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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: The Discovery of Rich Gas Hydrate Accumulations in
Sand Reservoirs in the Gulf of Mexico – Results from
DOE-Chevron Joint Industry Project Drilling
1AOA Geophysics
2U.S. Department of Energy
3U.S. Geological Survey
4Minerals Management Service
5Columbia University
6Schlumberger
7Chevron
In April and May of 2009 the Gulf of Mexico Gas Hydrate Joint
Industry Project realized its second field program (Leg II) with
the semi-submersible Helix Q4000 drillship. The three-week,
$11.5MM expedition drilled seven logging-while-drilling (LWD)
holes at three sites to test a variety of geologic/geophysical models
for the occurrence of gas hydrate in sand reservoirs in the deepwater
Gulf of Mexico. Over 17,000 ft of sedimentary section were
logged using a state-of-the-art bottom-hole assembly. The three
sites drilled were Walker Ridge (WR) Block
313, Green Canyon
(GC)
Block
955, andAlaminos Canyon (AC)
Block
21. The program
was completed on-time and under budget. The locations for
JIP Leg II drilling were the result of an integrated geological and
geophysical prospecting approach that considered direct
geophysical evidence for gas hydrate-bearing strata in the context
of evaluation of indicators for gas sourcing, gasmigration pathways
to the shallow section, and occurrence of sand reservoirs within
the gas hydrate stability zone. High saturation gas hydrate
deposits in sands were found, where predicted, in four of five
holes at two sites, WR 313 and GC 955. The third site, AC 21,
indicated low to moderate gas hydrate saturation in extensive
shallow sands. The full research-level LWD assembly deployed for
Leg II collected gamma-ray, neutron and density porosity,
neutron spectroscopy data, as well as full azimuthal resistivity
and acoustic velocity, including both compressional and
shear-wave measurements.
Leg II was clearly a high-risk proposition, despite the drilling of a
large number of industry wells in the deepwater Gulf of Mexico,
there had been only one prior instance (at Alaminos Canyon
Block
818) in which gas hydrate had been reported to occur in
sand. Nonetheless, the potential for gas hydrate at high saturation
in sands was large; an assessment conducted under the leadership
of the Minerals Management
Service produced a mean
estimate of 6,700 tcf gas-inplace
in gas hydrate-bearing
sands in the deepwater Gulf of
Mexico. Perhaps the primary
scientific objective of the Leg
II program was to provide
ground truth data to test the
soundness of the prospecting
techniques developed with the
JIPs site selection team. One
part of this approach was predrill
inversions of seismic data
to estimate areal variations in
gas hydrate saturation.
The two wells in WR 313, in the Terrebonne Basin, confirmed the pre-drill models. The main gas hydrate target s were approximately 2,700 ft below the seafloor. The first well, WR 313 G, had a predrill prediction of 57% gas hydrate in the target sand. LWD indicated a net of ~30 ft of sand containing gas hydrate with a saturation of 70% at the target horizon. The second well approximately 0.6 miles east and updip, WR 313 H, tested a similar but stratigraphically deeper target. The pre-drill saturation was 53% at the primary target. Results show two lobes of very clean sand with over 90% gas hydrate saturation in the upper lobe. Saturations in the lower lobe ranged from 50% to 60%. In addition, both holes revealed a shallow unit with 350-500 ft of grain-displacing fracture filling gas hydrate in clays beginning approximately 600 ft below the sea floor.
Three holes were drilled in GC Block
955 just outboard of
the Sigsbee Escarpment, where a wide and thick late
Pleistocene channel complex has been fractured and uplifted by
a shallow salt stock. A highly faulted four-way closure with
numerous amplitude anomalies at the base of gas hydrate
stability is near to but west of the channel axis. The first well GC
955 I was closest to an industry well that penetrated thick sands.
As expected, the “I” well encountered a thick sand section but the
sands contained primarily water with only a few feet of potential
gas hydrate.
The next two wells, GC 955 H and GC 955 Q, targeted the sand at the four-way closure approximately 0.7 miles proximal to the youngest well-preserved channel axis in the depositional sequence. The LWD data obtained at this location indicate over 100 ft of gas-hydrate-bearing zones within a single sand-rich unit with saturations estimated to be over 70%. This accumulation is overlain and underlain by, and most surprisingly, interbedded with gas-hydratefree, water-bearing sands. In addition to the gas hydrate in the target sand, fracture fill gas hydrate was detected in the clay-prone section above the target. GC 955 Q is believed to have encountered at least 50 ft of highly saturated gas hydrate sand at the target, but drilling was aborted because of a potential gas hydrate dissociation event and subsequent gas flow. The LWD data, however, show complex acoustic responses and are still being analyzed before a confident interpretation of the pore fill in the drilled interval can be offered. However, it appears that gas hydrate occurrence, at the GC 955 site is highly complex, both in the sands and in the overlying clays, and is potentially complicated by fault-controlled compartmentalization and related lateral variations in gas delivery, thermal gradients, pore-water salinities, and other phenomena.
The two wells drilled in Alaminos Canyon Block
21 (AC21), in
the vicinity of the Diana Field development, confirmed the predrill
prediction of potential extensive occurrence of gas hydrates
in shallow sand reservoirs at relatively low (
The expedition demonstrated the ability to reasonably predict
gas hydrate occurrence through seismic data in the absence of
pre-drill well data. At WR313, the model linking aligned phase
reversals at multiple levels with gas-hydrate bearing sands at the
base of gas hydrate stability (BGHS) was confirmed.
Furthermore, initial results suggest that gas hydrate has the
potential to fully saturate reservoirs well above the BGHS, with
the primary control being occurrence
of reservoir quality facies. In addition,
unexpected findings, such as the complex
nature of the gas hydrate occurrence at
GC Block
955 and the discovery of the
extensive, strata-bound shallow hydrate
occurrence at WR
Block
313, raise exciting
new questions.
The DOE and the JIP are committed to making these data publically available as soon as possible to support a wide range of scientific studies. The initial reports will be published shortly at http://www.netl.doe.gov/ MethaneHydrates/JIPLegII-IR/
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