Clay smear: Review of mechanisms and applications

AbstractClay smear is a collection of fault processes and resulting fault structures that form when normal faults deform layered sedimentary sections. These elusive structures have attracted deep interest from researchers interested in subsurface fluid flow, particularly in the oil and gas industry. In the four decades since the association between clay-smear structures and oil and gas accumulations was introduced, there has been extensive research into the fault processes that create clay smear and the resulting effects of that clay smear on fluid flow. We undertake a critical review of the literature associated with outcrop studies, laboratory and numerical modeling, and subsurface field studies of clay smear and propose a comprehensive summary that encompasses all of these elements. Important fault processes that contribute to clay smear are defined in the context of the ratio of rock strength and in situ effective stresses, the geometric evolution of fault systems, and the composition of the faulted section. We find that although there has been progress in all avenues pursued, progress has been uneven, and the processes that disrupt clay smears are mostly overlooked. We highlight those research areas that we think will yield the greatest benefit and suggest that taking these emerging results within a more process-based framework presented here will lead to a new generation of clay smear models

Relations between mud volcanoes, thrust deformation, slope sedimentation, and gas hydrate, offshore north Panama

SeaMARC II swath-mapping and migrated seismic reflection data show a high concentration of mud volcanoes in the primary sediment depocentre along the lower slope of a thrust belt, offshore north Panama. The mud volcanoes are 0.4-2.0 km wide, Sonar reflectivity (backscattering), sediment cores, and seismic stratigraphic relations indicate that the depocentre contains thick sequences of basinal turbidites which are ponded between the anticlinal ridges. The ridges are composed of the deformed turbidites of the Colombian basin and exhibit a strong bottom-simulating reflector (BSR), apparently associated with a gas hydrate layer. Based on the concentration of mud volcanoes along the crests of the anticlinal ridges in the depocentre and the structural position of the BSR, we suggest that folding along the deformation front, sediment ponding leading to differential loading, methane migration and accumulation in the anticlines, and gas hydrate formation are important factors in the development of mud volcanoes in this region.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28727/1/0000553.pd

The dating of shallow faults in the Earth's crust

Direct dating of ductile shear zones and calculation of uplift/exhumation rates can be done using various radiometric dating techniques. But radiometric dating of shallow crustal faulting, which occurs in the crust's brittle regime, has remained difficult(1-4) because the low temperatures typical of shallow crusted faults prevent the complete syntectonic mineral recrystallization that occurs in deeper faults. Both old (detrital) and newly grown (authigenic) fine-grained phyllosilicates are thus preserved in shallow fault zones and therefore their radiometric ages reflect a mixture of both mineral populations. Also, the loss of Ar-39 during neutron irradiation in dating of clay minerals can produce erroneously old ages. Here we present a method of characterizing the clay populations in fault gouge, using X-ray modelling, combined with sample encapsulation, and show how it can be used to date near-surface fault activity reliably. We examine fault gouge from the Lewis thrust of the southern Canadian Rockies, which we determine to be similar to 52 Myr old. This result requires the western North America stress regime to have changed from contraction to extension in only a few million years during the Eocene. We also estimate the uplift/exhumation age and sedimentary source of these rocks to be similar to 172 Myr.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62567/1/412172a0.pd

