Page 45 - The Petroleum System From Source to Trap

2. Petroleum System Logic as an Exploration Tool 37

1.0 below the red unconformity. Other gas indications were
observed down to economic basement. This section
0.8
u contains much coal and attains a thermal maturity suffi­
e o.e cient to generate significant volumes of gas. Shell's inter­
"' pretation of these observations was that the probability
"' 0 .4
::E of finding commercially significant oil reserves in the
0.2 Stuart subbasin was very low. Thus, tracts in the area
were given speculative bids in case the COST-2 well was
15 20 25 30 35 unrepresentative or something was overlooked in the
CARBON NUMBER interpretation.

Figure 2.18. Normal paraffin distributions expressed as Summary of the Oil Potential of the
milligrams of each normal paraffin per grams TOC. Open St. Lawrence Subbasin
circles: the waxy shale sample as received (0.7% R0) The COST-1 well was favorably located to penetrate
picked from cuttings at 9690--9750 ft depth. Closed circles: any source rocks deposited in the St. Lawrence subbasin.
the same sample after heating for 6 days at 330'C. Minor oil shows in samples were observed in the
Triangles: model kerogen containing 70% vitrinite +
1 � !0 liptinite + 1 3 % alginite. thermally mature section, and Rock-Eva! pyrolysis
S1/TOC observations suggested that minor amounts of
oil had been generated. A model was set up for the most
favorable case for oil accumulation on a prospect. From
confirms our conclusion from the SifTOC data that the this model, the most favorable prospect ("A" in Figure
oil generation potential of the macerals visually identi­ 2.11) was estimated to have the potential for a recover­
fied as lipid kerogen was much less than that of type II able volume of approximately 20 million bbl of oil. This
kerogen. The final step in our evaluation of the oil volume is below the minimum needed for development.
potential of the Norton basin was to calculate the volume This result, combined with the probability that there
of oil that the waxy shale interval might supply to the would be an inadequate thickness of reservoir rocks in
prospect judged to have access to the largest volume of the St. Lawrence subbasin, led Shell to make only specu­
oil from this source. This prospect is identified by the lative bids in the area.
letter "A" in the St. Lawrence subbasin (Figure 2.11). The
measured pyrolysis yield on the picked cuttings was Results
0.9% by weight. Based on the model of 70% vitrinite, 17% A total of six exploratory wells were drilled by others
liptinite, and 13% alginite, we estimated the total oil yield to evaluate Norton basin prospects. One well, Area cx:::s­
of the waxy shale at complete conversion to be 0.8% by Y-0435 No. 1, was drilled on a prospect for which the
weight. To create the source rock thickness, this oil yield hydrocarbon fetch area was near the well. Some oil
was optimistically applied to the full 60-ft thickness from shows were reported from this well, but none were
which the waxy shale was derived. The prospect worthy of a drill-stem test. Since the hydrocarbon
receiving the maximum oil charge from this source was potential of this prospect could not be predicted from
estimated to acquire about 1 1 0 million bbl of oil. either COST well, observations in the Area OC5-Y -0435
Assuming favorable efficiency factors for migration to well are not relevant to our evaluation and will receive
and recovery from the trap, this best prospect for oil no further attention.
might have a recoverable volume of 15-20 million bbl. No exploratory wells were drilled on prospects in or
This volume is far below the minimum required for adjacent to the St. Lawrence subbasin so we lack results
development in the Norton basin. to compare to our prediction that oil migration from the
basin fill sequence in this subbasin is quite small.
Summary of the Oil Potential of the Stuart Five exploratory wells were drilled on four prospects
Subbasin adjacent to the Stuart subbasin (Figure 2 . 1 9 ) . The
The COST-2 well was favorably located to penetrate locations of these exploratory wells are shown on the
any source rocks deposited above the red unconformity structural contour map in Figure 2.20 drawn at the top of
in the Stuart subbasin. Thermal maturity for oil genera­ an important reservoir rock unit. Reservoirs and seals
tion was reached at about 9000 ft, and the red unconfor­ penetrated in each well are shown relative to sea level in
mity was reached at about 1 1 , 950 ft subsea. Rock-Eva! Figure 2.21. None of the wells found a hydrocarbon accu­
pyrolysis S1 /TOC observations on sidewall cores mulation, and all of the wells were plugged and
indicated that oil had not been expelled from this mature abandoned without drill-stem tests. We will go beyond
source rock interval. Absence of oil shows in samples these two facts and look for evidence that oil migrated
confirmed this interpretation. A trace of oil was observed out of the deep part of the Stuart subbasin to adjacent
in the mud at 1 1 , 825-11,830 ft. There was insufficient prospects. All of the wells encountered sandstones above
evidence for an oil source rock of significant thickness basement overlain by a sealing rock interval (Figure
near this depth. Accordingly, it was assumed that a thin 2.21). Three of the wells, Cascade CX:::S-Y-0398, Yellow
source rock of high lipid content had expelled an Pup OCS-Y-0497, and Chugach OCS-Y-0425, were
insignificant amount of oil at this depth. A major gas appropriately located to be on the migration path of
show was observed at about 12,200 ft in the section hydrocarbons moving updip from the area containin g