supercharged, low impedance,
sulfur content is less than 0.3%. The pristane-phytane laterally drained petroleum system.