on the shape and velocity of fluid filled fractures in the earth summary in this paper i present a model to estimate the shape and velocity of slowly ascending buoyancy driven fluid filled fractures the model considers elastic deformation linear fracture mechanics and fluid flow an attempt is made to incorporate the effect ofthe 2 d fluid pattern on the viscous pressure drop most other models assume that the viscous pressure drop can be approximated by flow through a channel with constant width although the form of the fracture is known to deviate from such a simple shape the 2 d flow in my model has an important consequence for the mechanism of buoyancy driven fracture propagation it predicts a large pressure gradient at the tail of the propagating fracture indicating that the tail of the fracture is most important in hindering the fracture propagation a singularity at the tail of the fracture can be avoided when a small amount of fluid trails in the channel left behind the propagating fracture the trailing and decoupling of fluids at the tail seems to be accompanied by small flow and shape instabilities which is indicated by the jerky movement of the tail observed in propagating air filled fractures in solidified gelatine and by numerical boundary element solutions of the coupled flow deformation fracturing problem by comparing the predictions for propagation velocities with laboratory observations of buoyancy driven fracture propagation in gelatine i derive a non dimensional effective thickness at the tail of the fracture for which trailing of fluids may occur the model is applied to earth relevant problems such as oil and water filled fractures in pressurized sediments or magma filled dykes in the lithosphere which are discussed in the paper

magma extraction from the mantle wedge at convergent margins through dikes a parametric sensitivity analysis 1 we address under what conditions a magma generated by partial melting at 100 km depth in the mantle wedge above a subduction zone can reach the crust in dikes before stalling we also address under what conditions primitive basaltic magma mg 60 can be delivered from this depth to the crust we employ linear elastic fracture mechanics with magma solidification theory and perform a parametric sensitivity analysis all dikes are initiated at a depth of 100 km in the thermal core of the wedge and the moho is fixed at 35 km depth we consider a range of melt solidus temperatures 800 1100c viscosities 10 100 pa s and densities 2400 2700 kg m 3 we also consider a range of host rock fracture toughness values 50 300 mpa m 1 2 and dike lengths 2 5 km and two thermal structures for the mantle wedge 1260 and 1400c at 100 km depth and 760 and 900c at 35 km depth for the given parameter space many dikes can reach the moho in less than a few hundred hours well within the time constraints provided by u series isotope disequilibria studies increasing the temperature in the mantle wedge or increasing the dike length allows additional dikes to propagate to the moho we conclude that some dikes with vertical lengths near their critical lengths and relatively high solidus temperatures will stall in the mantle before reaching the moho and these may be returned by corner flow to depths where they can melt under hydrous conditions thus a chemical signature in arc lavas suggesting partial melting of slab basalts may be partly influenced by these recycled dikes alternatively dikes with lengths well above their critical lengths can easily deliver primitive magmas to the crust particularly if the mantle wedge is relatively hot dike transport remains a viable primary mechanism of magma ascent in convergent tectonic settings but the potential for less rapid mechanisms making an important contribution increases as the mantle temperature at the moho approaches the solidus temperature of the magma

extension subsidence and thermal evolution of the los angeles basin a two dimensional model abstract the los angeles basin is a narrow deep basin containing large oil fields its formation began in the middle miocene and its subsidence has been extraordinarily rapid we suggest that although the basin is in a predominantly strike slip tectonic environment the subsidence is largely a result of crustal extension simple one dimensional extension models ignoring the lateral flow of heat are inadequate to explain the rapidity of the subsidence we use a two dimensional numerical method to model the thermal evolution of the basin based on our model studies the central southwest northeast and palos verdes blocks are inferred to have been extended by 178 69 20 and 27 respectively the average crustal extension for the whole basin is 66 the lateral flow of heat out of the central block into adjacent cooler blocks accounts for its rapid subsidence during the early stage the deepest sediments are predicted to have been heated to about 210 c hydrocarbon maturity calculations indicate that the upper miocene sediments reached oil maturity a little over 4 m y b p no sediment is predicted to have achieved natural gas maturity

timing and velocity of petroleum migration in the los angeles basin petroleum in the los angeles basin took a relatively short time to migrate through carrier beds from the deep basin to present day reservoirs we use a numerical model to reconstruct beginning in miocene time subsidence sedimentation compaction thermal evolution and fluid migration in the basin s central block the modeling indicates that the heat flow is currently near the continental average 1 5 hfu but was higher 2 hfu during the early evolution of the basin the thermal history predicted by the model suggests deeply subsided source rocks of late miocene age began to generate oil about 2 2 ma in the simulations compaction slowly drives groundwater from mature source beds toward reservoir rocks these slow rates combined with the low solubility of petroleum in water preclude the possibility that oil migrated by a miscible process instead the oil must have moved as a phase separate from the groundwater the buoyant force acting on the oil phase along the steeply dipping carrier beds was as

