Introduction
Figures 1-4
This paper discusses the history of the
subsalt
petroleum exploration play (SPEP) in the northwestern
Gulf
of
Mexico
. Before oil was first discovered in the late 1800's,
mankind had explored for resources and knowledge in many ways. It
took visionary leadership, coupled with technology, intelligence,
and funding to drill the first well in rural Pennsylvania, just as
it had taken Columbus, Magellan, and Lewis and Clark to discover new
worlds. Over fifty years later, these same ingredients came together
again to step offshore into the
Gulf
of
Mexico
, where the world's
offshore petroleum industry was born.
After 45 years of petroleum exploration
and development in the offshore
Gulf
of
Mexico
, over 35,000 wells
have been drilled, with over 9 billion barrels of oil and in excess
of 100 trillion cubic feet of gas having been discovered and
produced (Melancon et al, 1994). Statistical probability theory
suggests that such large reserves, having been produced over such a
time period, might preclude the discovery of additional giant
fields. However, even in a mature United States petroleum basin,
such as the offshore
Gulf
of
Mexico
(Figures
1, 2,
3, and 4),
new oil and gas fields are still being discovered with the
application of new technology, which contributes significantly to
enhanced economic feasibility and new plays. The SPEP confirms that
potential, and its growth evolves from the most recent large
commercial play in the
Gulf
, the Plio-Pleistocene play of the
1970-80's. During the 1970's, the Plio-Pleistocene play in the south
additions of offshore Louisiana and southeast Texas was the emerging
trend, with over 3 billion barrels of oil reserves and 40 trillion
cubic feet of gas reserves discovered, developed, and produced after
1973, nearly 25 years after initial overall basin exploration began
(Melancon et al, 1994).
As is the case with many new plays, the
geotechnical leads to the
subsalt
play were hidden in the wells of
this earlier Plio-Pleistocene trend. For many years, explorers had
drilled one to two miles deep looking for oil and gas sands, but in
several hundred wells, they found salt instead. Conventional
geologic models indicated that salt was mostly reshaped as vertical
diapirs rooted to autochthonous salt deposits. Throughout the
1970-80's, explorers had drilled around the flanks of these salt
diapirs and frequently encountered these salt flanks by drilling
through their edges as well. However, seismic acquisition and depth
migration processing technology were advancing to the point that by
the early 1980's explorationists became convinced that they could
better image the flanks of these diapirs, as well as beneath their
crests where shallow salt bases could be inferred. Conceptually,
this meant that thick stratigraphic sections of sand and shale could
exist underneath the salt sheets and might be of tremendous untested
petroleum potential. However, seismic technology did not yet image
the
subsalt
structural/stratigraphic horizons clearly enough to
spark a play.
Subsalt
Exploration
Beginning in 1983, and extending through
the 1980's, an average of one well each year was drilled that
penetrated many different salt sheets, both intentionally and
unexpectedly. Although many salt diapir overhang wells had been
drilled over the decades preceding 1980 onshore and offshore
Gulf
Coast, perhaps the earliest definitive salt sheet penetration
occurred as an unanticipated result of drilling for structure to
depths beyond good seismic resolution.
Between 1980 and the present (1995), over
30 wells drilled through/or into varying thicknesses of salt.
Several of these wells have drilled through large, thick horizontal
sheets, several miles away from their edges (Figure
4). Specifically,
subsalt
tests that demonstrate the drillable
exploratory potential of the regional horizontal salt sheets, as
well as, in several cases, the presence of significant clastic
subsalt
stratigraphic sections are listed below.
Return to top.
