Alaminos Canyon Research Articles

A Miocene Nannofossil Biostratigraphic Case Study: Alaminos Canyon Block 627 and Mississippi Canyon Block 555, and Sedimentation Rates in the Gulf of Mexico

The Miocene sediments of 2 deep—water boreholes from the northern Gulf of Mexico, from Alaminos Canyon (AC) Block 627 and Mississippi Canyon (MC) Block 555, have been biostratigraphically analyzed using calcareous nannofossils, revealing changes in sedimentation rates and depositional environments between these 2 areas. High nannofossil abundance values and low sedimentation rates generally recorded in the Alaminos Canyon region suggest a condensed section during much of the Miocene, associated with a basinal environment. Mississippi Canyon exhibits lower nannofossil abundance and higher sedimentation rates compared to Alaminos Canyon during the majority of the Miocene. Increased sediment volumes are largely attributed to input from the Mississippi River. Sedimentation rate was calculated for both sites. In the AC Block 627 borehole, sedimentation rate ranged from 13 to 107 m/million years (my), and in the MC Block 555 borehole, it varied from 11 to 914 m/my. One major anomaly was a low observed sedimentation rate in the lowest portion of the section at MC Block 555, in the interval defined between the extinctions of Triquetrorhabdulus carinatus and Dictyococcites bisectus. This may reflect a hiatus or possible fault which has shortened the section.

E&P Notes (February 2022)

E&P Notes (February 2022)

Shear wave velocity prediction based on LSTM and its application for morphology identification and saturation inversion of gas hydrate

Compressional wave velocity (Vp) and shear wave velocity (Vs) are important geophysical parameters for studying gas hydrate systems. However, the Vs parameter is not commonly used in logging-while-drilling because of the high costs of acquiring it. To address this issue, we propose a long short-term memory (LSTM) neural network for predicting a complete shear wave velocity curve at wells in Alaminos Canyon Block 21 (AC 21) based on the limited measurement of Vs logs. The LSTM method performs better than the LSF method when comparing the Vs curves generated by them with the measured one. The measured Vp and predicted Vs curves were applied to determine the distribution of hydrates using the ratio of Vp increment to Vs increment, and we projected velocity samples into the cross plot of modelled Vp and Vs by assuming pore- and fracture-filling hydrate reservoirs to identify the hydrate morphologies. The pore- and fracture-filling gas-hydrate-bearing reservoirs were proved at 541–633 feet below sea floor (fbsf) and 1200–1500 fbsf in hole AC 21-A, respectively. However, the gas hydrate saturations estimated using the joint usage of Vp and Vs at shallow and deep depths are less than 10% (at most 30% locally) and 10%–20%, respectively. Because less Vs data is measured, a transfer learning technique is introduced to estimate a Vs curve at well AC 21-B. Comparing velocity samples with the theoretical velocity curves, the morphology of the hydrate in well AC 21-B is observed to be a horizontal fracture-filling type, and its gas hydrate saturation is approximately 10%–20% at the depth intervals of hydrate occurrence. According to our numerical calculations, the hydrate occurrence at well AC 21-B shows a better continuity than in well AC 21-A. • A LSTM model is developed to predict a complete VS curve in JIP wells. • Morphologies and distributions of gas hydrates are determined. • Gas hydrate saturation is estimated based on the jointly usage of Vp and Vs data. • transfer learning to predict a Vs curve and further analyse gas hydrates.

E&P Notes (July 2021)

E&P Notes (July 2021)

Microbial diversity and activity in seafloor brine lake sediments (Alaminos Canyon block 601, Gulf of Mexico).

The microbial communities thriving in deep-sea brines are sustained largely by energy rich substrates supplied through active seepage. Geochemical, microbial activity, and microbial community composition data from different habitats at a Gulf of Mexico brine lake in Alaminos Canyon revealed habitat-linked variability in geochemistry that in turn drove patterns in microbial community composition and activity. The bottom of the brine lake was the most geochemically extreme (highest salinity and nutrient concentrations) habitat and its microbial community exhibited the highest diversity and richness indices. The habitat at the upper halocline of the lake hosted the highest rates of sulfate reduction and methane oxidation, and the largest inventories of dissolved inorganic carbon, particulate organic carbon, and hydrogen sulfide. Statistical analyses indicated a significant positive correlation between the bacterial and archaeal diversity in the bottom brine sample and NH4+ inventories. Other environmental factors with positive correlation with microbial diversity indices were DOC, H2 S, and DIC concentrations. The geochemical regime of different sites within this deep seafloor extreme environment exerts a clear selective force on microbial communities and on patterns of microbial activity.

