Abstract The continental margin of the northern Gulf of Mexico is characterized by a very complex morphology due to the interactions between sedimentary and halokinetic processes. Sediments deposited on the margin during the last glaciation provide an excellent opportunity for the study of depositional processes of fine-grained sediments in a structural complex deep-water environment. This study is based on detailed analysis of long sediment cores and high-resolution geophysical data from two areas of the northern Gulf of Mexico: Bryant and Eastern Canyon Systems and the Atlantis Discovery. At least three sedimentary provinces existed on the continental margin during the last glaciation. The Mississippi Canyon and Fan resulted from the building of the Mississippi River delta at the shelf-margin during the last glaciation. Turbidity currents flowing through the Mississippi Canyon at this time contributed to the continued development of the preexisting Mississippi Fan. The Texas and Louisiana continental slope province characterized by numerous intraslope basins. Deposits from the last glaciation consist of hemipelagic sediments interbedded with finegrained (silty-clay to clayey-silt) turbidites that thin and fine downdip. These deposits were produced by low-density turbidity currents that resulted from the depositional segregation (deposition of the coarsest material at the most proximal locations) of large turbidity currents initiated on the outer shelf and/or upper continental slope. Thick (up to 40 m), structureless mud deposits (unifites) on the floors of intraslope basins most likely resulted from the partial deposition of long-lasting (1.5-3 months), low-density turbidity currents. The continental rise and lower continental slope province of the northwest Gulf of Mexico has fine-grained, layered sediments that are siltier than those of the fine-grained turbidites from the continental slope province, and occasionally reveal a lenticular to wavy nature. The layered sediments are interpreted as deposits of turbulent sediment flows whose site of deposition is controlled by westward flowing bottom currents. The currents pirate the finer-grained sediment load (upper part and tails) of turbidity currents flowing through the Mississippi Canyon to the Mississippi Fan and relocate them within this province. Turbidity currents were most abundant between 32-28.5 ky BP and 28.5-21.5 ky BP. The first time interval coincides with the development of a major deglaciation event that led to highly increased river discharges. The second time interval coincides with the drop of the sea level to the shelf edge, and the production of shelf margin deltas during the last glacial maximum.

Abstract Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer-grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales. Studies on these deposits have indicated that they are often comprised of graded fine silt laminae sandwiched between films of clay minerals, quartz dust and organics. To date, little attention has been given to their characteristics resulting from different depositional processes. Internal characteristics, based on the different types of transport and the resulting structures after compaction and diagenesis, are poorly understood. Depending on the transport- deposition process, the architecture of the deposit will have different 3D extents and continuity. Stratigraphic prediction of the position and dimensions of the fine-grained beds can indicate whether the deposits will be a barrier or a baffle to fluid flow or a possible reservoir for natural gas. Minor variations in depositional style and other characteristics can result in more differences than presently assumed.

Abstract Shales and mudrocks are important to all aspects of hydrocarbon exploration and production. They commonly form source and seal rocks, and in some areas they are important reservoir rocks. In reservoir development, they may control production performance by compartmentalizing or baffling reservoir sandstones. Often, significant shales are beneath seismic resolution or detection limits, so their significance is unrecognized during early field development. Shale stochastic modeling is a useful technique for predicting production performance in reservoirs where shales and mudrocks are present. Future research should be directed toward (1) understanding why shales and mudrocks vary in their capacity to seal or compartmentalize strata, (2) developing recognition criteria to differentiate various shale types, and (3) enhancing their detection on seismic reflection records.