Figures 5-40
![](thumbs/f05.jpg) ![](thumbs/f06.jpg) |
Mermentau #1 (1980)
(Figures 5 and
6) |
![](thumbs/f07.jpg) ![](thumbs/f08.jpg) |
Garden Banks 171 #1
(1984) ( Figures 7 and
8) |
|
West Cameron 505 #1
(1984) |
![](thumbs/f09.jpg) ![](thumbs/f10.jpg) ![](thumbs/f11.jpg) |
South Marsh Island
200 #1 (1986) (Figures 9,
10, and 11) |
![](thumbs/f12.jpg) ![](thumbs/f13.jpg) |
Vermilion 356 #1
(1988) (Figures 12 and
13) |
|
Lake Washington #1
(1990) |
|
Mississippi Canyon
211 #1 (1990) |
![](thumbs/f14.jpg) ![](thumbs/f15.jpg) |
Bay Marchand 4 #1
(1991) (Figures 14 and
15) |
![](thumbs/f16.jpg) ![](thumbs/f17.jpg) |
Garden Banks 165 #2
(1992) (Figures 16 and
17) |
![](thumbs/f18.jpg) ![](thumbs/f19.jpg) ![](thumbs/f20.jpg) ![](thumbs/f21.jpg) |
Ship Shoal 349 #1
(1993) (Figures 18,
19, 20,
and 21) |
![](thumbs/f22.jpg) ![](thumbs/f23.jpg) |
South Marsh Island
169 #1 (1994) (Figures 22 and
23) |
![](thumbs/f24.jpg) ![](thumbs/f25.jpg) ![](thumbs/f26.jpg) ![](thumbs/f27.jpg) ![](thumbs/f28.jpg) |
South Timbalier 260
#1 (1994) (Figures 24,
25, 26,
and 27 and
28 [for ST259#1]) |
![](thumbs/f20.jpg) ![](thumbs/f21.jpg) |
Ship Shoal 349 #2
(1994) (Figures 20 and
21) |
![](thumbs/f29.jpg) ![](thumbs/f30.jpg) |
Vermilion 349 #1
(1994) (Figures 29 and
30) |
![](thumbs/f20.jpg) ![](thumbs/f31.jpg) ![](thumbs/f32.jpg) |
Ship Shoal 360 #2
(1994) (Figures 20,
31, and 32) |
![](thumbs/f33.jpg) ![](thumbs/f34.jpg) |
Ship Shoal 250 #1
(1994) (Figures 33 and
34) |
![](thumbs/f28.jpg) ![](thumbs/f35.jpg) |
South Timbalier 289
#1 (1995) (Figures 28,
35) |
![](thumbs/f07.jpg) ![](thumbs/f36.jpg) ![](thumbs/f37.jpg) |
Garden Banks 127 #1
(1995) (Figures 7,
36, and 37) |
![](thumbs/f38.jpg) ![](thumbs/f39.jpg) |
Vermilion 308 #1
(1995) (Figures 38 and
39) |
![](thumbs/f40.jpg) |
Mississippi Canyon
292 #1 (1995) (Figure 40) |
|
South Timbalier 308
#1 (1995). |
Return to top.
However, most of the wells drilled since
1993 cannot be detailed in this paper, due to confidentiality of
data as a result of their recent operational activity.
Additional wells have penetrated the
flanks/edges of regional salt sheets or have encountered small,
remnant salt sheets, either adjacent to, above, or isolated from the
larger sheets. Wells encountering salt sheets in these structural
environments were Ship Shoal 366 #2 (1983), Green Canyon 98 #1
(1984), Green Canyon 39 #1ST1 (1984), Mississippi Canyon 400 #1 and
2 (1985), East Breaks 170 #1 (1985), East High Island A-374 #1
(1985), Green Canyon 152 #1 STH1 (1985), Vermilion 412 #1 (1987),
Eugene Island 385 #A-12 and #B-4 (1988), Green Canyon 184 #A-12
(1990), and Garden Banks 260 #1STH2 (1992).
Significantly, several wells have
penetrated salt withdrawal surfaces, or so called "welds", where
salt is interpreted to have been emplaced as lateral sheets over
clastic sedimentary sections, and then subsequently withdrawn or
evacuated, primarily due to downloading. Several wells are
interpreted to have penetrated these types of surfaces. Example
wells are listed below.
Figures 41-43
|
South Marsh Island
153 #3 (1973) |
|
South Marsh Island
119 #1 (1981) |
|
Vermilion 292 #1
(1983) |
![](thumbs/f41.jpg) ![](thumbs/f42.jpg) |
Eugene Island 324 #1
(1984) (Figures 41 and
42) |
|
Ewing Bank 790 #1
(1986) |
|
South Marsh Island
148 #1 (1988) |
|
Garden Banks 128
(1994) |
![](thumbs/f21.jpg) ![](thumbs/f43.jpg) |
Ship Shoal 368 #1
(1995) (Figures 21,
43). |
There are probably many more that exist
but are not mentioned herein, primarily because the interpretation
of a salt weld on a seismic section or in a well log is not nearly
as definitive as is a salt sheet.
The
Decade of the 1980's
Beginning in September of 1980, the
Peltex #1 Mermentau Mineral Land Co. in Cameron Parish, onshore
Louisiana (Figures 5 and
6), was plugged as a 21,500 ft test
well that drilled through 85 feet of salt between -20,330 ft and
-20,415 ft. A total of 1085 ft of Oligocene-aged
subsalt
clastic
section was drilled, encountering thin sands, before the well was
abandoned. This marked the first penetration of what has become
commonly known as a
Gulf
Coast allochthonous salt sheet, and the
presence of thin
subsalt
sands was encouraging.
The existence of the salt sheet in the #1
Mermentau well (Figures 5 and
6) was not widely known, and especially
unknown to the offshore exploration industry. However, in
preparation for the first area wide sale, scheduled for May 1983,
Gulf
Oil Corporation assembled a team to generate prospects for
lease sale purchase that would focus on frontier plays, and
principally
subsalt
. At that sale in May 1983,
Gulf
bought several
subsalt
or
subsalt
weld prospects, all but two of which were never
drilled by
Gulf
. Most of these have subsequently been purchased and,
in some cases, drilled by other operators during the 1990's. South
Timbalier 259/260 (Teak discovery) (Figures
24, 25, 26,
27, and 28) and Vermilion 308 (South
Anna prospect) (Figures 38 and
39) are two of those lost opportunities
never drilled by
Gulf
Oil. The takeover of
Gulf
Oil in the
mid-1980's terminated internal pursuit of this play after the two
dry holes were drilled in 1984. The subsequent owners of these
Gulf
leases relinquished them back to the government in late 1987, and
the blocks were then purchased by new operators at later lease
sales.
