Sigsbee Escarpment Research Articles

Case Study: Statistical Risk to a Drill Center From Shallow-Water Flow

Development projects in a number of deepwater provinces face the risk of shallow-water flow (SWF) from overpressured water-bearing sands. Typically found 1,000 to 3,000 ft below the mudline, these sands are often drilled before riser installation and can flow against a saltwater gradient. In extreme cases, vigorous flow will cause borehole rugosity or cavities from sand mining. After casing installation, poorly isolated SWF sands can flow, causing subsidence and casing buckling in both the afflicted well and its offsets. This article describes the evolution of SWF discovery, analysis, and mitigation on BP’s Atlantis project, located in 6,900 ft of water in southern Green Canyon. The earliest seismic images of Atlantis had shown a possible SWF sand at 9,200 ft, some 2,300 ft below the mudline, that generally covered the entire prospect. Drilling revealed a pore pressure of 9.2 ppge for this zone, which was successfully cased by the first four wells on the prospect. The fifth well, GC 743 #4, was spudded in July 2002. Drilling progressed below the 36-in. jet string at 7,100 ft toward the next planned casing point at 10,250 ft. At 8,400 ft, a 100-ft sand was drilled that had not been seen on seismic or in previous wells. The sand flowed against salt water during drilling, and at least some degree of sand mining occurred. After “pumping and dumping” to casing point at 10,250 ft, pad mud was spotted and the drillstring was retrieved to run 20-in. casing. The 20-in. shoe hit a ledge or bridge at the 8,400-ft sand, buckled above the wellhead, and parted. GC 743 #4 was cemented and abandoned after unsuccessful fishing attempts. A replacement well, GC 743 #6, was drilled 195 ft away with an additional string of 26-in. casing set above the newly identified 8,400-ft sand. Surprisingly, the sand was absent in this well, as shown in Fig. 1. With the data from these two wells, recently acquired high-resolution seismic revealed vague images of a steeply banked sand-filled river channel meandering through DC-1, the primary Atlantis drill center. But seismic resolution was inadequate to map the exact boundaries of the sand. Bathymetry and proximity to the Sigsbee Escarpment prevented relocation of the drill center. With sanction approaching, an additional string of 26-in. casing, top set above the newly problematic 8,400-ft sand, was included for each of the 15 remaining wells. Atlantis was sanctioned in November 2002 with the casing design shown in Fig. 2. A slimmer version, shown in Fig. 3, was under consideration at the time to reduce high “pump and dump” mud volumes experienced in GC 743 #4. The sanction work scope provided for a continuous batch-setting program for the remaining wells.

High-resolution geological AUV survey results across a portion of the eastern Sigsbee Escarpment

High-resolution geophysical data were acquired for an investigation across a portion of Sigsbee Escarpment using an autonomous underwater vehicle (AUV Hugin 3000), which allowed mapping of the seabed and near-seafloor features in detail and large two-dimensional data sets to be collected in a short time in deep water over rugged terrain. Complex seafloor structures are revealed in the survey area. These seafloor structures include a graben fault zone, rugged escarpment faces, slump deposits, and erosional furrows. Geological morphologies occurring in the survey area are associated with salt tectonics, gravitational driven failure, and ocean bottom-current activities. The Sigsbee Escarpment in the survey area is marked by an abrupt scarp on the order of 700 m (2300 ft) and a prominent increase in seafloor gradients as much as 30°. The Sigsbee Escarpment in the center and west of the survey area is generally scalloped, representing retrogressive slumps. The escarpment face is characterized by narrow and sharp ridges and numerous gullies. In the east of the survey area, the escarpment appears to be upturned, tilted, and eroded. A graben fault structure, representing a suture zone possibly associated with the joining of the two underlying salt sheets, is observed in the north-central survey area. In front of the escarpment, on the continental rise, a series of longitudinal furrows and slump deposits have been interpreted. The slump deposits at the base of the escarpment form aprons of sediment consisting of displaced and mixed sediments primarily of clay.

