analysis of directional wave fields using x band navigation radar the aim of this work is to present an approach to describe complex sea states including the ones consisting of superpositions of swell and wind sea components using a nautical radar in x band as a remote sensing technique in the present article the inversion method to obtain the spectral representation of the wave fields is described the method is applied to analyse data obtained from simulation techniques as well as from measurements obtained during oceanographic campaigns in the bay of biscay and north sea

geochemical relationships of petroleum in mesozoic reservoirs to carbonate source rocks of jurassic smackover formation southwestern alabama algal carbonate mudstones of the jurassic smackover formation are the main source rocks for oil and condensate in mesozoic reservoir rocks in southwestern alabama this interpretation is based on geochemical analyses of oils condensates and organic matter in selected samples of shale norphlet formation haynesville formation trinity group tuscaloosa group and carbonate smackover formation rocks potential and probable oil source rocks are present in the tuscaloosa group and smackover formation respectively extractable organic matter from smackover carbonates has molecular and isotopic similarities to jurassic oil although the jurassic oils and condensates in southwestern alabama have genetic similarities they show significant compositional variations due to differences in thermal maturity and organic facies lithofacies organic facies reflect different depositional conditions for source rocks in the various basins the mississippi interior salt basin was characterized by more continuous marine to hypersaline conditions whereas the manila and conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter petroleum and organic matter in jurassic rocks of southwestern alabama show a range of thermal transformations the gas content of hydrocarbons in reservoirs increases with increasing depth and temperature in some reservoirs where the temperature is above 266 f 130 c gas condensate is enriched in isotopically heavy sulfur apparently derived from thermochemical reduction of jurassic evaporite sulfate this process also results in increased h2s and co2 in the gas and depletion of saturated hydrocarbons in the condensate liquids thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature because some deep 18 000 ft or 5 5 km and hot 320 f or 160 c smackover and norphlet reservoirs contain low sulfur petroleum

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

a study on the spectral models for waves in finite water depth spectral models for waves in finite water depths are developed based on an extension of the wallops spectrum huang et al 1981a for the deep water waves it is found that for intermediate water depth stokes wave expansions offer a good approximation in this case the spectral function is controlled by three parameters the significant slope the nondimensional depth kh and the peak frequency n0 for the shallow water waves solitary and cnoidal wave models have to be used the controlling parameters now reduce to two again that is the ursell number ur 2 kh 3 and the peak frequency the resulting spectral models are designed with a specific emphasis on but not limited to the energy containing range of the spectrum they are also not limited to any particular sea state they offer a possible explanation of the variations in the spectral slope observed by previous investigators comparisons with observational results from both laboratory and field show good agreements

appleton field case study eastern gulf coastal plain field development model for upper jurassic microbial reef reservoirs associated with paleotopographic basement structures appleton oil field located in escambia county alabama was discovered in 1983 through the use of two dimensional seismic reflection data the field structure is a northwest southeast trending paleotopographic ridge comprised of local paleohighs the field produces from microbial reef boundstones and shoal grainstones and packstones of the upper jurassic smackover formation because appleton field is approaching abandonment owing to reduced profitability an integrated geoscientific study of the field structure and reservoir was undertaken to determine whether drilling additional wells in the field would extend the productive life of the reservoir the conclusion from the integrated study which included advanced carbonate reservoir characterization three dimensional geologic visualization modeling seismic forward modeling porosity distribution analysis and field production analysis was that a sidetrack well drilled on the western paleohigh should result in improved oil recovery from the field the sidetrack well was drilled and penetrated porous smackover reservoir near the crest of the western paleohigh the well tested 136 bbl oil day begin page 1700