The first
Gulf
Oil prospect to be drilled
was a
subsalt
weld test adjacent to a large regional salt sheet in
Eugene Island 324. This well was drilled through a salt weld surface
initially interpreted to have been encountered around 10,000 ft in
depth. The well was drilled ahead to 15,492 ft before being plugged
and abandoned in March 1984. Several thin,
subsalt
sands were
encountered in both a thick Pliocene and a thin Miocene section
penetrated below salt.
Spudding within days of the first
Gulf
Oil test in fall of 1983, Ship Shoal 366 Placid #2 was permitted as
a 13,000 ft apparent hydrocarbon indicator (HCI) or bright spot test
well, that unexpectedly encountered two thin salt sheets before
bottoming in a thick salt layer at a depth of only 8203 ft.
Seismically, 2-D time data cannot easily resolve the individual salt
sheets penetrated from -7040 ft to -7281 ft (241 ft), -7329 ft to
-7355 ft (26 ft?), nor the 513 ft+ of salt at total depth. Of
special note, the authors believe that wells encountering clastic
zones between salt sheets, as in this well, could be penetrating
sedimentary inclusions seen in some
subsalt
wells and in several
coastal salt domes.
Simultaneously in the spring of 1984,
Garden Banks 171 Marathon #1 (Figures 7
and 8) unintentionally encountered a
1110 ft thick salt sheet (with sedimentary inclusion) between -8400
ft and -9510 ft, before reaching total depth of 10,597 ft STH.
Nearly 1000 ft of
subsalt
clastic section was drilled through mostly
shales of Neogene age. Although encountering a salt sheet, this well
marked the second of many wells drilled throughout the 1980's that
unexpectedly encountered salt while drilling for another HCI or
"bright spot".
Another mid-year 1984 test occurred,
presumably by accident, in 853 ft of water in the Green Canyon 98
Conoco #1. This well drilled to a depth of 13,159 ft, where
drillpipe was stuck, causing the well to be subsequently junked and
abandoned. Drilled on the flank of a large diapired salt sheet, this
test encountered 1380 ft of salt between -10,320 ft and -11,700 ft
and penetrated a small oil sand beneath the sheet. Rather than a
traditional overhang, this salt feature is interpreted as being a
withdrawing salt feeder, as is the case with most of the flank salt
penetrations described in this report. Significantly, a thin oil
sand was encountered just below the salt base, so this may be one of
the first hydrocarbon shows observed in the
Subsalt
Play.
A few months later in the fall of 1984,
Gulf
Oil finished drilling an 18,500 ft intentional
subsalt
test in
West Cameron 505 #2 that penetrated a deep 1690 ft thick salt sheet
between -13,900 ft and -15,590 ft and found neither oil or gas, nor
any significant reservoir sand in the 2820 ft of mid-Miocene clastic
section below. This well marked the end of
Gulf's
two well effort,
for corporate takeover problems preempted further
subsalt
exploration. This also marked the only true
subsalt
well that
Gulf
Oil drilled since the Eugene Island 324 #1, which had been spud
several months earlier, penetrated a weld surface directly adjacent
to a salt sheet.
Another late 1984 salt test was the Green
Canyon 39 Placid #1, and like the Ship Shoal 366 Placid #2, it
encountered a thin, 140 ft thick salt sheet between -11,675 ft and
-11,815 ft, before penetrating 185 ft of additional salt from
-12,005 ft to a total depth of 12,294 ft. Again, like the other
Placid test, it is not clear whether the intermediate shale zone
between the two salt zones is a 190 ft thick sedimentary inclusion
within one sheet or simply a depositional shale zone between two
sheets. The 2-D seismic does not permit resolution of this question,
but detailed biostratigraphic analysis might give tangible results
for both of these wells.
The next four wells drilled were similar
style tests to the Green Canyon 98 #1. These (1985)
subsalt
flank
tests penetrated salt withdrawal feeders. Mississippi Canyon 400
Amoco #1 was drilled to a depth of 16,000 ft, encountering 3450 ft
of salt sheet flank from -10,170 ft to -14,160 ft and then
penetrating 1840 ft of
subsalt
clastic section, containing only one
thin sand. Moving a short distance back from the salt sheet with an
additional test, Mississippi Canyon Amoco #2 was drilled to a total
depth of 17,010 ft, encountering the same salt sheet with a thinner
thickness of only 1290 ft from -14,230 ft to -15,220 ft, and then
penetrating 1700 ft of
subsalt
clastic section without encountering
a similar sand. Presumably, these wells were intended as HCI tests
on the flank of a salt structure, but proved to be
subsalt
flank
tests instead.