High-resolution geological AUV survey results across a portion of the eastern Sigsbee Escarpment

High-resolution geological AUV survey results across a portion of the eastern Sigsbee Escarpment

Structural evolution of the Frampton growth fold system, Atwater Valley-Southern Green Canyon area, deep water Gulf of Mexico

The Frampton growth anticline is part of the Atwater Valley-Southern Green Canyon frontal fold belt in the deep-water northeastern Gulf of Mexico. The anticline is located basinward of the allochthonous Sigsbee Salt sheet near the Sigsbee Escarpment. The timing and mechanisms of the formation of the frontal fold have been investigated using palinspatically restored depth sections and by the analysis of the growth stratal architecture preserved on fold limbs. The Frampton anticline is cored by autochthonous Middle Jurassic Louann salt and its western limit is bounded by the Green Knoll diapir. The fold geometry varies along strike, from a symmetric box-fold in the east, to a breached detachment fold in the west. Small-wavelength salt pillows formed during the Late Jurassic-Cretaceous in response to an early contractional deformation. These precursor structures controlled the geometry of Tertiary age folding. The Green Knoll diapir, west of the Frampton anticline, influenced deformation, leading to complex interactions between segments of the fold system and associated thrust faults. A landward-vergent thrust fault accommodated shortening adjacent to the diapir whereas folding was the main mechanism of deformation in the eastern part of Frampton anticline. Analysis of growth strata indicates that the detachment anticline developed according to a progressive limb rotation kinematic model with minor hinge migration. The fold evolution model proposed in this study could be used as an analogue for less well imaged, hydrocarbon-bearing growth folds in the deep-water province of the Gulf of Mexico, particularly those that are partially obscured by overlying allochthonous salt sheets.

Slope and Roughness Statistics of the Northern Gulf of Mexico Seafloor With Some Oceanographic Implications

We analyzed 11 cross-slope and six along-slope bathymetric profiles over the continental slope of the northern Gulf of Mexico using statistical and time series techniques. Linear regressions account for over 93% of the water depth variability in nine north-south profiles; the remaining profiles follow quadratic polynomials accounting for over 92% of the variability. Seafloor gradients from the linear fits are generally ≤1°, but local gradients can reach ≅16° near the Sigsbee Escarpment (SE), which is smaller than previously documented. Seafloor roughness elements reach 13-300 m, with most <100m. Such rough bottoms could affect waves with wavelengths of tens of kilometers but not waves of hundreds of kilometers. Water depth power spectra are red (having the most energy at scales ≤10 km) and exhibit a k-2 dependence. Power spectra of short-scale gradients are near constant at scales >0.02 cpkm, implying a white noise process, and overall, these spectra exhibit an exponential dependence. Oceanographically, the slopes are large enough for topographic β-effects to dominate over the planetary β-effect, which allows approximating the topographic Rossby waves (TRWs) dispersion in terms of the Brunt-Vaisala frequency and bottom gradients. The steep SE can sustain minimum periods of ~18 d, which agrees with observed periods. Bottom trapping caused by stratification should be effective only for short waves, but observations suggest that bottom trapping is independent of wavelength. This discrepancy can be explained by the fact that the Gulf of Mexico can be approximated as a two-layer ocean, and TRWs are bottom trapped regardless of wavelength. The critical frequency and slope show that only diurnal and inertial frequencies (at this latitude) could be inducing strong vertical mixing on the study area. The initial conjecture that cyclonic eddies with diameters of 40-150 km are generated by flow-topography interaction was not upheld because the resonance conditions are not met. Finally, the analysis reveals that fluids inside basins cannot escape.