the shikotan earthquake of october 4 1994 lithospheric earthquake a large m w 8 2 earthquake occurred off shikotan is one of the kurile is on october 4 1994 we inverted 32 body wave records to determine the rupture pattern using an iterative deconvolution method the mechanisms of the subevents were allowed to vary during rupture the source parameters obtained are the location of the initial break 43 48 n 147 40 e the centroid depth 56 km strike dip rake 49 75 125 for the total source the seismic moment m o 2 6 10 21 nm m w 8 2 source time duration t 42 s the average rupture velocity 2 5 km s we also determined the mechanism using long period love and rayleigh waves from 14 stations the solution for a finite source distributed over a depth range from 0 to 90 km is strike dip rake 54 76 129 with mo 2 3 10 21 nm in good agreement with that from body waves referring to the extent of the aftershock area and the subevent distribution we estimated the fault area s 120 60 km the average slip d 5 6m and the stress drop 11 mpa we computed synthetic waveforms as well as static displacements using either the steep or the low angle plane as the fault plane and found that the steep dip fault model fits the data better our result the mechanism large centroid depth high stress drop strongly suggests that the 1994 shikotan earthquake is a lithospheric earthquake an intra plate event that ruptures through a substantial part of the subducting oceanic lithosphere this type of lithospheric earthquake is relatively common

behaviour of the oceanic lithosphere at subduction zones plastic yield strength from a finite element method abstract using the finite element method to study the deflection of the oceanic lithosphere at subduction zones shows that deflection of an elastic plastic model agrees with observed bathymetry data except for 2 examples kermadec and mariana where the lithospheric behaviour remains elastic seaward of the trench seven of the cases we studied could be easily fitted and enabled us to compute an effective yield strength the average value of which is about 3 8 to 4 8 kbar and seems to be the same in every case

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

controls on reservoir development in a shelf carbonate upper jurassic smackover formation of alabama hydrocarbon reservoirs of the upper jurassic smackover formation in alabama are predominately oolitic and pelletal dolostone pore systems are dominated by moldic and secondary intraparticle pores intercrystalline pores or mixtures of these pore types all smackover reservoirs in alabama have been strongly affected by early cementation dissolution of calcium carbonate allochems and dolomitization marine phreatic cement occluded primary interparticle porosity in much of the smackover reservoirs in alabama dolomitization of the smackover in alabama included penecontemporaneous early burial and late deep burial episodes early burial dolomite predominates fabric selective dolomitization yielded reservoirs strongly influenced by both depositional fabric and diagene is nonselective dolomitization yielded reservoirs with intercrystalline pore systems shaped primarily by diagenesis porosity evolution was controlled regionally by level of thermal exposure mode of dolomitization and proximity to the wiggins arch thermal exposure is inversely related to porosity but the relationship is weak r2 0 5 fabric selective dolostone is slightly more porous than nonselective dolostone when averaged over the entire study area averages of 18 1 and 15 1 respectively p 0 0001 but nonselective dolostone is more porous at a given level of equivalent vitrinite reflectance smackover fields on the north flank of the wiggins arch are unusually porous given their level of thermal maturity and are unusual in other ways as well local porosity variation was controlled by depositional fabric early cementation dissolution and burial compaction and cemen ation regional permeability variation cannot be explained using existing data permeability is locally controlled by pore throat size the effects of dolomite crystal size distribution early cementation fracturing and burial compaction and cementation pore throat size exhibits the strongest overall correlation with permeability r2 0 54 permeability and porosity are strongly correlated locally but the regional correlation is weak

regional jurassic geologic framework of alabama coastal waters area and adjacent federal waters area abstract to date numerous jurassic hydrocarbon fields and pools have been discovered in the cotton valley group haynesville formation smackover formation and norphlet formation in the tri state area of mississippi alabama and florida and in alabama state coastal waters and adjacent federal waters area petroleum traps are basement highs salt anticlines faulted salt anticlines and extensional faults associated with salt movement reservoirs include continental and marine sandstones limestones and dolostones hydrocarbon types are oil condensate and natural gas the onshore stratigraphic and structural information can be used to establish a regional geologic framework for the jurassic for the state coastal waters and adjacent federal waters areas evaluation of the geologic information along with the hydrocarbon data from the tri state area indicates that at least three jurassic hydrocarbon trends oil oil and gas condensate and deep natural gas can be identified onshore these onshore hydrocarbon trends can be projected into the mobile area in the central gulf of mexico and into the pensacola destin dome and apalachicola areas in the eastern gulf of mexico substantial reserves of natural gas are expected to be present in alabama state waters and the northern portion of the mobile area significant accumulations of oil and gas condensate may be encountered in the pensacola destin dome and apalachicola areas

