are there large bathymetric highs near 73 n 139 w in the arctic basin comment on acoustic backscattering from the basin and margins of the arctic ocean by i dyer et al

surface wave mapping of the crust and upper mantle in the arctic region pure path rayleigh wave group velocities for 24 regional grids inside the arctic have been obtained from 74 mixed path measurements the vertical shear wave velocity structures under these 24 grids were then determined according to their pure path group velocity dispersion data this result indicates that the crustal shear wave velocity in the arctic region is generally lower than the worldwide average in addition a typical mid ocean ridge structure was clearly shown under the nansen ridge we also found an abnormally high upper mantle shear wave velocity under the alpha ridge and makarov basin and low velocity beneath the canadian arctic islands in general our inversion result agrees with the known tectonics in the arctic region to further check the validity of our result we have computed synthetic rayleigh waves and a comparison with our observations shows very good agreement

subaqueous evaporites of the buckner member haynesville formation northeastern mobile county alabama abstract the lower part of the buckner anhydrite member of the haynesville formation upper jurassic was deposited as shallowing upward cycles of subaqueous to subaerial deposits on the north flank of the wiggins arch in northeastern mobile country alabama the unit studied conformably overlies the smackover formation and is generally evaporite dominated the buckner anhydrite averages about 35 m 115 ft in thickness and has been buried to depths of 5 5 km 18 000 ft in spite of this deep burial it has suffered little deformation since alteration of gypsum to anhydrite subaqueous evaporites in the buckner are dominated by bedded massive bedded mosaic bedded nodular bedded nodular mosaic and mosaic structural types buckner cycles begin with the selenite facies in which bedded nodular mosaic and bedded mosaic structural styles often display a preferential vertical alignment of anhydrite nodular pseudomorphs after gypsum selenite crystals were precipitated on the sediment surface possibly below wave base but within the photic zone gypsarenite which vertically succeeds selenite is a laminated deposit in which ripples and low angle cross beds indicate clastic deposition above wave base the upper part of this facies may have been subaerially exposed stromatolite domes formed in relatively quiet water areas subaqueous evaporites of this unit commonly grade upward into a generally poorly laminated anhydrite often containing burrow structures detrital silt and possible root structures this may have been a gypsite facies which formed in the zone of soil moisture often this facies is located stratigraphically just below probable storm deposits and tidal flat sabkha deposits which accumulated under subaerial conditions these deposits are located at the top of the cycle subaerial conditions are indicated by desiccation cracks an influx of oxidized red silt and clay and erosional scours there probably was not a significant change in water depth at the transition from the smackover formation to the buckner anhydrite subaqueous buckner evaporites were deposited conformably over subtidal deposits at the top of the smackover the formation of microcrystalline dolomite at the top of the smackover probably occurred below the sediment water interface relative thickness of cycles facies correlations and the frequency and duration of subaerial exposure indicate the relative paleotopograpic position of various locates in the area buckner cycles were controlled by the rate of evaporite aggradation versus sea level rise and or rate of subsidence

stratigraphy and source potential of the smackover formation in the northern manila embayment southwest alabama abstract petrographic studies of conventional cores and correlation of electric logs indicate that upper smackover hydrocarbon reservoir facies are more widely distributed in the northern manila embayment than previously recognized the embayment does not appear to have been a separate basin during early smackover deposition but rather was an extension of the mississippi salt basin a carbonate shelf and starved basin topography developed in the manila embayment during deposition of the smackover formation and is reflected in the distribution of smackover lithofacies fine grained clastics are abundant in shelf and backshelf environments source rock geochemistry of the manila embayment appears to be atypical of the eastern smackover trend because of a strong terrestrial influence although the upper smackover is organic rich potential for generation of crude oil is limited by abundant terrestrially derived kerogen and thermal immaturity the updip lower smackover nearest the paleoshoreline also appears to be characterized by terrestrial kerogen thermal maturity and source potential for crude oil in the lower smackover increase southward this contributes to explaining the clustered distribution of crude oil discoveries downdip in proximity to the thermally mature lower smackover source facies

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

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

the structures of the alpha ridge arctic ocean and iceland faeroe ridge north atlantic comparisons and implications for the evolution of the canada basin abstract based on organic fragments found in a sediment core retrieved from the eastern alpha ridge it is concluded that 75 ma ago this part of the ridge was an archipelago of forested islands which was formed above sea level about 95 ma ago comparison of the alpha ridge with the iceland faeroe ridge ifr reveals remarkably similar morphological geological and geophysical characteristics except that the ifr crust is 7 km shallower and lacks the ridges and troughs of the alpha ridge it is concluded that the alpha ridge is not a hotspot trace but like the ifr was formed by funneling of volcanic material from a nearby hotspot into a spreading centre it is suggested that the block faulted nature of the alpha ridge is due to a collapse of the hot lithosphere which at the time of formation was too weak to support the massive accumulation of plume material whereas the lithosphere under the ifr was sufficiently strong to support the thinner crust the canada basin may have evolved in two stages in the late jurassic the southwestern canada basin opened in the latter part of the early cretaceous the whole alaska siberian plate including the southwestern canada basin and a continental chukchi borderland and mendeleev ridge started to rotate away from the canadian polar margin it is postulated that the icelandic hotspot then located under ellesmere island channeled plume material into the spreading axis forming the alpha ridge channeling and spreading stopped after a rotation of 35 formation by seafloor spreading implies the youngest part of the ridge is near the centre the oldest parts at either end indeed the oldest sediment core of the arctic ocean was recovered from the ellesmere end of the alpha ridge

natural gas plays in jurassic reservoirs of southwestern alabama and the florida panhandle area abstract three jurassic natural gas trends can be delineated in southwestern alabama and the florida panhandle area these include a deep natural gas trend a natural gas and condensate trend and an oil and associated natural gas trend trends are recognized by hydrocarbon types basinal positions and relationships to regional structural features within these natural gas trends eight distinct natural gas plays can be identified plays are recognized by characteristic hydrocarbon traps and reservoirs the deep natural gas trend includes the mobile bay area play that is characterized by structural hydrocarbon traps associated with salt tectonism and norphlet sandstone reservoirs at depths exceeding 20 000 feet the natural gas and condensate trend includes the mississippi interior salt basin play the mobile graben play the wiggins arch complex play and the pollard fault system play the mississippi interior salt basin play is typified by salt related structural and combination hydrocarbon traps and smackover dolomitized oolitic oncolitic and peloidal grainstone and packstone reservoirs at depths of approximately 16 000 feet the mobile graben play is exemplified by salt induced structural hydrocarbon traps and smackover dolostone and norphlet sandstone reservoirs at depths ranging from 12 400 to 18 400 feet the wiggins arch complex play is characterized by structural and combination hydrocarbon traps associated with stratigraphic pinch outs and salt flow these traps are salt related and occur along the flanks of paleohighs associated with the wiggins arch complex smackover dolostone reservoirs at depths ranging from 16 100 to 18 400 feet are typical of this play the pollard fault system play is typified by salt induced structural hydrocarbon traps and reservoirs at depths of approximately 15 000 feet these reservoirs are smackover dolomitized oolitic and peloidal grainstones and packstones and norphlet sandstones the oil and associated natural gas trend includes the gilbertown and west bend fault systems play the foshee fault system play and the basement ridge play the gilbertown and west bend fault systems play is exemplified by salt related structural or combination traps and smackover dolomitized oolitic oncolitic and peloidal grainstone and packstone reservoirs and norphlet sandstone reservoirs at depths ranging from 11 000 to 14 000 feet the foshee fault system play is characterized by structural and combination hydrocarbon traps related to salt movement and smackover dolomitized peloidal grainstone and packstone and norphlet sandstone reservoirs at depths of approximately 15 000 feet the basement ridge play which is typified by structural and combination traps associated with the conecuh and pensacola decatur ridge complexes and smackover oolitic and peloidal grainstone and packstone and algal boundstone and haynesville sandstone reservoirs at depths ranging from 11 800 to 15 500 feet has potential for significant undiscovered natural gas

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

evaporite depositional fabrics in the deeply buried jurassic buckner formation alabama abstract the upper jurassic buckner formation contains evaporites anhydrite buried to depths of up to nearly 6 000 m in southwestern alabama although these evaporites and the associated sediments have been deformed and mineralogically altered some primary depositional textures and fabrics remain this establishes the buckner formation as the most deeply buried succession of evaporites from which primary features have been reported a cored section of the buckner formation contains repetitive depositional sequences from 1 5 m to 5 5 m in thickness of 1 dolostone facies shallow restricted lagoon overlain by 2 nodular anhydrite facies sabkha within the nodular anhydrite facies layers containing nodules surrounded by siliciclastic sediment and finely crystalline dolomite are interpreted as having formed by displacive growth of gypsum or anhydrite at or above a sabkha water table other intervals within the nodular anhydrite facies consist of vertically oriented and elongated nodules thought to be pseudomorphs after gypsum that originally formed by subaqueous crystal growth each of the buckner formation sequences represents the outbuilding of an arid shoreline with progradation of sabkha evaporites and siliciclastics over lagoonal carbonates in a manner similar to the holocene sequences of the persian gulf and the northwest gulf of california the buckner sequences that contain the vertically elongated anhydrite nodules differ from the normal persian gulf record of supratidal sabkha deposition in that these pseudomorphs are interpreted as having formed as gypsum in temporary ponds of a sabkha proper a gypsum pan

predicting bathymetry from geosat erm and shipborne profiles in the south atlantic ocean abstract portions of two geosat erm altimeter tracks and corresponding suborbital shipboard gravity and bathymetry profiles in the south atlantic ocean were analyzed one across the walvis ridge about 1100 km long and the other in the brazil basin about 2300 km long together these profiles sample those types of sea floor topography which dominate the gravity signature at wavelengths of 20 to 300 km the walvis ridge is a massive aseismic ridge and the brazil basin profile crosses both an old seamount emplaced at the time the crust was young and a very young mid plate volcano both profiles cross fracture zones after the gravity and bathymetry profiles were split into subprofiles various cross spectral characteristics could be determined by fft techniques analysis showed that observed admittance is not well constrained by either an airy type or flexural compensation models across the walvis ridge but those over the brazil basin can be readily explained by an airy type model with a mean crustal thickness of about 20 km a theoretical filter was then designed based on a priori geological knowledge and used to predict bathymetry from the high passed gravity geoid anomalies not surprisingly the predicted bathymetry shows more detailed and correct short wavelength 20 300 km features than those predicted from the historical data base as represented e g by the dbdb5 gridded bathymetric model for areas where historical shipboard bathymetry measurements are widely spaced longer than about 10 km for single beam data but where some regional geologic information is available such as the relative ages of mid plate volcanoes and crust bathymetry predicted from altimetric data can be used to upgrade regional bathymetrie data bases on which regional geologic geophysical understanding depends