A similar test well was also being
drilled over 100 miles to the west, as the East Breaks 170 Amoco #1.
Drilled to a depth of 17,500 ft, this well penetrated a 250 ft thick
salt withdrawal feeder from -16,075 ft to -16,325 ft before
penetrating 1100 ft of
subsalt
clastic Miocene section, without any
significant reservoir quality sandstone.
Later in 1985, the Green Canyon 152
Marathon #1 STH1 well was sidetracked from its original non-salt
wellbore and penetrated 1130 ft of salt from -11,350 ft to -12,470
ft before drilling to an eventual total depth of 16,281 ft MD
(14,133 ft TVD). The well penetrated 1623 ft of
subsalt
clastic
section dated as Pliocene with several reservoir quality
sandstones.
Also, in the fall of 1985, a 15,000 ft
test was drilled as the East High Island A-374 Mobil #1 well
encountered a unique thin salt with limited areal extent. A 250 ft
salt sheet was penetrated from -7575 ft to -7825 ft with over 5000
ft of Pliocene shales below the sheet, followed by nearly 2000 ft of
Miocene shales to total depth. This salt sheet remnant had been
detached from a larger more extensive sheet and, as a result,
probably was interpreted as an HCI because of its limited extent and
thickness.
An even greater HCI disappointment was a
well upon which the senior author was the project geologist for the
operator. In early 1986, Diamond Shamrock drilled a 13,500 ft test
near the edge of the outer continental shelf in 475 ft of water, on
South Marsh Island 200. This well unintentionally drilled into a 990
ft thick salt sheet at -8730 ft, but the partnership agreed to drill
through the salt and found base of salt at -9720 ft. We then
encountered a thick, water-filled 1000 ft thick reservoir sand
between -11,540 ft and -12,540 ft, of early Pleistocene age.
Sidewall core and porosity log analysis confirmed superb reservoir
rock properties with permeabilities approaching 2000 millidarcies
and porosities in excess of 30% within most of this fine-grained
sandstone. This proved conclusively that thick reservoir rock
potential could be found underneath salt sheets and in deepwater
paleoenvironments, as well. As was the case with several previous
wells, an HCI had been misinterpreted, and salt had proven to be the
actual cause. This had become an increasingly common occurrence
throughout the 1980's and into the 1990's, as well. Like the SMI 200
#1, many of the upcoming wells were to be drilled as interpreted
HCI's, only to penetrate salt at the objective target level. Most
significantly from this well, however, a thick, massive sandstone
section, deposited in a deepwater marine paleoenvironment, was
encountered
subsalt
, and new evidence for significant reservoirs was
demonstrated conclusively.
Late in 1986, less than 20 miles west of
the SMI 200 well (Figures 9,
10, and 11),
the Vermilion 412 Mobil #1 was drilling, again presumably for a deep
salt flank HCI, when it penetrated a 1315 ft thick salt layer from
-7705 ft to -9020 ft before drilling ahead to 10,496 ft MD (10,160+
TVD) where drillpipe was stuck in Pleistocene aged shale. Like Green
Canyon 152 Marathon #1 STH, the salt sheet in this well is not
likely to be the primary salt withdrawal source of the large sheet
to its west, but may represent a secondary horizontal salt layer
from the primary sheet, emplaced later than the primary sheet.
Purchasing leases on its "Pooh" prospect
in early 1985, Amoco began a dedicated program of
subsalt
exploration that continues to this day (1995). The Vermilion 356
Amoco #1 (Pooh prospect) well (Figures 12
and 13) began in August of 1987 and
drilled to a depth of 17,000 ft after penetrating 2100 ft on the
western edge of a thick salt sheet between -8400 ft and -10,500 ft,
less than 20 miles north of the SMI 200 sand-rich well. Most
significantly, the well encountered nearly 5000 ft of Pliocene, not
Pleistocene, aged clastic section below salt followed by 1500 ft of
upper Miocene aged clastic section, as well. A 600 ft thick zone of
segregated sands was encountered between 14,400 ft and 15,000 ft in
the lower Pliocene, with intermittent thin sands encountered in the
older Miocene section to total depth. Again, reservoir quality rock
properties had been established for this deepwater marine
paleoenvironment; yet hydrocarbon occurrence was still proving to be
elusive, due principally to well location selection based on
evolving seismic processing. This significant, intentional
subsalt
well was abandoned in early 1988.
