Page 56 - The Petroleum System From Source to Trap

48 Smith

size. Exploratory drilling confirmed these predictions. structural trap on the Schlee dome. A review of the
Two kinds of evidence were particularly helpful in the available data showed that migration to this trap was
evaluation of oil potential. Rock-Eva! pyrolysis measure­ ensured from a thick, matured, undrilled stratigraphic
ments of thermally mature samples indicated that a section. The critical remaining condition that needed to
negligible amount of oil had been expelled from the be satisfied was the presence of either oil- or gas-prone
potential source rocks. This interpretation was supported source rocks within this thick stratigraphic section. An
by the weakness of the oil shows in porous rocks within adequate petroleum charge was predicted to be available
the thermally mature section. because favorable environments for source rock deposi­
Quantitative analyses of the processes of hydrocarbon tion were inferred for this thick, undrilled interval from a
generation, migration, and accumulation were required geologic model derived from reflection seismic data.
to determine which prospects might be gas productive Postsale drilling discovered no petroleum. Although the
and to estimate the size of any gas accumulations. The prediction was wrong in this case, this example provides
gas was generated in a sequence of coal beds a good illustration of the use of petroleum system logic
interbedded with paludal shales and siltstones and to define the critical factor in an exploration evaluation
fluvial sandstones. Significant migration losses were problem and of the approach used to assess the critical
estimated to occur in the fluvial sandstones. Supporting factor.
evidence for these losses came from the observation of Petroleum system logic follows from the fundamental
gas shows in these fluvial sands in the COST well. The processes that control the creation of petroleum deposits.
Stuart subbasin example illustrates the importance of Because of this fundamental basis, petroleum system
making a quantitative evaluation whenever there is a logic is applicable to all petroleum exploration problems.
possibility that petroleum accumulation size will be The examples chosen for review in this chapter are
limited by the available charge. intended to illustrate how petroleum system logic can be
The use of petroleum system logic in the evaluation of applied to a diverse set of problems encountered in
an exploration opportunity often allows the explorer to frontier area exploration.
reduce the evaluation problem to the careful assessment
of a single factor. For example, often the presence of
adequate reservoir and seal rocks and a trapping config­
uration can be demonstrated with a high level of Acknowledgments I thank Shell Oil Company for giving me
certainty. To complete the evaluation using petroleum permission to publish this paper which was based on work
system logic requires demonstrating that a source rock carried out in Shell Oil Company Exploration Divisions. A
attained maturity at the right time and expelled hydro­ large number of Shell Oil and Shell Development personnel, far
carbons into a migration path leading to the accumula­ too numerous to list, contributed to the OCS sale preparations
tion position. There are four factors in this part of the described in this paper and to the research and development
evaluation: source rock existence and type, source rock work that preceded the applications described here. Of all these
maturity, migration path, and timing. Study of the partic­ people, I wish to single out four individuals for particular
ular geologic characteristics may lead to the conclusion credit. The leadership of R. E. McAdams, Vice President
that the probability of success is dominated by the Exploration, Shell Oil Company, 1957-1970, was instru­
outcome of one of the factors. The final evaluation is then mental in the creation of the environment in which organic
reduced to the assembly and careful interpretation of the geochemical research could be effectively incorporated into the
evidence regarding this one factor. exploration program. In addition, Mr. McAdams was an
Two of the examples presented in this chapter illus­ inspiring personal leader to me and many others in the Shell
trate the use of petroleum system logic to reduce the Oil exploration organization. The research contributions of G.
evaluation problem to the consideration of a single T. Phillippi, Archie Hood, and Pierre Mommessin provided a
factor. In both of these ex amples, the presence of sound technical foundation. Finally, I thank Vic Aipperspach
adequate reservoir rock, seal rock, and structural config­ for his assistance in resurrecting the Alaska Division work on
uration for trapping was ensured, but in each case there the Norton basin, and I give special thanks to Karee Kimbro for
was an uncertainty regarding a facet of the hydrocarbon assuming the major responsibility for the graphics.
generation process. The first example was taken from the
1968 Offshore Texas lease sale. The exploration opportu­
nity was a major growth fault trend with no previous References Cited
drilling. The major unknown was whether oil would be
present in the new trend as opposed to gas. The critical
factor thus became the type of source rock expected in Allan, U. S., 19 89, Model for hydrocarbon migration and
the depth intervals that could provide such an oil charge. entrapment within faulted structures: AAPG Bulletin, v.
8
Examination of the evidence bearing on the question led 73, n. 7, p. 0 3-811.
to the conclusion that oil should not be expected. Subse­ Desautels, D. A., 1988, Exploration evaluation of hydrocarbon
potential from Norton basin, Alaska: Bulletin des Centres
quent drilling proved the validity of this conclusion.
The second example was taken from the 1976 de Rescherches Exploration-Production Elf-Aquitaine, v.
4-
5 31.
12,n. 2,p. 51
Baltimore Canyon sale. The problem was the estimation Dow, W. G., 19 8 2 , Geochemical analysis of Norton Sound
of the amount and type of hydrocarbons that were COST No. 2 well, Alaska: Houston, TX, Robertson
expected to accumulate in a well-defined large volume Research (U.S.), 33 p.