facies heterogeneity and depositional models of a ladinian middle triassic microbial dominated carbonate ramp system catalan coastal ranges ne spain abstract the ladinian sedimentary successions record a major transgressive pulse of northeastern iberia during the middle triassic this record is arranged in two transgressive regressive t r sequences formed by two fault block microbial and shoal dominated carbonate ramps the main factor controlling the creation of accommodation space is the differential rapid syn rift subsidence which induces the generation of palaeogeographical barriers that played a key role in the sedimentary architecture and microbial development the facies architecture of these ramps was reconstructed by means of facies analysis and sequence stratigraphy t r sequence 1 corresponds to a fault block carbonate ramp system dominated by microbialites and oolitic shoals t r sequence 2 represents a fault block carbonate ramp system mainly characterised by lagoons and shoals the diverse types of microbialites stromatolites and thrombolites are widely represented in the inner and middle ramp settings reaching a width of 45 km and an average thickness of 40 m t r sequence 1 contains the most noticeable microbial deposits stromatolites in the inner ramp up to 7 m thick and thrombolites in the middle ramp up to 40 m thick t r sequence 2 is mainly characterised by the occurrence of oolitic stromatolites in the inner ramp and internal shoals and sheltered lagoons in the middle ramp microbialites exhibit a fabric selective ancient porosity which is currently occluded by coarse calcite and quartz cements the most common types of ancient porosity are interlaminar vuggy and mouldic grain dissolution and evaporite replacement the studied carbonate ramps can be used as an analogue for ancient microbialite reservoirs in rapidly subsiding depositional areas with a high diversity of microbial deposits formed in restricted to open marine conditions with a wide array of ancient porosity and a well known sequence stratigraphic context

mantle convection with strong subduction zones because mantle viscosity is temperature dependent cold subducting lithosphere should be strong which implies that the rapid localized deformation associated with subduction should strongly resist plate motions due to computational constraints the deformation of a subducting plate cannot be accurately resolved in mantle scale convection models so its effect on convection is difficult to investigate we have developed a new method for implementing subduction that parametrizes the deformation of the oceanic lithosphere within a small region of a finite element grid by imposing velocity boundary conditions in the vicinity of the subduction zone we enforce a geometry for subduction producing a slab with a realistic thermal structure to make the model dynamically consistent we specify a rate for subduction that balances the energy budget for convection which includes an expression for the energy needed to deform the oceanic lithosphere as it subducts this expression is determined here from a local model of bending for a strong viscous lithosphere by implementing subduction in this way we have demonstrated convection with plates and slabs that resemble those observed on earth but in which up to 40 per cent of the mantle s total convective resistance is associated with deformation occurring within the subduction zone this additional resistance slows plate velocities by nearly a factor of two compared to models with a weak slab for sufficiently strong lithosphere the bending deformation slows surface plates sufficiently that they no longer actively participate in global scale convection which occurs instead beneath a sluggish lid by introducing a low viscosity asthenosphere beneath the oceanic plate we demonstrate that small scale convection at the base of oceanic lithosphere may limit plate thickness and thus the resistance to bending and cause plate velocities to depend on the strength of the bending lithosphere rather than on the viscosity of the underlying mantle for a cooling earth the effective lithosphere viscosity should be nearly constant but the mantle viscosity should increase with time thus subduction resisted convection should produce nearly constant plate velocities and heat flow over time which has implications for the earth s thermal evolution we estimate that this style of convection should apply if the effective viscosity of the bending lithosphere is greater than about 10 23 pa s but only if some mechanism such as small scale convection prevents the bending resistance from stopping plates altogether such a mechanism could be fundamental to plate tectonics and earth s thermal history