oil and gas developments in western canada in 1989 there were 5513 wells drilled in western canada during 1989 30 fewer than in 1988 total meterage drilled was 6 459 797 m for an average well depth of 1172 m exploratory drilling in 1989 dropped by 13 and development drilling by 40 from 1988 levels british columbia was the only area in western canada with increased activity the industry continued to acquire more acreage for less total bonus at provincial crown sales throughout western canada during 1989 3 564 827 ha were acquired for a total bonus of 484 872 000 footnote 5 although the total disposition of crown lands decreased only 4 the average bonus dropped to 136 ha down 26 from 1988 in alberta the most active exploration areas were the southern and eastern mesozoic gas play the western deep devonian gas play the oil plays on and adjacent to the peace river arch and the middle devonian carbonate oil plays in the northwest in british columbia the most active exploration areas were the mesozoic and paleozoic gas plays in the northeastern portion of the province for the first time in 5 years crown drilling rights were issued within the fraser valley in the southwest there was a focus on gas exploration in the province as pipeline reversal was implemented permitting access to canadian and california markets through the alberta transportation network in manitoba exploration and development activity continued to decline in northern canada exploratory drilling success offering of crown lands for exploration approval of gas exports and indications that many of the native lands claims may be approaching settlement all provided exploration encouragement for the next decade exploration activities occurred throughout the northwest territories from the offshore beaufort to the southern border in saskatchewan active exploration areas were shallow mesozoic gas plays in the western portions of the province and deep paleozoic oil plays in the south

subsurface dissolution of heavy minerals frio formation sandstones of the ancestral rio grande province south texas abstract oligocene sandstones frio formation of south texas deposited by the ancestral rio grande contain a heavy mineral assemblage that is markedly altered even in the shallowest sample examined approximately 1 km depth the weight fraction of detrital heavy minerals in the modern rio grande is approximately 1 whereas subsurface frio sandstones contain as little as 0 1 or less heavy minerals modern rio grande sand contains a diverse and unstable heavy mineral assemblage dominated by pyroxenes amphiboles and an assortment of iron titanium oxides minor components include apatite garnet sphene epidote and zircon heavy minerals in subsurface oligocene sandstones include abundant apatite zircon and titanium oxide among detrital components minor iron rich garnet also survives in the subsurface to depths of about 3 km other very stable but also very minor heavy minerals include tourmaline chromite spinel and monazite locally the heavy mineral fraction in the frio is dominated by authigenic components such as siderite pyrite or barite authigenic sphalerite is also observed among the heavy minerals feldspars and rock fragments suggest that sediment deposited by the ancestral rio grande was more strongly dominated by volcanic detritus than is the modern river it is reasonable to infer that the initial heavy mineral assemblage of the frio formation was likewise more unstable than that which appears today in the rio grande thus heavy mineral loss determined by comparing subsurface frio sandstones with the sand in the modern rio grande is conservative comparison of sr and nd concentrations in modern rio grande sands and subsurface frio sandstones suggests that heavy mineral destruction in the subsurface does not cause major perturbations of whole rock isotopic systems involving these elements

diagenesis of the maxon sandstone early cretaceous marathon region texas a diagenetic quartzarenite abstract environments of deposition climate detrital mineral composition and stratigraphic position within a thick sequence of carbonate rocks were major controls on the diagenesis of maxon sandstones fluvial deltaic sands which were subjected to calichification and other pedogenic processes lost all macroporosity prior to burial most sands underwent compaction average of 14 porosity loss minor cementation by quartz and kaolinite possibly during invasion of meteoric water introduced during a low stand of sea level followed by extensive cementation and grain replacement by calcite c and o isotopic values of calcite suggest that carbonate was introduced by formation waters derived from adjacent limestones mass balance calculations on the loss and gain of chemical components during diagenesis show there was a major loss of silica from loss of feldspars and major gains in cao and co2 in calcite cement the overlap of calcite cement on detrital quartz grains gives the impression in thin section that the replacement of quartz by calcite was extensive however thin section and sem views of samples leached in hc1 show that even partial replacement of quartz is rare and is limited largely to the edges of tiny quartz overgrowths ignoring intraformational clasts most maxon sandstones now are quartzarenites q86f1r3 however compensating conservatively for feldspar grains replaced by calcite sands originally were subarkoses q86f10r3

the concept of joint saturation and its application two of the major joint driving mechanisms are joint normal stretching and poroelastic shrinkage and these lead to joint sets commonly associated with structural bending and natural hydraulic fracturing respectively regardless of joint driving mechanism joint infilling is a nonhomogeneous poisson process in the presence of stress shadows through probability modeling we show that in all cases joint spacing is best fit with gamma distributions the shape parameter of the best fit gamma distribution to joint spacing data is a quantitative means to assess the extent of saturation which is represented in a new parameter the joint saturation ratio jsr to test the utility of jsr we call upon published structural bending joint data elk basin lilstock and rives plate bending experiment the shape parameters for these well developed structural bending joints are equal to around three corresponding to a jsr of approximately 30p using the same analysis on the spacing of natural hydraulic fractures collected from outcrops in the gas prone devonian sections of the appalachian basin we find that natural hydraulic fractures differ in two aspects from structural bending joints first the joint spacing is proportional to bed thickness in bedded rocks but not in gas shale sections second the joint saturation of natural hydraulic fractures is generally lower than in well developed structural bending joints thus the jsr is a means to distinguish the joint driving mechanism and to represent joint saturation level independent of bed thickness effects it can be used to distinguish natural fractures from drilling induced fractures and to improve the fracture network modeling