Upper Jurassic Tithonian-centered source mapping in the deepwater northern Gulf of Mexico

Long the subject of speculation, the origin, distribution, and quality of Mesozoic source beds in the deepwater Gulf of Mexico (GOM) are now open to analytical study and hypothesis. We have developed new maps and concepts for organic richness and lithofacies patterns of the primary Upper Jurassic oil-prone source rock interval spanning the Kimmeridgian to Lower Berriasian in the northern GOM. This interval, previously referred to as the Tithonian-centered source, includes the Haynesville and Bossier shales, which lie within supersequences representing second-order transgressive and high-stand systems tracts, respectively. A newly developed gulf-wide Cotton Valley-Bossier paleogeographic map based on a novel paleotectonic model for the Mesozoic provides the framework for this source mapping study. Organic richness averages up to 4.7% and 6.5% total organic carbon for the Kimmeridgian and Tithonian-Lower Berriasian supersequences, respectively, based on the log overlay [Formula: see text] technique and increases toward the basin center. Lithofacies-sensitive geochemical parameters from reservoired oils and oil seeps tied to the Tithonian-centered oil family demonstrate several potential entry locations for siliciclastic sediments into the Tithonian distal calcareous environment. The region from Garden Banks to Alaminos Canyon appears to be the locus of siliciclastic mixing with carbonates in the deepwater and link with updip evidence of a wide progradational clastic apron sourced by the paleo-Mississippi river. This siliciclastic input may contribute to the potential for improved oil quality by lowering the content of sulfur incorporated into kerogen moving southwest from Ewing Bank-Mississippi Canyon to Alaminos Canyon. Ultimately, enhanced Upper Jurassic source deposition ended with a ventilation event whereby more oxygenated, deeper water entered the northern GOM. Declines in calculated total organic carbon occurring later to the west during the earliest Cretaceous (Berriasian) indicate the opening of the gateway between the GOM and the central Atlantic possibly related to the termination of seafloor spreading in the eastern GOM.

Integration of seismic and gravity data — A case study from the western Gulf of Mexico

A 3D gravity model was developed in the western Gulf of Mexico in the East Breaks and Alaminos Canyon protraction areas. This model integrated 3D seismic, gravity, and well data; it was constructed in support of a proprietary seismic reprocessing project and was updated iteratively with seismic. The gravity model was built from seismic horizons of the bathymetry, salt layers, and the acoustic basement; however, the latter was only possible to map in seismic data during the latest iterations. In addition, a deep layer representing the Moho boundary was derived from gravity and constrained by public-domain refraction data. A 3D density distribution was derived from the seismic velocity volume using a modified Gardner equation. The modification comprised imposing a depth dependency on the Gardner coefficient, which is constant in the classic Gardner equation. The modified coefficient was derived from well data in the study area and public-domain velocity-density data sets. The forward-calculated gravity response of the composed density model was then compared with the observed gravity field, and the mismatch was analyzed jointly by a seismic interpreter and a gravity modeler. Adjustments were then made to the gravity model to ensure that the resultant salt model was geologically reasonable and supported by gravity, seismic, and well data sets. The output of the gravity modeling was subsequently applied to the next phase of seismic processing. Overall, this integration resulted in a more robust salt model, which has led to significant improvements in subsalt seismic imaging. The analysis of the regional trend in the observed gravity field suggested that a stretched continental crust underlay our seismic reprocessing area, with an oceanic-continental transition zone located to the southeast of our reprocessing region.