on the use of the autocorrelation method for estimating the depth to the magnetic basement the depth to the top of magnetic dykes can be estimated from total field aeromagnetic data using the relation between the depth to magnetic sources and the autocorrelation function of magnetic data by using synthetic anomalies we show that in the ideal case depth can be determined to an accuracy of 10 or better when the anomaly sources are two dimensional dykes however the estimated depths depend on the width of the dykes the estimated depth is about 0 6 times the actual depth to the top of thin dykes and around the true depth for thick dykes having width to depth ratio around 3 the depth is considerably overestimated for very thick dykes e g contacts which is a special case of the thick dyke thus the autocorrelation method requires that the width to depth ratio of the dyke is estimated independently to correctly estimate the depths alternatively it must be assumed that the width to depth ratio for the two dimensional source body is between 1 5 and 4

diagenesis of devonian reefal carbonates in the oscar range canning basin western australia abstract the oscar range part of the extensive devonian reef complex of the northern canning basin is a large atoll 80 10 km in which frasnian and famennian late devonian reefs grew around an emergent island of precambrian metasedimentary rocks many sediments in the oscar range had high depositional porosities but this porosity was largely occluded by diagenetic alteration diagenesis included marine cementation dolomitization pressure solution burial cementation and karstification field mapping petrography cathodoluminescence and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected oscar range sediments earliest cementation occurred in the marine environment where reef margin and associated subfacies developed abundant non luminescing radiaxial fibrous and microcrystalline calcite cements based on convergent covariant isotopic trends marine cements had a characteristic late devonian oxygen signature of 4 5 0 5 pdb and a carbon signature of 2 0 0 5 pdb in well cemented parts of the reef complex synsedimentary neptunian fissures formed and were filled with cement and sediment cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes marine diagenesis was followed by marine burial and possible meteoric phreatic cementation and dolomitization marine cements are overgrown by non luminescing scalenohedral cements that grade into banded bright and non luminescing blocky calcite cements that are common on the platform but are missing in many reef margin and marginal slope deposits banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform the reef complexes however were generally drowned reefs with little chance for prolonged meteoric exposure this sedimentologic evidence suggests that banded luminescing cements may have formed in the marine burial environment the next cement generation consists of non luminescing non ferroan bloc y calcites that occur throughout the reef complex stratigraphic relationships combined with isotopic results 18o 6 0 to 9 0 13c 2 0 suggest that this spar is a burial cement resulting from late devonian through early carboniferous burial the reef complex was exhumed and eroded in the late carboniferous dedolomite dolo moldic porosity pisolitic calcrete crusts iron oxide stained blocky cements and deep 200 m sinkholes formed at the unconformity surface non ferroan irregularly banded moderate to bright luminescing blocky calcite precipitated throughout the reef complex unconformity related cements calcrete crusts and dedolomites have distinctive isotopic signatures that show little variation in oxygen 7 0 to 9 0 but have a broad range of carbon values 1 5 to 8 0 highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface in post carboniferous time most of the remaining porosity was filled with non ferroan bright luminescing blocky calcite that formed by pressure solution reprecipitation during burial isotopic analyses 18o 8 0 to 12 0 13c 1 5 support a burial origin for this generation of cements these cements fill primary porosity tension gashes along stylolites and hairline fractures along which hematite is leached a final generation of ferroan homogeneous dull to non luminescing blocky calcite cement formed in a late burial event that according to isotopic evidence 18o 10 5 to 15 0 13c 2 0 to 9 0 was probably related to movement of basinal brines through the reef complexes

geology of the argentine continental shelf and margin from aeromagnetic survey new aeromagnetics show a complex of magnetic patterns and source depths of the north and central argentine continental and adjacent oceanic crust the 40 km wide positive magnetic g anomaly reflects an end rift associated volcanic edifice at the continental margin m4 mo and cretaceous quiet zone oceanic crust are identified the colorado magnetic discontinuity forms the southern boundary of the precambrian la plata craton to the south the continental crust of the patagonia platform is composed of amalgamated late precambrian through end paleozoic weakly magnetic sedimentary prisms the sw patagonia platform crust shows a strong high frequency magnetic pattern associated with mesozoic volcanics while zones of mid crustal level intrusives and some volcanics on the outer shelf are expressed as a broad higher amplitude magnetic pattern

detailed 3 d depositional architecture of late jurassic carbonate anhydrite cycles brightling mine weald basin uk abstract quantifying the geometries of evaporite deposits at a 13 c and 18 o isotope analysis the stratigraphic interval contains five facies in stratigraphic order these include supratidal porphyritic nodular evaporite shallow subtidal peloidal packstone with evaporite and two overlying rhythmic sequences of intertidal microbial laminite subtidal shale and subtidal laminar marl capped by nodular anhydrite the interpreted environment of deposition is a supratidal sabkha subject to periodic flooding in which intertidal tidal flat facies and subtidal shallow marine facies laterally passed into the evaporative sabkha the cycles are interpreted as meter scale shoaling upward sequences likely controlled by localized high frequency changes in relative sea level and or sabkha hydrology spatial patterns in the geometries of key stratigraphic surfaces reveal a subtle depression towards the central western region of the mine seam the variation in stratal geometries is interpreted as paleotopography and is a function of individual or composite processes related to dissolution eolian processes and coastal erosion these observations indicate a similar mode of deposition to the modern day sabkha of the persian gulf we conclude that the dynamic process of evaporite deposition led to subtle stratigraphic heterogeneities and changes in bed thicknesses but largely continuous lateral bedding at an interwell scale

thirtyone formation permian basin texas structural and lithologic heterogeneity in a lower devonian chert reservoir chert and carbonate reservoirs of the early devonian thirtyone formation comprise a major regional reservoir in the permian basin with more than 700 mbbl production to date and roughly an equal amount of potentially producible reserves remaining most major pools in the thirtyone formation were discovered several decades ago and are now in relatively advanced stages of enhanced recovery with significant variation apparent in response to waterflooding among different fields as shown by comparison between dollarhide and three bar fields reservoirs display significant heterogeneity due to structure and to depositional diagenetic factors affecting porosity development most studies of these fields to date have tended to emphasize depositional and diagenetic factors as the reason for observed variation in enhanced recovery performance however structural factors may be of nearly equal importance enhanced recovery at dollarhide field for example has been extremely successful with waterflood recoveries more than double those under primary production and a tertiary co2 flood expected to do nearly as well this extraordinary level of success is understood as a result of a combination of reservoir character and compartmentalization with the latter especially related to division of the field into several fault block portions mapping of these fault blocks and their related structures has been greatly advanced by a recent 3 d three dimensional seismic survey ultimate recovery at dollarhide is projected at 62 of original oil in place in comparison production history at three bar field shows a lower level of overall recovery yet a very similar increase between primary and secondary production reservoir heterogeneity related to depositional diagenetic factors appears more pronounced at three bar field than at dollarhide field but faulting is also significant with respect to division of the field into separate blocks with distinct productive histories a 3 d seismic survey may provide important discriminating data useful for a future co2 flood or other tertiary recovery project at three bar field

use of volume based 3 d seismic attribute analysis to characterize physical property distribution a case study to delineate sedimentologic heterogeneity at the appleton field southwestern alabama u s a abstract in this paper we illustrate the use of 3 d seismic attribute studies for predicting the distribution of physical properties in the subsurface using a data set consisting primarily of digital logs and seismic data we show how correlations can be made between seismic attributes and physical properties porosity and how those relationships can be exploited to predict the distribution of the property of interest in three dimensions the results of these studies 1 provide quantitative site specific 3 d models of physical properties that are of more use for applied studies than qualitative 2 d models commonly derived from facies modeling or sequence stratigraphic analysis 2 are generally more geologically reasonable than studies based on geostatistics alone 3 can provide sedimentary geologists with fundamental insights into depositional and or diagenetic controls on the distribution of properties of interest 4 need to be rigorously evaluated by integrating other types of data and analyses and 5 are best thought of as supplementing rather than replacing conventional geologic analyses the concepts and methods we illustrate may have application in various branches of sedimentary geology our study area is appleton field in southern alabama at this location we predict the 3 d distribution of porosity in carbonates of the upper jurassic smackover formation using a probabilistic neural network and a combination of four attributes our results suggest that porosity was best developed and preserved in thrombolite facies of the reef front

predicting permeability from porosity using artificial neural networks permeability values in a borehole are predicted by an artificial neural network from the porosity values at the same depths the network used in this study employs an architecture called backpropagation that is good at making predictions the traditional approach for permeability prediction is regression analysis in regression analysis the relationship between porosity and permeability is assumed to be known in reality the functional form of this relationship i e the model equation is unknown in contrast the neural network approach assumes no functional relationship six wells from big escambia creek jurassic smackover carbonate field in southern alabama were used to test predicting permeability from porosity using a neural network porosity and spatial data alone were used to predict permeability because these data are readily available from any hydrocarbon field three scenarios were performed in each one a subset of the six wells was used for a training set one well for calibration and one or two wells were used for prediction for each scenario simple linear regression was also used to predict permeability from porosity the neural net predicted permeability much better than did regression in one scenario in the other two scenarios the two methods performed equally well the neural net predicted permeability accurately using minimal da a but other kinds of information e g log or core derived lithologic information are easily incorporated if available in addition compartmentalization of carbonate reservoirs may be recognizable by this approach

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

rock fabric controls on pore evolution and porosity permeability trends in oolitic grainstone reservoirs and reservoir analogs although the general influence of rock fabric on porosity and permeability k within carbonates is well documented if and how pore evolution and k scatter quantitatively relate to depositional fabric remains poorly constrained this project empirically explores this uncertainty within oolitic grainstones from a range of geologic ages and diagenetic histories to understand depositional sediment pore relationships and how they can evolve with lithification integrating data from point counting digital image analysis nuclear magnetic resonance and core analysis of holocene pleistocene pennsylvanian and mississippian oolitic grainstones reveals quantitative relations among rock fabric pores and petrophysical parameters oolitic grainstones of similar sedimentology taken from distinct diagenetic scenarios display a unique combination of pore size shape spatial distribution and k character within each scenario pore attributes and k are correlated more closely with grain size sorting and type than with cementation and compaction collectively these results are interpreted to suggest that sedimentology controls the trends or variability within an oolitic succession but that diagenesis defines the absolute values of pore attributes and petrophysical parameters these findings suggest that petrophysical variability within oolitic reservoirs may closely follow sedimentologic trends which may be predictable within a stratigraphic framework

sequence stratigraphic analysis of jurassic and cretaceous strata and petroleum exploration in the central and eastern gulf coastal plain united states the formulation of an integrated sequence stratigraphic and biostratigraphic framework is fundamental in the design of an effective strategy for petroleum exploration in a sedimentary basin for the interior salt basins of the gulf coastal plain of the united states that are filled primarily with mesozoic postrift nonmarine to marine siliciclastic and carbonate deposits a sequence stratigraphic approach using transgressive regressive t r sequences and integrated with biostratigraphic information has utility as a method for establishing such a framework the sequence stratigraphy established for upper jurassic and cretaceous strata is used to categorize petroleum reservoirs in the central and eastern gulf coastal plain transgressive aggrading eolian fluvial and coastal sandstone facies of the t r sequences include highly productive hydrocarbon reservoirs in the eastern gulf coastal plain productive reservoirs in the central and eastern gulf coastal plain include regressive infilling fluvial to nearshore marine sandstone facies and nearshore marine shelf ramp and reef carbonate facies transgressive backstepping nearshore marine facies include highly productive reservoirs in the central gulf coastal plain these transgressive and regressive facies are recognized by their wireline log patterns and seismic reflection configurations knowledge of the diagnostic wireline log signatures and seismic reflection characteristics assists in the detection of exploration targets

