Liu, X., and P. B. Flemings, Dynamic response of oceanic hydrates to sea level drop, Geophys. Res. Lett., 36, L17308, 2009, doi:10.1029/2009GL039821, 
During sea-level drop, water and gas pressures within oceanic hydrate systems can exceed the total vertical stress and this can drive slope failure and gas venting. We investigate this behavior with a multi-phase fluid and heat flow model. During sea-level drop, fluid pressures drop much less than the total stress due to both the high gas compressibility and hydrate dissociation. In permeable sediments, hydrate dissociation, water expulsion and gas mobility combine to induce underpressure and downward water flow from the seafloor. This study provides a causal mechanism for slope failure and fluid exchange that occur in hydrate systems during sea-level fall.
Moore, J. C., G. J. Iturrino, P. B. Flemings, and D. E. Sawyer, Data report: Stress orientations from borehole breakouts, IODP Expedition 308, Ursa area, Mississippi Fan, Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2009, doi:10.2204/iodp.proc.308.212.2009,
Borehole failures are a conspicuous feature of the logging-while-drilling resistivity images at Integrated Ocean Drilling Program Sites U1322 and U1324. Failures appear as irregular zones of low resistivity on opposite sides of the well bore (resembling traditional breakouts) and also as zones of high resistivity flanked by narrower low-resistivity intervals. The failures show a consistent east–west trend at both sites and with depth in each. The inferred SHmin directions are 85°–265° and 91°–271°, respectively, at Sites U1322 and U1324. SHmax at Sites U1322 and U1324 is oriented subparallel to the overall southerly slope of the Gulf of Mexico slope in this region. At Site U1322, SHmin is perpendicular to en echelon extensional fractures along the margin of a submarine landslide.
Nelson, H. M., P. B. Flemings, J. T. Germaine, and B. E. Dugan, Data report: Radiography and X-ray CT imaging of whole core from IODP Expedition 308, Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2009, doi:10.2204/iodp.proc.308.213.2009,
We completed 151 radiographs and 12 X-ray computed tomography (CT) scans from whole cores taken during Integrated Ocean Drilling Program (IODP) Expedition 308. In Brazos-Trinity Basin IV, 29 radiographs and 1 X-ray CT scan were taken at Sites U1319 and U1320. In Ursa Basin, 122 radiographs and 11 X-ray CT scans were taken at Sites U1322 and U1324. These radiographs and X-ray CT scans were completed to select undisturbed portions of the core for experiments and to qualitatively assess the presence of inclusions and variation in the whole-core soil samples from the expedition. The imaging was performed at three locations: Pennsylvania State University, Massachusetts Institute of Technology, and Rice University.
Sawyer, D. E., P. B. Flemings, B. E. Dugan, and J. T. Germaine, Retrogressive failures recorded in mass transport deposits in the Ursa Basin, Northern Gulf of Mexico, J. Geophys. Res., 114, B10102, 2009, doi:10.1029/2008JB006159,
Clay-rich mass transport deposits (MTDs) in the Ursa Basin, Gulf of Mexico, record failures that mobilized along extensional failure planes and transformed into long runout flows. Failure proceeded retrogressively: scarp formation unloaded adjacent sediment causing extensional failure that drove successive scarp formation updip. This model is developed from three-dimensional seismic reflection data, core and log data from Integrated Ocean Drilling Project (IODP) Expedition 308, and triaxial shear experiments. MTDs are imaged seismically as low-amplitude zones above continuous, grooved, high-amplitude basal reflections and are characterized by two seismic facies. A Chaotic facies typifies the downdip interior, and a Discontinuous Stratified facies typifies the headwalls/sidewalls. The Chaotic facies contains discontinuous, high-amplitude reflections that correspond to flow-like features in amplitude maps: it has higher bulk density, resistivity, and shear strength, than bounding sediment. In contrast, the Discontinuous Stratified facies contains relatively dim reflections that abut against intact pinnacles of parallel-stratified reflections: it has only slightly higher bulk density, resistivity, and shear strength than bounding sediment, and deformation is limited. In both facies, densification is greatest at the base, resulting in a strong basal reflection. Undrained shear tests document strain weakening (sensitivity = 3). We estimate that failure at 30 meters below seafloor will occur when overpressure = 70% of the hydrostatic effective stress: under these conditions soil will liquefy and result in long runout flows.