Geophysical signatures for low porosity can mimic natural gas hydrate: An example from Alaminos Canyon, Gulf of Mexico

AbstractNatural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity. The same geophysical signatures occur, however, in low‐porosity sand. We investigate the possible occurrence of natural gas hydrate in a sand interval in Alaminos Canyon Block 21 (AC 21) in the Gulf of Mexico, drilled by the U.S. Gas Hydrate Joint Industry Project. The sand interval has an increase in resistivity (~2.2 Ω m) and a strong peak and trough at the top and bottom of the sand on exploration seismic, which has been interpreted as natural gas hydrate. We reexamine the logging data and construct a new reservoir model that matches the measured resistivity, the high‐density sublayers in the sand, and the surface seismic trace. Our model shows that the sand interval in AC 21 is most likely water saturated; and the slight increase in resistivity, higher‐measured density, and the seismic amplitudes are caused by a reduction in porosity to ~30% in the sand interval relative to a porosity of ~42% in the surrounding marine muds. More broadly, we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf, though it could be as shallow as 600 mbsf for high‐porosity sands. In any case, the similar geophysical signatures for water‐saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide.

Methane Flux and Authigenic Carbonate in Shallow Sediments Overlying Methane Hydrate Bearing Strata in Alaminos Canyon, Gulf of Mexico

In June 2007 sediment cores were collected in Alaminos Canyon, Gulf of Mexico across a series of seismic data profiles indicating rapid transitions between the presence of methane hydrates and vertical gas flux. Vertical profiles of dissolved sulfate, chloride, calcium, magnesium, and dissolved inorganic carbon (DIC) concentrations in porewaters, headspace methane, and solid phase carbonate concentrations were measured at each core location to investigate the cycling of methane-derived carbon in shallow sediments overlying the hydrate bearing strata. When integrated with stable carbon isotope ratios of DIC, geochemical results suggest a significant fraction of the methane flux at this site is cycled into the inorganic carbon pool. The incorporation of methane-derived carbon into dissolved and solid inorganic carbon phases represents a significant sink in local carbon cycling and plays a role in regulating the flux of methane to the overlying water column at Alaminos Canyon. Targeted, high-resolution geochemical characterization of the biogeochemical cycling of methane-derived carbon in shallow sediments overlying hydrate bearing strata like those in Alaminos Canyon is critical to quantifying methane flux and estimating methane hydrate distributions in gas hydrate bearing marine sediments.

Subsurface gas hydrates in the northern Gulf of Mexico

The northern Gulf of Mexico (GoM) has long been a focus area for the study of gas hydrates. Throughout the 1980s and 1990s, work focused on massive gas hydrates deposits that were found to form at and near the seafloor in association with hydrocarbon seeps. However, as global scientific and industrial interest in assessment of the drilling hazards and resource implications of gas hydrate accelerated, focus shifted to understanding the nature and abundance of “buried” gas hydrates. Through 2005, despite the drilling of more than 1200 oil and gas industry wells through the gas hydrate stability zone, published evidence of significant sub-seafloor gas hydrate in the GoM was lacking. A 2005 drilling program by the GoM Gas Hydrate Joint Industry Project (the JIP) provided an initial confirmation of the occurrence of gas hydrates below the GoM seafloor. In 2006, release of data from a 2003 industry well in Alaminos Canyon 818 provided initial documentation of gas hydrate occurrence at high concentrations in sand reservoirs in the GoM. From 2006 to 2008, the JIP facilitated the integration of geophysical and geological data to identify sites prospective for gas hydrate-bearing sands, culminating in the recommendation of numerous drilling targets within four sites spanning a range of typical deepwater settings. Concurrent with, but independent of, the JIP prospecting effort, the Bureau of Ocean Energy Management (BOEM) conducted a preliminary assessment of the GoM gas hydrate petroleum system, resulting in an estimate of 607 trillion cubic meters (21,444 trillion cubic feet) gas-in-place of which roughly one-third occurs at expected high concentrations in sand reservoirs. In 2009, the JIP drilled seven wells at three sites, discovering gas hydrate at high saturation in sand reservoirs in four wells and suspected gas hydrate at low to moderate saturations in two other wells. These results provide an initial confirmation of the complex nature and occurrence of gas hydrate-bearing sands in the GoM, the efficacy of the integrated geological/geophysical prospecting approach used to identify the JIP drilling sites, and the relevance of the 2008 BOEM assessment.

