Abstract Direct simple shear experiments on mud samples from 0 to 15 mbsf (metres below seafloor) in the Ursa Basin (northern Gulf of Mexico) document that stress level impacts shear strength and pore pressure during failure. As burial depth increased (from 7.35 to 13.28 mbsf), cohesion decreased (from 12.3 to 6.5 kPa) and the internal friction angle increased (from 18° to 21°). For a specimen from 11.75 mbsf, an increase in maximum consolidation stress (from 45 to 179 kPa) resulted in an increase in the shear-induced pore pressure (from 29 to 150 kPa); however, the normalized peak shear stress decreased (from 0.37 to 0.25). Our results document that consolidation at shallow depths induces a positive feedback on pore-pressure genesis. For resedimented samples, which lack a stress history, cohesion was 3.6 kPa and the internal friction angle was 24°. As the maximum consolidation stress increased (from 40 to 254 kPa) on resedimented samples, the shear-induced pore pressure increased (from 22 to 203 kPa), whereas the normalized peak shear stress decreased (from 0.32 to 0.25). Our experiments showed that resedimented samples have similar strength and failure behaviour to intact samples. By constraining pore pressure, strength and initial stress state, we gain a better insight into slope-failure dynamics. Therefore, our experiments provide constraints on strength and shear-induced pore pressure at the onset of shallow failure that could be included in slope-failure and hazard models.

Abstract The Oxfordian Smackover Formation is generally acknowledged to be a hydrocarbon source for numerous reservoirs in the Gulf of Mexico, both onshore and offshore. More than 25 wells in the eastern Gulf of Mexico have penetrated the Smackover since 2003. Offshore, the Smackover consists predominantly of limestone and shale containing thin organic layers. Immediately above the lower Smackover is a widespread shale marker. This thin shale is correlated as the base of the upper Smackover Formation, which consists of interbedded shale and limestone. This study will demonstrate that the lower Smackover Formation in the eastern Gulf of Mexico (Mississippi Canyon and De Soto Canyon offshore areas) is composed of a series of seven units that occur in the same sequence in virtually every well in which the lower Smackover has been encountered. Although the seven individual units can be resolved readily with the proper wireline suite, each has a sub-seismic thickness. The overall thickness of the lower Smackover is about 300 +/-100 feet. Unlike the lower Smackover, the surrounding Mesozoic formations, from Cotton Valley to Norphlet, vary greatly in thickness in the eastern Gulf. The initial correlations of the units in the lower Smackover were made by comparing the gamma ray, resistivity, and density log patterns with the computed mineralogy of Elemental Capture Spectroscopy (ECS) wireline logs. It was immediately obvious that the same sequence of beds/units was present in the lower Smackover in well after well. Within the lower Smackover Formation is a conspicuous zone characterized by iron-bearing minerals having a matrix density in excess of 3.0 g/cm 3 throughout. However, X-Ray Diffraction (XRD) data from rotary sidewall cores was necessary to validate the mineralogy. Because the mineralogy of the ECS log is a model-based calculation from the elemental concentrations of iron, calcium, aluminum, etc,. rather than a direct measurement, the modeled mineralogy can be inaccurate as was the case in the bottom two units. Mineralogy of the seven units has been verified by XRD analyses, albeit from a limited number of rotary sidewall cores obtained in only five wells. The top three units are limestones which vary in carbonate, clay, and pyrite content. The fourth and fifth units contain significant amounts of high density minerals, particularly siderite and pyrite. The sixth zone is dominated by anhydrite. The seventh unit is a hematite-rich shale and its base is an unconformity. Although wireline data are plentiful, analysis of the seven units within the lower Smackover is hampered by the limited amount of rock data and the complete lack of whole core. Many depositional and geochemical questions suggested by the unusual mineralogy and sequence of beds remain unanswered.