reservoir characterization modeling and evaluation of upper jurassic smackover microbial carbonate and associated facies in little cedar creek field southwest alabama eastern gulf coastal plain of the united states an integrated field study of the microbial carbonate and associated reservoirs at little cedar creek field in southwest alabama eastern gulf coastal plain of the united states provides an excellent opportunity to examine the spatial distribution of the sedimentary petrophysical and productivity trends in microbial reservoirs this study includes characterizing the sedimentary petrophysical and hydrocarbon productivity characteristics of microbialites developing a three dimensional geologic reservoir model and evaluating the hydrocarbon potential of these reservoirs the lower reservoir comprises subtidal thrombolitic boundstone associated with microbial buildups oriented in a southwest to northeast direction over an area that encompasses 32 mi2 83 km2 these buildups developed in clusters in the western central and northern parts of the field and attained thicknesses of 43 ft 13 m the clusters are separated by interbuildup areas of microbialites of 7 9 ft 2 3 m in thickness that are overlain by a thick section of nonreservoir microbially influenced lime mudstone and wackestone porosity in the microbial reservoirs includes depositional constructed void intraframe and diagenetic solution enhanced void and vuggy pore types this pore system provides for high permeability and connectivity in the reservoir beds permeability ranges to as much as 7953 md and porosity to as much as 20 the microbial boundstone beds have high potential as hydrocarbon flow units however the buildup areas are separated by interbuildup areas associated with a thick section of low permeability to nonreservoir beds that serve as potential baffles or barriers to flow much of the 17 2 million bbl oil produced from the field is from the microbial lithofacies the results from the little cedar creek field study have application in the design of improved development strategies for other fields producing from microbial carbonate reservoirs

on the use of laser profilometry for ocean wave studies airborne laser profilometer measurements of wave skewness and spectra are presented the measurements were made during high wind and sea state conditions in the north atlantic the sea states ranged from 4 5 to 8 7 m and varied from swell to wind wave dominated situations skewness values for the five cases ranged from 0 08 to 0 28 and compare favorably with other existing measurements also the phillips and wallops spectral models show excellent agreement with profilometer spectra of developing seas for one case where the seas were fully developed the pierson moskowitz spectrum provides the best representation

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

constraints on subducting plate strength within the kermadec trench four specially designed surveys parallel to the kermadec trench allow localized estimates of plate strength within the subducting pacific plate to be made the transfer function between topography and gravity is estimated for five trench parallel ship tracks at distances of 25 110 km from the trench axis we find a clear reduction in the magnitude and peak wavelength of the transfer function from the outer rise to the trench axis the change in the transfer function indicates a decrease in plate strength and is consistent with a reduction in flexural rigidity by 3 5 orders of magnitude or a decrease in effective elastic thickness by more than 15 km such a large magnitude decrease in the effective elastic strength suggests that the plate has little or no elastic strength within the trench and that viscous stresses play an important role in transferring slab pull forces to the subducting plate and regulating plate speeds in subduction zones

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

origin of dolostone reservoir rocks smackover formation oxfordian northeastern gulf coast u s a formation of regionally extensive dolostone reservoir rocks in the smackover can be understood despite the possible effects of recrystallization geochemical and petrographic data suggest that dolomitization took place in 1 seawater seepage 2 reflux 3 near surface mixed water 4 shallow burial mixed water and 5 deeper burial environments which overlapped in time and space to form a platform scale dolostone body composed of a complex mixture of dolomites seawater seepage and reflux dolomitization occurred in the near surface penecontemporaneously with deposition of the smackover and overlying haynesville formations dolomitization by seawater seepage occurred within an oolite grainstone sill which separated an intraplatform salt basin from the open sea seawater flowed landward through the sill in response to evaporitic drawdown of brines in the isolated intraplatform basin isolation of the salt basin occurred during the oxfordian when the shoreline retreated from the conecuh embayment dolomite located at the top of the smackover enriched in 18 o suggests additional dolomitization by reflux of hypersaline brines reflux occurred as buckner coastal sabkhas prograded over smackover oolite grainstone shoreface deposits vugs lined with shallow burial calcite and dolomite cements indicate flushing of the smackover grainstone aquifer with fresh water freshwater intrusion probably occurred following sea level lowstands during the late jurassic and early cretaceous leaching in the proximal portion of the freshwater aquifer produced excellent limestone reservoir rocks in the updip smackover dolomitization in the contemporaneous downdip mixed connate freshwater zone formed dolostone reservoir rocks with depleted isotopic compositions consistent with a shallow burial mixed water origin