Seismic imaging of migration pathways by advanced attribute analysis, Alaminos Canyon 21, Gulf of Mexico

Potential accumulations of gas hydrates in Alaminos Canyon Block 21 (AC21) in the Gulf of Mexico are thought to occur in a shallow sand-rich interval, stratigraphically separated from sources of free gas below the base of the gas hydrate stability zone (BGHSZ), by an intervening thick layer of clay- and silt-rich sediments. Availability of sufficient gas charge from depth, in addition to local biogenic sourcing is considered key to the formation of gas hydrates in the GHSZ. Implicitly, a detailed understanding of geometries associated with fault and fracture networks in relation to potential gas migration pathways can provide additional confidence that seismic amplitude anomalies are related to gas hydrate accumulations. Delineation of fault and fracture systems from high resolution seismic data in and below the gas hydrates stability zone (GHSZ) was performed using an automated algorithm—Ant Tracking. The capturing of small-scale detail has particular significance at AC21, revealing a pervasive network of typically small-extent discontinuities, indicative of fracturing, throughout this intervening clay- and silt-rich layer of mass-transport deposits (MTDs). Ant Tracking features appear to correlate, to some extent, with potential gas hydrate accumulations, supporting the concept that fracturing possibly provides migration pathways albeit via a tortuous, complex path. This study demonstrates that the Ant Tracking attribute, in conjunction with detailed seismic interpretation and analysis, can provide valuable evidence of potential gas migration pathways.

Electrical anisotropy of gas hydrate-bearing sand reservoirs in the Gulf of Mexico

We present new results and interpretations of the electrical anisotropy and reservoir architecture in gas hydrate-bearing sands using logging data collected during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II. We focus specifically on sand reservoirs in Hole Alaminos Canyon 21 A (AC21-A), Hole Green Canyon 955 H (GC955-H) and Hole Walker Ridge 313 H (WR313-H). Using a new logging-while-drilling directional resistivity tool and a one-dimensional inversion developed by Schlumberger, we resolve the resistivity of the current flowing parallel to the bedding, R‖ and the resistivity of the current flowing perpendicular to the bedding, R⊥. We find the sand reservoir in Hole AC21-A to be relatively isotropic, with R‖ and R⊥ values close to 2Ωm. In contrast, the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic. In these reservoirs, R‖ is between 2 and 30Ωm, and R⊥ is generally an order of magnitude higher.Using Schlumberger’s WebMI models, we were able to replicate multiple resistivity measurements and determine the formation resistivity the gas hydrate-bearing sand reservoir in Hole WR313-H. The results showed that gas hydrate saturations within a single reservoir unit are highly variable. For example, the sand units in Hole WR313-H contain thin layers (on the order of 10–100cm) with varying gas hydrate saturations between 15 and 95%. Our combined modeling results clearly indicate that the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic due to varying saturations of gas hydrate forming in thin layers within larger sand units.

Seismic Geomorphology and Geometric Evolution of a Single Base-of-Slope Deepwater Channel in the near Seafloor of Alaminos Canyon, Northwestern Deep Gulf of Mexico

The main goal of this article is to investigate the seismic geomorphology and geometric evolution of the single base-of-slope deepwater channel within a depositional sequence in Alaminos Canyon area of northwestern deep Gulf of Mexico. High resolution 3D seismic data was used to characterize the stratigraphic architecture evolution of deepwater deposits within 300 milliseconds interval below the sea floor. The seismic geomorphologic feature of deepwater channel was fast interpreted by combining seismic cross sections and stratal slices rendered by domain transformation. The surface wrapping interpretation results of the deepwater channel indicate that the near seafloor channel evolved from earlier wider and straight channel to later relatively narrow and sinuous channel within a depositional sequence. The channel exhibits vertical aggrading pattern in updip confined setting and changes to appear lateral migrating pattern in downdip unconfined setting through time. The spatial and temporal evolution of this deepwater channel in this study represents the typical evolution of the channel in base-of-slope deepwater setting and can be used as analog to predict and characterize the deepwater hydrocarbon reservoir at exploration depth in this area or other similar settings around the world.