reservoir characteristics of devonian cherts and their control on oil recovery dollarhide field west texas approximately 70 million bbl of oil have been produced from the chert dominated thirtyone formation at dollarhide field the thirtyone formation is devonian in age and contains two reservoir intervals an upper dolomite and a lower chert separated by nonporous limestone the chert reservoir contains approximately 83 of the original oil in place ooip and consists of two different facies laminated microporous chert and burrowed chert the laminated microporous chert was apparently deposited as sponge spicule sands grainstones in channels and fans on the slope of the tobosa basin the burrowed chert facies was apparently deposited as burrowed mixtures of sponge spicules siliceous mud and carbonate mud in broad slope environments between fans and channels spicules are thought to be derived from disaggregation of siliceous sponges living on the slope early marine and or meteoric diagenesis dissolved many sponge spicules and reprecipitated the silica as microcrystalline chalcedony and quartz resulting in spicule moldic porosity and microporosity in the originally siliceous strata the burrowed chert underwent small scale differential compaction that produced short discontinuous fractures 1 10 mm in length and 0 1 0 5 mm in width production from the field has been related to amounts and characteristics of the pore systems the thickness of the porous chert ranges laterally from 0 to 80 ft 0 24 m and pore volume fh ranges from 0 to 20 pore ft 0 6 pore m the laminated microporous chert is very homogeneous and has high porosity 25 35 and uniform permeability 5 30 md areas dominated by laminated microporous chert had moderate primary recovery approximately 200 000 bbl of oil per well excellent waterflood production 1 2 5 begin page 36 million bbl of oil per well and poor 20 ac 8 1 ha infill production 20 000 bbl of oil per well the burrowed chert has more heterogeneous porosity 5 30 and permeability 1 100 md the dolomite also has heterogeneous pore networks that have porosities of 3 15 and measured permeabilities of less than 1 200 md wells having substantial amounts of porous dolomite and or burrowed chert had moderate to good primary recovery 200 000 300 000 bbl of oil per well moderate waterflood recovery 300 000 1 100 000 bbl of oil per well and moderate 20 ac 8 1 ha infill recovery 50 000 100 000 bbl of oil per well oil recovery from the co2 flood is expected to average approximately 250 000 bbl per well having slightly higher recovery 300 000 bbl in areas that have thick laminated microporous chert and or burrowed chert oil recovery from the homogeneous laminated microporous chert during the co2 flood is improved by large amounts of residual oil and good sweep efficiency but decreased by the previously efficient sweeping of original mobile oil by the waterflood the co2 recovery from the heterogeneous burrowed chert is improved by significant amounts of unswept mobile oil but decreased by poorer sweep efficiency as a field primary recovery is estimated at 13 of ooip waterflood at 30 of ooip infill at 3 5 of ooip and co2 flood at 11 of ooip

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

new geophysical results from the south western eurasian basin morris jesup rise gakkel ridge yermak plateau and the fram strait the international multi ship expedition arctic 91 was able to collect a seismic transect between the morris jesup rise and the yermak plateau these conjugate plateau structures in the south western part of the eurasian basin bound one of the slowest mid

toward mitigating the double itcz problem in ncar ccsm3 1 the appearance of a spurious inter tropical convergence zone south of the equator in the eastern and central equatorial pacific in addition to the observed one north of the equator is a common problem in coupled global climate models the present study investigates this double itcz problem in the ncar ccsm3 it shows that use of a modified zhang mcfarlane convection scheme significantly mitigates the double itcz problem in boreal summer this has a profound impact on the simulated sea surface temperature through cloud radiative forcing feedback both the warm bias in the southern itcz region and the cold bias in the cold tongue over the equator are reduced examination of the time series of precipitation sst and surface energy fluxes shows that depending on the convection parameterization used double or single itcz emerges quickly within the first few months after the model start

reservoir geology of devonian carbonates and chert implications for tertiary recovery dollarhide field andrews county texas 1 devonian strata of the thirtyone formation at dollarhide field andrews county texas originally contained 144 million bbl of oil in place with 18 6 million bbl produced by primary recovery 39 3 million bbl projected to be produced by secondary recovery waterflood and 27 4 million bbl projected to be produced by tertiary recovery co 2 flood devonian strata contain five main lithologies from bottom to top 1 carbonate chert mudstone deposited in a basinal environment 2 burrowed chert dolomite 10 35 ft or 3 11 m thick which accumulated as siliceous spiculitic wackestone and carbonate mud in a low energy slope environment 3 laminated tripolitic chert 0 65 ft or 0 20 m thick deposited as a sponge spicule sand in submarine channels and fans on the slop 4 bioclastic limestone 47 92 ft or 14 28 m thick which accumulated as crinoidal grainstones in moderate to high energy outer shelf environments and 5 upper dolomite 50 90 ft or 15 27 m thick with dolomitized peloidal grainstones deposited in high energy shallow subtidal to intertidal environments and thin chert beds deposited in supratidal environments deposition of the 180 ft thick 55 m reservoir section occurred during a basinward progradation of shelf and slope facies porosity within devonian strata is controlled mainly by depositional facies porosity occurs in two stratigraphic zones separated by the tight bioclastic limestone the lower porosity zone laminated tripolitic chert and part of the burrowed chert dolomite 0 100 ft or 0 30 m thick is relatively homogeneous not stratified with high porosity commonly 25 35 moderate permeability 5 20 md and short fractures causing elevated permeabilities near the bottom the lower reservoir is thick in fan shaped and elongate areas where sponge spicules were deposited on submarine fans and in submarine channels the upper porosity zone 0 90 ft or 0 27 m thick is within the upper dolomite and is heterogeneous with stratified porosity that bifurcates coalesces and pinches out in several di ferent intervals the relative homogeneity and moderate permeability of the lower reservoir should help achieve a uniform sweep during co 2 flooding higher permeabilities near the base of the lower reservoir should minimize co 2 override thereby improving sweep efficiency the thinner more heterogeneous less continuous more stratified more permeable pay in the upper reservoir will result in more rapid and less uniform co 2 sweep

depth to magnetic source analysis of the arctic ocean region abstract approximately 400 000 line kilometers of high quality low level arctic aeromagnetic data collected by the naval research laboratory the naval oceanographic office and the naval ocean reseach and development activity from 1972 through 1978 have been analyzed for depth to magnetic source this data set covers much of the canada basin the alpha ridge the central part of the makarov basin the lincoln sea the eurasia basin west and south of the 55 e meridian and the norwegian greenland sea north of the jan mayen fracture zone the analysis uses the autocorrelation algorithm developed by phillips 1975 1978 and based on the maximum entropy method of burg 1967 1968 1975 the method is outlined examples of various error analysis techniques shown and final results presented where possible magnetic source depth estimates are compared with basement depths derived from seismic and bathymetric data all major known bathymetric features including vesteris bank and the greenland molloy and spitsbergen fracture zones as well as the mohns knipovich and nansen spreading ridges and the alpha cordillera appear as regional highs in the calculated magnetic basement topography shallow basement was also found under the northeastern yermak plateau the morris jesup rise and under the southern greenland ellesmere island end of the lomonsosov ridge regional magnetic source deeps are associated with such bathymetric depressions as the canada makarov amundsen nansen greenland and lofoten basins more localized magnetic basement deeps are found over the molloy f z deep and over the mohns knipovich and nansen rift valleys a linear magnetic basement deep follows the extension of nares strait through the lincoln sea toward the morris jesup rise suggesting the continuation of the nares strait or wegener f z into the lincoln sea a sharp drop in the regional magnetic source depths to the southeast of the alpha ridge suggests the alpha ridge is not connected to structures in northwest ellesmere island as previously postulated from high altitude aeromagnetic collected by canadian workers a regional deep under the east greenland shelf west of the greenland escarpment suggests the presence of 5 10 km of post paleozoic sediments

oxygen isotope composition of holocene dolomite formed in a humid hypersaline setting holocene dolomite forming in supratidal carbonate sediments of bonaire netherlands antilles is precipitated from hypersaline fluids in which the mg ca ratio is elevated by evaporation and gypsum precipitation kinetic effects and water vapor exchange between the humid atmosphere and hypersaline brines limit the 18o values of the brines to between 0 7 and 2 1 standard mean ocean water the mean 18o value of dolomite is 1 0 peedee belemnite n 8 which is lower than values of dolomite precipitated from hypersaline brines in less humid environments bonaire dolomite is also isotopically lighter than dolomites interpreted to have formed in normal marine waters and mixed marine and or meteoric waters calculated fractionation between dolomite and calcite formed from bonaire brines is between 1 5 and 3 5 the high end of this range is consistent with previously proposed mineralogic fractionations of 3 1 and 3 2 between dolomite and calcite

prediction of sediment thickness in the norwegian greenland sea from gravity inversion 1 we explore the feasibility of inverting freely available altimetry derived gravity grids for sediment thickness on oceanic crust a four layer earth model is adopted assuming that the gravity signal from the base of the sedimentary layer can be fully isolated from the observed free air gravity the following additional data are required 1 shipboard bathymetry for reliable bouguer correction 2 oceanic crustal ages to correct for gravity anomalies related to the thermal history of lithospheric breakup and seafloor spreading and 3 some wide angle seismic observations of sediment thickness a fixed relationship between the base of the sedimentary layer and the moho is also required because of limited gravity resolution at large depths the final inversion setup searches for three control parameter values density of uppermost sediments density of oceanic crystalline crust and thermal diffusivity of mantle rock that minimize misfit between the earth model and observations applied to the norwegian greenland sea the inversion successfully predicts locations of 20 25 km wide depocenters and crystalline crustal highs yet 1 km thickness errors are common even in well constrained areas the largest errors are associated with underplated crust active mid ocean ridges and the continent ocean boundary indicating locally incorrect assumptions of mantle thermal structure and crystalline crustal thickness the thicknesses of 4 km thick submarine fans are underpredicted probably because of too low sedimentary densities in the earth model if these limitations are acceptable or do not apply because of simple crustal structure gravity inversion may be used for mapping regional sedimentary patterns e g in the arctic ocean