Several minor, seemingly unintentional
subsalt
tests were drilled during 1988. In an existing field in
Eugene Island 385 and 371, Union Texas drilled the #A-12 and #B-4
wells. The #A-12 penetrated 60 ft of salt between -12,450 and
-12,510 ft and stopped at 14,104 ft MD (12,687 ft TVD), less than
100 ft below the base of the sheet. The #B-4 test penetrated 1875 ft
of salt in #B-4 between -7842 ft and -9717 ft stopping at 13,038 ft
MD (9881 ft TVD), less than 50 feet below the base of the sheet.
Neither well drilled far enough into the
subsalt
clastic section to
provide significant stratigraphic data.
Near the end of 1988, the Green Canyon
184 Conoco #A-12 also encountered a remnant salt sheet, penetrating
300 ft of salt from -8300 ft to -8600 ft before drilling ahead to a
depth of 12,564 ft MD (12,448 ft TVD). Over 3700 ft of
subsalt
shales and sands were encountered by this well, including two thin
oil pay sands near 10,300 ft and 10,600 ft in the thick
subsalt
Pliocene section.
Return to top.
The
Decade of the 1990’s
Figures 44-45 (Mississippi Canyon 211)
![](thumbs/f44.jpg) ![](thumbs/f45.jpg) |
Figures 44-45
(Mississippi Canyon 211) |
|
South Marsh Island
153 #3 (1973)
South Marsh Island
119 #1 (1981) |
Late 1989 saw the spudding of the first
of many intentional
subsalt
wells to come. The State Lease #12878
Amoco #1 well was drilled in the Lake Washington Field - Plaquemine
Parish, in the southeastern onshore Louisiana to a depth of 21,241
ft. After penetrating 4075 ft of salt sheet between -9350 ft and
-13,425 ft, it encountered nearly 8000 ft of
subsalt
clastic section
that contained thin, but a consistent series of sandstone beds that
occurred in the lower half of this deep wellbore. Again, high
quality reservoir rock was encountered but was water bearing;
however, the first significant
subsalt
oil discovery was preparing
to spud as this Lake Washington well was approaching total depth in
early 1990. One tremendous difference between this well and the
soon-to-be-announced discovery well was the water depth. Had the pay
section in this next well been discovered in a land-drilled
wellbore, it might have been the first commercial discovery in the
play.
A depth of 4352 ft of water is nearly a
mile below the surface of the
Gulf
of
Mexico
. As the 1990's began,
Exxon spud a wildcat in this water depth to test a
subsalt
prospect
below a "shallow" salt sheet. Although in deep water, the
Mississippi Canyon 211 Exxon #1 (Mickey Prospect) penetrated a 3030
ft thick salt sheet from -5750 ft to -8774 ft, barely 1400 ft below
the ocean bottom (Figures 44 and
45). Only 2000 ft below the base of
salt, a thin gas-filled sand was encountered near 10,700 ft, and
then four additional thin, oil-filled sands were penetrated below
12,500 ft before a total depth of 14,670 ft was reached. Exxon
issued a press release in May of 1991 announcing a discovery of
between 100-200 million barrels from five pay sands between 10,000
ft and 13,000 ft. Clearly, a significant discovery had been made.
Core analysis and wireline log data demonstrated quality reservoir
rock with good permeabilities and porosities containing good oil and
gas shows. Although not offset or delineated at this date, the
Mississippi Canyon 211 Exxon #1 well represents the first
significant occurrence of hydrocarbons beneath a salt sheet. Had the
prospect been located on the continental shelf, rather than the
continental slope, commerciality would have probably been immediate,
and rapid development would have ensued. Nonetheless, the
significance of this discovery should not be understated.
By the spring of 1991, Louisiana was
experiencing the next wildcat for
subsalt
objectives. In only 36 ft
of water, the Bay Marchand 4 Amoco #1 well was drilled rapidly to a
total depth of 18,277 ft after encountering a 4340 ft thick salt
sheet between -9820 ft and -14,160 ft, and drilling a very shaly
section to total depth (Figures 14 and
15).
In the fall of 1991, Chevron drilled an
important well just off the edge of the continental shelf in 724 ft
of water, as the Garden Banks 165 Chevron #2 (Figures
16 and 17).
This well was a great technical drilling achievement from the
standpoint that it penetrated nearly 7000 ft of salt and encountered
a thick, quality sandstone reservoir rock below 15,000 ft, before
reaching total depth at 18,000 ft. Penetrating 6950 ft of the salt
sheet from -5765 ft to -12,715 ft, this well established the
important fact that
subsalt
wells could penetrate over a mile of
thick salt and still drill at least a mile below its base. Nearly
250 ft of high porosity, high permeability sandstone reservoir rock
was encountered between 15,200 ft and 15,900 ft, as well as several
thin sands below it. Another old preconceived idea had been
disproven by this well: now, very thick salt sheets could be
penetrated by the drill bit, and thick clastic sections could be
drilled to deep depths below them. When this critical well was
finally plugged and abandoned in April of 1992, drillers throughout
the play now had an excellent type well for planning and
implementing the
subsalt
drilling programs that were only now
starting to be designed, and geoscientists had another excellent
example of superb reservoir rock beneath salt.