Architecture of gas-hydrate-bearing sands from Walker Ridge 313, Green Canyon 955, and Alaminos Canyon 21: Northern deepwater Gulf of Mexico

Logging-while-drilling data acquired during the 2009 Gulf of Mexico (GoM) Gas Hydrate Joint Industry Project Leg II program combined with features observed in seismic data allow assessment of the depositional environment, geometry, and internal architecture of gas-hydrate-bearing sand reservoirs from three sites in the northern Gulf of Mexico (GoM): Walker Ridge 313, Alaminos Canyon 21, and Green Canyon 955. The site descriptions assist in the understanding of the geological development of gas-hydrate-bearing sands and in the assessment of their energy production potential. Three sand-rich units are described from the Walker Ridge site, including multiple ponded sand-bodies representing turbidite channel and associated levee and terminal lobe environments within the Terrebonne basin on the lower slope of the GoM. Older units display fewer but greater-reservoir-quality channel and proximal levee facies as compared to thinner, more continuous, and unconfined sheet-like sands that characterize the younger units, suggesting a decrease in depositional gradient with time in the basin. The three wells in the Green Canyon 955 site penetrated proximal levee sands within a previously recognized Late Pleistocene basin floor turbidite-channel-levee complex. Reservoirs encountered in GC955 exhibit thin-bedded internal structure and complex fault compartmentalization. Two wells drilled in the Alaminos Canyon 21 site tested a large, shallow, sand unit within the Diana mini-basin that exhibits steep lateral margins, non-sinuous elongate form, and flat base with hummocky upper surface. These features suggest deposition as a mass-transport deposit consisting of remobilized sand-rich turbidites or as a large basin-floor fan that was potentially eroded and buried by later-stage, mud-prone, mass-transport deposits.

Anisotropic models to account for large borehole washouts to estimate gas hydrate saturations in the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Alaminos Canyon 21 B well

Through the use of 3-D seismic amplitude mapping, several gas hydrate prospects were identified in the Alaminos Canyon (AC) area of the Gulf of Mexico. Two locations were drilled as part of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) in May of 2009 and a comprehensive set of logging-while-drilling (LWD) logs were acquired at each well site. LWD logs indicated that resistivity in the range of ∼2 ohm-m and P-wave velocity in the range of ∼1.9 km/s were measured in the target sand interval between 515 and 645 feet below sea floor. These values were slightly elevated relative to those measured in the sediment above and below the target sand. However, the initial well log analysis was inconclusive regarding the presence of gas hydrate in the logged sand interval, mainly because large washouts caused by drilling in the target interval degraded confidence in the well log measurements. To assess gas hydrate saturations in the sedimentary section drilled in the Alaminos Canyon 21 B (AC21-B) well, a method of compensating for the effect of washouts on the resistivity and acoustic velocities was developed. The proposed method models the washed-out portion of the borehole as a vertical layer filled with sea water (drilling fluid) and the apparent anisotropic resistivity and velocities caused by a vertical layer are used to correct the measured log values. By incorporating the conventional marine seismic data into the well log analysis, the average gas hydrate saturation in the target sand section in the AC21-B well can be constrained to the range of 8–28%, with 20% being our best estimate.