Sea-floor sediment distribution in the Gulf of Mexico

Carbonate content, smear-slide analysis and diffuse reflectance spectrophotometry were utilized to determine modern sediment composition and distribution throughout the Gulf of Mexico. In all, 186 core top and grab samples distributed throughout the Gulf were analyzed. Reflectance spectra were taken from thick smear slides from the near ultraviolet, through the visible, and into the near infrared. The first derivatives of the percent reflectance data were subjected to factor analysis producing factors that grouped covarying first-derivative wavelengths. Factors were interpreted by comparison to first-derivative curves for known sediment components and minerals. Interpretation was aided by the mapping of both calcium carbonate content and smear-slide sediment classes. The most easily interpreted factor solution was produced by analyzing only the visible region of the spectrum and extracting seven factors which explained ∼98% of the cumulative variance. These factors, in order of their relative importance, are interpreted as (1) marl and calcareous clay, (2) glauconite, (3) kaolinite, (4) organic matter, (5) phosphorite, (6) hematite, and (7) goethite. Some factor maps are consistent with known sources of fluvial sediment input; for example, kaolinite is deposited off rivers draining the southeastern US. Other factor maps are related to the origin of the material in the factor, glauconite, for example, being confined to low sedimentation regions of the outer shelf. The most unusual observation concerns the distribution of hematite, which appears to be transported from the rivers of south Texas, primarily the Rio Grande, across the shelf then eastward downslope along the base of the Sigsbee Escarpment. This eastward transport seems to be explainable only by transport in bottom currents flowing along the base of the Sigsbee Escarpment.

Measurements and modeling of the shallow pore pressure regime at the Sigsbee Escarpment: Successful prediction of overpressure and ground-truthing with borehold measurements

Editor's note: The material in this paper was prepared and presented (OTC Paper 15201) at the 2003 Offshore Technology Conference, 5–8 May in Houston, Texas, U.S., and is published with permission. Shallow overpressures and shallow water flow (SWF) are significant hazards to deepwater drilling and facilities. In this paper we demonstrate how a combination of geomorphology, hydrogeology, and seismic interpretation has been successfully used in infer shallow overpressures, to develop a testable hypothesis of pore pressure distribution, and to delineate where to collect in-situ data. In-situ pore pressure measurements provide ground-truth data that can validate the model and be used to evaluate the hazards that internally driven failure might pose for facilities development. Several slope failures on the Sigsbee Escarpment in the Mad Dog and Atlantis field areas show a geomorphology distinct from “top-down” slope failures. Specifically, these slumps have steep, amphitheater-shaped headscarps, shallowly sloping bases, sharp inflection points with the surrounding seafloor at both the top and bottom of the headscarp, a linear trend in map view, and linear side walls. These geomorphic characteristics suggest that the slumps are formed by internally driven failure, and that the slumps grow retrogressively by headward migration. Within the Mad Dog and Atlantis field areas, Mad Dog Slump 8 and Atlantis Slump A are the best examples of this morphology; in this paper we will focus our attention on Mad Dog Slump 8 due to its proximity to planned facilities. The types of slope failures discussed in this paper are found on many margins worldwide. Their characteristic morphology indicates that the primary mechanism of slope failure is due to internal forcing (overpressure, weakening), rather than by downslope erosional flow. Internally driven slope failures can provide clues about the shallow overpressure regime, and provide exploration targets. Because the ocean is at constant head, and …

ABSTRACT: Comparison of Latest Quaternary Depositional Processes of the Texas-Louisiana Intraslope-Basin Province and the Deep GOM Basin Floor Seaward of the Sigsbee Escarpment

ABSTRACT: Comparison of Latest Quaternary Depositional Processes of the Texas-Louisiana Intraslope-Basin Province and the Deep GOM Basin Floor Seaward of the Sigsbee Escarpment

The effect of allochthonous salt evolution and overpressure development on source rock thermal maturation: a two-dimensional transient study in the northern Gulf of Mexico Basin

Sequential two-dimensional forward modelling of source rock maturation along a north–south 600 km megaregional profile across the northern Gulf of Mexico Basin illustrates the coupled influence of allochthonous salt evolution and overpressure development on source rock maturation. Model results indicate that allochthonous salt evolution strongly affected local and regional source rock maturation, while overpressure development had a minor impact. Within the lower slope portion of the profile (south of the Sigsbee escarpment to the Tabular Salt/Minibasin province), localized thermal lows associated with salt stock development locally retarded the maturation of source rocks, while the slow development of overpressures effected little influence on the maturation. Within the centre of the profile (offshore Louisiana shelf), however, the evolution of allochthonous salt sheets and shallow salt structures locally and regionally lowered the oil generation window. The timing of overpressures in this region, related to the progradation of the Cenozoic marine depocentre, may have accelerated the maturation of middle Eocene source rocks, although the deeper Turonian source rocks had reached maturation at the time of greatest overpressure development. In the northern portion of the profile (southern Louisiana), the Terrebonne salt sheet lowered the oil generation window and retarded the maturation of the middle Eocene source rocks.

