Barry C. McBride

Sequence Stratigraphy of Pliocene and Pleistocene Turbidite Systems, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

Neogene turbidite systems are major reservoirs in the northern deep Gulf of Mexico. Few publications have described the stratigraphic variations or the three-dimensional geometries of these turbidite systems in detail; hence, an understanding of the stratigraphic characteristics of the producing sands is important for deep-water exploration in the Gulf of Mexico and similar basins worldwide. This study focuses on the northern Green Canyon and central Ewing Bank protraction (lease) areas, where the Pliocene-Pleistocene turbidite systems were mapped using an integrated exploration database. Interpretation of 10,000 km of two-dimensional seismic, 185 well logs, and biostratigraphy from 180 wells allowed us to define the regional sequence stratigraphic framework for this area and potential areas for future exploration. A complex Pliocene-Pleistocene geologic evolution of the area is indicated by the seismic and geologic facies, depositional rates, nature of turbidite systems, and sand content. Significant sand deposits (basin-floor fans) were deposited in most sequences and directly overlie sequence boundaries. Salt tectonics and faulting greatly influenced the loci of these fansx92 deposition. Large, thick fans fill entire salt-withdrawal minibasins at the base of the Pliocene sequences. In the Pleistocene sequences, where the rate of salt withdrawal was less, smaller and thinner fans were deposited downdip of faults and adjacent to shallow salt bodies. Channel systems, interbedded with overbank shales, constitute most of the sediments in the sequences. The older sequences contain more channels and sandier channel fills than the younger sequences. Analysis of all sequences indicates a complex depositional history where significant sands were deposited where abrupt decreases in bathymetric gradient are associated with salt tectonics or faulting.

The Evolution of Allochthonous Salt Systems, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf Of Mexico

The discovery that major episodes of subhorizontal, allochthonous salt flow have occurred in the Gulf of Mexico Basin requires a means of quantifying the evolution of allochthonous salt and associated structures to conduct both basin and petroleum systems analyses. Sequential structural restorations of allochthonous salt systems provide an evolving structural framework for integrating stratigraphic, geophysical, and geochemical data sets. In this study, interpretation of more than 10,000 km (6200 mi) of multifold seismic data, and sequential restoration of eleven profiles, were used to determine the geometry and evolution of allochthonous salt structures within Ewing Bank and northern Green Canyon protraction areas. The results illustrate the complex geometry of the multilevel salt system and the types of interactions between counterregional and salt-stock canopy models of allochthonous salt system evolution. Sedimentary loading is accommodated by salt sheet extrusion, gravity spreading, gravity gliding, extension, salt evacuation, and contraction. Salt geometry commonly changes dramatically through time because it provides much of the accommodation for sediments and absorbs much of the extension and contraction within its overburden. The positioning and kinematics of extensional and contractional structures are controlled by salt body geometries, salt system interactions, and, most importantly, the topography of the base salt or equivalent salt weld. The structural restorations also constrain the timing of salt sheet and salt weld formation and document the positive correlation among sedimentation rates, salt flow, and structural deformation. Cross-sectional salt area generally decreases through time in areas of salt evacuation and minibasin formation, but increases in sections crossing growing salt bodies. Three-dimensional restoration is required to determine the three-dimensional kinematics and balance of allochthonous salt tectonics.

The Effect of Allochthonous Salt on the Petroleum Systems of Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

The northern Green Canyon/Ewing Bank region, northern Gulf of Mexico basin, contains the Oxfordian-Neogene (.), Tithonian-Neogene (.), Albian-Neogene (.), Turonian-Neogene (.), and Eocene-Neogene (.) petroleum systems. The systems encompass 42 fields or discoveries in the study area and include four subsalt discoveries. Essential elements of the systems include source shales of Oxfordian, Tithonian, Albian, Turonian, and Eocene age; Neogene siliciclastic turbidite reservoirs; allochthonous salt; and overburden strata ranging in age from Jurassic to Quaternary. The petroleum systems of the area are significantly affected by the evolution of allochthonous salt. The high thermal conductivity of salt retards the thermal maturation of subsalt petroleum source rocks and causes late generation and migration from them. Most traps were formed during the Pliocene-Pleistocene, and the generation-migration-accumulation of petroleum ranges from early Miocene to the Holocene. The critical moment of peak oil generation for each source varies spatially and temporally as a function of the overlying sediments and allochthonous salt evolution. The impermeability of salt prevents vertical petroleum migration and causes migration pathways to be deflected laterally up the dip of base salt. Where salt welds form, petroleum migration is unimpeded and continues vertically. By integrating predictions of potential source rocks, structural restorations, thermal maturation modeling, regional salt maps, and petroleum systems logic, we can determine petroleum migration pathways and zones of concentration. All 42 fields or discoveries within the study area are associated with predicted zones of paleosubsalt petroleum concentration. Present-day salt geometries do not delineate many of these zones because of salt weld formation during the Pleistocene. This generation, migration, and accumulation technique enables geoscientists to focus their exploration efforts toward areas with a greater probability of success.

