Page 34 - The Petroleum System From Source to Trap

26 Smith

Petroleum system logic as used in exploration means the
development of an integrated interpretation of the
processes of petroleum generation, migration, and accu­
mulation in order to evaluate an exploration opportunity
or prospect. In the ideal situation, this integrated inter­
pretation is developed from a geologic description that
includes the distribution in space and time of source
rocks, reservoir rocks, seal rocks, and overburden rocks.
In a frontier area, the geologic description is typically
derived from reflection seismic data calibrated to strati­ Gll.F OF MEXICO
graphic control provided by outcrop or sea bottom
samples and to wells using a limited amount of rock 1968 TEX.&.S OFFSHORE SALE
samples and wireline log data. Paleontologic evidence � AREA OFFERED A LL GAS
� EXIST�G FIELDS -
from available samples and seismic stratigraphy are used -.- MI\JOR FAULT TflENDS
in the development of the geologic description of the
area.
In the ideal situation where the geologic description is
essentially complete, the first step in applying petroleum
system logic is to develop an understanding for the
particular frontier area of where and when hydrocarbons
are generated. This understanding leads directly to the Figure 2.1. Area offered in 1968 Offshore Texas sale.
choice of possible migration paths for these hydrocar­ Corsair trend indicated by brackets.
bons and then to reservoirs with access to petroleum.
Finally, for these particular reservoirs, entrapment situa­
tions having the proper timing are located. The Norton those prospects believed to have traps at depths less than
Sound example illustrates two applications of petroleum 10,000 ft. Shell's regional maps indicated the presence of
system logic in a situation where this approach could be approximately 1 7 different structural features with
followed. trapping potential in the Corsair trend within the 1968
The explorer in a frontier area must often deal with an sale area.
incomplete geologic description. By applying petroleum
system logic to such a case, the explorer can identify the Prediction of Hydrocarbon T y pe
critical unknown parameters and can set up a program
designed to make the necessary evaluation. Often the The two parameters having the greatest impact on the
critical unknown parameters arise in connection with the value of a Corsair trend tract were the volume of hydro­
hydrocarbon generation process. The examples taken carbons trapped on the tract and the type of hydrocar­
from the 1968 Offshore Texas sale and the 1976 Baltimore bons in the traps. The economics of oil versus gas were
Canyon sale are of this type. such that, for a given trap volume, oil was worth about
seven times as much as gas. Thus, our petroleum system
work focused on predicting the likelihood of oil versus
CORSAIR TREND: 1968 OFFSHORE gas on Corsair trend tracts.
TEXAS SALE Notice from Figure 2.1 that the only field in the upper
Miocene trend in 1968 was a gas field. This field (Shell's
Geologic Setting Buccaneer gas field) has small oil rims in some reser­
The Corsair trend is a major growth fault trend of late voirs. The ultimate reserves were estimated to be 26
Miocene age on the offshore Texas shelf. Figure 2.1 million bbl of oil and condensate and 736 bcf of gas for a
shows the location of the Corsair trend along with other ratio of 34 bbl of liquids per million ft3 of gas. Also note
information pertinent to the 1968 Offshore Texas sale. No in Figure 2.1 that all the production from the lower
wells had been drilled in the Corsair trend prior to the Miocene updip of Corsair along the coastline was gas.
1968 sale. Figure 2.2 shows the geologic characteristics The question we wished to answer given the presence of
inferred for the Corsair trend from seismic data and well gas along strike and updip was whether the source rock
control north and east of the trend. Seismic velocity data or thermal maturity conditions could change enough to
indicated the presence of hydropressured conditions and create an oil province in the Corsair trend. Our work in
thus sandstone reservoir rock in the upper part of the onshore Texas had identified mature, oil-prone source
upper Miocene throughout the Corsair trend. It was rocks of Eocene, Cretaceous, and Jurassic ages. We had
anticipated that traps would be created along growth been unable to find any oil-prone source rocks in the
faults when upper Miocene reservoirs were faulted interbedded sandstone and shale sequences deposited
against a shale seal. Structural closure occurs on the during the Oligocene-Miocene. We inferred that the oil
down-dropped side where the reservoir rock, overlain present in Oligocene-Miocene reservoirs in the onshore
by a seal rock, rolls into an upper Miocene growth fault. area of Texas had migrated vertically from Eocene or
The Corsair trend area offered for sale encompassed older source rocks. We reasoned that the transition from