loss of provenance information through subsurface diagenesis in plio pleistocene sandstones northern gulf of mexico abstract subsurface alteration of plagioclase potassium feldspar and a variety of nonopaque heavy minerals has substantially modified the detrital composition of plio pleistocene sediments in the northern gulf of mexico albitization of plagioclase and dissolution of unstable grains are the two principal mechanisms of grain alteration grain alteration increases progressively with depth and has taken place without the influence of significant amounts of meteoric fluids the degree to which strictly subsurface grain alteration has affected these young sediments suggests that grain assemblages in older rocks should be viewed most skeptically with regard to their state of preservation

new insights on natural hydraulic fractures induced by abnormally high pore pressures in this paper i identify a mechanism by which high pore pressures acting alone can cause natural hydraulic fractures and keep them open for extended time periods such fractures may occur in tectonically extensional neutral or compressional settings although the fractures orientations will be governed by the regional stress field they can form and remain open without a horizontal stress differential large enough to induce regional scale dilation rather these fractures are a result of compression or shrinkage of the host rock s solid constituents grains as such grain shrinkage caused by a pore pressure increase is analogous to grain shrinkage caused by temperature decreases a process that also causes extension fractures in rocks i call these fractures hydra lic shrinkage fractures to distinguish them from other types of hydraulic fractures that may be induced by intrusive fluids other than the dominantly present pore fluid i present sample calculations to demonstrate the theoretical basis for these fractures and include various examples to show how fracture potential and orientation are affected by the host rock s mechanical properties and different geological processes these examples indicate that pore pressure magnitude cannot be used alone as a fracture criterion instead fracture potential and orientation depend on the geologic processes causing the excess pore pressures and the mechanical properties of the unfractured rock in general vertical shrinkage fractures are favored and should occur at lower pore pressures in less compressible rocks buried in basins with lower geothermal gradients

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

weathering of charnockites and sediment production in the catchment area of the cauvery river southern india abstract in the cauvery catchment area charnockites are making high hills compared to gneisses in situ static weathering of a charnockitic boulder shows lesser extent of chemical weathering relative to that developed into a profile this difference is seen both in clay mineral composition their ree patterns and in the a cn k and a cnk fm triangular plots based on the chemical changes relative to ti three groups of elements are recognized ba si k na sr ca and mg show increasing depletion as the weathering intensity increases in listed order al remained relatively immobile whereas fe followed by mn cr ni ree and p show a variable extent of enrichment with increasing chemical weathering the sediment occurring on the foothills of the charnockite hills have a very uniform composition and texture without any depositional features reworking of these sediments in a first order stream has caused only minor compositional changes by the local segregation of heavy minerals on the channel bed and by the removal of small amount of clay materials as suspended load the ree pattern of the reworked channel bed sand is nearly identical to that of the channel wall sediments but with lower by 25 abundance although the rocks are 2500 ma and now located in tropical latitudes and probably had experienced several periods of warm and wet climate the soils and the sediments derived from them are chemically very immature suggesting their exposure to denudational processes very recently by tectonic uplift

magnetic mineralogy of sandstones from the lunde formation late triassic northern north sea uk origin of the palaeomagnetic signal the minerals responsible for the magnetic properties of the late triassic lunde formation have been examined using a combination of magnetic mineral extraction microscopy and rock magnetic measurements the magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis by a process of differential preservation and modification of the original detrital minerals the magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material which contains most of the magnetic minerals paramagnetic minerals both of depositional origin and produced during diagenesis are largely responsible for the magnetic susceptibility a suite of residual ferrimagnetic oxides chromite and mn substituted magnetite is responsible for the remanence properties in grey sandstones because most of the originally deposited magnetic fe ti oxides have been removed by reducing conditions during diagenesis however a small number of reddened horizons preserve a higher proportion of original detrital fe ti oxides ferrimagnetic ilmenite ferrimagnetic chromite and magnetite although authigenic hematite has replaced most of the original mineralogy in the reddened horizons magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal particularly in horizons which have a low content of discrete detrital ferrimagnetic grains either through diagenetic dissolution or paucity of the silt and clay fraction within the grey sandstones the triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates with a contribution from detrital ferrimagnetic chromite a model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed

phenomenological understanding of poroelasticity via the micromechanics of a simple digital rock modelmicromechanics basis of poroelasticity

