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: We have examined the Neogene stratigraphic successions recovered from six wells located along a present-day middle neritic (current depositional depth 92 m) to upper bathyal depth (current depositional depth 482 m) transect oblique to the shelf/slope margin in the northern Gulf of Mexico (GOM) using calcareous plankton biostratigraphy. The latter were used to conduct stratigraphic interpretation of the sections and to determine their completeness. We establish that all sections vary considerably in thickness and completeness, depending on depth of deposition, as estimated from benthic foraminiferal analysis, which shows that depositional depth at the six sites changed little through the Neogene. The shallowest section (~90-m estimated depositional depth through the Neogene) is the thinnest with the most complete Upper Miocene–Pleistocene record, whereas the deepest section (~600– 800-m estimated depositional depth) is the thickest but also contains the least complete Pliocene–Pleistocene record. The Upper Miocene to Pleistocene sediments deposited between ~200- and 500-m estimated depositional depth exhibit a characteristic allostratigraphic architecture, with sedimentary units bounded by unconformities associated with 1- to 2-Myr hiatuses that vary little along the transect. We integrate the stratigraphic architecture along our local transect in the regional Cenozoic depositional framework in the GOM of Galloway and coauthors and establish that the allostratigraphic units (AUs) correspond well with several of the genetic and seismic sequences delineated. We interpret the depth-related increase in thickness of the Upper Miocene–Pleistocene AUs in light of the sedimentary processes discussed by these authors. However, our interpretation differs considerably from theirs based on our documentation of temporally incomplete sections in the wells. The sedimentary pattern in Well-3 (~200-m estimated depositional depth) is quite different from that at nearby Well-1 (~100–600-m estimated depositional depth), although very similar to the wells further west, even though the distance between Well-3 and Well-6 is about four times that between Well-3 and Well-1. We note also that the stratigraphic pattern in Well-1 changed ~8 Ma, from highly discontinuous before to remarkably continuous after. We have found no clear evidence that glacio-eustasy shaped the Neogene stratigraphic record in the study area. Therefore, we question whether glacio-eustasy was the primary forcing mechanism on stratigraphic architecture in the GOM beyond the shallow part of shelves and propose that salt tectonics may have been a more prominent factor in controlling accommodation. An allostratigraphic architecture was described earlier from the De Soto Canyon northeast of the GOM transect, where the AUs and their boundaries were shown to match, respectively, the seismic sequences and surfaces on the nearby Florida margin. We therefore consider the AUs along the GOM transect as corresponding as well to seismic sequences and therefore to parts of depositional sequences. Based on this, we review notable difficulties in characterizing seismic features (sequences and surfaces) in concrete stratigraphic records and recommend a greater awareness of the temporal significance of unconformities, many of which are associated with multimillion-year hiatuses.

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.