the western svalbard margin 74 80 n abstract a regional study of the continental margin between the senja and molloy spitsbergen fracture zones reveals that the transition from continental to oceanic crust occurs in a narrow zone beneath the outer shelf and uppermost slope the postulated continent ocean boundary appears to be fault related consisting of sheared and rifted segments the marginal structures are compatible with a plate tectonic model in which the southern greenland sea opened along a northeasterly propagating plate boundary in the eocene whereas the northern greenland sea started opening in the early oligocene the main structure at the margin is the hornsund fault zone which probably reflects an old zone of weakness rejuvenated in the tertiary first by shear and later by extensional movements in the early tertiary local transpressional and transtensional components along the plate boundary are associated with the spitsbergen orogeny emplacement of belts of high density oceanic crust and tectonism in the western barents sea a complex volcanic rifted margin characterized by the bjornoya marginal high links the predominantly sheared margin segments on either side the main ridge like segment of the hovgaard fracture zone was originally part of the spitsbergen margin in a regional sense the hornsund fault zone demarcates the eastern boundary of the tertiary sedimentary wedge which reaches a total thickness of more than 7 km there appears to have been a considerable increase in deposition of sediments the last 5 6 my depocentres located seaward of the east west fjord systems and submarine depressions indicate a relationship between late cenozoic glaciations and high sedimentation rates

contrasting styles of reservoir development in proximal and distal chert facies devonian thirtyone formation texas the lower devonian thirtyone formation of west texas and new mexico is one of the largest chert reservoir successions in the world having accounted for more than 750 million bbl of oil production as much as 650 million bbl of additional mobile oil remains in these reservoirs making this play an important target for further exploitation a major limitation on the recovery of this remaining oil resource is an appreciation of the controls on reservoir development and heterogeneity although all thirtyone chert reservoirs have much in common they can be divided into proximal and distal settings each of which is characterized by distinct depositional geometries and styles of reservoir heterogeneity proximal reservoirs represented by three bar field are composed of a single thick sheetlike chert unit which extends for hundreds of square miles heterogeneity in these reservoirs which were formed by strike parallel deposition on a gently sloping outer platform during regional transgression is primarily a function of faulting fracturing and dissolution of associated carbonate along unconformities small scale bed scale heterogeneity also exists within the tabular chert body resulting from variations in silica deposition and diagenesis between and among beds by contrast distal reservoir successions typified by university waddell field comprise thin vertically stacked and laterally discontinuous chert intervals whose origin is a function of transport and deposition of siliceous sediments as debris flows and turbidites flow units in these reservoirs are thin 10 20 ft 3 6 m and separated vertically and laterally from one another by low permeability mud rich siliceous sediments and hemipelagic deposits the distribution of flow units is the result of both paleotopography and sea level cyclicity chert units are most abundant in transgressive and early highstand legs of sea level rise fall cycles and display offset stacking suggestive of topographically controlled reciprocal sedimentation faults and fractures appear to begin page 8 be relatively minor contributors to reservoir heterogeneity in distal reservoirs

mineralogical control on porosity type in upper jurassic smackover ooid grainstones southern arkansas and northern louisiana abstract original mineralogy of ooids affects both their diagenetic history and therefore the reservoir quality of ooid lithologies previous studies have shown that ancient ooid cortices were composed either of aragonite mg calcite or calcite this study based on fabrics and geochemical analyses of smackover oolites suggests that both aragonite and calcite ooids were deposited on the upper jurassic smackover shelf of southern arkansas and northern louisiana smackover ooids exhibit both a blocky calcite spar fabric and a radial calcite fabric grainstones with blocky calcite spar ooids and oomoldic porosity have a higher strontium content than those with a radial calcite fabric the high strontium has a jurassic marine isotopic signature supporting the interpretation that the blocky cal ite spar is an early diagenetic fabric after aragonite radial calcite ooids were originally calcitic the ooids accumulated in a series of linear bars parallel to shore ooids with original aragonite mineralogy are older and were deposited in the north the younger more southern bars consisted of calcite ooids aragonite ooids in the north were susceptible to dissolution and formation of moldic porosity during freshwater diagenesis moldic porosity is generally accompanied by low permeability calcite ooids in the south however were less susceptible to dissolution in meteoric waters hence preserved porosity is interparticle porosity and permeability is generally higher porosity types in the upper smackover of arkansas and louisiana reflect both the original mineralogy of the ooids as well as their diagenetic history

northwestern margin of the east siberian sea russian arctic seismic stratigraphy structure of the sedimentary cover and some remarks on the tectonic history abstract multichannel seismic reflection data acquired by mage in 1990 provide the first view of the geological structure of the east siberian sea between 76 5 80n and 152 164e this northwestern part of the east siberian margin may be considered within the context of a passive transform model bordering a late mesozoic early cenozoic makarov basin of the arctic ocean the continental basement is formed by paleozoic and mesozoic rocks which were folded in the late mesozoic the sedimentary cover includes cretaceous aptian to cenozoic terrigeneous deposits the total thickness of the sedimentary cover varies from a few tens of metres to 8 9 km generally increasing northward at least 10 sedimentary units are correlated and stratigraphically identified from the seismic data three structural sequences cretaceous aptian to santonian campanian to lower paleocene and cenozoic are distinguished in the sedimentary cover they may correspond to three main stages of deep margin evolution continental breakup 125 80 ma drift 80 60 ma and post drift 60 ma to present the present morphology of the northern shelf and deep margin is the result of continental terrace progradation and gravity driven slope processes

diurnal patterns of rainfall in northwestern south america part i observations and context one of the rainiest areas on earth the panama bight and pacific western littoral of colombia is the focal point for a regional modeling study utilizing the fifth generation pennsylvania state university ncar mesoscale model mm5 with nested grids in this first of three parts the observed climatology of the region is presented the seasonal march of rainfall has a northwest southeast axis with western colombia near the center receiving rain throughout the year this study focuses on the august september season the diurnal cycle of rainfall over land exhibits an afternoon maximum over most of south and central america typically composed of relatively small convective cloud systems over some large valleys in the andes and over lake maracaibo a nocturnal maximum of rainfall is observed a strong night morning maximum of rainfall prevails over the coastal ocean propagating offshore and westward with time this offshore convection often takes the form of mesoscale convective systems with sizes comparable to the region s coastal concavities and other geographical features the 10 day period of these model studies 28 august 7 september 1998 is shown to be a period of unusually active weather but with a time mean rainfall pattern similar to longer term climatology it is concluded that the rain producing processes during this time period are likely to be typical of those that shape the seasonal climatology

how well do cmip5 models simulate the low level jet in western colombia the choco jet is an important atmospheric feature of colombian and northern south america hydro climatology this work assesses the ability of 26 coupled and 11 uncoupled amip global climate models gcms included in the fifth phase of the coupled model intercomparison project cmip5 archive to simulate the climatological basic features annual cycle spatial distribution and vertical structure of this jet using factor and cluster analysis we objectively classify models in best worst and intermediate groups despite the coarse resolution of the gcms this study demonstrates that nearly all models can represent the existence of the choco low level jet amip and best models present a more realistic simulation of jet worst models exhibit biases such as an anomalous southward location of the choco jet during the whole year and a shallower jet the model skill to represent this jet comes from their ability to reproduce some of its main causes such as the temperature and pressure differences between particular regions in the eastern pacific and western colombian lands which are non local features conversely worst models considerably underestimate temperature and pressure differences between these key regions we identify a close relationship between the location of the choco jet and the inter tropical convergence zone itcz and cmip5 models are able to represent such relationship errors in worst models are related with bias in the location of the itcz over the eastern tropical pacific ocean as well as the representation of the topography and the horizontal resolution

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

the yermak plateau formed at a triple junction the yermak plateau is an arcuate marginal plateau located seaward of northwestern spitsbergen the plateau is divided into two zones based on bathymetric trends and magnetic characteristics that have roughly coincident boundaries north of latitude 82 n where the bathymetric contours trend ne the magnetic amplitudes are large whereas the southern portion strikes nnw and exhibits a subdued magnetic signature beneath the northern section of the plateau crustal refraction velocities of 5 0 km s 1 underlain by 7 2 km s 1 are recorded while farther south velocities of 4 3 6 0 and 8 0 km s 1 are measured the 6 0 km s 1 velocity is similar to ones reported on the continental shelf south of spitsbergen dredged material gravity heat flow and seismic reflection and refraction data indicate the northern region of the plateau is oceanic and formed in conjunction with the morris jesup rise in contrast the southern section is thinned continental crust formed by the initial rifting process at a triple junction north of greenland plate reconstructions for anomalies 24 and 13 are consistent with this interpretation little space is required for the yermak plateau at anomaly 24 because it is composed of oceanic and thinned continental crust the development of the yermak plateau and the morris jesup rise paired aseismic ridges took place when the north american eurasian and greenland plates were moving independently

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

dolomitization of holocene mg calcite supratidal deposits ambergris cay belize dolomitized crusts and associated sediments 845 2925 yr old on ambergris cay belize central america compose a significant portion of the holocene sediment section on many supratidal flats the dolomitic sections are as thick as 0 7 m and contain an average of 70 calcic dolomite dolomite occurs as a replacement of high mg calcite micrite matrix and allochems and as passively precipitated cements isotopic analyses suggest that dolomitization appears to be promoted primarily by reactions of the magnesian calcite supratidal sediments with essentially near normal marine waters the effects of seasonal influxes of meteoric and hybrid fluids however are also clearly evident in the crusts as indicated by isotopes dolomite crystal etching the selective leaching of aragonite and the local loss of mg 2 from the host micrite and included skeletal fragments the amounts rapid rates of dolomitization and initial mg calcite mineralogy of the sediments make the ambergris cay supratidal flats exceptional in the holocene and this area may be a model for the genesis of some ancient peritidal dolomites

rifting in the northern norwegian greenland sea thermal tests of asymmetric spreading the analysis of heat flow seismic and topographic data collected in the northern norwegian greenland sea reveals an asymmetric evolution of the eurasian and north american plates these data are compared to predictions from three kinematic models of extension which produce asymmetry about the knipovich ridge 1 uniform asymmetric pure shear 2 lithospheric simple shear and 3 rift ridge jumping the data are consistent with a range of deformation scenarios from one ridge jump occurring at about 25 m y after the initiation of spreading to continuous asymmetric extension the simple shear model can match the data only when a detachment fault dips more steeply than 45 under svalbard tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models when the mohns ridge propagating to the east encountered the preexisting northward trending spitsbergen shear zone the direction of ridge propagation shifted to the north being influenced by the abrupt change in the regional stress field at the shear zone ridge axis intersection as a result the nascent knipovich ridge entered into and propagated along the shear zone consequently formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended the high level of deviatoric stress about the mohns and knipovich ridge intersection may cause a gradual eastward migration of the knipovich ridge resulting in multiple zones of magma intrusion off axial bands of high heat flow and volcanism located along the barents sea and svalbard margin along the yermak plateau and on continental svalbard just southeast of the yermak plateau may be evidence for this migration propagation from the nansen ridge may have entered the same shear zone from the north explaining the creation of the small molloy ridge and off axis volcanism on the yermak plateau

framework of hydrocarbon generation and destruction in eastern smackover trend laminated lime mudstones of the lower member of the jurassic smackover formation are significant source rocks for crude oil across mississippi alabama and florida the source facies was deposited in an anoxic and perhaps hypersaline environment that preserved algal derived kerogen the distribution of kerogen along laminations of depositional origin and along stylolites of diagenetic origin resulted in efficient expulsion of crude oil with increasing thermal maturity crude oil initially emplaced in reservoirs was cracked to yield gas condensate and then methane rich in nonhydrocarbon gases such as hydrogen sulfide carbon dioxide and nitrogen early destruction of methane was driven by thermochemical sulfate reduction the thermal maturity framework of the smackover rend explains the distribution of hydrocarbon discoveries and suggests areas previously overlooked by exploration