Meanwhile, Garden Banks 260 field
(Baldpate Prospect) had been announced as a significant conventional
discovery for Amerada Hess. However, the Garden Banks 260 Amerada
Hess #1 STH 2 proved to be a
subsalt
sheet flank test well,
encountering an apparent 1607 ft thick salt sheet edge between
-13,856 ft and -15,463 ft before penetrating two thick,
water-bearing sands from -18,358 ft to -18,688 ft and then reaching
total depth of 20,086 ft MD (18,845 ft TVD) in late spring of 1993.
Moving the Play Forward
In mid-1993, several significant
subsalt
tests were to be drilled. Roughly half a dozen companies had
purchased leases on prospective
subsalt
acreage during the late
1980's and early 1990's. Investing significant effort in attempts to
prepare these prospects using advanced depth migration processing
techniques, these acquisitions represent the dawn of an accelerated
period of
subsalt
drilling activity. Over those past two years,
industry drilled no fewer than 15
subsalt
sheet or weld tests with
one commercial discovery announced to date, and others speculated
(Figure 4). Since the Minerals
Management Service government regulatory agency does not release
wellbore survey data publicly for two years from suspension of well
operations, less detailed data is available from these wells with
which to demonstrate
subsalt
concepts. However, a few leases have
expired since operations ceased, and under federal guidelines it is
required that this data be released. Consequently, where possible
this report will detail the results.
Exploration Efforts Produce Commercial Result
The most significant result of
subsalt
drilling to date (1995) is the discovery announced in October 1993
at the Ship Shoal 349 Phillips #1 "Mahogany" well (Figures
18, 19,
20, and 21,
in only 372 ft of water depth (Harrison et al., 1995). Although
minimal amounts of data have been released, this well was drilled to
a total depth of 16,500 ft and tested at flow rates of up to 7256
BOPD and 9.9 MMCFGPD at a FTP of 7063 psi through a ½ in. choke from
a
subsalt
sandstone reservoir. Based on interpretation of 2-D
seismic data, this well appears to have penetrated a salt base below
a top of salt of -7230 ft encountered in an old well drilled on the
block in the 1980's by another operator. Since the late 1993
announcement, Phillips drilled two additional wells directionally
from the surface location of the discovery and announced both of
these wells as successful, as well. In late summer 1994, the Ship
Shoal 349 Phillips #2 (Mahogany #2) well (Figures
20 and 21)
drilled to a depth of 19,108 ft MD (18,603 ft TVD), and the operator
announced a test that flowed at the rate of 4366 BOPD and 5.3
MMCFGPD with a FTP of 6287 psi through a 20/64 in. choke. The Ship
Shoal 359 Phillips #2 (Mahogany #3) was drilled to a depth of 19,665
ft MD (18,308 ft TVD), and operations were suspended in spring of
1995. They immediately began Ship Shoal 359 Phillips #3 (Mahogany
#4), and then Phillips announced plans to develop the Mahogany
Field. Construction of a platform capable of handling 45,000 BOPD
and 100 MMCFGPD, was announced with initial production expected at
22,000 BOPD and 30 MMCFGPD to be commenced by the end of 1996. These
projected rates demonstrate the tremendous potential of the
subsalt
play and suggest a significant reserve size for the Mahogany Field.
No well drilled to date has had as significant an impact on
energizing the play as has the discovery well for this new field.
Activity Accelerates Rapidly
Figure 46 (Garden Banks 128)
![](thumbs/f46.jpg) |
Figure 46
(Garden Banks 128)
|
Over 60 miles due west of the Mahogany
well, another
subsalt
test reached total depth of 18,020 ft, at the
end of 1993. Based on 2-D time seismic data, South Marsh Island 169
Amoco #1 (Mattaponi Prospect) (Figures 22
and 23) was drilled on the eastern edge
of a large salt sheet that extends over 10 miles to the west where
it was penetrated over five years earlier by the Vermilion 356 Amoco
#1. No additional information has been released by the operator, but
the well was plugged and abandoned with several openhole plugs set
in the wellbore. Sometimes, openhole plugs can be evidence of zones
of hydrocarbon occurrence, but not in all cases.
About the same time as the Mahogany
activity in late 1993, the South Timbalier 260 Phillips #1 (Teak
Prospect) well (Figures 24,
25, 26,
and 27) began in 295 ft of water and
drilled to a total depth of 16,610 ft. As was the case with the
Mahogany block, an old well drilled by a previous operator
encountered a top of salt near 10,500 ft. Thus, 2-D time seismic
data interpretation proved to be sufficient to define a top of salt
seismic reflector. Phillips announced this as a discovery in the
summer of 1994 with three zones testing hydrocarbons. The first zone
produced at an average rate of 413 BOPD with 673,000 CFGPD and 463
BWPD flowing at 2120 psi on a 16/64 in. choke. A second zone flowed
an average rate of 276 BOPD with 1.096 MMCFGPD at 4200 psi on a
10/64 in. choke. More significantly was the test on a third zone
that tested up to 3742 BOPD with 5.988 MMCFGPD on a 22/64 in. choke
flowing at 7220 psi. As of this report; no additional data have been
released by the operator, nor has any further operational activity
occurred. The well was reportedly plugged and abandoned due to
mechanical difficulty. Phillips did receive a suspension of
production classification from the MMS prior to the April 1995 lease
expiration; so additional operational activity is anticipated.
