The
Gulf of Mexico subsalt play spans a vast area
on the Texas-Louisiana Outer Continental
Shelf and slope. This region is defined by the areal extent of
tabular salt bodies. These tabular salt bodies are not continuous
throughout the entire play outline.
Additional subsalt potential also exists
beneath leaning or detached salt diapirs to the north, and beneath salt
massifs to the south of the traditional subsalt play outline. Other
hydrocarbon accumulations also exist above and between tabular salt bodies
of the subsalt play. A detailed discussion of subsalt exploration is provided in
the MMS
2000 National Assessment (OCS Report 2001-087 CD release).
A summary of the
National Assessment is available online.
A list of non-proprietary and
publicly announced subsalt wells and a
map outlining the areal extent of the
subsalt play in the Gulf of
Mexico are included in this
publication. Other
non-proprietary well data and information can be
downloaded from the MMS website.
What
is the Subsalt Play?
The subsalt play is characterized by
allochthonous (removed from its depositional location) tabular salt that commonly occurs in tongues, sheets, nappes, or canopies (Montgomery and Moore, 1997). These salt bodies were
once part of a deeper Jurassic (approximately 170 million years before
present) layer known as the Louann Salt (mother salt). Sedimentation on
top of the mother salt layer caused deformation of the salt and its
overlying sediments. Sometimes this deformation resulted in near-vertical
"salt feeders" or "salt stocks" that flowed laterally into sheets as they
approached equilibrium at or near the seafloor. The salt feeders are
sometimes pinched off, resulting in isolated allochthonous salt sheets
above the mother salt layer.
Thick sediment accumulations lie
between the mother salt and the allochthonous salt sheet. Structural and stratigraphic traps of
reservoir quality
sediments are the target of subsalt hydrocarbon exploration. Allochthonous salt sheets mask the
underlying (subsalt) structures and stratigraphy, but sophisticated
computer processing of seismic data makes subsalt imaging possible.
Significant Subsalt Discoveries in the Gulf of Mexico
Until the mid 1980’s, it was a common
paradigm that, once tabular salt was encountered, the potential for deeper
hydrocarbon accumulations was nonexistent. Several of the early subsalt wells
were accidental subsalt penetrations. Operators were sometimes drilling to
test anomalous seismic reflectors and found salt where they expected
hydrocarbons.
The first subsalt well drilled in the
Gulf of
Mexico Outer Continental Shelf
was Placid Oil Company's Ship Shoal 366 well (OCS-G-05588 #2) in 1983.
This well drilled through two thin salt sheets before being plugged and
abandoned in a third salt body. A total of only 295 feet of subsalt
sediments was penetrated between three salt bodies.
Over the next several years, sporadic
subsalt drilling continued. One of the highlights was Diamond Shamrock's
1986 well in South Marsh Island 200 (OCS-G-07719 #1), which penetrated a
990-foot thick salt sheet and encountered a massive 1,000-foot thick
reservoir-quality sandstone below salt. The potential for world-class
reservoir sands became apparent, but the absence of subsalt hydrocarbon
accumulations still limited drilling activity.
In 1990, Exxon cleared the next hurdle
when it discovered subsalt oil and gas in its Mississippi Canyon 211
(OCS-G-08803 #1) well. This prospect, nicknamed "Mica," encountered hydrocarbons below a 3,300-foot
thick salt sheet in 4,000 feet of water and it took over a dozen years to
bring the project to production.
Subsalt drilling continued at a slow
but steady pace until the well publicized discovery of the "Mahogany"
field in 1993, operated by Phillips Petroleum and located in Ship Shoal
349 (OCS-G-12008 #1) . This field, which began production in early 1997,
was the first commercial subsalt oil development in the Gulf of Mexico.
Mahogany has produced over 24 million BOE from its wells as of June 2005.
Following the Mahogany discovery,
subsalt leasing and drilling activity increased dramatically. In 1994,
Shell Offshore, Pennzoil, and Amerada Hess announced a significant
discovery in Garden Banks 128 (OCS-G- 11455 #1), nicknamed "Enchilada."
Enchilada has combined reserves estimated at 400 Bcf of gas and 25 million
bbl of oil/condensate (DeLuca, 1999), and was brought on line in July of
1998. In 1995, Shell and partners announced another discovery at Garden
Banks 127 (OCS-G- 11454 #1), the “Chimichanga” prospect. The well was a subsalt followup to the Enchilada discovery. The Enchilada/Chimichanga
was the second “commercial” subsalt discovery, which began producing in
July of 1998.
Late in the year 1995, Texaco and
Chevron announced a major discovery in Mississippi Canyon 292 (OCS-G-08806
#1), the "Gemini" prospect. Estimated recoverable reserves for the field
are 250-300 Bcf of gas and 3-4 million bbl of condensate (DeLuca, 1999). Production
through a subsea system commenced in June of 1999.
Anadarko and Phillips in 1996
announced the discovery of the "Agate" field in Ship Shoal 361
(OCS-G-14514 #1). Agate is now producing through a tie-back into the
neighboring Mahogany platform.
Anadarko, along with partner BHP, also
in 1996, announced a second subsalt discovery in Vermilion 375
(OCS-G-14427 #1), nicknamed "Monazite." The discovery well revealed
multiple pay zones, but because of problems during testing, the hole was
plugged and abandoned.