Observations of high speed deep currents in the northern Gulf of Mexico

Recent measurements in water depths of 2000 m in the vicinity of the Sigsbee Escarpment, south of New Orleans, show the presence of recurring high speed current episodes, some of which had velocities greater than 85 cm/s only 10 m above the local bottom. In the Gulf of Mexico, currents above about 1000 m are not locally coupled with the relatively depth‐independent currents in the deeper layers. Topographic Rossby waves, with periods of approximately 10 to 14 days, appear to control the dynamics of the lower water‐column.

Submarine landslide geomorphology, US continental slope

Abstract The morphometric analysis of submarine landslides in four distinctly different tectonic environments on the continental slopes of Oregon, central California, Texas, and New Jersey provides useful insight into submarine process, including sediment transport mechanisms and slope stability. Using Geographic Information System (GIS) software, we identify landslides from multibeam bathymetric and GLORIA sidescan surveys based solely on surficial morphology and reflectivity. This method provides useful data in a time- and cost-efficient manner. We measure various aspects of the failures, including landslide area, runout distance, and headscarp height, along with the slope gradient of the runout zone, the failure's scar, headscarp, and adjacent slopes. The largest failures of the four study areas occur in the Gulf of Mexico, adjacent to Mississippi Canyon, and between salt withdrawal basins. Smaller landslides occur within the basins, and at the base of the Sigsbee Escarpment. These smaller landslides tend to have higher headscarps than the larger ones, and often have cohesive material at the base, suggesting a stronger rheology. Oregon has the steepest local slopes, but surprisingly few large failures for a seismically active margin (especially in the north), implying that slope angle and seismic activity may not be the most important slope stability controls. The California continental slope is heavily incised, which makes failure isolation difficult. Most of the landslides occur within the larger canyons (Vizcaino, Pioneer, Monterey) and adjacent to a pock mark field in the Point Arena basin. The majority of landslides offshore New Jersey occur on the open slope between Lindenkohl and Carteret Canyons. Morphometric statistics give us insight into where mass movements occur, how big they are likely to be, their relative importance as sediment transport mechanisms, and the overall slope stability of a given margin. Most landslides occur on slopes less than 10°. Curiously, the steepness of the slope adjacent to the failure tends to be inversely proportional to the runout length. In both California and Oregon, slope failures tend to make the local slope steeper, whereas failures in the Gulf of Mexico and offshore New Jersey will tend to make the local slope less steep. Landslides with rubble beneath the scar are mostly smaller than those without, are deep seated, and make the slope steeper. We use the ratio of headscarp height to runout length as a measure of the failure's dynamic rheology. This ratio in the submarine case is orders of magnitude less than subaerial landslides. Hydroplaning of the failed mass may be responsible for the very long runout lengths. These morphometric relationships give us important insight into landslide dynamics and process in different sedimentary and tectonic environments.

ABSTRACT: Massive Bed-Forms, Mega-Furrows, on the Continental Rise at the Base the Sigsbee Escarpment, Northwest Gulf of Mexico

Abstract Some of the largest deep-water sedimentary bed-forms ever found in the world oceans were recently discovered along the base of the Sigsbee Escarpment in the northwest Gulf of Mexico. Deep-Tow high-resolution geophysical surveys on the continental rise south of the Sigsbee Escarpment have revealed the existence of mega-furrows, ~10 m deep and ~30 m wide, spaced ~100 m apart that occupy an area on the Continental Rise 20 to 40 km wide. A single furrow within a field of furrows in the Green Knoll Diapir area can be traced over a distance of 150 km. The furrows are located at the base of the Sigsbee Escarpment from 90° to 93° West Longitude, from north of Green Knoll to the area just west of Bryant Canyon. The presence of furrows, longitudinal bed-forms, suggests that unidirectional bottom currents up to 100 cm/sec may exist at the base of the Escarpment. The furrows are inferred to be formed by helical secondary circulation. Allen (1968) produced sedimentary furrows in the lab and found a hierarchy of bed forms ranging from uniform furrows at low flow conditions to meandering furrows and flute casts at the highest flow conditions. All the conditions described by Allen are present in the area due south of Bryant Canyon on the continental rise. In terms of hazards relating to deepwater hydrocarbon exploration and production, the formation of mega-furrows demonstrates the existence of previously undocumented high-velocity currents at the base of the escarpment. End_of_Record - Last_Page 410-------