Abstract The eastern Mississippi Canyon area has been largely a Miocene oil and gas province in which recent discoveries in the Jurassic Norphlet Formation have been made. This paper focuses on a nascent Cretaceous play targeting the eastern Tuscaloosa fan comprised of large symmetric and asymmetric structures created by an expulsion-rollover system in the pre-Miocene interval. The top of the Cretaceous interval is found between 15,000’ and 27,000’, is up to 15,000’ thick, and is underlain by a mature Tithonian source rock. The play extends downdip from the Cretaceous shelf edge and the reservoir is interpreted to be the equivalent of the Tuscaloosa Formation of onshore South Louisiana. This paper will examine the idea that the central Cretaceous basin is in the optimal zone for the trend of appropriate subsurface temperatures, depth, and significantly expanded reservoir section in the Upper and Lower Cretaceous intervals.

Abstract The eastern Mississippi Canyon area has long been known as a prolific Miocene oil and gas province. Organic rich shales and marls of Tithonian and Oxfordian age strata are considered the major source rocks in the region. Recently however, attention has been directed towards the intervening siliciclastic Cretaceous strata as an enticing exploration target. This presentation focuses on the duality of the Cretaceous section as both a “first carrier” for hydrocarbons expelled from Late Jurassic source rocks and as a potentially significant “container” for hydrocarbon accumulations as well. Examined are the petroleum system elements and processes of eastern Mississippi Canyon. Emphasis is placed on the relative roles of vertical versus lateral secondary hydrocarbon migration from source to trap. Fluid properties from sampled accumulations help constrain model-derived estimates of thermal maturation, migration timing, loss en route and potentially available hydrocarbon yield. The “carrier/container” nature of the Cretaceous is characterized through multiple scenarios that address: Subsurface pressure relationships, bed-seal and fault properties; as well as seismically derived lithology and facies distributions. Fetch area constrained source rock expulsion through the Cretaceous is modeled, taking into account known Miocene accumulations. The results of such analyses lend insight into hydrocarbon potential within the Cretaceous.

Abstract This study provides an assessment of two source-to-sink sediment routing systems of the Early Cretaceous and highlights sedimentologic changes that occurred in response to major tectonic reorganization of the eastern Gulf of Mexico during the Valanginian-Hauterivian stages. Depth-imaged 2D and 3D seismic data, well log correlation, sand grain size, and detrital zircon U-Pb data obtained from the Valanginian intervals of the cores of a key well, facilitates source-to-sink analysis of Early Cretaceous deep-water deposits, as well as construction of a new depositional model of Hosston equivalent-siliciclastics previously investigated only in the western Gulf of Mexico onshore areas. U-Pb dating of detrital zircon grains suggests that Hosston siliciclastics observed in the 200-km-long base-of-slope sandy progradational delta-fed apron at the Florida Escarpment originated in a peninsular Florida source terrane – the Ocala Arch. Interpretation of 3D seismic data with nearby well control also allows conclusions to be drawn about the Appalachian-sourced Hosston fan system in Mississippi Canyon. This Appa-lachian-sourced sandy fan is believed to have terminated updip of a series of salt-related asymmetric expulsion rollovers, although we know sediment accommodation in these inverted basins was not confined to the Valanginian-Hauterivian age Hosston interval and extended from the Jurassic Cotton Valley-Bossier supersequence to the Late Cretaceous Navarro-Taylor supersequence. Two plausible models of Appa-lachian-sourced fan length are considered, incorporating calculations of salt rafting to estimate a best-case scenario fan length of 90-km, while a more probable fan geometry is determined from seismic observations and well control, yielding a Valanginian-Hauterivian submarine fan of 70-km length. The study presents a new paleogeographic model, with special focus on the eastern Gulf of Mexico and the interpreted sand-prone fan and progradational delta-fed apron. It also provides a robust model for source to sink transport during a critical phase of Gulf of Mexico basin evolution. The shorter fan length calculated in this study suggest the majority of asymmetric expulsion rollovers in Mississippi Canyon are either sandstone-poor or were sourced from a different, likely younger, source-to-sink system ( e.g. , Late Cretaceous, Cenomanian-Turonian-age Tuscaloosa fluvial system).