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

subsurface fractures and their relationship to stress history in east texas basin sandstone abstract subsurface fractures were sampled with more than 400 m of core from the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 600 m thick that was deposited in the gradually subsiding gulf coast basin depth to the top of the formation in east texas now ranges between 1800 and 2900 m fractures in sandstone exhibit features that are due to intermittent fracturing and sealing crack seal which suggest that the fractures are the result of episodes of fluid pressure in excess of hydrostatic correlation of the vein mineral precipitation sequence with the diagenetic history and the results of previous 18 o studies of quartz cement indicates that fractures propagated at depths of between 900 and 1500 m during the migration of quartz precipitating fluids fracture permeability provides a mechanism for the passage of large fluid volumes recorded by the precipitation of extensive quartz cement in the formation fractures stopped propagating before maximum burial depth was achieved during approximately 1500 m of subsequent burial fractures either remained partly open or were sealed by postkinematic ankerite and clay minerals

heavy mineral assemblages in sandstone intrusions panoche giant injection complex california u s a excellent exposure from part of the panoche giant injection complex in the san joaquin valley is used to examine provenance characteristics of sandstone intrusions with respect to two parent sandstone units that are known to feed the sand injection complex the succession is part of the upper mesozoic to lower tertiary great valley group and was deposited in a deepwater part of an evolving deep water forearc basin the section examined is mudstone dominated and the sand injection is constrained to have occurred in the danian sandstones in the dosados member moreno fm are identified as the main parent unit on the basis of total heavy mineral assemblage compositions and varietal studies of selected minerals tourmaline garnet titanite apatite and zircon fluidized sand is emplaced in turbulent flow conditions creating high velocity inter grain collisions evidence of comminution and diminution of minerals that are less hard than quartz is documented using indices for the relative hardness tah and durability tad of heavy minerals preferential settling of high density zircon relative to lower density tourmaline produces density controlled variations of zircon tourmaline upward through the injection complex heavy mineral dissolution occurred in the most permeable sandstone intrusions and is believed to record the effects of mid eocene deep weathering when subtropical climate prevailed in the study area detrital heavy mineral assemblages which are dominated by titanite and garnet record erosion of the sierran metamorphic terrane with mafic and alkaline plutonic rocks zircon with u pb ages of c 140 160 ma and c 90 110 ma consistent with earlier independent analyses record erosion of sierran granitoids on the paleo seafloor enrichment of ca amphibole and epidote is indicative of sierran provenance concurrent with sand extrusion the presence of na amphibole in the uhalde sandstone supports earlier work that suggested sediment input from obducted seafloor to the west

geochemistry of the 2 51 ga old rampur group pelites western himalayas implications for their provenance and weathering the late archean volcano sedimentary succession of the rampur group rg represents the oldest superacrustal rocks exposed in the himalayas the sedimentary component is pelite dominated in the lower part and siliciclastic with associated mafic volcanics dominated in the upper part indicating shallowing of the basin over time the bulk chemistry of the rg pelites shows illite chlorite control that gets diluted with increase in quartz content they have a general similarity with the post archean shales but a slightly higher average sio2 and lower al2o3 contents and severe depletion in cao na2o sr and ba relative to the north american shale composite ree patterns are highly fractionated with la yb n ratio ranging from 8 8 to 18 6 hree show slight fractionation average gd yb n b2 and all the samples have large negative eu anomalies average 0 61 la yb n decrease with increase in al2o3 na2o but lack any relationship with k2o thereby discounting clay mineral control hree have a positive correlation with tio2 th y and nb but not with p2o5 and zr the increase in hree seems likely due to the increased mobilization of hree enriched phase s during intense weathering high zr and th contents and th ni la sc and la yb n ratios as well as sc th sc relations favor a dominantly felsic source with only a minor contribution from mafic rocks the most likely source lithology compatible with the southerly paleocurrent direction and first cycle sediment textural features of the rg sediments appears to be the banded gneissic complex bgc of the northern indian shield the same conclusion is reached when rg data are plotted in al2o3 cao na2o k2o a cn k compositional space the data define a trend which indicates unweathered plagioclase k feldspar source composition similar to that of the bgc pre metasomatized high chemical index of weathering as well as the high plagioclase index of weathering for a majority of the samples conforms to the world wide warm moist climate prevalent during the time of their deposition 2001 elsevier science b v all rights reserved