Schneider, J., P. B. Flemings, B. E. Dugan, H. Long, and J. T. Germaine, Overpressure and consolidation near the seafloor of Brazos-Trinity Basin IV, northwest deepwater Gulf of Mexico, J. Geophys. Res., 114, B05102, 2009, doi:10.1029/2008JB005922,
Pore water overpressures (u*) within mudstones beneath Brazos-Trinity Basin IV (deepwater Gulf of Mexico, offshore Texas) are greater than 70% of the hydrostatic vertical effective stress (σ′ vh ) [λ* = 0.7 = (u*/σ′ vh )]. These results are compatible with recent observations that suggest sedimentation rates in this region are rapid (6 mm/a). We compare the petrophysical properties and pore pressures within a 127-m-thick package of mudstone penetrated at two locations: Integrated Ocean Drilling Program (IODP) sites U1319 and U1320. Site U1319 is at the margin of Brazos-Trinity Basin IV, whereas Site U1320 lies at its center, beneath 180 m of turbidite fill. Experimentally derived preconsolidation stresses and an in situ pore pressure measurement record overpressure at Site U1319 and Site U1320 (λ* ∼ 0.2 to 0.8 and λ* ∼ 0.8, respectively). We use these data to define an average vertical effective stress gradient. Assuming that void ratio (e) is proportional to the log of vertical effective stress (σ′ v ), we predict pore pressures (u) throughout the mudstone at both sites using bulk density data. Overpressures are greater at Site U1320 due to rapid deposition of the overlying turbidites. However, a large fraction of the overpressure induced by the turbidite load applied at Site U1320 has dissipated by drainage into the overlying basin fill. High overpressures near the seafloor drive shallow fluid flow, reduce slope stability, and may explain large submarine landslides.
Flemings, P. B., H. Long, B. E. Dugan, J. T. Germaine, C. John, J. H. Behrmann, D. E. Sawyer, and the IODP Expedition 308 Scientists, Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Lousiana, Gulf of Mexico, Earth Planet. Sci. Lett., 269, 309-325, 2008, doi:10.1016/j.epsl.2007.12.005,
Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70% and 60% of the hydrostatic effective stress () in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone, and sandstone. The penetrometer measurements did not reach the in situ pressure at the end of the deployment. We used a soil model to determine that an extrapolation approach based on the inverse of square route of time () requires much less decay time to achieve a desirable accuracy than an inverse time (1/t) extrapolation. Expedition 308 examined how rapid and asymmetric sedimentation above a permeable aquifer drives lateral fluid flow, extreme pore pressures, and submarine landslides. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. Reduced overpressure at depth at Site U1324 suggests lateral flow (drainage) whereas high overpressure at Site U1322 requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2°) offshore from the Mississippi delta.
Flemings, P. B., H. Long, B. E. Dugan, J. T. Germaine, C. John, J. H. Behrmann, D. E. Sawyer, and the IODP Expedition 308 Scientists, Erratum to 'Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Lousiana, Gulf of Mexico', Earth Planet. Sci. Lett., 274, 269-283, 2008, doi:10.1016/j.epsl.2008.06.027,
The publisher regrets that when the above article was printed, there were a series of errors in the text. The full correct article is printed on the following pages. We apologize for any inconvenience this may have caused. The Publisher. Available online 3 August 2008.
Gilhooly, W. P., S. A. Macko, and P. B. Flemings, Data report: Isotope compositions of sedimentary organic carbon and total nitrogen from Ursa basin (Sites U1322 dnU1324) and Brazos-Trinity basin #4 (Sites U1319 and U1320), deepwater Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.208.2008,
Organic carbon and total nitrogen stable isotopes are reported for sediments drilled during Integrated Ocean Drilling Program Expedition 308. Brazos-Trinity Basin IV sediments exhibited a broad range in organic carbon 13C ranging between –27 and –20 (13C average = –24.1) compared to Ursa Basin sediments that are generally more depleted in 13C (13C average = –25.7). Bulk 15N values across all basins ranged from –2.7 to 8.2 (15N average = 3.7) and showed no obvious trend. The relative contribution of marine and terrestrial detrital material deposited within these sediments was inferred by comparing isotopic compositions to C/N values. The significant contribution of inorganic nitrogen (Nbound average 75%), as estimated from total organic carbon/total nitrogen plots, likely lowered the observed C/N values.
Long, H., P. B. Flemings, J. T. Germaine, D. M. Saffer, and B. E. Dugan, Data report: Consolidation characteristics of sediments from IODP Expedition 308, Ursa Basin, Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.203.2008,
We conducted temperature and pore pressure measurements using the Davis-Villinger Temperature-Pressure Probe and the temperature/dual pressure probe penetrometers during Integrated Ocean Drilling Program Expedition 308. In Ursa Basin, 18 measurements were used to determine that the geothermal gradient at Site U1324 is bilinear. The temperature gradient is 18.6°C/km in lithostratigraphic Unit I and 16.7°C/km in Unit II. Based on nine measurements at Site U1322, the geothermal gradient is 21.9°C/km. In Brazos-Trinity Basin IV, the geothermal gradient at Site U1320 is 23.1°C/km. In Ursa Basin, significant overpressures (overpressure ratio = ~0.7) are observed in the sediments above ~200 meters below seafloor (mbsf) at Sites U1322 and U1324. At Site U1324, pore pressure decreases with increasing depth between 200 and 300 mbsf. Below 300 mbsf and within lithostratigraphic Unit II, overpressure is approximately constant (~1 MPa). Unit II is composed of silty claystone interbedded with beds of silt and very fine sand. In Brazos-Trinity Basin IV, only two penetrometer deployments were made and the data are inconclusive.