Perdido Startup: Flow Assurance and Subsea Artificial Lift

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 21716, ’Perdido Startup: Flow Assurance and Subsea Artificial Lift Performance,’ by H.S. Littell, J.W. Jessup, and W.W. Schoppa, Shell International E&P Inc., and M.R. Seay and T.D. Coulon, Shell E&P Company, originally prepared for the 2011 Offshore Technology Conference, Houston, 2-5 May. The paper has not been peer reviewed. The Perdido regional host is in the ultradeepwater Gulf of Mexico (GOM) and produces from two Lower Tertiary horizons in multiple drill centers—some directly under the host spar, and others offset up to 7 miles and in water depths between 7,800 and 9,600 ft. The commissioning and early-production phases of the development provided opportunities to validate Perdido subsea hydraulic models and operational methods that were considerably different from the typical approach used by major oil companies in deep water. Introduction The Perdido project encompasses the Great White, Tobago, and Silvertip fields and is 250 miles south of Galveston, Texas, in Alaminos Canyon 857, only 6 miles from the Mexico maritime border. The fields all are being developed with subsea wells tied back to the host, which has full processing capabilities and exports gas and oil by pipeline. In Fig. 1, the spar is shown as a hub with three flowlines from offset drill centers back to the manifold system underneath the host. All of the production flowlines lie completely on the seafloor; there are no flowline risers to the spar. The manifolds under the host connect the offset wells and the local producing wells to subsea separation and artificial-lift systems using electrical submersible pumps (ESPs). The host nameplate capacity is 100,000 BOPD, 40,000 BWPD, and 200 MMscf/D, with seawater-injection capability of 80,000 BWPD. In an effort to understand fluid behavior and risks, and to design for operational flexibility, extensive fluid sampling, analysis, and property evaluations were conducted during the exploration and appraisal phase. The Great White, Silvertip, and Tobago fields initially are being developed in two oil-bearing horizons that are (relatively) low temperature and low pressure, while exhibiting distinct fluid properties. Upper Wilcox fluids are fairly typical of light GOM oils, while Oligocene fluids are a heavily biodegraded, heavy, viscous oil. Hydrate management presents a significant challenge for the deepwater environment, and hydrates are the primary flow-assurance solids risk for Perdido. Reservoir temperatures are low, gas/oil-ratio (GOR) values are potentially high, and significant expansion cooling in risers as a result of the large pressure drop is a dangerous reality. The hydrate-formation region is especially large for the Upper Wilcox fluid, with a hydrate-dissociation pressure of 160 psig at 40°F, accompanied by particularly aggressive hydrate-formation kinetics. Both fluids demonstrated solid-hydrate-plug formation in laboratory-test cells, even at lower pressures and moderate subcooling. The project leveraged the effect of subsea separation, which presents a beneficial shift of the hydrate curve up to 10 to 20°F for the liquid stream because of degassing, as well as elimination of the extreme gas- expansion cooling effect that would be experienced in a naturally flowing riser.

Changing redox conditions at cold seeps as revealed by authigenic carbonates from Alaminos Canyon, northern Gulf of Mexico

Anaerobic oxidation of methane in anoxic sediments at cold seeps often leads to formation of authigenic carbonates close to the seafloor along continental margins. Recent work, however, indicated that the redox conditions in sediments may vary to some degree during seepage activity. In order to shed new light on the extent of this variability, authigenic carbonates from Alaminos Canyon lease block 645 of the northern Gulf of Mexico have been characterized by means of inorganic and organic geochemistry. The carbonates were collected from seep deposits representing various seafloor morphologies, including extensive pavements, mounds, fractured carbonate slabs surrounded by dense bivalve shells, and vestimentiferan tubeworm colonies. The deposits almost entirely consist of aragonite. The δ 18O values of aragonite vary from + 2.6 to + 5.8‰ V-PDB, suggesting precipitation in slight disequilibrium with the surrounding pore fluids. The δ 13C values of aragonite between − 33.9 and − 20.4‰ V-PDB agree with variable amounts of carbonate derived from oxidation of thermogenic methane and crude oil. Methane was primarily oxidized in an anaerobic process as revealed by the presence of 13C-depleted molecular fossils of methane-oxidizing archaea (δ 13C values as negative as − 118‰) and sulfate-reducing bacteria (δ 13C values as negative as − 97‰), the syntrophic partners in the anaerobic oxidation of methane. The observed inventories of molecular fossils in the authigenic carbonates mirror those of known consortia of anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria, namely the ANME-2/ Desulfosarcina/ Desulfococcus ( DSS) and ANME-3/ Desulfobulbus ( DBB) consortia . In contrast, the same carbonates exhibit shale-normalized rare earth elements patterns that all display real negative Ce anomalies (Ce/Ce* < 0.78), suggesting that at least temporarily oxic conditions prevailed. The episodic occurrence of oxic conditions is confirmed by the presence of molecular fossils of aerobic methanotrophic bacteria, including 4α-methylcholesta-8(14),24-dien-3β-ol, and two bacteriohopanepolyols, aminotetrol and aminotriol. The δ 13C values of the biomarkers of aerobic methanotrophs are as negative as − 58‰ and are consequently less 13C-depleted than the molecular fossils of the prokaryotes performing anaerobic oxidation of methane, a pattern in accord with culture experiments. Overall, our results suggest that redox conditions at cold seeps are variable. This variability probably reflects changes in seepage flux. The combination of an inorganic and an organic geochemical approach used here is promising to better assess the variability and diversity of past fluid and gas expulsion at seeps.