improving recovery from mature oil fields producing from carbonate reservoirs upper jurassic smackover formation womack hill field eastern gulf coast u s a reservoir characterization modeling and simulation were undertaken to improve production from womack hill field eastern gulf coast united states this field produces oil from upper jurassic smackover carbonate shoal reservoirs these reservoirs occur in vertically stacked heterogeneous depositional and porosity cycles the cycles consist of lime mudstone and wackestone at the base and ooid grainstone at the top porosity has been enhanced through dissolution and dolomitization porosity is chiefly interparticle solution enlarged interparticle grain moldic intercrystalline dolomite and vuggy pores dolostone pore systems and flow units have the highest reservoir potential petroleum trapping mechanisms include a fault trap footwall uplift with closure to the south against a major west southeast trending normal fault in the western area a footwall uplift trap associated with a possible southwest northeast trending normal fault in the south central area and a salt cored anticline with four way dip closure in the eastern area potential barriers to flow are present as a result of petrophysical differences among and within the cycles as well as the presence of normal faulting reservoir performance analysis and simulation indicate that the unitized western area has less than 1 mmstb of oil remaining to be recovered and that the eastern area has 2 3 mmstb of oil to be recovered a field scale reservoir management strategy that includes the drilling of infill wells in the eastern area of the field and perforating existing wells in stratigraphically higher porosity zones in the unitized western area is recommended for sustaining production from the womack hill field

model for how microbial methane generation can preserve early porosity in dolomite and limestone reservoirs in some dolomite and limestone hydrocarbon reservoirs protection from cementation is a primary factor in porosity preservation we present a model in which methanogens methane producing microorganisms produce methane gas ch4 g that outgasses from solution in pore space creating a two phase system that reduces effective hydraulic conductivity k protecting pore space from cementation methanogens have been implicated in dolomite formation and can generate methane to fill pore space of a model near surface carbonate sediment 37 5 primary porosity with ch4 g in 180 to 4650 yr depending on nutrient levels gas generation results in occlusion of the aqueous phase from pore spaces and throats creating a two phase flow regime reducing the effective hydraulic conductivity of model marine carbonate sand by about 50 from 2 8 to 1 2 cm day in as little as 55 yr therefore reducing aqueous fluid flow necessary for cementation despite rapid burial and complete cessation of methanogenesis effective hydraulic conductivity could take more than 100 000 yr to return to its original value if methanogenesis continues with burial the effective hydraulic conductivity is reduced to zero after 250 yr dolomites can preserve more primary porosity with depth than other carbonates we propose that in addition to increased structural resistance a biogenic model exists for porosity preservation in dolomites that is linked to the activity of methanogens this model represents specific end member cases and illustrates the effect of methane buildup in relationship to the extent of reservoir diagenesis