petrology and geochemistry of siliciclastic rocks of mixed feldspathic and ophiolitic provenance in the northern apennines italy provenance in clastic sedimentary rocks in the northern apennines is determined by integrated petrographic and geochemical criteria thus mafic poor arkoses in the gottero formation senonian paleocene deposited prior to the alpine orogeny were derived from granitic provenance in the sardinian corsican massif in the gottero formation chemical criteria distinguish between very low input of detrital mafic material and co c and ni added entirely by adsorption on clay minerals post alpine orogeny feldspathic sandstones in the macigno formation mid oligocene lower miocene and cervarola formation lower miocene reflect a more diverse provenance of granitic metamorphic ophiolitic and sedimentary rocks in the apennines and alps the macigno and cervarola rocks are distinguished by serpentine gabbro and diabase fragments and higher chlorite contents than in the gottero rocks chemically the macigno and cervarola rocks contain distinctive signatures expressed in higher mgo co cr and ni contents of the mafic ultramafic detritus a wider range and more scattered cr and ni variation and higher niggli ti for a given niggli al alk content the mgo co cr and ni abundances indicate up to 10 ultramafic ophiolitic detritus in the macigno and cervarola sandstones and up to 17 ophiolitic detritus in associated shales estimated by comparison of mixtures of feldspathic sands with ligurian peridotites up to 10 basaltic ophiolitic detritus in these sediments is suggested by cao and fe2o3 total abundances petrographic data indicate that a major part of the detritus in the macigno and cervarola formations is from alpine granitic basement of calc alkaline granite to tonalite plutons and gneisses but apart from higher ba in macigno and cervarola sandstones than in the gottero sandstones this is difficult to identify chemically the trace elements cr ni ba rb ce la y co cu ga pb and zn generally have positive correlations with niggli al alk a chemical surrogate measure of sheet mineral clays micas and chlorites content whereas sr which is associated with carbonate and feldspar shows negative correlation with al alk an olistostrome unit contains the most weathered detritus but otherwise shows similar chemical variation to the other formations methods to 1 identify separately mafic and ultramafic inputs 2 recognize in shales co cr and ni derived from ophiolitic sources and non ophiolitic sources 3 estimate the minimum mafic ultramafic proportion in a sediment may have general application elsewhere in sedimentary and metasedimentary rocks

pore pressure prediction in gas hydrate bearing sediments of krishna godavari basin india abstract predrill estimate of pore pressure pp is important for well design especially in areas containing gas hydrate bearing sediments here we report the pressure distribution in the gas hydrate bearing sediments in krishna godavari k g basin predicted from the seismic data along two lines passing through the site 10 of expedition 01 exp 01 conducted by the indian national gas hydrate programme nghp a series of elastic parameters namely p wave velocity vp s wave velocity vs density vp vs p impedance zp and s impedance zs were derived from seismic data using pre stack inversion first the pp estimated from a sonic transit time log was calibrated with pressure core measurements in the depth interval of 1060 1280 m its magnitude is found to be ranging from 10 5 to 12 9 mpa where the vertical stress varies from 10 7 to 13 4 mpa within the same depth interval this estimated pp was next treated as a target log desired output and seismic attributes resulting from pre stack seismic inversion were used as input parameters in the training of a multi layered feedforward network mlfn finally we use the trained network to generate a subsurface pp distribution map along two 2d multichannel seismic profiles line x and line y passing through nghp01 10 within a time interval of 1420 1620 ms corresponding to a depth interval of 1060 1280 m this corresponds to sedimentary column from 22 to 242 mbsf our results demonstrate that the pressure remains hydrostatic within the gas hydrate bearing sediments and is mostly above hydrostatic below the gas hydrate bearing zone

structural evolution of shale diapirs from reactive rise to mud volcanism 3d seismic data from the baram delta offshore brunei darussalam two areas of shale diapirism in a deltaic setting were examined using 3d seismic data from offshore brunei to determine the origin evolution and geometry of shale diapirs the study areas cover parts of the egret and ampa fields within the baram delta province the ampa diapir consists of a small bulge of the mobile setap shale around which lies a zone of chaotically disrupted country rock reflections at the crest of this chaotic zone vertical cylindrical shale pipes arise that end in a wider head the diapir heads wear halos of high amplitude reflections probably due to the presence of methane gas the egret data show the same features except that the bulge of the source layer is not detected due to low reflectivity and chaotic reflections at the base of the overburden the bulging of the setap shale on the ampa data is interpreted as modified reactive diapirism in response to differential loading the initial reactive diapir shape was modified by lateral or oblique upward migration of fluidized shales into the hanging wall of the fault controlling the reactive diapir this probably occurred as a network of dykes and sills the presence of some preserved bedding is indicated by weak but coherent reflectivity from the area initially defined as chaotic shale the spreading of overpressure and the injection of shale into the overburden is thought to be a late phenomenon possibly late pliocene or early pleistocene and the mobility of the shale is attributed to overpressure increase due to gas generation within the setap shale and maybe the lower part of the overburden gas migration perhaps ahead of the diapir intrusion probably facilitated the mud injection the ampa and egret diapirs formed in two diapiric phases an early reactive diapirism and a later active diapirism but the phases have little in common they are seperated by about 9 million years of inactivity and differ in process and probably differ in degree of overpressure in overpressure generating mechanism and in properties of the diapiric mud