Although not as actively exploited as the Mahogany discovery, the
Teak discovery is significant as well, because of the amounts of
hydrocarbons recovered from below a salt sheet, proving again the
significant productivity and potential of the
subsalt
play.
As the testing was being conducted on the
Teak prospect in early spring of 1994, no fewer than five wells were
near spudding with the intent of testing
subsalt
objectives. At the
same time, in early March of 1994, industry paid hundreds of
millions of dollars at a federal lease sale for dozens of
prospective leases in the
subsalt
play. It was a very competitive
sale with many participants and sizable amounts bid.
Vermilion 349 Anadarko #1 (Mesquite
Prospect) (Figures 29 and
30) began in early March of 1994, and
drilled to a total depth of 16,146 ft without encountering any
hydrocarbons. It penetrated a 2460 ft thick salt sheet between -9550
ft and -12,010 ft, as well as over 4000 ft of
subsalt
clastic
section. A thick sandy interval between 13,300 ft and 13,700 ft was
encountered; the well was plugged and abandoned, and the lease
allowed to expire. The occurrence of quality reservoir rock in a
thick clastic section is a significant result of this unsuccessful
test.
Barely three miles to the west of the
Mahogany discovery, the Ship Shoal 360 Unocal #2 (Rhino Prospect)
(Figures 20,
31, and 32) began in early May and
drilled to a total depth of 19,000 ft. A suspension of production
(SOP) was obtained by the operator despite having reported no open
hole plugs below the last casing depth of 16,985 ft; so it is
probable that the qualifying pay zones are behind casing, although
the operator did plug and abandon the well without indicating
mechanical difficulty. Additional operational activity may be
anticipated by the SOP lease extension granted by the MMS past the
summer 1994 lease expiration date; thus far (1995) wellbore data has
not been released on this well either.
Nearly twenty five miles north-northwest
of Mahogany, the Ship Shoal 250 Japex #1 well (Figures
33 and 34)
was drilled to a depth of 17,750 ft after penetrating a 956 ft thick
salt sheet from -12,246 ft to -13,202 ft before encountering 4447 ft
of clastic section with intermittent sandstone reservoir rock to
total depth. Like the SS 360 Unocal well, this well began just
before the five year lease expiration date, and since no open hole
plugs were set, the lease expired 90 days after it was plugged in
September, and the wireline log data was released in early 1995.
Ten miles to the south-southwest of the
ST 260 Phillips "Teak" discovery, the South Timbalier 289 CNG #1
(Cypress Prospect) (Figures 28,
35) began in the last days of its five
year lease. Drilled to a total depth of 18,034 ft, the well
penetrated 920 ft of a thick salt sheet from -12,083 ft to -13,003
ft before encountering 4934 ft of
subsalt
clastic section containing
thin, consistent reservoir quality sandstones. The operator plugged
and abandoned this wellbore after setting two open hole plugs below
the last casing depth of 13,980 ft, presumably across two
hydrocarbon shows from 14,030 ft to 14,170 ft and 15,150 ft to
15,160 ft, and the lease has since expired.
The fifth
subsalt
(weld?) test well spud
in the first half of 1994 occurred barely five miles south of the
Mahogany discovery. Ship Shoal 368 Amerada Hess #1 and its two
sidetracks (Citation Prospect) (Figures 21,
43) drilled to a total depth of 16,400
ft MD (15,774 ft TVD), where the well was junked and abandoned in
early 1995. No additional data has been released on this well, and
the lease was still held by 90-day operations, as of press time in
early summer of 1995, extending the June 1994 expiration date.
According to a partner's quarterly report to shareholders, another
possible
subsalt
discovery may have been made on Garden Banks 128
Shell #1 (Enchilada Prospect) (Figure 36,
46), but no other details have been
released. This well drilled to a depth of 18,454 ft MD (17,477 ft
TVD) in water depth of 718 ft. Open hole plugs were set below the
last casing depth of 12,549 ft, and the well suspended. As of press
time, this lease was past its five year primary term, but had been
classified as part of a unit.
As of this writing, five additional
subsalt
tests had spudded in the last quarter of 1994 and the first
half of 1995, and were drilling toward their objective depths.
