Page 55 - The Petroleum System From Source to Trap

2. Petroleum System Logic as an Exploration Tool 47

It is evident from the nature of the problem and the presence of migrating oil. However, oil shows were
quality and quantity of data available for the presale lacking in all three wells. Therefore the evidence from the
evaluation that our estimates of the volumes of expelled postsale drilling of these prospects is in agreement with
gas and of the volumes of gas retained in the fluvial Shell's presale evaluation that a significant oil charge was
sandstones are subject to large uncertainties. In partic­ unlikely.
ular, our values for the volumes of expelled gas should The second aspect of our application of petroleum
be viewed as estimates of the median value, with about a system logic to the Norton basin was the development of
tenfold range from the 5% to 95% probability values. a complete petroleum system analysis of the four drilled
Even given this uncertainty, these estimates made in prospects adjacent to the Stuart subbasin. Essentially all
advance could provide a warning that there was a signif­ of the hydrocarbon charge available to these prospects
icant risk with the gas charge. These estimates could was judged to be gas expelled from the pre-red coals
have indicated that the prospects adjacent to the Stuart during the latter part of the basin fill period. These coals
subbasin were unlikely to have important gas reserves are distributed throughout a thick sedimentary package
even though there were two large structures in a and are interbedded with fluvial sandstones and fluvial
geologic setting that contained reservoir and seal rocks. plus paludal siltstones and shales. We assigned fetch
areas to each prospect so that the gas available to each
Summary of Norton B a sin Example prospect and the losses along the migration path could
be estimated. From these estimates it was concluded
The Norton basin is a complex consisting of three using presale data that gas accumulations were unlikely
subbasins. For the presale evaluation, two COST wells to exist on two of the prospects and that the volumes of
were available, COST -1 in the St. lawrence subbasin and gas available to the other two prospects were small
COST -2 in the Stuart subbasin. Our application of relative to the potential trapping capacity of the
petroleum system logic was restricted to the areas in and prospects.
around these two subbasins and started with an assess­ The results obtained in the five exploratory wells
ment of the probability of finding significant oil reserves drilled on these four prospects after the sale were in
in either area. agreement with these predictions made from the
A major unconformity, the red unconformity, petroleum system analysis. Gas accumulations were
occurred at approximately 12,000 ft in both wells. An undetected on any of the prospects. The possibility of
abrupt increase in Ro to 1.0% occurred at this unconfor­ onlap traps on the flanks of the two prospects believed to
mity. The section below the red unconformity contained have received some gas was confirmed by the strati­
coals. Because of this increase in R0 to 1.0%, we graphic information acquired during the drilling. Volu­
concluded that any potential for oil generation in the pre­ metric estimates made using the observed reservoir para­
red section was used up and that the oil was lost prior to meters indicated that, because of the locations chosen for
the deposition of the post-red basin fill sequence. Where the three wells drilled on these two prospects, failure to
the red unconformity was buried deeply enough under find accumulations of the estimated size was easily
the basin fill sequence to increase the Ro above 1.0%, a explained. Therefore, the results obtained from the
second episode of gas generation and expulsion was postsale drilling of the five wells on four prospects are
assumed to have taken place. consistent with predictions made from a petroleum
Organic matter is present throughout the basin fill system logic using the COST-2 well and reflection
sequence in both COST wells and is mature for oil gener­ seismic data.
ation at depths greater than about 9000 ft. In this mature
interval, the organic matter is a mixture of lipid and
humic kerogens. Rock-Eval pyrolysis data and the poor CONCLUSIONS
quality of oil shows indicated that there had been an
insignificant amount of oil expelled from the kerogen in The use of petroleum system logic in exploration
the mature basin fill sediments in either well. We means reliance on integrated interpretations of the
estimated that a subcommercial volume of oil may have processes of petroleum generation, migration, and accu­
been expelled from this sequence in the central part of mulation for the evaluation of exploration opportunities.
the St. Lawrence subbasin where the overburden rocks The accuracy of such an interpretation increases when
are thickest. The COST-2 well lacked intervals with a the quality and amount of pertinent data increase.
high enough lipid content to expel oil. Where the over­ However, even in a frontier area having little well
burden rock is thickest in the Stuart subbasin, expulsion control, it is possible to make a reliable evaluation of an
of a minor amount of gas and condensate may have exploration opportunity using petroleum system logic.
occurred from the basin fill sequence. Because of the This conclusion was illustrated by the successful applica­
small amount of this gas and condensate, it was most tion of petroleum system logic to two aspects of the eval­
likely lost along the migration path between the gas uation of prospects in the Stuart subbasin area of the
expulsion region and the prospects. Norton Sound in offshore Alaska. Using the COST well
Five wells have been drilled on four different in the Stuart subbasin for stratigraphic control and a grid
prospects adjacent to the Stuart subbasin. Three wells on of reflection seismic lines, predictions were made that the
three different prospects were properly located and prospects would have little or no oil and that some
provided a good stratigraphic section for observing the prospects would have gas accumulations of moderate