carbon and oxygen isotopic composition of holocene reefal carbonates carbon and oxygen isotopic compositions have been determined for holocene reefal carbonate grains and cements in samples from enewetak bikini bermuda and belize to identify components that exhibit systematic isotopic fractionation relationships these findings are compared with fractionation relationships determined from experimentally precipitated low magnesium calcite lmc aragonite and high magnesium calcite hmc to determine if these relationships are observed in naturally precipitated carbonates compositional variation within individual samples is extreme up to 6 in 13 c pdb and 3 in 18 o pdb experimental fractionation relationships fail to predict the range of observed compositions aragonite cements are enriched 1 0 to 1 4 in 13 c and 1 5 to 2 0 in 18 o relative to calcite equilibrium compositions hmc cements are enriched by 0 3 to 0 4 in 13 c and by 1 8 to 1 9 to 18 o for carbon these results are in agreement with the experimental work of rubinson and clayton for oxygen our results suggest that fractionation factors of both aragonite and hmc may have been underestimated skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co occurring skeletal carbonates

upper jurassic depositional systems and hydrocarbon potential of southeast mississippi abstract upper jurassic sedimentation in southeast mississippi was controlled by eustatic sea level fluctuations and locally modified by salt tectonism and basement structure the present study using conventional core data and geophysical logs indicates that a stable carbonate platform developed along the updip margin of the mississippi interior salt basin the basin was partially barred from the main gulf of mexico water mass by the wiggins uplift and became evaporitic during the upper jurassic the smackover and haynesville formations are separate sedimentologic packages deposited during two distinct sea level cycles high energy carbonates extended across the northern basin margin as sedimentation kept pace with subsidence and slowing sea level rise near the close of each transgression an effective shelf margin barrier did not develop lime mud generally accumulated under subtidal conditions within the interior salt basin however the basin was sufficiently shallow for wavebase sedimentation to occur on crests of salt cored bathymetric highs resulting in localized reservoir facies to the south eolian conditions initially prevailed on the wiggins uplift later as the wiggins subsided subaqueous shoals fringed and capped individual basement islands eustatic sea level fluctuations and salt movement controlled diagenesis and porosity formation limestone porosity dominates in upper smackover platform carbonates with a transition from secondary moldic porosity updip to preserved primary porosity downdip moldic intercrystalline and vuggy dolomite porosity is developed on the crests of intermediate and high amplitude salt highs and on the wiggins uplift jurassic source rocks are lower smackover laminated lime mudstones migration into adjacent reservoirs postdated formation of porosity and the growth of salt anticlines the most common trap type a large potential norphlet smackover gas play extends along the southern flank of the wiggins uplift salt anticlines within the interior basin remain viable targets small oil discoveries should continue in stratigraphic traps subtle salt structures and basement blocks on the platform

pore networks in holocene carbonate sediments abstract permeabilities of holocene carbonate sediments from florida and the bahamas determined for 74 samples by water flow rates in a variable head permeameter range from 0 6 to 57 000 millidarcies porosities range from 40 to 78 percent these parameters are related to depositional texture as follows table the muddiest sediments i e the finest grained have the highest porosities but lowest permeabilities this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks even as young as pleistocene high porosity and low permeability in sediments show a strong correlation with percentage of fines 62 m from capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 m in diameter at least after drying measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation the calculated rates would require excessive time to produce the degree of cementation seen in late pleistocene rocks of florida and the bahamas the amounts of rainfall and evaporation are the most important factors in the degree of cementation

links between topography moisture fluxes pathways and precipitation over south america the andes cordillera plays a role in driving moisture and heat from tropical onto subtropical south america it forces the development of a lee side trough that covers most of western argentina and a low level jet that maximizes over paraguay eastern bolivia and northern argentina and is tightly linked to precipitation variability over much of central and southeastern south america its steep slopes and the large zonal gradients in topography between the equator and 40 s are misrepresented in climate simulations using global climate models gcm with resolutions coarser than about 100 km since they naturally have a poor representation of the andes and related circulation features this paper analyses the impact of varying artificially the altitude of the andes cordillera in a gcm as well as increasing the horizontal resolution to study how these variations determine moisture fluxes and precipitation over selected regions of south america results show that the height of the andes is crucial in shaping moisture fluxes pathways onto subtropical south america all year long in particular the low level jet is only simulated when the andes heights are doubled at the same time the relationship between the andes shape and the location of the bolivian high in summer is also discussed in terms of precipitation the lowest bias in the simulations is achieved when the horizontal resolution is increased while in particular near the andes foothills the simulated annual rainfall is largely determined by the mountains shape

petrography and diagenesis of silcrete from kuwait arabian gulf abstract silcretic duricrust is found capping scattered isolated hills in the southern desert plain of kuwait it occurs in genetic association with surface and subsurface calcrete dolocrete and gypcrete deposits two main types of silcrete were found quartzitic silcrete and massive silcrete the first was developed by precipitation of microcrystalline drusy quartz cement in the intergranular pores of sandstones whereas the second type was formed by replacement of calcrete dolocrete and freshwater limestone by microcrystalline quartz silica precipitation took place in two stages in the first primary silicification caused the development of the main silcrete mass which is usually composed of microcrystalline quartz syntaxial quartz overgrowth and quartzine spherulites a second stag of secondary silicification was responsible for the precipitation of length fast chalcedony and megaquartz in cavities the field occurrence and the petrographic characteristics of the silcrete in kuwait provide support for the folk and pittman 1971 hypothesis on the replacement of evaporite and carbonate by quartzine and lutecite calcretization of kuwait group clastic deposits which resulted in complete or partial dissolution of siliciclastic grains and precipitation of carbonate minerals is likely to have provided soluble silica for groundwater a diagenetic model for the studied silcrete is proposed in which both vertical and lateral translocation of soluble silica in groundwater is considered silcrete has played an important role in the landscape development of the southern ar a of kuwait

distribution of hydrocarbon source potential in the jurassic smackover formation abstract there are regionally consistent differences in source potential between the upper and lower members of the smackover formation measurements of total organic carbon and visual kerogen assessments indicate that the lower smackover has source potential for crude oil from texas to florida source potential is largely restricted to the laminated lime mudstone facies of the lower smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline algal derived amorphous kerogen is concentrated in laminations of depositional origin carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites we suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement the upper smackover serves mainly as a reservoir rock and other jurassic formations lack source potential

geomagnetic polarity reversal model of deep tow profiles from the pacific jurassic quiet zone the jurassic magnetic quiet zone jqz contains magnetic lineations but their low amplitudes make correlation and interpretation difficult part of the problem is the separation of source and sensor for old deep ocean crust we increased anomaly amplitudes by collecting magnetic data along two deep tow profiles over western pacific jqz lineations a magnetic polarity reversal timescale was constructed by matching deep tow anomalies with a simple rectangular block magnetization model for oceanic crust the polarity sequence covers 11 m y 156 167 5 ma and contains 88 pre m29 polarity chrons extending to chron m41 a limitation of this model is its poor representation of the oldest anomalies upward continued to sea level on deep tow profiles these anomalies have both long and short wavelength components but only the latter are easily modeled on a datum close to the source an alternative polarity model was constructed to match the anomalies upward continued to sea level this model retains only 44 of the deep tow model polarity chrons because of short wavelength attenuation by upward continuation because of the inferred periods and magnetization contrasts we think many of the short wavelength anomalies represent paleofield intensity fluctuations in contrast polarity reversals have been documented by prior magnetostratigraphic work for the younger part of the timescale covered by our model thus our data may show a transition from a geomagnetic field behavior dominated by intensity fluctuations to one dominated by reversals

the equatorial energy balance itcz position and double itcz bifurcations the intertropical convergence zone itcz migrates north south on seasonal and longer time scales previous studies have shown that the zonal mean itcz displacement off the equator is negatively correlated with the energy flux across the equator when the itcz lies in the northern hemisphere energy flows southward across the equator and vice versa the hemisphere that exports energy across the equator is the hemisphere with more net energy input and it is usually the warmer hemisphere but states with a double itcz straddling the equator also occur for example seasonally over the eastern pacific and frequently in climate models here it is shown how the itcz position is connected to the energy balance near the equator in a broad range of circumstances including states with single and double itczs taylor expansion of the variation of the meridional energy flux around the equator leads to the conclusion that for large positive net energy input into the equatorial atmosphere the itcz position depends linearly on the cross equatorial energy flux for small positive equatorial net energy input the dependence of the itcz position on the cross equatorial energy flux weakens to the third root when the equatorial net energy input or its curvature become negative a bifurcation to double itcz states occurs simulations with an idealized aquaplanet general circulation model gcm confirm the quantitative adequacy of these relations the results provide a framework for assessing and understanding causes of common climate model biases and for interpreting tropical precipitation changes such as those evident in records of climates of the past

origins of gypsum in deep carbonate reservoirs implications for hydrocarbon exploration and production gypsum can form penecontemporaneously in carbonate sediments and it may be altered to anhydrite during burial some workers have suggested that gypsum dehydration is complete at burial depths of 3500 ft 1067 m however gypsum has been documented at depths as great as 13 100 ft 3993 m two examples of deeply buried gypsum illustrate contrasting origins gypsum in the permian san andres formation in the permian basin of west texas at a depth of 6000 ft 1829 m occurs as anhydrite nodules with an outer rind of gypsum suggesting that anhydrite was rehydrated to gypsum by interaction with interstitial brines in contrast gypsum in the carbonate beds of the cretaceous ferry lake anhydrite of the mississippi interior salt basin at 13 100 ft 3993 m occurs as gypsum nodules with a rind of anhydrite suggesting that the gypsum is primary and in the process of dehydration to anhydrite these contrasting origins are likely a function of evaporite and carbonate stratigraphy in the permian basin a thick section of carbonate rocks is overlain by a relatively thin section of evaporites in the mississippi interior salt basin carbonates are interbedded with multiple evaporite beds which inhibit circulation of interstitial brines the burial temperature pressure and salinity of these formations suggest that gypsum could be preserved as primary or secondary gypsum the presence of gypsum in reservoir rocks can affect porosity calculations especially calculations using neutron porosity logs which measure bound water of hydration in gypsum as porosity identification of gypsum at the depth of 13 100 ft 3993 m indicates that log estimates of porosity in some deep carbonate reservoirs containing gypsum could have significant porosity and water saturation measurement errors