Long, H., P. B. Flemings, B. E. Dugan, J. T. Germaine, and D. Ferrell, Data report: Penetrometer measurements of in situ temperature and pressure, IODP Expedition 308, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.203.2008,
We conducted temperature and pore pressure measurements using the Davis-Villinger Temperature-Pressure Probe and the temperature/dual pressure probe penetrometers during Integrated Ocean Drilling Program Expedition 308. In Ursa Basin, 18 measurements were used to determine that the geothermal gradient at Site U1324 is bilinear. The temperature gradient is 18.6°C/km in lithostratigraphic Unit I and 16.7°C/km in Unit II. Based on nine measurements at Site U1322, the geothermal gradient is 21.9°C/km. In Brazos-Trinity Basin IV, the geothermal gradient at Site U1320 is 23.1°C/km. In Ursa Basin, significant overpressures (overpressure ratio = ~0.7) are observed in the sediments above ~200 meters below seafloor (mbsf) at Sites U1322 and U1324. At Site U1324, pore pressure decreases with increasing depth between 200 and 300 mbsf. Below 300 mbsf and within lithostratigraphic Unit II, overpressure is approximately constant (~1 MPa). Unit II is composed of silty claystone interbedded with beds of silt and very fine sand. In Brazos-Trinity Basin IV, only two penetrometer deployments were made and the data are inconclusive.
Sawyer, D. E., R. Jacoby, P. B. Flemings, and J. T. Germaine, Data report: Particle size analysis of sediments in the Ursa Basin, IODP Expedition 308 Site U1324 and U1322, northern Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.205.2008,
We conducted particle size analyses on 340 samples from Integrated Ocean Drilling Program Expedition 308 Sites U1324 (246 samples) and U1322 (94 samples) in the Ursa Basin (Gulf of Mexico) and found two characteristic lithologies: silty clay and clayey silt. Silty clays are composed of ~60% (±10%) (by weight, ~40% silt-sized particles by weight, and <1% sand-sized particles by weight. Clayey silts are generally composed of ~30% clay-sized particles by weight, 65%–70% silt-sized particles by weight, and 0%–5% sand-sized particles by weight. Site U1322 is dominated by silty clays with little particle size variation throughout the cored interval (0–235 meters below seafloor [mbsf]). At Site U1324, both lithologies occur where the lowermost section (~360–608 mbsf) is dominated by clayey silt and the uppermost section (0–360 mbsf) is dominated by silty clay.
Sawyer, A. H., P. B. Flemings, D. Elsworth, and M. Kinoshita, Response of submarine hydrologic monitoring instruments to formation pressure changes: Theory and application to Nankai advanced CORKs, J. Geophys. Res., 113, B01102, 2008, doi:10.1029/2007JB005132,
We describe the response of a compressible submarine hydrologic monitoring instrument to formation pressure changes in low-diffusivity rock. The measured pressure depends on the frequency of the pressure signal, the hydraulic diffusivity, and the wellbore storage. The Nankai advanced circulation obviation retrofit kits (ACORKs) (offshore Japan) record tide-induced formation pressure changes with small amplitudes (<10% of seafloor amplitudes) and large phase shifts (>25°). The pressure measurements occur in thick, homogeneous, compressible, low-permeability sediment, where in situ tidal pressure responses should approximate the seafloor tidal signal. A wellbore storage of 2 × 10−8 m3 Pa−1 can explain many of the observed tidal responses, given the hydraulic diffusivities of the monitored intervals. A reduced permeability around the wellbore of 1000-fold and a wellbore storage of 10−11 m3 Pa−1 can also reconcile the data. Our analysis suggests that ACORK screens in the Lower Shikoku Basin facies have a critical frequency on the order of 5 × 10−8 Hz (equivalent to a period of 250 days); higher-frequency formation pressure signals will be distorted in the pressure record. Within the Lower Shikoku Basin facies the time for this monitoring system to record 90% of an instantaneous pressure change is on the order of 10 d. We suggest that the ACORK instrument compliance contributes to, but does not fully explain, the small tidal amplitudes and large phase shifts recorded at the least permeable monitoring intervals.