getting more for less the unusual efficiency of fluid flow in fractures although it is recognized that the opening of fractures changes with changing fracture spacing it is a common practice to consider only the effects of loading and rock properties and to assume that opening of all fractures is independent of fracture spacing thus one would expect that the more fractures contained in a rock mass the greater the fluid flow rate using the cubic law and the finite element method we have investigated the volumetric flow rate through a number of equally spaced fractures in a fractured rock layer as a function of the ratio of fracture spacing to layer thickness under a remote extension results show that there is an optimum value for the ratio of fracture spacing to layer thickness that yields the maximum flow rate this value is independent of the fluid properties the head gradient the applied extension the internal fluid pressure and the overburden stress

petrology and geochemistry of diagenetically altered tuffaceous rocks from the middle triassic of central spain abstract petrographical and geochemical analysis of altered tuffaceous rocks of the middle triassic from the iberian range in central spain shows that pedogenic processes and subsequent diagenesis have strongly modified the original mineralogy and textures these rocks are related to a magmatic episode that occurred at the buntsandstein muschelkalk unconformity in this region during the evolution of the iberian trough the tuffaceous rocks accumulated in saline ponds in an open and terminal flood plain setting their alteration is typified by neoformation of k feldspar and illite precipitation of chert and carbonate cements and replacements were also important processes resulting in intercalated chert beds and nodules much of the silica appears to be derived from extensive devitrification and weathering of glassy volcanic ash x ray diffraction analysis microscopic observations and edx microanalysis reveal that the chert consists of various types of chalcedony tridymite micro and megaquartz illite k feldspar and small amounts of albite in some of the nodules dolomite ankerite is also common enclosing anhydrite and barite relicts which are partially silicified different levels of ammonium enrichment have been found in each petrofacies type tuffs and tuffaceous rocks contain from 27 to 326 ppm mean 139 of nh4 underlying arkosic sandstones with authigenic k feldspar contain nh4 ranging from 48 ppm to 191 ppm mean 124 the nh4 was probably derived from decay of organic matter in the evaporitic environment of deposition and its incorporation into feldspar and illite as they formed from unstable volcanic glass sanidine plagioclase and vitric volcanic fragments this transformation could have occurred via the intermediate phases of smectite and zeolites the tuffaceous rocks also have a high content of b ranging from 61 ppm to 170 ppm mean 101 however high levels of b are not positively correlated with high levels of nh4 in the samples both b and nh4 can be contained in either k feldspar or illite but it is not possible to know how much is present in each mineral without separating the minerals comparison of spidergrams of the tuffaceous rocks with those in the underlying arkosic sandstones indicates similar patterns with positive anomalies in la sm and y and negative in nb p ti and yb although the tuffaceous rocks are slightly richer in total ree these data suggest that the distribution of ree must also be controlled by weathering and diagenetic reactions and cannot be used for discrimination between volcanic and nonvolcanic sandstones

tidally driven groundwater flow and solute exchange in a marsh numerical simulations 1 tidal fluctuations drive groundwater flow in salt marsh sediments this flow could cause significant chemical exchange across the sediment water interface and could affect marsh ecology numerical models of a generalized tidal creek and marsh were constructed to calculate flow patterns and solute exchange between the marsh and creek the governing equation for saturated unsaturated flow was modified to account for tide related changes in total stress groundwater flow occurred primarily in the creek bank even when the marsh platform was inundated at high tide for marsh sediments with a hydraulic conductivity of 10 4 m s 1 groundwater ages in simulations lasting 60 days were on the order of days near the creek bank and increased to 50 60 days with distance into the marsh the volume of water that discharged between high and low tide was 0 22 0 31 m3 per meter length of channel which for a creek drainage density of 0 012 m 1 corresponds to 10 14 l m 2 d 1 sediment permeability and capillarity were important controls on flow and groundwater age in the marsh sediment compressibility affected groundwater age for compressible sediments representative of mud but not for sediments with lower compressibilities representative of sand simulations were relatively insensitive to dispersivity a comparison of simulation results with other estimates of groundwater exchange from the north inlet south carolina suggest that tides could drive observed exchange there only if the pleistocene sands underlying muddy marsh sediments outcrop within the tidal range

relationships between pore pressure stresses and present day geodynamics in the scotian shelf offshore eastern canada a geomechanical study of the mesozoic and tertiary sediments beneath the scotian shelf shows that two major weak layers exist at depth the overpressured unit at 4000 m and the basal argo salt which is ductile and diapiric a consistent relationship is observed between the overpressures and the stress state in the basin first the smaller horizontal principal stress increases dramatically just above the onset of overpressuring and secondly borehole breakouts indicators of stress anisotropy decrease in abundance within the overpressured zone neither tectonic shear nor rapid burial can explain these observations however overpressuring by fluid generation will result in horizontal stress increase and eventually lead to an isotropic stress state compelling evidence is found for hydrocarbon generation being a key cause of overpressuring in the scotian shelf present day horizontal stress orientations in the sediments overlying the overpressured zone are aligned with contours of equal overpressure but not with the traces of tertiary listric normal faults or with the shelf edge this implies that contemporary stresses in these sediments are causally related to overpressuring the upward transmission of lithospheric plate movements and associated stress is unlikely because the ductile argo salt effectively detaches the sediments from basement instead the rocks of the scotian shelf are attempting to slide down the geomechanically weak overpressured zone in the directions indicated by the breakouts but they are prevented from doing so by the bowl shaped basement structure in the region