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records

Experiments on Rapid Deposition of Sand from High-velocity Flows

Observations of sand transport and rapid deposition by strong sand-laden surges in an open channel, by means of high-speed motion pictures, revealed that the dominant effect is a process by which a laminar sheared layer, with sand concentrations approaching the threshold for immobilization by grain interlocking, develops as suspended sand becomes concentrated near the base of the flow. The laminar sheared layer climbs vertically with time as sediment is progressively immobilized at the base of the layer and added at the top from the overlying turbulent flow. The mobility of the laminar sheared layer is probably enhanced by continuous upward flow of interstitial water that is trapped within the layer as the layer accumulates. As the flow weakens and the concentration of suspended sand in the flow decreases, the laminar sheared layer thins and/or becomes immobilized, giving way to the familiar weak traction transport on a well-defined immobile sand bed. Particles reside in the laminar sheared layer only briefly, and move only a short distance before immobilization; the laminar sheared layers seem not to share essential features with traction carpets. Much of the coarser, structureless lower most parts of thick turbidites might have been emplaced by processes not unlike those described here. Our results suggest that the entire turbidity current need not be of extremely high density for an initial deposit to be formed rapidly from extremely high near-bed sediment concentrations. Résumé Des observations réalisées à l'aide de caméras à hautes vitesses sur le transport et la sédimentation rapide provoqués par de forts courants déferlants et à fortes charges de sables en écoulement chenalisé, ont montré que l'effet dominant est le développement, à la base de la coulée, d'une couche laminaire cisaillée ayant une concentration de particules à la limite du seuil de l'immobilisation par congestion des particules. Avec le temps, cette couche cisaillée se déplace verticalement vers le haut, au fur et à mesure que les sédiments s'immobilisent à la base de la couche et que d'autres s'y ajoutent à partir de la couche turbulente du dessus. L'expurgation incessante de l'eau interstitielle par le tassement des sédiments du fond augmente probablement la mobilité de la couche laminaire cisaillée. Lorsque le courant s'affaiblit et que la concentration du sable en suspension diminue, la couche laminaire cisaillée s'amincit et/ou s'immobilise et est remplacée par une couche de transport plus faible caractérisée par la traction sur un fond sableux immobile bien défini. Les particules ne font que transiter brièvement dans la couche laminaire cisaillée et ne se déplacent que sur de faibles distances avant de s'immobiliser ; les couches laminaires cisaillées et les couches de traction ne semblent partager aucune caractéristiques communes essentielles. Il est possible qu'une bonne portion des couches basales non-structurées des turbidites aient été mises en place par un processus de ce genre. Les résultats de nos études montrent qu'il n'est pas nécessaire que la charge sédimentaire de l'ensemble d'un courant de turbidité soit d'une très grande densité pour entraîner rapidement la formation d'un premier dépôt à partir d'une couche à forte densité longeant le fond

Gas geochemistry of the Mobile Bay Jurassic Norphlet Formation: Thermal controls and implications for reservoir connectivity

The Mobile Bay gas field is located offshore Alabama in the northern Gulf of Mexico. Production is from eolian dunes of the Jurassic Norphlet sandstone at depths exceeding 6100 m (gt20,000 ft) and temperatures greater than 200degC. Reservoir connectivity and compositional variation, including the distribution of nonhydrocarbon gases (H2S and CO2), are critical factors in production strategy. To evaluate the controls on compositional variation and connectivity, detailed molecular and isotopic analyses were conducted for 29 wells. Analysis of volatiles in fluid inclusions suggests that the field was originally filled with oil that subsequently cracked to gas. In addition to the thermal destruction (cracking) of oil, the process of thermochemical sulfate reduction (TSR) continues to destroy the remaining hydrocarbons through oxidation of gas and reduction of sulfate to form H2S and CO2. The variable extent of the TSR process at Mobile Bay results in a wide range of hydrocarbon and H2S compositions. Condensates are almost exclusively composed of diamondoids whose composition appears controlled by H2S concentrations. In contrast to hydrocarbon and H2S contents, CO2 concentrations are relatively constant throughout the field. Carbon isotopic ratios for CO2 correlate positively with those for wet-gas hydrocarbons but are heavier than expected for CO2 originating from hydrocarbon oxidation via TSR. The narrow range of CO2 contents and heavy isotope ratios suggests that CO2 is regulated by water-rock equilibration and carbonate precipitation. The destruction of the hydrocarbon gas and mineralization of the carbon dioxide product create a volume reduction and an associated drop in reservoir pressure. This process creates several internal sinks (or exits) that may control the spill direction for gas in the field