Abstract Sediment cores and high-resolution CHIRP seismic data were collected on the inner shelf adjacent to Atchafalaya Bay, Louisiana to examine the evolution of the newly forming mud delta associated with the Atchafalaya River lobe of the Mississippi deltaic plain. 210 Pb accumulation rates from sediment cores show maximum sedimentation rates (10-20 cm/yr) are concentrated on the innermost shelf (<6 m water depth) immediately seaward of the Pt. Au Fer shell reef at the bay mouth. Rates decrease rapidly offshore to 8-10 m water depth, where seismic profiles show modern deposits pinch out adjacent to shoals formed by erosional remnants of older Holocene deltaic deposits. Alongshore, rates remain relatively high to the west (along the chenier coast of West Louisiana) following the trend of coastal currents. The wedge-shaped prodelta reaches 2.5 m in thickness and is gas-charged adjacent to the Atchafalaya dredge channel on the shelf. In areas where accumulation rates exceed ~1 cm/yr, the prodelta muds take the form of cm-scale interlaminations of silty sands (proximal) or silts (distal) and clayey silt layers. At accumulation rates of ~0.5-1 cm/yr, primary fabric is partially destroyed by macrofaunal burrows. In seaward areas, 10-25 m water depth, where modern sediments are accumulating but rates are low, clayey silt and silty clay deposits are completely homogenized by burrowing activity. Sediment dispersal paths can be traced seaward of the bay mouth and westward, along the direction of prevailing coastal currents, using coarse silt content of the mud delta. This indicates that coarse silts behave as individual particles and are preferentially sorted, while fine-medium silts are contained within flocs with clay-size mineral grains, and show no preferential sorting downdrift from the source.

Abstract Recent studies of fluvial-marine sediment dispersal have demonstrated that, under conditions of adequate sediment supply and intense turbulence in the bottom- boundary layer, wave-enhanced gravity flows can develop on shelves with gradients <0.7°. These conditions can exist seasonally on many continental shelves proximal to river mouths. Therefore, such flows (and resultant deposits) are probably more widespread than previously thought. Examples from a high-energy active-margin setting (Eel Shelf, California) and a lower energy passive margin (Atchafalaya River/Louisiana Inner Shelf) indicate that similar bedding can be produced by high- concentration near bed flows in apparently contrasting depositional settings, due to the occurrence of similar physical processes in the bottom boundary layer. Because such hyperpycnal flows can carry much more sediment mass than can buoyant plumes of suspended sediment, and can move at velocities on the order of cm s -1 , such benthic sediment flows on shelves could represent a significant and previously underestimated transport mechanism for fine sediment across continental shelves.

Abstract

Abstract A predictive model to estimate the distribution, sealing capacity and petrophysical properties of shale seals and flow barriers will significantly reduce the risks associated with hydrocarbon exploration and exploitation. Such a sequence stratigraphy-based predictive model must be grounded in outcrop and field analogs, such as this examination of the sealing capacity, petrophysical properties and distribution of Upper Cretaceous Lewis marine shales in two wells from south-central Wyoming. The measured sealing capacity of these shales varies with textural and compositional factors that allow division of the Lewis Shale depositional sequence six argillaceous microfacies. Each microfacies displays distinctive compositional and petrophysical properties and occupies a well-defined sequence stratigraphic position including transgressive, highstand, and condensed section deposits, with characteristic seal and seismic properties. The microfacies, in order of greatest seal capacity to least, are phosphatic shales, pyritic fissile shales, silty shales, silty calcareous shales, silty calcareous mudstones, and bioturbated argillaceous siltstones. The most promising seals, the phosphatic and pyritic shales, belong to the condensed section and uppermost transgressive systems tract. The phosphatic shale is also characterized by the highest content of both total organic carbon (TOC) and authigenic minerals. Interestingly, neither of these two high sealing capacity microfacies shows more detrital clay than other microfacies. The microfacies with lower sealing capacities belong to the highstand systems tract and are generally poorer in iron-rich minerals than the better sealing microfacies. Petrophysical properties, including high bulk density, shear velocity, Young’s modulus and shear modulus, distinguish the best sealing microfacies from highstand systems tract microfacies with poorer seal capacity. This correspondence between sealing capacity and petrophysical properties suggests that seismic data may have good potential as a tool for seal evaluation.