The Evolution of Allochthonous Salt Along a Megaregional Profile Across the Northern Gulf of Mexico Basin

Sequential restorations of a north-south megaregional cross section across the north-central Gulf of Mexico Basin from east-central Louisiana to the abyssal plain define a dynamic, complex history of sedimentation, salt flowage, and salt evacuation. Proprietary composite seismic profiles (590 km), 33 wells, and published depth-to-basement maps were used to constrain the section in depth. Thirteen sequential structural restorations, incorporating both decompaction and isostatic subsidence (thermal and tectonic), were then constructed from the Late Cretaceous to the Holocene. The restorations highlight and constrain a protracted history of deformation that is primarily controlled by gravity and the progradation of Cenozoic sediments over salt. Early stages of the tectonic history of the northern Gulf of Mexico Basin were related to differential thermal subsidence resulting from Early-Middle Jurassic rifting. During the Cenozoic, the evolution of the basin was dominated by the influx of large clastic depocenters, which caused the basinward evacuation of autochthonous Jurassic salt. Salt extrusion from the autochthonous layer was accomplished by inclined salt bodies, which flowed into salt glaciers or sheets near the sea floor. Evacuation of allochthonous salt layers provided significant sediment accommodation, and unusually thick sedimentary sections were deposited, such as the Terrebonne Trough of southern Louisiana (3-7 km of Miocene strata). Salt sheet formation and evacuation occurred progressively basinward through time in response to basinward shifts of major Cenozoic sedimentary depocenters. As a salt sheet neared complete evacuation, the underlying autochthonous salt layer would begin to evacuate, providing additional sediment accommodation that caused autochthonous salt flowage basinward, and the formation of the next allochthonous salt sheet basinward of the depocenter. The area of autochthonous salt progressively decreased through time and currently represents at most 45% of its maximum along this transect. The total area of salt through time was more stable, with variations of only 30% from its maximum. This relationship is a function of lateral salt flow into and out of the plane of section, and possible salt dissolution. The restorations indicate that very little translation or extension (1.46%) occurred at the autochthonous salt level during the evolution of the basin. The majority of translation or extension occurred above allochthonous salt sheets (25%) and was compensated laterally by salt flow. Displacements above allochthonous salt sheets were driven by gravitational instabilities caused by the slope gradient.

Evaluating the petroleum systems of the northern deep Gulf of Mexico through integrated basin analysis; an overview

Exploration and development activity has increased significantly during the past 5 years in the northern deep Gulf of Mexico. This activity has been caused by several factors, including significant discoveries in deep water (>1500-ft water depth), outstanding reservoir performance in some of these discoveries, expiration of 10-year leases originally purchased in the mid-1980s, innovative production techniques, and new federal royalty relief. Exploration and production has occurred in three general exploration subprovinces: present shelf, deep water, and the subsalt that extends from shelf into upper slope. Each subprovince consists of slightly different geology and, subsequently, different economic scenarios. This paper introduces the geologic setting for a portion of the outer shelf and upper to middle slope region in the northern Gulf of Mexico. The following eight papers demonstrate how the petroleum systems of the deep Gulf of Mexico can be analyzed by using an integrated approach. This issue of the Bulletin includes papers that describe the petroleum geology of the northern Green Canyon and Ewing Bank region: petroleum fields and discoveries, sequence stratigraphy, biostratigraphy, three-dimensional seismic stratigraphic interpretation, structural geology using restorations, interaction of salt tectonics and sedimentation, and geothermal modeling and path migration prediction.

The effects of salt evolution, structural development, and fault propagation on Late Mesozoic-Cenozoic oil migration: A two-dimensional fluid-flow study along a megaregional profile in the northern Gulf of Mexico Basin