burial diagenesis and porosity evolution upper jurassic smackover arkansas and louisiana the jurassic upper smackover ooid lime grainstone of the central gulf coast has been a major hydrocarbon exploration target for nearly half a century study of the geologic and sedimentologic framework petrography two phase fluid inclusions and trace element and stable isotope chemistry indicates the following diagenetic history the upper smackover is a blanket ooid grainstone ranging in thickness from 300 to 400 ft 91 to 122 m and covering at least 4 000 sq mi 10 400 sq km in the northern part of the arkansas shelf a regional meteoric water system was established in the grainstone shortly after deposition and prior to significant burial this meteoric water system gave way to conditions which were increasingly more marine toward the south near the louisiana arkansas state line the fresh to marine hydrologic gradient is responsible for the strong precompaction regional diagenetic overprint that eventually controlled available diagenetic pathways for the unit during burial three diagenetic zones are present 1 a northern zone dominated by oomoldic porosity and preburial spar cementation 2 a sou hern zone characterized by compaction effects late subsurface cement primary porosity preservation and late secondary subsurface porosity generation and 3 a transitional zone having characteristics common to both the northern and southern zones during the shallow burial phase compaction became increasingly more important to the south in zones dominated by marine connate fluid whereas rock integrity and a secondary oomoldic porosity system had already been established in the northern zone as burial increased brine originating in the underlying louann salt reached the shelf grainstone and began to displace the original connate fluid the brine triggered the inversion to calcite of remaining grains and cement composed of unstable mineralogy when the louann brine had displaced the original connate fluid caso4 replacement as gypsum and anhydrite was common across all three diagenetic zones also precompaction fine cement associated with oomoldic porosity was recrystallized and reequilibrated with the concentrate brine localized replacement dolomitization may well have developed throughout the burial history of the sequence but was most commonly postcompactional postcompaction poikilitic calcite cement and baroque dolomite were precipitated from the louann brine under conditions of elevated temperature and pressure as subsidence continued organic material from the basinward equivalents of the upper smackover lime grainstone arrived on the shelf and catalyzed so4 reduction resulting in pyrite and galena replacement in the southern and transitional zones as hydrocarbon maturation continued in the fine grained equivalents of the upper smackover h2s and co2 by products of these processes began to increase in the brine of the southern and transitional zones ultimately causing regional dissolution of existing carbonate and significantly enhancing preserved porosity porosity in the northern zone was established early and preserved through moderate burial although porosity is well developed permeability has been largely destroyed by recrystallization dolomitization is a prerequisite for economic reservoir development in the northern zone porosity in the southern zone is solution enhanced preserved primary intergranular porosity compaction is the most important porosity destruction process compaction in the absence of an early diagenetic overprint is affected primarily by grain type and secondarily by size and sorting pellets and oncolites compact easily yet destroy porosity during burial ooids and rhodolites resist compaction during burial therefore are related to porosity preservation grain type distribution as well as size and sorting are controlled by depositional environment processes with the most favorable areas for porosity development concentrations of ooids and rhodolites associated with early salt related structural movement porosity distribution in the southern zone therefore can be outli ed with the aid of geophysical end page 597 techniques porosity in the transitional zone is early secondary or preserved primary intergranular or a combination porosity in this zone therefore is highly unpredictable and a strong potential exists for diagenetic traps unrelated to structure or original depositional environment documentation of significant deep subsurface secondary porosity possibly associated with hydrocarbon maturation opens the potential for hydrocarbon exploration in deeply buried carbonate dominated basins this would include carbonate rocks heretofore thought to be well below the depth of carbonate porosity maintenance because of lithostatic load

characterization of reservoir lithologies within subunconformity pools pekisko formation medicine river field alberta canada the pekisko formation in western canada is a third order sequence comprised of an open marine grainstone through restricted carbonate mudstone succession truncation of the pekisko along an unconformity edge prior to the jurassic and several periods of incision from early jurassic early cretaceous have formed an intricately sculptured subcrop belt in the medicine river field oil is trapped in discrete pools close to the unconformity edge reservoir facies comprise three lithologies medium crystalline dolostone with intercrystalline and vuggy porosity forms the reservoir in an elongate dolostone body that has replaced grainstone fine crystalline dolostone is facies selective within a lime mudstone unit in which the common reservoir rock is bioturbated dolomudstone with intercrystalline porosity grainstones have secondary porosity formed though leaching of microcrystalline calcite allochems and intergranular cements reservoir quality is assessed from consideration of orthogonal permeability values kmax maximum horizontal permeability k90 horizontal permeability at 90 to the maximum and kv vertical permeability obtained from conventional whole core analyses sedimentary lamination primarily affects the kv and the presence of fine grained lamination dictates that kv 3k90 and kv k90 from this relationship the fracture density in all three lithologies appears to be similar low and variable porosity and permeability values in grainstones indicate that the fracture system is poorly connected to the matrix despite leaching and fracturing at the unconformity edge reservoir distribution closely follows patterns of facies that were susceptible to dolomitization future exploration potential lies in fine crystalline dolostone bodies that may form stratigraphic traps downdip from the unconformity edge

an eolian facies within the upper jurassic smackover formation tchula lake field mississippi the late jurassic upper smackover formation in mississippi is primarily composed of an overall regressive sequence of marine carbonate ramp deposits in central mississippi however carbonate deposition was interrupted by a large influx of siliciclastic sediment tchula lake field is unique not only because it produces gas and condensate from dolomitic quartz sandstone deposited in a mixed siliciclastic carbonate environment but also because it produces from what we interpret to be an eolian dune sand facies throughout the gulf coast basin no previous literature has cited an eolian facies within the smackover formation several characteristics of the producing zone sandstones have led to interpretations of this unit as an eolian coastal dune facies deposited on and around a jurassic paleohigh facies and paleoenvironmental interpretations are based on compositions and textures described in cores and thin sections the eolian facies consists of fine grained dolomitic quartz sandstone with a mean porosity of 10 20 and permeability of 0 800 md the sand grains are rounded to subrounded and well sorted the sandstone exhibits a sequence of upward steepening planar laminations capped at the top by a set of small scale wedge planar cross laminations the sharply defined parallel lamina are less than 5 mm thick and are commonly inversely graded and contain probable vadose meniscus carbonate cements this facies is interpreted to have been deposited in a backshore coastal dune environment the unit is characterized by low angle 20 degrees dip climbing translatent strata commonly produced by the migration of wind ripples along a dune no fossils or trace fossils are found within this facies eolian deposited sandstone is a potential reservoir rock along the updip limit of the smackover formation in west central mississippi other paleohighs along the jurassic shoreline may have undergone similar eolian deposition during a regional sea level fall eolian deposited sandstones on these paleohighs should have begin page 486 similar reservoir properties as the sandstone at tchula lake field the updip smackover formation in west central mississippi is an underexplored horizon and eolian deposited reservoirs have strong potential as a new exploration target the sea level fluctuation event associated with deposition at tchula lake field also has implications for exploration in upper jurassic sedimentary rocks across the entire united states gulf coast basin and prompts a reevaluation of these reservoirs

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

chemical constraints and origins of four groups of gulf coast reservoir fluids discussion the authors take to task stoessell and moore s 1983 review of some previous ideas on the origin of gulf coast formation fluids taking specific exception to their interpretation of data from the edwards formation six points are emphasized edwards brines are significantly enriched in bromide relative to chloride over seawater evaporated to halite saturation edwards brines have significantly higher k c1 and ca mg c1 ratios than seawater evaporated to halite saturation edwards brines cannot achieve molar ca mg ratios of 16 by dolomization progressive stabilization of metastable dolomite by updip fluid cannot raise the magnesium content of edwards water in the absence of dolomite dissolution the volume of brine contained in the cretaceous section exceeds any reasonable volume of hypothetical connate mesozoic brine and isotopic differences between edwards brines and jurassic seawater evaporated to halite saturation are extreme and therefore preclude recognition of any primary jurassic component in the current brines

petrofacies analysis of carbonate rocks example from lower paleozoic hunton group of oklahoma and texas a petrophysical classification of hunton group dolomitized carbonates lower paleozoic in the anadarko basin of oklahoma and texas is based on porosity mercury recovery efficiency median pore throat size normalized pore throat size the shapes of capillary pressure curves and lithology five petrophysical facies petrofacies include i porous and permeable dolostones ii skeletal grainstones and packstones iii low porosity low permeability dolomitized oomoldic ooid grainstones iv cemented ooid grainstones cherts and cherty carbonates and v lime mudstones and tight fine grained dolostones petrofacies i porous dolostone is the most variable this variability reflects the inherent variability of dolostones and the probability that dolomitization of t e hunton group occurred in more than one episode four subpetrofacies include ia crystalline and sucrosic dolostones ib vuggy dolostones iva cemented ooid grainstones and ivb cherts and cherty mudstones and wackestones the petrofacies form mappable subsurface units that can be used to predict the distribution of petroleum reservoirs e g petrofacies i and ii and of potential sealing units petrofacies iii iv and v and to identify potential stratigraphic traps petrofacies and depositional lithofacies are discordant indicating that diagenetic processes did not respect lithofacies boundaries the best reservoir rocks sucrosic fossiliferous dolostones of subpetrofacies ia and crinoidal grainstones of petrofacies ii have both high porosities mean 5 2 6 9 and high mercury recovery efficiencies mean 30 7 36 1 these rocks are characterized by moderately sized throats median 2 6 8 8 m mesokurtic to platykurtic pore throat size distributions and intermediate or gently sloping cumulative mercury intrusion capillary pressure curves higher porosity rocks with low mercury recovery efficiencies mean porosity 11 8 re 25 for example vuggy and moldic dolostones of subpetrofacies ib would not perform well during primary oil recovery but would be good candidates for enhanced oil recovery for reservoir rocks that are oil wet capillary pressure dat must be interpreted differently than is done in this paper most petrofacies can be distinguished by the shapes of capillary pressure curves alone however bivariate plots of petrophysical parameters also allow discrimination between petrofacies the negative log of median pore throat size plotted vs either porosity or mercury recovery efficiency provides the most readily interpretable graphic method of assigning samples to petrofacies such plots are particularly useful in highlighting samples that are lithologically similar to but petrophysically different from samples typical of a particular petrofacies

zonation and geochemical patterns of burial calcite cements upper smackover formation clarke county mississippi abstract cathodoluminescence cl trace element concentrations and isotope compositions of luminescently zoned poikilitic and mosaic calcite cements of the upper smackover formation are studied in the harmony pachuta creek east nancy goodwater and garland creek fields in clarke county mississippi these calcites precipitated during burial in a depth range of 100 m to 3 km a temperature range of 30 100 c and a time span of 60 my the calcium carbonate for calcite cementation was primarily derived locally by pressure dissolution of the smackover formation luminescent patterns are consistent in all samples within each field however calcites from individual fields differ in number of zones and cl intensity there is no compelling evidence that equivalent zones in separate fields precipitated simultaneously it is therefore unwise to correlate individual zones or groups of zones as time horizons from one field to another magnesium content of all calcites ranges from about 500 1000 ppm in the oldest zones and increases to about 3000 4000 ppm toward the younger zones the low mg2 content of the older zones is the result of precipitation from waters with low mg2 ca2 ratio typical of subsurface waters the increase in mg2 toward the younger zones may have resulted from precipitation over time at progressively higher temperatures and the increase in dmg2 from a pore water whose mg2 ca2 ratio was declining strontium varies from 100 300 ppm and does not exhibit systematic variation across zones but increases with increasing mn2 content low sr2 content of the calcites is the result of very slow precipitation at high temperatures from a water with low sr2 ca2 ratio co variation of sr2 with mn2 may reflect preferential sr2 substitution for ca2 in order to reduce lattice distortion caused by the substitution of mn2 for ca2 non luminescing to dull zones contain less than 30 ppm mn2 while bright zones have 300 1500 ppm mn2 iron is nearly absent in all zones regardless of cl intensity low mn2 content of older zones is attributed to calcite precipitation in a reduced environment where mn2 was not available the increase in mn2 content in younger zones is the result of importation of mn2 through local conduits from the underlying norphlet formation coupled with an increase in dmn2 due to a progressive increase in temperature during burial absence of fe2 in all zones may have been the result of precipitation of iron sulfides when fe2 and mn2 bearing waters came into contact ith h2s generated by organic maturation of the lower smackover formation 18o decreases and 87sr 86sr becomes more radiogenic toward the younger zones while 13c remains constant the lower 18o and more radiogenic 87sr 86sr composition of the younger zones suggest calcite precipitation at higher temperature and influx of radiogenic strontium from the underlying norphlet formation the absence of any cl quenchers lack of any cl activators other than mn2 and geologic and geochemical considerations suggest that cl intensities were primarily controlled by the mn2 ca2 ratio of pore waters and temperature