Abstract The effect of shale fabric on the primary migration of oil is seen through scanning electron microscope analysis of oil morphology and microfracture network in two oil source rocks. SEM micrographs of oil produced by hydrous pyrolysis experiments from two well known source rocks (the Devonian-Missisipian Woodford Shale, Oklahoma and the Jurassic Kimmeridge Clay Formation, Dorset, England) show the following sequence of initial oil formation and movement: (1) within the shale matrix oil is initially generated in the Woodford Shale at 350°C for 1 day heating and in the Kimmeridge Clay Formation at 300°C for 3 days heating; (2) initial oil generation and microfracture formation occur at the same stage; (3) oil droplets continue to move along a pressure gradient out of the matrix into adjacent open microfractures; and (4) ribbons of oil fill the microfractures and move along the fracture network. The primary migration phase through the source rock appears to continue as oil is expelled. These observations highlight the importance of the argillaceous microfabric in influencing the primary migration of oil.

Abstract Permeability is the property or capacity of a porous rock, sediment, or soil for transmitting a fluid; it is a measure of the relative ease of fluid flow under unequal pressure ( Bates and Jackson, 1980 ). The most significant characteristic of mudstones, siltstones and shales are their extremely low permeability. Fine-grained sediments have some of the lowest permeability of any natural occurring mineral, rock or sediment. The porosity of a fine-grained sediment is the major factor that controls the permeability of sediment. The relationship between porosity and permeability of a fine-grained sediment is one of nature’s largest contrasts, one that covers 13 to 15 orders of magnitude. Determining the permeability of fine-grained sediments, that are virtually impermeable, is a difficult accomplishment and can only be achieved accurately by certain geotechnical measurements, such as the consolidation test. Consolidation tests performed on a multitude of fine-grained sediments from the Gulf of Mexico resulted in the determination of the relationships between porosity and permeability of a fine-grained sediment.

Abstract The Cretaceous interval in the Maracaibo consists mainly of fine-grained clastic and carbonate sediments deposited during a period of passive margin development. Stratigraphically, the early Cretaceous Apon Formation of Aptian age contains interbedded rich organic dolomitic and calcareous shale and black bituminous limestone (Mercedes, Tibu, Guaimaros and Machiques Members). Not only do these constitute important source and reservoir rocks in the southwest region of the Maracaibo Basin, but also represent maximum flooding surfaces within the overall depositional characteristics in the Basin. To the north, the Apon along with the Lisure and Maraca Formations of the Aptian-Albian Cogollo Group, consisting mostly of limestone and fine-grained sandstone, represent a shallow to middle shelf environment transgressive systems tract. In the south, the Cretaceous Capacho, consisting of black shale and the La Luna Formation comprised of interbedded calcareous shale and black cherty limestone, were deposited during a period of relative sea level rise. The La Luna Formation is the primary source rock in the basin as well as a reservoir in places where microfractures produce the necessary permeability for hydrocarbon flow. Numerous geochemical studies performed in the Maracaibo Basin provide evidence for the existence of multiple stratigraphic intervals containing hydrocarbon generating organic-rich shale and calcareous mudstone within the Cretaceous. Integrated reservoir studies and production data indicate that some of the same intervals are excellent oil and gas producers where fracture systems exist. The above-mentioned stratigrahic and depositional characteristics are responsible for making the Maracaibo Basin one of the most prolific producers of hydrocarbons in the world.