contributions of compaction and aquathermal pressuring to geopressure and the influence of environmental conditions reply luo and vasseur 1992 presented a useful methodology of simplifying the complex equations that describe basin compaction and thereby directly assessing the contribution of aquathermal effects in the development of abnormal pressures their methodology leads to the conclusion that aquathermal pressuring plays only a small role in the development of overpressures in sedimentary basins although this conclusion follows from the cases studied it is possible to conclude otherwise using 1 a more general model of sediment compaction and 2 material properties that are more representative of conditions at depth the primary factor behind objection to the conclusion in luo and vasseur 1992 is that is fundamental equations which formed the basis for the simplifying methodology do not account fully for conditions in which a sediment s effective stresses are reduced we emphasize here that we do not claim that thermal effects are the dominant cause of overpressures rather our calculations suggest that they are not negligible in all cases there is a complex interplay of the various mechanisms that cause excess pore water pressures no one mechanism of overpressuring dominates all others for all situations because thermal effects not only cause higher excess pore pressures but also reduce effective stresses thermal effectsmore will be minimized by default in calculations that do not account for effective stress reduction the resulting reduced effective stresses also have important implications for geological questions such as acoustic velocities and fracture potential less

geochemistry of the floodplain sediments of the kaveri river southern india abstract the floodplain sediments of the kaveri river southern india derived from archean gneissic and charnockitic source regions show interbedding of silty 4 4 7 and sandy units 1 4 3 7 the geochemistry of silty beds is remarkably uniform at a given location and over a lateral distance of nearly 250 km the sandy beds have more variable chemical compositions yet are comparable to those of silty beds except for the diluting effect of quartz silty sediments retain the geochemical signature of prominently exposed source rocks for almost all elements and provide evidence of tectonic instability in the source region the floodplain sediments contain all size grades sand silt and clay which may have resulted in minimizing the biases imposed on suspended and bedload sediments due to sorting the low chemical index of alteration cia the dominance of unweathered primary minerals and the minor proportion of smectitic clay all suggest that the region has been subjected to little chemical weathering this is possible if the region has undergone recent uplift exposing fresh archean rock to surface denudation the formation of fertile farmland along the kaveri river course and its delta is related to these recent geological processes

submarine groundwater discharge and solute transport under a transgressive barrier island abstract many recent investigations of groundwater dynamics in beaches employed groundwater models that assumed isotropic numerically convenient hydrogeological conditions real beaches exhibit local variability with respect to stratigraphy sediment grain size and associated topographic profile so that groundwater flow may diverge significantly from idealized models we used a combination of hydrogeologic field methods and a variable density saturated unsaturated transient groundwater flow model to investigate sgd and solute transport under cabretta beach a small transgressive barrier island seaward of sapelo island georgia we found that the inclusion of real beach heterogeneity drove important deviations from predictions based on theoretical beaches cabretta beach sustained a stronger upper saline plume than predicted due to the presence of a buried silty mud layer beneath the surface infiltration of seawater was greater for neap tides than for spring tides due to variations in beach slope the strength of the upper saline plume was greatest during spring tides contrary to recent model predictions the position and width of the upper saline plume was highly dynamic through the lunar cycle our results suggest that field measurements of salinity gradients may be useful for estimating rates of tidally and density driven recirculation through the beach finally our results indicate that several important biogeochemical cycles recently studied at cabretta beach were heavily influenced by groundwater flow and associated solute transport

mechanical compaction of deeply buried sandstones of the north sea abstract sandstones experience mechanical compaction when the overburden load exceeds the compressive strength petrographic evidence is rarely sufficient to determine the timing of mechanical compaction it is often assumed from indirect evidence such as regional porosity depth trends that mechanical compaction is a process that occurs exclusively during shallow or intermediate burial mechanical compaction without grain fracturing may occur at depth following pervasive framework grain dissolution and or if anomalously high porosity has been preserved due to the presence of small amounts of cement we describe examples from the fulmar sandstone formation of the central graben north sea that experienced late stage mechanical compaction following sponge spicule dissolution and microcrystalline quartz cementation deep burial mechanical compaction involving grain crushing may occur if the rate of grain contact quartz dissolution and or quartz overgrowth development cannot compete with the rate of stress increase at grain contacts some rotliegendes sandstones of the southern north sea that have been buried to 4 5 km offer a good example where the suppression of chemical compaction due to the presence of grain coating clays resulted in pervasive grain fracturing mineral veins are frequently associated with sandstones that have experienced pervasive mechanical compaction during deep burial these may reflect the sudden development of overpressure resulting from the transfer of load to the fluid during collapse of the sandstone framework

