Page 33 - The Petroleum System From Source to Trap

Magoon, L. B, and W. G. Dow, eds., 1 9 94, The petroleum
s y stem-from source to trap: AAPG Memoir 60.

Chapter 2

P e troleum System L o g i c as an

Explorati n T o ol in a Frontier Settin g
o

John T. Smith

Shell Developme11t Compa11y
Housto11, Texas, U.S.A.

Abstract

Petroleum system logic is the thought proces required to develop an integrated interpretation of
the proceses of petrleum generation, migration, and acrumulation. It is illustrated here in frontier
area exploration with examples taken from three offshore sales. Details of the application of
petroleum system logic vary widely depending on the nature of the exploration problem and the
data available. The application of petrleum system logic often allows the explorer to reduce the
evaluation problem to the careful assessment of a single factor. The first two examples are of this
type. The thir example is a comprehensive evaluation illustrating the quantitative treatment of the
proceses of hydrocan generation, migration, and accumulation.
The critical problem in the first example (1986 Offshore Texas) was the prediction of petroleum
type in a new growth fault trend. The presece of gas in the new trend was correctly predicted using
petrleum system logic to extrapolate information bearg on hydrocarbon type from adjacent previ­
ously explored areas. In the second example (1976 Baltimore Canyon), the critical problem was
predicting a petrleum charge in a previously unexplored area. Reservoirs, seal, trap, and ease of
migration from a thick, mature stratigraphic secton were ensured for the Schlee dome. An adequate
petroleum charge was predicted to be available because favorable environments for source rock
deposition were inferred from a geologic model derived from reflection seismic data. Postsale
drilling discovered no petrleum and demonstrated the risk inherent in this mode of prediction.
The thir example was taken from the 1983 Norton Sound sale. In part I of this example, the
critical problem was determig the likeood of an oil charge in the area. A reliable answer was
anticipated because the detennination was based on analyses of samples obtained from favorably
located wells that penetrated the whole sedimentary section at a thermally mature location. The
most usef evidence was Rock-Eval pyrolysis measuent of the amount of oil generated in the
thermally mature section and oil shows in porous rocks in the thermally mature section. These
indicated that a negligible volume of oil had migrated out of the mature section. This prediction has
been confirmed by drilling. Part II of this example is a comprehensive evaluation of the Stuart
subbasin, where the proceses of hydrocn generation, migration, and acrumulation were quanti­
fied using rock data from a COST well. The failure of five exploratory wells drilled on four prospect
around the Stuar subbasin to find any gas acrumulations is explained by this evaluation.
These examples demonstrate that all pertinent data should be considered and that proper inter­
pretation of hydrocarbon shows is often important. When there is a possibility of a limited
petrleum char, quantitative evaluation of the proceses of hydrocarbon generation, migration,
and acrumulation should be considered to aid in prospect or play evaluation.

INTRODUCTION Sound. In each case, the conclusions drawn from the
application of petroleum system logic played a major
This chapter illustrates the use of petroleum system role in Shell's evaluation. The characteristics assigned to
logic in frontier area exploration by describing three each example and the ensuing interpretation based on
examples taken from the author's experience as a petroleum system logic were derived from Shell's work
geochemical consultant to Shell Oil Company. These carried out in preparation for the particular offshore sale.
examples are from the following offshore sales: 1968 In each case, the postsale drilling results are compared to
Offshore Texas, 1976 Baltimore Canyon, and 1983 Norton the presale predictions.

25