Vermilion 308 Amoco #1 (South Anna) (Figures
38 and 39)
is projected to a total depth of 20,000 ft in 205 ft of water,
Garden Banks 127 Shell #1 (Chimichanga) (Figures
36 and 37)
is projected to a total depth of 16,000 ft in 622 ft of water,
Garden Banks 216 Amerada Hess #1 STH1 (Penn State Deep) is projected
to a total depth of 20,714 ft MD (18300 ft TVD) in 1611 ft of water,
South Timbalier 308 Marathon #2 (North Lobster) is projected to a
total depth of 18,899 ft MD (16,200 ft TVD) in 564 ft of water, and
Mississippi Canyon 292 Texaco #1 (Gemini) (Figure
40) is projected to a total depth of
14,572 ft in 3393 ft of water. All five of these wells appear to be
pre-planned to penetrate a salt sheet and drill ahead to greater
depths, based on interpretation of 2-D time seismic data.
Predictably, 3-D depth migrated seismic data has presumably been
applied to each prospect to high grade them to drillable status. It
is interesting to note that of these five wells, four of them are
drilling in over 500 ft water depth, suggesting very large projected
reserve potential to justify the significantly higher costs of
deepwater development. This is reflective of a simple fact: that the
play will be pursued both on and seaward of the outer continental
shelf, since the large horizontal salt sheets extend off the shelf
into the deepwater areas of the slope.
Conclusions
Throughout the remainder of the 1990's,
offshore explorers will continue to aggressively pursue this play
because the profit potential is so tremendous, given the reserve
size, existing infrastructure, advancing technology, and attractive
water depths of the northern
Gulf
of
Mexico
.
Subsalt
sandstone
reservoir quality has been repeatedly found to be high porosity,
high permeability, and high pressure, setting the stage for
tremendous production capacity from future discoveries. Thick
sequences of primarily Pliocene, as well as some Pleistocene and
Miocene, sandstone sections reflect
subsalt
deepwater
paleoenvironments and advance deepwater depositional models.
Additionally, industry is discovering that there is a critical
thickness to the interval drilled below salt. The first 1,000+ feet
below salt is a less competent section referred to by some as a
"gumbo zone", or more accurately; a "non-competent zone". Nearly a
dozen of the
subsalt
penetrations did not drill deep enough to see
beyond this section. It is critically important that future
subsalt
wells be drilled roughly 4000 ft - 5000 ft below the base of this
zone, so that adequate reservoir sandstone opportunities can be
encountered through the predictive cycles of sequence stratigraphy.
As advanced seismic acquisition and processing techniques provide
improvements in seismic image resolution and
subsalt
well control
continues to refine geologic concepts, geoscientific integration
will lead to giant discoveries in multiple style traps beneath the
horizontal salt sheets of the offshore
Gulf
of
Mexico
. The 1990's
will definitely be the "Decade of Discovery" for this potentially
giant, domestic petroleum play that is built on the experience of
the past, for the exploratory potential of the next century.
As explorers, we must never forget that
"Discovery" is seeing what everybody else has seen, and thinking
what nobody else has thought! For if we apply this precept to mature
petroleum basins, as in the offshore
Gulf
of
Mexico
, it is not
difficult to understand how we can find that "There is Life in a
"Dead Sea" After All"!!
Acknowledgments
The authors want to express their
appreciation to our managements and colleagues at Anadarko Petroleum
Corporation and TGS\CALIBRE Geophysical Company for their
encouragement and support in preparing this manuscript. Art Waterman
of Paleo Data contributed biostratigraphic data; the Offshore Oil
Scout Association contributed basic statistical data, and TGS and
Geco-Prakla contributed the seismic data displayed in our oral
presentation. Interpretations and opinions expressed by the authors
are their own and cannot be attributed to any organizations or other
individuals.
References
Brooks, R. O., and E.H. Denman, 1995,
Changing Styles of Horizontal Salt Bodies 1984-1994 (abs.): AAPG
Annual Convention Abstracts, p. 13A.
Brooks, R.O., and D.J. Hall, 1995,
New Structural Concepts for the Deep Miocene, Southern Louisiana
Shelf (abs.): AAPG Annual Convention Abstracts, p. 13A.
Brooks, R.O., and D.C. Moore, 1993,
Horizontal Components of
Gulf
of
Mexico
Salt Tectonics; in
AAPG Hedberg Research Conference on Salt Tectonics, Bath, England,
p. 1-4.
Harrison, H., D.C. Moore, and
P.Hodgkins, 1995, The Mahogany
Subsalt
Discovery: A Unique
Hydrocarbon Play, Offshore Louisiana (abs.): AAPG Annual Convention
Abstracts, p. 40A.
Melancon, J.M., S.M. Bacigalupi, C.J. Kinler, D.A. Marin, and M.T.
Prendergast, 1994, Estimated Proved Oil and Gas Reserves
Gulf
of
Mexico
, December 31, 1993: OCS Report MMS 94-0045, U. S. Department
of the Interior, Minerals Management Service,
Gulf
of
Mexico
OCS
Regional Office, New Orleans, Louisiana, August 1994, 50 p.
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