Page 43 - The Petroleum System From Source to Trap

, . .

2. Petroleum System Logic as an Exploration Tool 35
.
. Table 2.1 . OCS Sale 57-Norton Basin COST Well Shows
COST COST
N0. 1 N 0 . 2 Depth (ft) Shows
SL Cost-1
1 0 ,200 Streaming cut fluorescence
SEISMIC 10,250 Cut fluorescence
EVENTS . 1 0 ,91 0 Crush cut fluorescence
1 0 ,960-1 0,990 (core) Patchy stain and cut fluorescence
Cost-2
.. . 1 1 ,825-1 1 ,830 Trace free oil in mud
. . . . .
. . .. .. . .
2
• • • . 41.11 11 12, 1 o -1 2,220 Major gas show
. . ·-�- . .
·:·:·;�-�-·-:·: 1 2 ,1 90-14,500 Gas shows in several sandstones
� ::,
· · · • ···•···
' ,
)
:
:· : �:::·�· indicated by mud gas unit and
by neutron-density wireline logs
'
' ' ·· ' ' ·'
.�:�: ·: �·: .
• ¥"• • • •
·.·�·-,\':.
concept was that even if these coals contained lipid
i .·
.·
• .· . , . • .· . • .· . . . . material, most of the oil would have been lost prior to
i (t .·
deposition of the basin fill sequence. Thus, this show did
.
- not enhance the oil potential of the Stuart subbasin in
Shell's interpretation .
... • . .
Rock-Eval Pyrolysis Evidence
An indication of when enough oil has been generated
to allow oil expulsion can be derived from Rock-Eval
pyrolysis data. The Rock-Eval instrument measures the
REO amount of hydrocarbons evolved from a sample as the
sample temperature is increased from room temperature
to 500'C. The oil-like hydrocarbons evolved below 300'C
were present in the sample at the start of the analysis,
and their total amount is reported as the S1 peak. The
1 4 magnitude of the S1 peak is a measure of the oil content
of the sample. The hydrocarbons evolved above 300'C
1 5 are largely formed by pyrolysis of kerogen in the sample.
TO 1 4 ,693 TO 1 4 ,889 The total amount of these hydrocarbons formed by
pyrolysis is reported as the S2 peak and represents the
remaining hydrocarbon-generating potential of the
SEAL ROCK sample.
While the thermal maturity of a source rock is
.. . . . increasing and oil is being generated, the value of
.
,-·:·;_·_ .
·
-:�:-�:·: RESERVOIR S1 /TOC increases until oil expulsion starts. After oil
ROCK expulsion begins, the value of S1/TOC remains approxi­
mately constant over a limited depth interval and then
LOW PERM decreases with increasing depth and thermal maturity.
BEDS
For the cases we have studied, S1 /TOC must attain a
value in the range of 0.1-0.2 for oil expulsion to start.
Figure 2.17 shows the plot of S1 /TOC versus depth
Figure 2.16. Stratigraphy of the COST ·1 and COST -2 wells
of the Norton basin. for samples from the two COST wells. Recall that in these
wells, oil generation is expected to be significant at a
depth of about 9000 ft and to continue to about 12,000 ft.
However, the observation of some free oil in the mud at The values of StfTOC for the COST-2 sidewall core
11,825-11,830 ft demonstrates that some oil was expelled samples are low throughout. There is little indication of
in the COST -2 well. The significance of this show is significant oil generation from 9000 to 10,500 ft. Even at
discussed in the next section. 12,000 ft where oil generation is expected to be essen­
Robertson Research (U.S.) reported an oil show below tially complete, S1 /TOC is only about 0.04 compared to
the red unconformity at 12,240 ft in the COST-2 well about 0.1 required for the onset of oil expulsion. We
based on a rock extract. This oil show occurred close to interpret these results to mean that the intervals sampled
the major gas show observed on the mud log (Table 2.1). in COST"2 below 9000 ft are not capable of expelling oil
Both of these shows occurred below the red unconfor­ at any thermal maturity level. At high thermal maturity
mity and are believed to be derived from coals. Shell's (in the gas window), some liquids might be expelled