Abstract: Into the Unknown, A History of the Discovery of the Giant Fields of the Western Mississippi Fan Foldbelt, Gulf of Mexico

ver the last 15 years, several giant petroleum accumulations have been discovered in the Mississippi Fan foldbelt, deep- water Gulf of Mexico. In the mid 1980s, initial work was driven by interest in the nature of sections at the edge of, and immedi- ately inboard of, the Sigsbee Escarpment. Outboard and counter-regional dip beneath an allochthonous salt canopy was recognized in early speculative 2D seismic transects.

ABSTRACT: Timing of Source Rock Maturation in the Northern Gulf of Mexico Basin: Results from Thermal Modeling of a Regional Profile

Abstract Maturation modeling along a 600 km long restored cross section in the northern Gulf of Mexico Basin allows estimation of the timing of source rock maturation. Forty 1-D thermal models, at a spacing of 15-25 km, were constructed along the series of restored cross sections. The cross section extends from the Mississippi-Louisiana border to south of the Sigsbee Escarpment. Source intervals modeled along the profile are of Eocene-, Turonian-, Tithonian-, and Oxfordian-age; their areal extent and kerogen types were based on published reports. In northern Louisiana, Turonian source rocks reached peak oil generation (0.9% Ro) at 31 Ma. In the Oligo-Miocene detachment province (southern Louisiana), Eocene source rocks reached peak oil generation at 14-12 Ma. In the shelf and slope provinces, considerable variations exist in the maturation windows based on the presence of allochthonous salt, its rates of deformation, and its effect on heat flow. In the late Miocene-Pliocene province, Eocene source rocks reached the peak oil window at 14-5.5 Ma. The tabular salt-minibasin province comprises southern Green Canyon (Tithonian source) and northern Walker Ridge (Oxfordian source). In southern Green Canyon, peak oil was reached at 65-10 Ma. In northern Walker Ridge, peak oil was reached at 9.5 Ma. In the abyssal plain, the source rocks have not yet reached the peak oil window or reached it in the last 0.5 Ma.

Abstract: Mega-Furrows on the Continental Rise South of the Sigsbee Escarpment, Northwest Gulf of Mexico

Abstract: Mega-Furrows on the Continental Rise South of the Sigsbee Escarpment, Northwest Gulf of Mexico

Modeling deepwater reservoir analogs through analysis of recent sediments using coherence, seismic amplitude, and bathymetry data, Sigsbee Escarpment, Green Canyon, Gulf of Mexico

Coherence maps, extracted from high‐resolution 3-D seismic data at an interval just below the seafloor provide an effective tool for analysis of reservoir analogs in older depositional sequences. This paper defines the geometry and depositional setting of Pleistocene to Recent sediments deposited in front of, and behind, the Sigsbee Escarpment, in the southeastern corner of Green Canyon, Gulf of Mexico.

Abstract: Definition of Pleistocene to Recent Sediments along the Sigsbee Escarpment, 3-D Seismic Coherence, Amplitude and Bathymetry Data, Southeast Green Canyon, Gulf of Mexico, USA

Abstract: Definition of Pleistocene to Recent Sediments along the Sigsbee Escarpment, 3-D Seismic Coherence, Amplitude and Bathymetry Data, Southeast Green Canyon, Gulf of Mexico, USA

Abstract: Structural Overview of the Deepwater to Ultra Deepwater Northern Gulf of Mexico Area based on a Recent Regional Seismic Program

