Page 39 - The Petroleum System From Source to Trap

2. Petroleum System Logic as an Exploration T o ol 31

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0.4 0.6 0.8 1 . 0 1 . 5 2 3 2.0
VITRINITE REFLECTANCE - o/o UJ-11 27.5
2.5
Figure 2.9. Percentage of total gas yield expelled from
type Ill kerogen versus percentage vitrinite reflectance.
TIME OF TRAP FORMATION
Shell assigned a high probability to the presence of a
commercially significant volume of hydrocarbons in the
Lower Cretaceous reservoirs on the Schlee dome. Figure 2.1 0. Thermal maturity history at the COST -B-2 well
location. Each solid line shows the percentage vitrinite
reflectance versus time relationship for a particular
Results horizon.

It was a great surprise and disappointment when the
Schlee dome was drilled and found to be completely
water bearing. Five wells have tested the prime objective 1983 NORTON BASIN SALE
below the Upper Cretaceous seal within the area of
closure without a show of hydrocarbons. The Schlee The Norton basin is a complex of three sedimentary
dome petroleum system seemed to have all the ingredi­ subbasins located off the west coast of Alaska (Figure
ents required for economic success. There were 2.11). Two aspects of the evaluation of the Norton basin
thousands of feet of sedimentary rock deposited under are presented here. Part I discusses the evaluation of the
conditions favorable for source rock development and likelihood of a commercial oil charge in the Norton
having a thermal maturation history compatible with the basin, and part II covers the Stuart subbasin petroleum
timing of trap formation. There was a direct migration system. Both parts provide useful examples of the types
route from this thick stratigraphic section to a seismically of problems encountered in applying petroleum system
logic.
well-defined structural trap having an excellent reservoir Economic considerations led to the conclusion that
and seal combination. The only significant risk was in the gas, if discovered, would be virtually worthless from
existence of source rock within the fetch area. This risk such a remote area. Accordingly, a critical aspect of the
seemed minor because of the favorable depositional presale evaluation effort was to assess the probability of
setting and the thick package of sediments within which finding significant oil reserves in the basin. Shell
the source rock could lie. To explain the absence of concluded that the important source rocks in the Norton
hydrocarbons on the Schlee dome we must first assume basin area were gas prone. The work leading to this
that none of the transgressive shales in the Upper conclusion is described in part I. Once this conclusion
Jurassic were source rocks and, second, that the transi­ was reached, Shell found it unnecessary to carefully
tion from carbonate reef to oxidized red beds occurred evaluate the processes of hydrocarbon generation,
over a short distance in the Upper-Middle Jurassic migration, and entrapment required for rigorous applica­
section between 19,500 and 31,500 ft. When the presale tion of petroleum system logic.
evaluation was underway, this latter condition was not Exxon and Elf reached a different conclusion from
suspected because the COST-B-2 well had penetrated an that of Shell and estimated that there was a reasonable
Upper Jurassic-Lower Cretaceous section with coaly chance of finding significant oil reserves in or adjacent to
material distributed over an interval 7000 ft thick the Stuart subbasin (Desautels, 1988). As a result, they