Abstract Turbidite-related mudstones are an integral part of any submarine fan complex. The Permian Tanqua Karoo in southwestern South Africa is an excellent example. Thick-bedded mudstones, 10–50 m thick, subdivide the individual sand-rich fans within a fan complex. The 30–50 cm thick medium-bedded mudstones and 1–30 cm thick thin-bedded mudstones occur between the sandstone layers of a fan system and may act as baffles to fluid flow. Characteristics of thick-bedded shales are that the middle fan areas (leveed channels) show predominantly alternating fining-upward sequences of parallel laminated mudstones and mud-shales (∼50–60% clay) and clay-stones and clay-shales (∼80–95% clay). A sharp contact typifies the location of the contact between the mud and the overlying clay. Medium-bedded shales are typically more siliceous. They are comprised of cyclic fining-upward sequences of quartz-rich siltstones, mudstones and sometimes a clay-shale lamination. However, clay-shale preservation is rare. A thin-bedded siltstone (∼10–20% clay), up to 0.5 cm thick, predominates at the base of each sequence and is capped by laminations of mud-shale (∼35–60% clay), which are generally less than 5 mm thick. Thin-bedded shales are characterized by fining-upward sequences comprised of siltstone (∼20–30% clay), mud-shale (∼35–55% clay), and clay-shale lamina. The sequence is typically mud-rich and the mud-shale often expresses an amalgamated contact with the clay-shale material above. The limited thickness of thin-bedded shales and the siliceous nature of medium-bedded shales suggests that fluid and gas flow may be possible. Even minor tectonics result in fractures. The combination of the clays with some micas and organics suggest that many of these mudstones can be source rocks and gas reservoir at the same time.

Abstract An extensive field of mega-furrows has been recently discovered on the seafloor at the base of the Sigsbee Escarpment in the northwestern Gulf of Mexico. Deep-tow data acquired by Texas A&M University and 3-D seismic data supplied by WesternGeco show that the scale of these fields far exceeds anything previously observed in the world's oceans. These data allow individual furrows and the entire field to be resolved in unprecedented detail. The size of the furrows, variations in their morphology, and their orientation relative to large topographic features all provide a geological record of the long-term deepwater flow that produces the furrows. The furrows change geometries in a predictable pattern according to flume experiments of increasing current velocity over fine-grained sediments. Dives with the DSV Alvin confirm the erosion of furrows into fine-grained material and their modern geometries, including steep sides up to ∼70°. Data from near bottom current meters verifies that strong, previously unknown, currents do exist at the base of the Sigsbee Escarpment today. Additional paleo-furrow horizons have been identified below the seafloor and may provide a link to past bottom currents in the Gulf of Mexico. The furrows and associated environmental processes are dominant features of the northwestern Gulf of Mexico and similar features are expected to be found along the rises of most other ocean basins.

Abstract The long-term production history of some offshore and onshore Gulf Coast reservoirs reveals that gas production exceeds assigned reserves for no readily apparent reason. The distinct possibility exists that surrounding shales contribute significant quantities of gas during the reservoir lifecycle. Direct evidence for economic gas production from non-fractured shale intervals comes from the Devonian of the Appalachian Basin and from Tertiary-Pleistocene reservoirs in the Gulf of Mexico. Intergranular pores occur between detrital clay particles in true shales of the Devonian interval. The ability of this intergranular pore system to transmit gas (permeability) is controlled primarily by the microfabric of the shale. It is reasonable, therefore, to expect that Gulf Coast shales with similar internal characteristics will yield sufficient gas to impact reservoir economics. The Gulf Coast area contains significant proportions of sediments deposited in distal deltaic and deep water environments. These environments produce thick, fine grained “shale” intervals that, in reality contain numerous thin (<1 inch) laminations of porous and permeable silt and/or sand separated from one another by layers rich in clay minerals (true shales). Given a large number of silt/sand interbeds, sufficient permeability thickness can be developed in the interval to yield gas at high rates. Routine methods of log analysis fail to resolve the thin-bedding in these pay intervals, many of which are therefore bypassed. Reserve calculations can also be significantly effected by gas production from true shales in traditional reservoir settings (such as the Wilcox Formation). The amount of gas recovered in reservoirs developed in relatively thin sand bodies (generally <50ft) can be increased by gas migration from surrounding shales during production-related pressure depletion of the main reservoir body. Improved reserve calculations require that potentially productive shales are included in all aspect of reservoir evaluation, from petrophysics to simulation.

Abstract Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales To date, little attention has been given to their characteristics resulting from different depositional processes.