Sequential two-dimensional (2-D) forward modeling of fluid flow along a north-south, 600 km megaregional cross section across the northern Gulf of Mexico Basin illustrates the influence of structural, stratigraphic, and thermal evolution on oil generation patterns and migration paths. Twelve megaregional fluid-flow models, which span from the Late Cretaceous to the Holocene, were constructed for this study. Each model uses a sequential structural restoration and proprietary well data to constrain the structural and stratigraphic development of the study area and to calibrate production depths along the megaregional profile. These fluid-flow models specifically address the levels of influence that salt evolution, sedimentation, thermal history, and fault development induce on temporal oil migration patterns. Results from the sequential 2-D fluid-flow models across the northern Gulf of Mexico Basin indicate that allochthonous salt evolution and excess-pressure development from differential sedimentation strongly influenced Late Mesozoic-Cenozoic oil migration patterns along the entire megaregional profile. Within the lower slope part of the profile, early and fairly rapid maturation of source rocks was accompanied by slow elevation of excess pressures. As a result, oil migration in these regions was minimal, and the impact of salt evolution on the fluid flow was restricted to circulatory patterns at the base of salt stocks. Within the center of the profile (Begin page 1946) (offshore Louisiana shelf), however, the evolution of allochthonous salt and the formation of high excess pressures, coeval with the development of listric and normal faults, strongly impacted the oil migration patterns. Penetration of high excess-pressure regimes by both listric and normal faults directed fluid flow vertically upward along the fault systems. Upon encountering salt sheets, oil migration in these regions exhibited both divergent and convergent flow patterns, flowing laterally along the base of the salt. A similar scenario was observed in the northern part of the profile (southern onshore Louisiana), reflected by oil migration beneath the Terrebonne salt sheet. Upon evacuation of allochthonous salt in the central and northern regions of the profile, migration patterns were primarily lateral. When excess pressures in these regions exceeded 50 MPa, however, oil flowed vertically through the salt welds and along suprasalt faults. A more localized and well-constrained study of fault migration in the Oligocene-Miocene detachment province further suggests that faults are important factors as migration pathways, with episodic flow directing oil migration into observed shallow reservoirs.

Structural and Stratigraphic Controls on Petroleum Seeps, Green Canyon and Ewing Bank, Northern Gulf of Mexico: Implications for Petroleum Migration

Abstract Numerous petroleum seeps and chemosynthetic communities are present on the continental slope in the northern Gulf of Mexico. These features are associated with faults and salt highs and indicate recent petroleum migration. Thirty-three known petroleum seep and/or chemosynthetic communities in Green Canyon and Ewing Bank were studied in a sub-regional context to understand their stratigraphic and structural controls and their significance to the overall petroleum system. Results indicate that surface seeps occur associated with all different allochthonous salt systems present (ramp-fault systems, roho-systems, stepped-counterregional systems, and counterregional systems), and all four systems are efficient. Of the thirty-five fields and discoveries in the area, only six occur near (<4.8 km) seeps, suggesting that the seeps are indicative of an efficient and mature petroleum system, but not necessarily of proximal reservoirs. Petroleum seeps may be more related to poor trap and/or seal development. The relative rates of vertical migration of petroleum can be estimated based on the timing of salt weld formation. These estimates range from 0.0015 to 0.025 m/y.

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.

Sequence stratigraphic setting of Neogene turbidite systems, Green Canyon and Ewing Bank, northern Gulf of Mexico: Preliminary results

The Neogene strata of Green Canyon and Ewing Bank lease areas in northern Gulf of Mexico are a major exploration play. Most discoveries and future exploration targets are in Neogene turbidite systems (low stand systems tracts). The regional sequence stratigraphy for this area has been interpreted to help define the potential areas for future exploration. Data base consists of 10,000 km of multifold seismic data, well log data for 100 wells, biostratigraphy from 180 wells. Existing fields and sand bodies are characterized within a sequence stratigraphic frame-work. Major sand-prone intervals directly overlie the 3.8, 3.4, 1.9, 1.4, 1.1, and 0.8 Ma sequence boundaries. A stacked condensed section occurs in the area between 3.0 to 1.9 Ma; it separates the stratigraphy into two discrete sections. Few reservoir intervals develop in this interval, because of a lack of major sand source into the basin. In general, sediments below the major condensed section were considerably sandier than those sequences younger than the condensed section. Producing sands are from turbidite systems deposited in mid-bathyal water depths in the GC 205, 184 (Jolliet) 65 (Bullwinkle), Ewing Bank 826. Reservoirs vary from massive, blocky sands to thinly laminated sands with low resistivity, and are commonly associatedmorexa0» with seismic amplitude anomalies.«xa0less

Geometry and Kinematics of Thrust System Within Central Snowcrest Range: A Major Rocky Mountain Foreland Uplift and Exploration Target in Southwest Montana: ABSTRACT

The Snowcrest Range forms the southeastern limb of the Late Cretaceous, northeast-trending, southeast-verging Blacktail-Snowcrest uplift. Field mapping and regional balanced cross sections indicate the Snowcrest thrust system consists of two master thrust faults (Snowcrest and sub-Snowcrest thrusts), each involving a system of subsidiary thrust imbricates, splays, and shear zones. East of the Snowcrest Range, the Gravelly Range thrusts are attributed to this system as regional splays. The Snowcrest thrust dips approximately 39/degrees/NW and forms the western boundary of large-scale, uprift, and overturned folds within Phanerozoic rocks above the sub-Snowcrest (Greenhorn) thrust. The sub-Snowcrest thrust is buried beneath or within the synorogenic deposit of the Beaverhead group and is inferred to merge with the Snowcrest thrust in Precambrian basement rocks at depth. Small-scale structures indicate a significant component of right-lateral oblique slip occurred on the sub-Snowcrest thrust system along the central and southern portions of the range. Preliminary strain data indicate that the principal shortening was directed east-southeast.