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

diagenetic controls on reservoir development and quality smackover formation of southwest alabama abstract abstract smackover reservoirs in southwest alabama are much more complex than those encountered in the central and western portions of the gulf coast basin this heterogeneity is a product of a complex history of diagenetic modification the diagenetic history can be divided into five separate stages 1 marine stage extensive grain micritization and precipitation of minor amounts of fibrous cement 2 meteoric vadose stage extensive dissolution of aragonitic grains 3 meteoric phreatic stage precipitation of granular and blocky cements 4 brine reflux stage extensive dolomitization and sulfate emplacement associated with refluxing of buckner brines and 5 burial stage grain to grain compaction of lithologies not stabilized by early cements stylolitization precipitation of poikilotopic calcite and dolomite cements minor dissolution of calcite associated with the introduction into the formation of undersaturated fluids hydrocarbon migration and sulfate replacement and cementation the diagenetic sequence varies dramatically over short distances in southwest alabama reflecting variation in paleotopography the products of this complex diagenetic history are heterogeneous reservoirs which consist of one or some combination of primary interparticulate moldic dolomitic intercrystalline and vuggy porosity primary interparticulate porosity is less common in southwest alabama than in the central and western portions of the gulf coast basin largely because of extensive compaction or early cementation where present however interparticulate reservoirs are characterized by moderate to high porosity and good permeability moldic porosity is produced by early fabric selective dissolution of aragonitic allochems and is associated with areas of subaerial exposure moldic reservoirs commonly have high porosity but low permeability and are generally productive only when permeability is enhanced by other pore types dolomitic intercrystalline porosity is common in the smackover of southwest alabama intercrystalline porosity alone however is seldom of reservoir quality and hydrocarbon production from dolomitized lithologies is generally dependent upon the coexistence of moldic or vuggy porosity vuggy porosity is the product of late non fabric selective dissolution of calcite vuggy pores are produced by solution enlargement of earlier formed interparticulate moldic or intercrystalline pores vuggy reservoirs are by nature composite and are characterized by moderate to high porosity and good permeability

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise

evolution of a late jurassic carbonate evaporite platform conecuh embayment northeastern gulf coast u s a successive time slice lithofacies maps of the smackover haynesville depositional sequence constrained by marker beds an interpreted maximum flooding surface and interpreted sequence boundaries show an onlapping and retrograding sequence of lithologies characteristic of a transgressive systems tract tst the tst is followed by several prograding sequences characteristic of a highstand systems tract hst and stacked shelf margin wedges smw comparison of stratigraphic cross sections lithofacies maps and a paleotopographic map of the sub smackover transgressive surface shows that the distribution of lithofacies within the tst was strongly controlled by two types of sub smackover paleotopographic highs 1 norphlet dune fields and 2 eroded basement rock as th tst became progressively buried during deposition distribution of lithofacies within the hst and the smws was controlled less by these early paleotopographic features and more by 1 differential compaction around shoal water facies in the tst 2 early louann salt movement 3 basement related faulting and 4 freshwater influx along the basin rim

morphology and history of the kermadec trench arc backarc basin remnant arc system at 30 to 32 s geophysical profile microfossil and k ar data abstract knowledge of the time span of arc activity essential for correct tectonic reconstructions has been lacking for the kermadec arc system but is supplied in this paper through study of microfossils contained in dredge samples and k ar ages on dredged basalt clasts the kermadec system at south latitudes 30 to 32 in the southwest pacific comprises from west to east the colville ridge remnant arc havre trough backarc basin kermadec ridge active arc and kermadec trench site of west dipping subduction of pacific plate lithosphere beneath the australian plate data are presented from two traverses dredge magnetic single channel seismic across the whole system an important transverse tectonic boundary the 32 s boundary lies between the two traverse lines and separates distinct northern 32 25 s and southern 32 36 s sectors the northern sector is shallower and well sedimented with broad ridges and a diffuse backarc basin the southern sector is deeper with narrow ridges and steep escarpments facing inwards to a little sedimented rifted backarc basin the kermadec ridge slopes smoothly trenchward to a mid slope terrace forearc basin with minor sediment fill at 5 6 km water depth a steeper 10 24 and more rugged lower trench slope is mantled with new zealand sourced rhyolitic vitric mud diamictons containing locally derived mafic volcanic clasts one clast is of late miocene age k ar age 7 84 ma the arc kermadec ridge is capped by active volcanoes very young k ar ages