This interpretation project is based on an extensive 2-D seismic program (known in the industry as Phase 45) that consists of 60,000 line miles, covering the central and western parts of the deepwater and ultra deepwater northern Gulf of Mexico region. This paper concentrates on the central part of the project area (Garden Banks, Keathley Canyon, Green Canyon and Walker Ridge) covering 38,000 mi 2 and utilizing a 4 by 4 mile recently acquired seismic grid. Four horizons have been interpreted: top and base salt along with the Discoaster surculus (DS - approximate top Pliocene) and Discoaster berggrenii (MDB - approximate top Miocene) horizons. Abundant well data, in the form of electric logs, micropaleontology and velocity data, were available in the northern Green Canyon and Garden Banks regions. All interpretation was performed on a workstation. The mapping of these horizons shows how salt geometry changes moving from north to south. Around the continental shelf break a series of coalesced but discrete allochthonous sheets with significant intersalt basins occur. To the south a virtually continuous salt sheet, with numerous suprasalt basins, develops out to the edge of the present day northern Gulf of Mexico salt province, marked, in part, by the Sigsbee Escarpment. In these distal parts of the salt province, particularly in the Walker Ridge area, there is comparatively little sedimentary cover above the salt except where suprasalt basins have locally depressed salt. These basins have both symmetric and asymmetric geometries. The Mio-Pleistocene section is oilen railed above the allochthonous salt in these basins and seems to override deeper Mio-Pleistocene identified outboard of, and carried beneath, salt.

Uncertainty analysis of subsalt overpressure development in offshore Louisiana, Gulf of Mexico

The confining pressure exerted by the prograding Tertiary clastic wedge has caused extensive salt deformation in the Gulf of Mexico. The high rates of lateral salt motion from the Louisiana shelf break to the Sigsbee Escarpment are expected to have significant impact on the dynamic evolution of the surrounding sedimentary formations. The evolution of fluid pressure in supra- and subsalt sediments has been modeled in a series of pseudo-wells along a profile intersecting numerous salt sheets and structures offshore Louisiana. Based upon the estimated rates of salt movement, the timing can be determined for insertion/depletion of salt in the sediments as the salt has moved through, and by, sediments on its way basinward. An execution of a series of I-D fluid/flow compaction models has enabled estimation of the spatial variation in overpressure build-up with time; indicating that overpressure uncertainty is about 10–20% about a mean of about 40 atmospheres, with uncertainties on salt thickness and salt speed roughly comparable in importance in contributing to the uncertainty. The quantitative behaviors suggest that the speed of lateral salt insertion and the thickness of the salt are the main factors causing anomalous overpressure build-up. A quantification of the dynamic behaviors is significant when modeling the timing of potential trapping of hydrocarbons beneath salt sheets and of the subsalt overpressure to be expected in subsalt drilling.

The Engineering and Geological Constraints of the Intraslope Basins and Submarine Canyons of the Northwestern Gulf of Mexico: ABSTRACT

ABSTRACT Future hydrocarbon finds on the upper and lower continental slope and rise of Texas and Louisiana well necessitate innovative methods for the construction of platforms and pipelines in a very difficult engineering and complex geological environment. There are 105 intraslope basins and eight submarine canyons on the continental slope of the northwestern Gulf of Mexico, all of which may be prime targets for hydrocarbon production. Examinations of the physiographic, geophysical, and geotechnical characteristics of the intraslope basins of Pigmy and Vaca basins and the Alaminos submarine canyon are used as examples to typify the various engineering and geological constraints that are most likely to be encountered on the continental slope and rise and along the Sigsbee Escarpment in the northwestern Gulf of Mexico. High-resolution bathymetry identifies such constraints as high angle intraslope basin walls; walls that exceed 40 degrees are common. Sediment slumps and other instabilities, such as long-term sediment creep and other affects of halokinesis and contemporaneous faulting, are evaluated from high-resolution geophysics. The small canyons and large gullies that dissect the perimeter of Alaminos Canyon, which may be the results of both recent and old turbidity currents and debris flows, are structures that require engineering consideration in the implacement of seafloor structures in, near or down-slope of these features.