In 1997, Amerada Hess and Kerr-McGee
made a discovery at Garden Banks 215 (OCS-G-09216 #4), the “Conger” prospect.
Later in the year (1997), the same partners made another major discovery
in their "Penn
State" prospect in Garden Banks 216
(OCS-G-14224 #3). These were the only successful new field wildcats
reported during the year. The companies subsea completions tiebacked to their
Baldpate development in Garden Banks 260.
In July of 1998, Anadarko announced a
subsalt discovery at the "Tanzanite" prospect in Eugene
Island
346 (OCS-G-14482 #1). Reserves for Tanzanite are estimated at 140 million
BOE (Deluca, 1999). Tanzanite began producing in December of 2000.
Later in 1998, Anadarko also announced
the discovery of their "Hickory"
prospect in Grand Isle 116 (OCS-G-13944 # A-1). The discovery well,
drilled to a total depth of 21,600 feet, penetrated approximately 8,000
feet of salt, one of the thickest sections of salt ever drilled in the
Gulf of Mexico. Reserves are estimated at 40 million BOE (Deluca, 1999),
and first production came on line in late 2000.
The subsalt discoveries at Green
Canyon 562/563, “K2/Timon Prospect” (OCS-G-11075 #1 and G-11076 #1), in
1999 in 4,144 ft of water by Conoco, and at “Champlain” prospect, Atwater
Valley 63 (OCS-G-13198 #1) in 2000 in 4,385 ft of water by Texaco, have
been reported by the industry as containing reserves totaling 280 MMBOE.
Today, subsalt leasing and drilling
continue at a healthy pace. The tremendous excitement is generated by
several major discoveries in the ultra-deep (greater than 5,000 ft)
waters.
BP Exploration made significant
discoveries in water depths greater than 6,000 ft at Green Canyon 699,
the “Atlantis” prospect (OCS-G-15604 #1), in 1998 in 6,133 ft of water, and
at Green Canyon 826, the “Mad Dog” prospect” (OCS-G-09982 #1 OH & STK), in
late 1999 in 6,560 of water. Production from Mad Dog started in
January 2005; BP estimated Atlantis reserves as 575 million BOE.
The largest of the subsalt discoveries
was made by BP and ExxonMobil at Mississippi Canyon 778, the “Thunder
Horse” prospect, (OCS-G-09868 #1) in 6,050 ft of water in 1999. Thunder
Horse is one of the largest deepwater Gulf of Mexico discoveries to date, with
estimated reserves of 1 billion BOE. A second discovery, “North Thunder
Horse” at Mississippi Canyon 776 (OCS-G-09866 #1), was made in 2000 in
5,636 ft of water. Industry press releases report that the entire Thunder
Horse and North Thunder Horse field complex may have reserves of 1.5
billion BOE. Thunder Horse is scheduled to commence production in
late 2005.
REFERENCES:
Deluca, Marshall;
1999; "Forty-six wells designated Subsalt in the Gulf of Mexico", in
Offshore Magazine; January.
1999; pp. 50,52, 145.
Montgomery,
Scott L. and D. Moore, 1997. Subsalt Play,
Gulf of Mexico: A Review: Bulletin of the American
Association of Petroleum Geologists, vol. 81, p 871-896.
Subsalt Imaging
Geophysicists image subsurface
sediments by transmitting seismic (sound) waves through the earth and
analyzing the energy that comes back to the surface. Until the early
1990's, detailed imaging beneath salt features was rare because seismic
reflections are severely scattered and distorted by rugose salt surfaces
and by anomalous acoustic properties of salt.
A sophisticated computer processing
technique, called 3D depth migration
(seismic section above), revolutionized subsalt hydrocarbon
exploration. Although this technique has been around for decades, it was
rarely used because it requires powerful computer and sophisticated
software. Recent subsalt discoveries and technological advances, however,
have made depth migration economically feasible.
In the eighties (1980's), nearly all
seismic reflection data in the Gulf of Mexico were acquired as
two-dimensional (2-D) data. This technique provides geoscientists with an
image resembling a vertical slice through the earth. Since the early
nineties, most seismic data are acquired as three-dimensional (3-D),
providing an image resembling a cube cut from the earth. The 3-D cube
provides a dense grid of data, and the data are more accurately positioned
near salt bodies.
Conventional 3D time migration of
seismic reflection data is a computer processing technique that is usually
adequate for imaging geologic features in the
Gulf of Mexico.
The basic assumption of time migration is that the acoustic properties of
the subsurface layers do not have abrupt lateral variations. This
assumption breaks down near salt bodies because acoustic waves travel much
faster through salt than through surrounding sediments. Because of these
velocity variations, conventional time migration is very poor in correctly
positioning, or even imaging, subsalt seismic events. Depth migration
takes into account vertical and lateral velocity variations in the
subsurface, creating more accurate and improved subsalt imaging. Depth
migration can be performed either before or after summing (stacking) the
seismic offset traces.
In the recent
years, companies have undertaken a major effort to enhance the seismic
imaging of salt bodies. Extensive sets of speculative 3-D seismic data
covering most of the Gulf of Mexico area with subsalt potential were
acquired and processed in the 1990’s. Exploration companies could now
cost-effectively buy and manipulate these 3-D data sets to improve their
ability to define prospects below the salt bodies.