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

an introduction to chert reservoirs of north america jim rogers has been studying mid continent and rocky mountain hydrocarbon reservoirs for 46 years often concentrating on pennsylvanian and mississippian carbonate rocks and cherts after graduate school at the university of michigan jim spent 15 years at the shell university studying with mentors r n ginsburg e a shinn r h nanz and h a bernard and prospecting in kansas and north dakota in 1970 he began conducting independent regional reservoir studies with several associates these studies were incorporated into his national geological services inc in 1979 jim continues to search for and write about prospective reservoirs while serving in various editorial positions and teaching geology in the caribbean mark w longman is a consulting geologist who specializes in the study of carbonate rocks and reservoirs he has worked on a wide variety of projects ranging from specific core and petrographic studies on individual hydrocarbon reservoirs and wells to regional studies covering entire basins states and countries his specialties include interpretation of carbonate depositional facies and diagenesis and tying these interpretations to wire line logs he received a b a degree from albion college michigan in 1972 and a ph d from the university of texas at austin in 1976 before becoming a consultant in 1984 he was a research geologist with cities service company s research laboratory in tulsa and later worked with coastal oil and gas corporation and buttercup energy in denver this volume represents part of the current work of more than 20 earth scientists who have studied hydrocarbon reservoirs in chert some in the petroleum industry will be surprised that many trillions of cubic feet of natural gas and hundreds of millions of barrels of oil have been and will be produced from these strange rocks despite this remarkable fact the literature on hydrocarbons and pore space in chert reservoirs is

depositional history of the smackover buckner transition eastern mississippi interior salt basin abstract shoaling upward cycles of the upper part of the smackover formation are locally capped by thin sabkha deposits adjacent to and on the crests of paleotopographic highs in the eastern mississippi interior salt basin misb these sabkhas are overlain by peritidal carbonates of the uppermost smackover relative sea level fell during late smackover progradation and sabkha deposition then rose again to deposit peritidal carbonates a sharp contact typically separates these peritidal carbonate deposits and the overlying massive buckner anhydrite in the misb evidence for a significant sea level change at the smackover buckner boundary is lacking smackover sabkha deposits in the misb consist of intercalated matrix dominated nodular anhydrite and anhydritic dolostone in which the proportion of anhydrite commonly increases upward they are overlain by micritic pelletal or oolitic peritidal dolostone smackover sabkhas formed on local paleotopographic highs concentrated along a north south trending ridge produced by movement of the louann salt because salt movement was variable in timing and intensity causing smackover sabkhas to be discontinuous and probably not everywhere the same age the carbonates that overlie the smackover sabkhas resemble typical smackover carbonates and include reservoir strata massive buckner anhydrite strata are predominantly subaqueous saltern deposits though peritidal evaporite deposits occur on the margins of the misb saltern deposits formed in a gypsum precipitating lagoon in the eastern misb this region was a silled basin sheltered from less saline water in the main part of the misb by a salt cored anticline capped by carbonate grainstone shoals in eastern mississippi saltern deposits are dominated by selenite and gypsarenite saltern deposits are locally interbedded with subtidal hypersaline to normal marine carbonate deposits which may record sea level fluctuations or changes in water circulation a saltern or evaporite lagoon formed in the eastern misb at the beginning of buckner time as a result of restriction of water influx into the eastern misb and resultant rapid increase in salinity to gypsum saturation the salinity increase was a chemical event hence the base of the massive anhydrite is a time plane and its extent approximates that of the buckner evaporite lagoon subaqueous evaporites of the basal buckner occupy a smaller area than do peritidal carbonates of the uppermost smackover suggesting that evaporative drawdown was a contributing factor in the deposition of the massive anhydrite smackover sabkhas are not physically connected to the subaqueous evaporite deposits of the buckner nor did they form in the same way saltern deposits thicken away from paleotopographic highs and have a blocky density log pattern smackover sabkhas typically have spiky density log patterns and thin away from paleotopographic highs

occurrence of subtidal dolomite in a hypersaline lagoon kuwait we report here the discovery of subtidal dolomite in a shallow hypersaline lagoon during a study of recent carbonate and evaporite sediments in the al khiran region of southern kuwait in the northern part of the arabian gulf fig 1 diagenetic dolomites are known from a variety of geographical locations and a spectrum of environmental conditions1 6 although many occurrences of subaqueous dolomite have been documented1 4 none has been reported from the subtidal areas of extensive recent marine carbonate environments the al khiran dolomites show complex polyhedral crystal habits and a composition which ranges from ca51mg49 to ca56mg44 we postulate that a partial removal of dissolved sulphate ions by bacterial reduction from the interstitial waters of the sediments the moderate mg2 ca2 ratios of the hypersaline waters and the occasional near schizohaline conditions in the lagoon may be responsible for the formation of this dolomite

pore facies of smackover carbonate reservoirs in southwest alabama abstract pore facies are rock units that are defined by certain proportions of pore types contain specific pore throat size distributions and exhibit certain consequent fluid flow properties pore facies may contain only one pore type but more typically are characterized by a combination of several pore types data used in this study consist of point counts of thin sections that are plotted on ternary diagrams whose apices are pore types ternary pore plots commercial porosity and permeability data from core analyses qualitative thin section descriptions and qualitative core descriptions two distinct but partially intergrading pore facies are recognized in the smackover of southwestern alabama these pore facies are defined based on the two most common pore types in the smackover particle moldic including secondary intraparticle and intercrystalline pores which together account for greater than 85 of total porosity in the smackover rocks assigned to the moldic pore facies exhibit similar depositional fabrics and experienced similar diagenetic processes for example this pore facies is most commonly composed of peloid and ooid grainstone some fabric selectively dolomitized modified by early cementation and particle dissolution by contrast in the intercrystalline pore facies destruction of primary fabrics by nonfabric selective dolomitization is almost the sole determinant of pore system characteristics the moldic pore facies dominates to the northwest choctaw western clarke washington counties the updip area and the intercrystalline pore facies is dominant to the south and east mobile monroe baldwin escambia and conecuh counties the downdip area pore facies distributions overlap in some areas e g western monroe county indicating that multiple pore facies occur in many smackover fields the two pore facies exhibit substantially different petrophysical characteristics the mean slope of regression lines of porosity on ln natural log permeability for the intercrystalline pore facies is 0 47 with a range of 0 19 to 0 90 the mean slope for the moldic pore facies is 0 22 with a range of 0 18 to 0 27 the higher slopes for the intercrystalline pore facies mean that permeability values may be more precisely predicted from porosity data in this pore facies the mean maximum permeability for the intercrystalline pore facies is 130 md the corresponding value is 91 md for the moldic pore facies high permeability fluid conduits are more common in the intercrystalline pore facies than in the moldic pore facies in addition all high permeability examples of the moldic pore facies contain substantial amounts of interparticle porosity and are found near the smackover subcrop the remainder of the moldic pore facies is characterized by permeability values substantially less than those of the intercrystalline pore facies porosity values are commonly higher in the moldic pore facies which has a range of mean porosity of 10 2 to 28 0 compared to 9 6 to 20 5 for the intercrystalline pore facies greater hydrocarbon volumes can be stored in reservoirs dominated by the moldic pore facies but connectivity is better in the intercrystalline pore facies mixtures between the two pore facies are moderately common mixed pore systems commonly resemble the intercrys talline pore facies in the slope of the regression line of porosity on ln permeability in strata with mixed pore systems isolated particle moldic pores are commonly connected by networks of intercrystalline pores which control transport of fluids out of particle moldic pores

influence of the andes mountains on south american moisture transport convection and precipitation mountain ranges are known to have a first order control on mid latitude climate but previous studies have shown that the andes have little effect on the large scale circulation over south america we use a limited domain general circulation model regcm3 to evaluate the effect of the andes on regional scale atmospheric dynamics and precipitation we present experiments in which andean heights are specified at 250 m and 25 50 75 and 100 of their modern values our experiments indicate that the andes have a significant influence on moisture transport between the amazon basin and the central andes deep convective processes and precipitation over much of south america through mechanical forcing of the south american low level jet llj and topographic blocking of westerly flow from the pacific ocean when the andes are absent the llj is absent and moisture transport over the central andes is mainly northeastward as a result deep convection is suppressed and precipitation is low along the andes above 50 of the modern elevation a southward flowing llj develops along the eastern andean flanks and transports moisture from the tropics to the subtropics moisture drawn from the amazon basin provides the latent energy required to drive convection and precipitation along the andean front large northerly moisture flux and reduced low level convergence over the amazon basin leads to a reduction in precipitation over much of the basin our model results are largely consistent with proxy evidence of andean climate change and have implications for the timing and rate of andean surface uplift

little ice age drought in equatorial africa intertropical convergence zone migrations and el nino southern oscillation variability high resolution analyses of the mg concentration in authigenic calcite in five cores from lake edward provide a water balance history of central equatorial africa spanning the past 1400 yr a high ratio of mg to ca mg indicates strong droughts in central africa during the little ice age a d 1400 1750 in contrast to records from lake naivasha kenya which suggest a wet little ice age this spatial pattern in africa likely arose due to coupled changes in the high latitudes the position of the intertropical convergence zone and the el nino southern oscillation enso system our results further suggest that the patterns and variability of twentieth century rainfall in central africa have been unusually conducive to human welfare in the context of the past 1400 yr

sediment thickness in the southern canada basin abstract multichannel seismic reflection data are used in conjunction with deep crustal seismic refraction data to estimate the thickness of sediments in the southern canada basin of the arctic ocean north of alaska the sediments are interpreted to be of hauterivian mid early cretaceous to holocene age comparison of the seismic reflection character of seismic reflections in the study area with that in other basins indicates that a base of sediment top of oceanic layer 2 reflection is not present above the depth at which the water bottom multiple obscures all deeper arrivals which is in conflict with the conclusions drawn from aeromagnetic refraction and other reflection studies seismic velocity structure determined from the reflection data indicates that the reflections above the multiple are from sedimentary strata in the absence of seismic reflection evidence for the top of layer 2 above the multiple we estimate total sediment thickness by using the layer 3 refractions and subtracting an average assumed layer 2 thickness from the top of layer 3 assuming that an average thickness of oceanic layer 2 1 4 km overlies layer 3 in the southern canada basin sediment thickness in the study area is estimated to range between 6 5 km where water depth is 3 8 km to greater than 11 km where the water depth is 2 km this is nearly double that of any previous estimates and should have a significant effect on calculations such as the age of canada basin regional heat flow and long term sedimentation rates