Perdido Development: Subsea and Flowline Systems

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 20882, ’Perdido Development: Subsea and Flowline Systems,’ by G.T. Ju, H.S. Littell, T.B. Cook, M. Dupre, K.M. Clausing, E. Shumilak, SPE, and W.W. Schoppa, Shell, and W.A. Blizzard, Seahorse Deepwater Technology, prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed. The Perdido development is in the western Gulf of Mexico in 7,817 ft of water. It is being developed with sub-sea technologies to mitigate key development challenges including extreme water depth, rugged seafloor terrain, low-pressure reservoirs, and aggressive hydrate-formation tendency. An overview of the subsea and flowline systems and the associated flow-assurance strategy is presented. The enabling technologies were a subsea-boosting system (SBS) and the surface blowout preventer (BOP) for drilling and completing subsea wells. Introduction The Perdido development includes the Great White, Silvertip, and Tobago fields and is in the Perdido basin and fold belt in the Alaminos Canyon Protraction Area. This area is 200 miles south of Freeport, Texas, USA, and 8 miles north of the Mexico maritime border. All three fields are developed with subsea wells tied back to the host, which is a spar with full offshore-processing capabilities and pipelines for export. Significant challenges include the following. Extreme water depth—7,800 to 9,600 ft, representing significant installation, operation, maintenance, and flow-assurance challenges. Problematic seafloor terrain—rugged seafloor terrain in this area includes geologic features such as fluid expulsion, seafloor erosion, and steep slopes. These features make the subsea-equipment layout and choice of host location extremely challenging.

Food-web structure of seep sediment macrobenthos from the Gulf of Mexico

The slope environment of the Gulf of Mexico (GOM) supports dense communities of seep megafaunal invertebrates that rely on endosymbiotic bacteria for nutrition. Seep sediments also contain smaller macrofaunal invertebrates whose nutritional pathways are not well understood. Using stable-isotope analysis, we investigate the utilization of chemosynthetically fixed and methane-derived organic matter by macrofauna. Biological sampling was conducted in three lower-slope GOM seep environs: Green Canyon (GC852, 1428m), Atwater Valley (AT340, 2230m), and Alaminos Canyon (AC601, 2384m). Infaunal δ13C and δ15N exhibited a broad range of values; most infauna appeared to be heterotrophic, although several taxa had very light δ15N and δ13C values, indicating possible reliance on chemoautotrophic symbioses. The lightest δ13C and δ15N values were observed in nematodes (δ13C=−54.6±0.1‰, δ15N=−6.1±0.2‰) and one gastropod (δ13C=−54.1‰, δ15N=−1.1‰) from Green Canyon. Mixing-model results indicated that sulfur-oxidizing Beggiatoa may be an important food source for seep infauna; the rate of utilization ranged from 60% to 100% at Green Canyon and Atwater Valley. The overall range in isotope values was similar across the three sites, suggesting that biogeochemical processes may be very similar in these geographically distinct areas.

Epizooic metazoan meiobenthos associated with tubeworm and mussel aggregations from cold seeps of the northern Gulf of Mexico

The abundance and higher taxonomic composition of epizooic metazoan meiobenthic communities associated with mussel and tubeworm aggregations of hydrocarbon seeps at Green Canyon, Atwater Valley, and Alaminos Canyon in depths between 1400 and 2800m were studied and compared to the infaunal community of non-seep sediments nearby. Epizooic meiofaunal abundances of associated meiobenthos living in tubeworm bushes and mussel beds at seeps were extremely low (usually <100ind. 10cm−2), similar to epizooic meiofauna at deep-sea hydrothermal vents, and the communities were composed primarily of nematodes, copepods, ostracods, and halacarids. In contrast, epizooic meiobenthic abundance is lower than previous studies have reported for infauna from seep sediments. Interestingly, non-seep sediments contained higher abundances and higher taxonomic diversity than epizooic seep communities, although in situ primary production is restricted to seeps.