Geology topics

Ann Cook

Publications and source records attributed to Ann Cook.

4 recordsLinked to original sources

USGS2022

Integrated geochemical approach to determine the source of methane in gas hydrate from Green Canyon Block 955 in the Gulf of Mexico

Massive volumes of gas are sequestered within gas hydrate in subsurface marine sediments in the Gulf of Mexico. Methane associated with gas hydrate is a potentially important economic resource and a significant reservoir of carbon within the global carbon cycle. Nevertheless, uncertainties remain about the genetic source (e.g., microbial, thermogenic) and possible migration history of natural gas incorporated into hydrate. Previous studies have primarily used the hydrocarbon molecular (CH 4 /C 2 H 6 +) and isotopic (δ 13 C-CH 4 , δ 2 H-CH 4 ) compositions of natural gas to address these uncertainties. However, hydrocarbon tracers are altered by mixing, oxidation, secondary methanogenesis, or fluid migration, which presents challenges when deciphering the mechanisms responsible for methane formation and accumulation. To evaluate the genetic source of natural gases from Green Canyon Block 955 (GC 955), east of the Sigsbee escarpment, we collected and analyzed samples from the first pressurized hydrate-bearing sediment cores collected from a coarse-grained hydrate reservoir in the Gulf of Mexico. Gas samples were analyzed for hydrocarbon gas (C 1 –C 5 ), major gas (e.g., N 2 , CO 2 ), and noble gas (He-Xe) abundance and isotopic (e.g., δ 13 C-CH 4 , δ 2 H-CH 4 , δ 13 C-CO 2 , δ 15 N-N 2 , 3 He/ 4 He, 4 He/ 20 Ne) compositions. We determined that natural gas in hydrates from this location are predominantly of primary microbial origin (conservatively at least 76%) and are formed by the hydrogenotrophic (CO 2 reduction) methanogenesis pathway. We also note increased thermogenic proportions (∼6%) in a hydrate-bearing layer below the main hydrate-bearing interval (separated by a 5-m water-bearing layer). Our results suggest that microbial methane may be abundant below the base of gas hydrate stability at GC 955.

Louisiana↗

USGS2020

Pressure coring operations during The University of Texas-Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition in Green Canyon Block 955, northern Gulf of Mexico

In May 2017, The University of Texas Hydrate Pressure Coring Expedition Gulf of Mexico 2-1 (UT-GOM2-1) drilled two adjacent holes in Green Canyon Block 955 in the deep-water Gulf of Mexico as part of The University of Texas at Austin and US Department of Energy Deepwater Methane Hydrate Characterization and Scientific Assessment. Expedition operations included testing two configurations of a rotary pressure-coring tool in a gas hydrate–bearing formation. In the first hole, an extended core barrel (cutting shoe) configuration of the Pressure Coring Tool with Ball Valve (PCTB-CS) was deployed, and in the second hole, the PCTB face bit configuration (PCTB-FB) was deployed. The PCTB-CS successfully recovered and maintained pressure for only one core out of eight deployments. A series of incremental modifications were made during and after the PCTB-CS deployment period that impacted the operations of the subsequent PCTB-FB deployments. Thus, in the second hole, the PCTB-FB successfully recovered and maintained pressure within the hydrate stability zone for 11 cores out of 13 deployments. The PCTB cored gas hydrate–bearing sandy silt interbedded with non–hydrate-bearing clayey silt within the main reservoir. The PCTB also recovered long intervals of unbroken, high-quality core with preserved sedimentary structures. We recovered one pressure core 130 m (437 ft) above the main hydrate reservoir in the silty clay. Pressure coring is the only available technology for recovering intact cores from sediment that is normally disturbed by gas expansion, dissolution, or dissociation; this allows a wide range of scientific measurements to be obtained with minimal disturbance to the core sediment fabric. Analysis of pressure cores has the potential to illuminate the in situ properties, gas saturation, and gas composition of a wide range of reservoirs including unconventional shale systems.

AAPG Bulletin↗

USGS2020

Pressure coring a Gulf of Mexico deep-water turbidite gas hydrate reservoir: Initial results from The University of Texas–Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition

The University of Texas Hydrate Pressure Coring Expedition (UT-GOM2-1) recovered cores at near in situ formation pressures from a gas hydrate reservoir composed of sandy silt and clayey silt beds in Green Canyon Block 955 in the deep-water Gulf of Mexico. The expedition results are synthesized and linked to other detailed analyses presented in this volume. Millimeter- to meter-scale beds of sandy silt and clayey silt are interbedded on the levee of a turbidite channel. The hydrate saturation (the volume fraction of the pore space occupied by hydrate) in the sandy silts ranges from 79% to 93%, and there is little to no hydrate in the clayey silt. Gas from the hydrates is composed of nearly pure methane (99.99%) with less than 400 ppm of ethane or heavier hydrocarbons. The δ 13 C values from the methane are depleted (−60‰ to −65‰ Vienna Peedee belemnite), and it is interpreted that the gases were largely generated by primary microbial methanogenesis but that low concentrations of propane or heavier hydrocarbons record at least trace thermogenic components. The in situ pore-water salinity is very close to that of seawater. This suggests that the excess salinity generated during hydrate formation diffused away because the hydrate formed slowly or because it formed long ago. Because the sandy silt deposits have high hydrate concentration and high intrinsic permeability, they may represent a class of reservoir that can be economically developed. Results from this expedition will inform a new generation of reservoir simulation models that will illuminate how these reservoirs might be best produced.

Texas, Louisiana↗

USGS2019

Clustered BSRs: Evidence for gas hydrate-bearing turbidite complexes in folded regions, example from the Perdido Fold Belt, northern Gulf of Mexico

We describe previously undocumented but extensive gas hydrate accumulations in the mouth of Perdido Canyon in the northern Gulf of Mexico. The accumulations are located within central parts of structural domes (four-way closures) and are characterized by stacked, high-amplitude bottom simulating reflections (BSRs) that we call clustered BSRs. Seismic data from Perdido Canyon show two clustered BSRs associated with turbidite sequences within two dome folds formed from tectonic folding and salt diapir rise. The northwestern (NW) and southeastern (SE) clustered BSRs have aerial extents of ~25 km2 and 50 km2, respectively. Well log data confirm gas hydrate occurs above the NW clustered BSR, within a 225 m-thick consistently high-resistivity interval that we interpret as gas hydrate in near-vertical fractures and turbidite sands. The SE dome is only drilled at the edge of the BSR; nevertheless, the well log data indicate that a 30 m-thick gas hydrate accumulation is present. Gas chromatographic logs in both domes suggest a gradual transition from predominantly microbial gas below the BSR (500–1000 meters below seafloor (mbsf)) to thermogenic gas at 1000–2000 mbsf. Based on the well log data and seismic stratigraphic analysis, we find gas hydrate is concentrated in fractures in marine mud, as well as in the pores of submarine fan turbidities, where saturations reach as high as 75%. An estimate of the total gas hydrate-bound gas volume at standard temperature and pressure is between 0.04 and 0.17 trillion cubic meters (TCM) assuming average hydrate saturation of 5-20% in a ~45 m thick turbidite sand unit above the Perdido Canyon BSR area. Measured BSR extent and gas volume estimates indicate that the NW and SE reservoirs are among the largest gas hydrate occurrences known in the Gulf of Mexico.

Perdido Fold Belt↗