Liu, X., and P. B. Flemings, Dynamic multiphase flow model of hydrate formation in marine sediments, J. Geophys. Res., 112, B03101, 2007, doi:10.1029/2005JB004227,
We developed a multicomponent, multiphase, fluid and heat flow model to describe hydrate formation in marine sediments; the one- and two-dimensional model accounts for the dynamic effects of hydrate formation on salinity, temperature, pressure, and hydraulic properties. Free gas supplied from depth forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation: Salinity and hydrate concentration increase upward from the base of the regional hydrate stability zone (RHSZ) to the seafloor, and the base of the hydrate stability zone has significant topography. In fine-grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing to free gas. This traps gas and causes gas pressure to build up until it exceeds the overburden stress and drives gas through the RHSZ. Gas chimneys couple the free gas zone to the seafloor through high-salinity conduits that are maintained at the three-phase boundary by gas flow. As a result, significant amounts of gaseous methane can bypass the RHSZ, which implies a significantly smaller hydrate reservoir than previously envisioned. Hydrate within gas chimneys lies at the three-phase boundary, and thus small increases in temperature or decreases in pressure can immediately transport methane into the ocean. This type of hydrate deposit may be the most economical for producing energy because it has very high methane concentrations (S h > 70%), located near the seafloor, which lie on the three-phase boundary.
Long, H., P. B. Flemings, and J. T. Germaine, Interpreting in situ pressure and hydraulic properties with borehole penetrometers in ocean drilling: DVTPP and Piezoprobe deployments at southern Hydrate Ridge, offshore Oregon, J. Geophys. Res., 112, B04101, 2007, doi:10.1029/2005JB004165,
Two borehole penetrometers, Fugro-McClelland's Piezoprobe and the Ocean Drilling Program's (ODP) DVTPP, were deployed 50 m below seafloor at Site 1244 on ODP Leg 204 to measure formation pressure at southern Hydrate Ridge, offshore Oregon. Pore pressure is interpreted to be hydrostatic and the sediment's coefficient of consolidation is interpreted to lie between 6.92 and 7.8 × 10−7 m2/s, which is in approximate agreement with laboratory measurements. The Piezoprobe pressure reaches 90% of dissipation 14 times sooner than the DVTPP. The observed and modeled pore pressure responses illustrate how penetrometer geometry impacts our ability to interpret in situ properties and demonstrate under what conditions these tools can be effectively used. Because of its narrow tip, the Piezoprobe disturbs a narrower zone than the DVTPP does. This generates a narrower zone of pressure increase around the piezoprobe, which dissipates much faster than the DVTPP. As consolidation proceeds, pressure dissipation of the Piezoprobe is retarded and forms a “bench,” or flat spot, on the dissipation curve. Owing to its distinct two-radius geometry, it is possible to apply a consistent method to estimate in situ pressure from partial dissipation record based on the position of the “bench.”
Sawyer, D. E., P. B. Flemings, R. C. Shipp, and C. D. Winker, Seismic geomorphology, lithology, and evolution of the Late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico, AAPG Bull., 91, 215-234, 2007, doi:10.1306/08290605190,
The interplay between sedimentation and erosion during the late Pleistocene in the Mars-Ursa region, northern Gulf of Mexico, resulted in a complex compartmentalized reservoir. Rapid deposition, directly downdip of the Mississippi River beginning about 70 k.y., quickly filled antecedent topography in the Mars-Ursa region with a thick accumulation of sand and mud called the blue unit. This permeable reservoir was rapidly and asymmetrically buried by thick, mud-rich levees of two channel-levee systems. Both systems plunged from north to south with a steeper gradient than the underlying blue unit. Rotated channel-margin slides present in both channel-levee systems rotated low-permeability, mud-rich levee deposits beneath the sand-rich channel fill. As a result of the channel-levee systems, the east-west hydraulic connectivity of the blue unit decreases progressively from north to south until its eastern and western halves become completely separated.
Trehu, A. M., P. B. Flemings, N. L. Bangs, J. Chevallier, E. Gracia, J. E. Johnson, C.-S. Liu, X. Liu, M. Riedel, and M. E. Torres, Feeding methane vents and gas hydrate deposits at south Hydrate ridge, Geophys. Res. Lett., 31, L23310, 2004, doi:10.1029/2004GL021286,
Log and core data document gas saturations as high as 90% in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas-saturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.
Mikada, H., M. Kinoshita, K. Becker, E. E. Davis, R. D. Meldrum, P. B. Flemings, S. P. S. Gulick, O. Matsubayashi, S. Morita, S. Goto, N. Misawa, K. Fujino, and M. Toizumi, Hydrogeological and geothermal studies around Nankai trough (KR02-10 Nankai Trough cruise report), JAMSTEC J. Deep Sea Res., 22, 125-171, 2003