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

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

moisture sources to the 2010 2012 anomalous wet season in northern south america during 2010 2012 northern south america experienced one of the strongest wet seasons during the recent decades with socio economic implications of wide proportions this period was characterized by two stronger than average la nina events one occurred in 2010 2011 and another less intense event in 2011 2012 this study focused on identifying the main moisture sources besides the eastern pacific for the anomalous wet season occurred in this region during 2010 2012 and their association with the la nina events observed in the equatorial pacific the results discussed here suggest that the main moisture sources to this anomalous wet season were the pacific ocean via the westerly flow of the choco jet and the caribbean sea via the weakening of the caribbean low level jet and the development of southward anomalies toward northern south america such changes appear to be associated not only to el nino southern oscillation enso driven sea surface temperature anomalies in the eastern pacific caribbean sea and north atlantic but also to enso independent variability in the atlantic ocean the latter is related to an enhanced atlantic meridional mode

norphlet formation upper jurassic of southwestern and offshore alabama environments of deposition and petroleum geology upper jurassic norphlet sediments in southwestern and offshore alabama accumulated under arid climatic conditions the appalachian mountains of the eastern united states extended into southwestern alabama to provide a barrier for air and water circulation during the deposition of the norphlet formation these mountains produced topographic conditions that contributed to the arid climate and they affected sedimentation norphlet paleogeography in southwestern alabama was dominated by a broad desert plain rimmed to the north and east by the appalachians and to the south by a developing shallow sea the desert plain extended westward into eastern and central mississippi norphlet sedimentation initiated as a result of basin subsidence accompanied by erosion of the southern appalachians norphlet conglomerates were deposited in coalescing alluvial fans in proximity to an appalachian source the conglomeratic sandstones grade downdip into red beds that accumulated in distal portions of alluvial fan and wadi systems quartz rich sandstones were deposited as dune and interdune sediments on a broad desert plain the principal source of the sand was updip alluvial fan and plain and wadi deposits wadi and playa lake sediments also accumulated in the interdune areas a marine transgression was initiated during the late phase of deposition of the norphlet formation resulting in the reworking of previously deposited norphlet sediments norphlet hydrocarbon potential in southwestern and offshore alabama is excellent six oil and gas fields already have been established petroleum traps discovered to date are primarily structural traps involving salt anticlines faulted salt anticlines and extensional fault traps associated with salt movement reservoir rocks consist primarily of quartz rich sandstones that are eolian wadi and marine in origin porosity is principally secondary dissolution with some intergranular porosity smackover algal carbonate mudstones were probably the source for the norphlet hydrocarbons jurassic oil generation and migration probably were initiated in the early cretaceous

pisoids in the jurassic smackover formation alabama florida evidence of marine vadose diagenesis abstract pisoids in the upper jurassic smackover formation of the conecuh embayment alabama florida exhibit textural characteristics of marine vadose diagenesis they occur in prograding repetitive shoaling upward sequences which are separated by truncations reflecting transgressive episodes at least eight such prograding cycles recur each sequence consists of a lower unit composed of fenestral textured pelletal mudstone and an upper unit of vadose pisolite the pisoids exhibit features indicative of non algal in place accretionary growth including isopachous multicoatings directional growth perched inclusions reverse grading leaching and brecciation and fracturing interpreted to be a result of subaerial exposure the pisoids formed at the expense of pre existing pelletal mudstone as indicated by progressive alteration and replacement of the original mudstone texture in interpisoid pores a fibrous cement coats the pisoids a first generation cement equant crystals of dolomite line the fibrous cement and the center of the former interparticle pore is filled with drusy crystals or megaspar of calcite these textures recall quaternary analogs in the persian gulf and the permian capitan reef complex of west texas new mexico in the persian gulf these features form in a hypersaline vadose diagenetic setting by comparison origin in an arid persian gulf type of environment is inferred

a depositional and sequence stratigraphic model for cold water spiculitic strata based on the kapp starostin formation permian of spitsbergen and equivalent deposits from the barents sea this article summarizes the lithostratigraphy of two sections through the kapp starostin formation on spitsbergen having contrasting distal and proximal depositional settings these outcrop sections are compared with an approximately age equivalent spiculitic unit penetrated in two exploration wells 800 km to the southeast on the finnmark platform eight main facies associations comprise these strata including limestones matrix rich matrix poor and mudstone siliciclastics shale and sandstone spiculite dark tight and light colored porous and phosphatized surfaces a sequence stratigraphic model is proposed wherein siliciclastic limestone intervals represent lowstands of relative sea level and the spiculites are mainly highstand deposits based on this model seven depositional cycles sequences are recognized in the distal high accommodation locality on spitsbergen akseloya four in the proximal locality dickson land and two on the finnmark platform a speculative model for correlation between these localities is presented wherein lateral amalgamation of sequences due to landward pinch out of some lowstand intervals results in recognition of fewer sequences in areas of lower accommodation on both dickson land and the finnmark platform an upward change in spiculite lithology from dark tight to light porous is shown and dickson land also shows an upward change in siliciclastic lithology from shale to glauconitic sandstone these changes record a long term shoaling trend approaching the permian triassic boundary this model provides a basis for interpreting the basin scale begin page 2062 distribution of lithofacies and predicting the occurrence of porous zones representing potential hydrocarbon reservoirs

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

thermal implications for the evolution of the spitsbergen transform fault abstract heat flow taken between svalbard and greenland reveal three thermal provinces 1 1 the molloy ridge within the spitsbergen transform 2 2 the yermak plateau 3 3 the northeastern margin of svalbard nordaustlandet the molloy ridge is a short spreading segment and the average heat flow is much above the sclater et al 1971 cooling curve but agrees with values from the norwegian greenland sea an additional zone of intrusion identified by heat flow lies to the northwest of the molloy ridge it straddles both the visible fracture zone and part of the yermak plateau a thermal boundary lies between the warm western segment of the yermak plateau and the shelf off nordaustlandet if the thermal subsidence of the western yermak plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m y approximately 20 m y younger than the northeastern segment of the plateau plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7 we postulate that the original transform is associated with the hornsund fault zone in response to deviatoric stress across the oblique ridge transform system the nansen ridge propagated southwestward aborting the old transform trace and shifted to its present position it is suggested that this propagation and migration of the ridge transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the western yermak plateau the propagation phenomenon coincides with large scale tertiary volcanic activity on svalbard readjustment and migration of the oblique transform is still taking place as the transform ridge system is liberated from continental constraints the migration rate will diminish as orthogonality is approached

burial diagenesis and thermochemical sulfate reduction smackover formation southeastern mississippi salt basin the smackover formation in the southeastern mississippi salt basin has been buried from 3 03 to 6 06 km and subjected to temperatures of 100 to 200c pre hydrocarbon migration burial diagenesis is characterized by precipitation of calcite cement followed by saddle dolomite and late anhydrite these phases formed at temperatures of less than 100c the oxygen isotopic composition of calcite ranges from minus 5 to minus 8 4 pdb and carbon isotopic composition ranges from 3 5 to 6 0 pdb the sulfur isotopic composition of late anhydrites ranges from 18 1 to 19 1 cdt post hydrocarbon migration burial diagenesis is dominated by replacement of previously formed late anhydrite by calcite or calcite plus elemental sulfur and precipitation of post bitumen calcite cement these diagenetic phases formed at temperatures above 150c and are present only in the deep part of the basin the oxygen isotopic compositions of calcites that replace anhydrite range from minus 3 0 to minus 4 0 pdb and carbon isotopic compositions range from minus 2 0 to 4 2 pdb the oxygen isotopic compositions of post bitumen calcites range from minus 3 4 to minus 5 6 pdb and carbon isotopic compositions range from minus 1 6 to minus 16 3 pdb two samples of elemental sulfur have sulfur isotopic compositions of 15 8 and 16 1 cdt the high temperature replacement of anhydrite by calcite more the presence of post bitumen calcite cement the sulfur isotopic composition of sulfates and elemental sulfur and high concentration of hydrogen sulfide indicate that rapid thermochemical reduction of solid sulfates is taking place in the deep part of the mississippi salt basin at temperatures as low as 150c less

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

regional variation in microscopic and megascopic reservoir heterogeneity in the smackover formation southwest alabama abstract quantitative rank measures of microscopic and megascopic reservoir heterogeneity are used to characterize the distribution of reservoir heterogeneity in smackover hydrocarbon fields and wildcat wells in southwest alabama microscopic reservoir heterogeneity h is 0 25 meanlnk 1 5lnk 3 megascopic heterogeneity mh is of reservoir intervals of high k reservoir intervals of of reservoir intervals where reservoir rock is defined as exhibiting permeability values 0 1 md and high permeability reservoir rock exhibits permeability values 1 0 md both mh and h are determined from core data the dykstra parsons coefficient dp is a measure of microscopic heterogeneity that is partially independent of h r2 0 428 all three of these parameters are primarily measures of vertical heterogeneity although averaging of wells within a field incorporates lateral heterogeneity in h and dp h and mh are distributed in contrasting but related patterns h generally decreases from northwest to southeast with the highest values found in the vicinity of the choctaw ridge complex north of the mississippi interior salt basin misb moderately high values typify the manila embayment and the conecuh ridge complex to the south whereas lower values are found in the misb on the north flank of the wiggins arch and in the conecuh embayment h values are high in the moldic pore facies and low in the intercrystalline pore facies the distribution of mh is roughly opposite to that of h mh values are high on the north flank of the wiggins arch on the conecuh ridge complex and in the conecuh embayment mh values are low near the choctaw ridge complex the conecuh ridge is unique because it is characterized by high values of both h and mh also the low relief north south trending salt cored anticline in western washington county is characterized by relatively high values of mh reservoirs belonging to the moldic pore facies tend to be homogeneous with respect to mh whereas reservoirs assigned to the intercrystalline pore facies are characterized by relatively high values of mh mh and h vary congruently with pore system characteristics controlled by depositional patterns dissolution and dolomitization and regional structural and paleogeographic trends this suggests that reservoir heterogeneity characteristics are controlled by structural and paleogeographic setting by depositional fabric and by diagenesis however because contours of h and mh are approximately normal to structure contours but parallel to smackover thickness contours it appears that depositional setting or paleogeography influenced reservoir heterogeneity more than did structural evolution the distribution of dp values is not related to pore facies distribution thus we conclude that dp is less useful for regional heterogeneity studies than is mh or h