denticles on chain silicate grain surfaces and their utility as indicators of weathering conditions on earth and mars abstract denticulated margins also known in older literature as sawtooth cockscomb or hacksaw terminations are a common feature of pyroxenes and amphiboles visible in optical and electron microscopes denticles are remnants of undissolved material that formerly constituted the walls between elongate etch pits the characteristic aqueous dissolution form of chain silicate minerals denticles occur widely in terrestrial near surface materials that have experienced low temperature aqueous alteration including chemically weathered regoliths soils in a variety of climatic and geomorphic settings sediments and sedimentary rocks ranges of dissolution forms and dimensions commonly tens of microns in length are identical on both pyroxenes and amphiboles in these materials denticles on terrestrial chain silicates are formed by low temperature aqueous alteration denticles on a soil mineral grain of unknown composition at the phoenix mars lander site and on terrestrial pyroxenes naturally weathered in terrestrial low temperature aqueous environments are broadly similar in size shape and orientation relationships between denticles however quantitative morphometric parameters vary among different terrestrially weathered pyroxenes and even between different images of compositionally identical pyroxenes from the same naturally weathered sample such variations are too large to allow simple identification of minerals of unknown composition from morphology alone nevertheless the overall morphological similarities are consistent with a low temperature aqueous corrosion origin of denticles at the phoenix mars lander site

faulting fracturing and in situ stress prediction in the ahnet basin algeria a finite element approach abstract many low efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures accurate and low cost information on basic fault and fracture properties orientation in particular is critical in reducing well costs and increasing well recoveries this paper describes how we used an advanced numerical modelling technique the finite element method fem to compute site specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties structural position and tectonic stress presented are the numerical results of two dimensional plane strain end member fem models of a hydrocarbon bearing fault propagation fold structure interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling however it still allows comparisons with the little available geological and geophysical data in all models the weak mechanical strength and flow properties of a thick shale layer the main seal leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement and results in flexural slip across the shale layer all models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression the stress regime for the formation of new fractures changes from compressional to shear with depth if pre existing fractures exist only sub horizontal fractures are predicted to open thus defining the principal orientation of effective reservoir permeability in models that do not include a blind thrust fault in the basement flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks the folding induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading models incorporating a blind thrust fault correctly predict the formation of secondary syn and anti thetic mesoscale faults in the basement and sediments of the hanging wall some of these faults cut reservoir and or seal layers and thus may influence effective reservoir permeability and affect seal integrity the predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure and proximity to failure these numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs

joint interaction with embedded concretions joint loading configurations inferred from propagation paths abstract the interaction between propagating joints and embedded concretions in a devonian black shale near seneca lake ny permits identification of the loading configurations responsible for two joint sets of different ages striking at nearly the same orientation the earlier set consists of systematic joints cut by later alleghanian joints of the appalachian plateau the later set consists of non systematic curving cross joints that abut these same alleghanian joints field evidence shows that concretions functioned as stiff inclusions in a compliant black shale as a consequence of this elastic contrast local perturbations in the remote stress field persisted around the concretions during burial tectonic deformation and exhumation these stress perturbations influenced joint propagation paths of both joint sets our conclusions about loading configurations are based on finite element modeling of the effect of the local stress perturbation on concretion modified joint propagation modeling shows that the local stress perturbation from a thermoelastic loading was responsible for deflecting cross joints away from concretions in a curved propagation path near the concretion this load configuration also led to arrest of cross joints before they penetrated the shale concretion interface at greater distances from the concretion the propagation path of cross joints was controlled by the contemporary tectonic stress field the interface between concretions and the surrounding shale was strongly bonded as indicated by the crossing of the interface by some of the systematic ene joints higher compressive stress levels within the concretions relative to the shale suppressed joint development in the concretion causing the arrest of those joints once they had driven across the interface and a short distance into the concretion numerical modeling shows that interface penetration by the systematic ene joints is consistent with a fluid load the same loading configuration postulated for the subsequent alleghanian joints the traces of the systematic ene joints align on opposite sides of concretions rather than curving toward the concretion as predicted by two dimensional models of the fluid load co planar traces are indicative of large planar joints propagating in plane around the concretion making it energetically inefficient for the crack front to curve as it enters the local stress perturbation near the concretion a fluid load for the systematic ene joints came from high pore pressure during the pre alleghanian stages of burial of the devonian catskill delta complex

numerical assessment of dissolution versus replacement in the subsurface destruction of detrital feldspars oligocene frio formation south texas abstract dissolution is the main process responsible for modifying detrital feldspar assemblages in the oligocene frio formation of south texas accounting for up to 80 of feldspar modification in completely altered assemblages this finding is supported through two independent calculations one based on an estimate of initial feldspar content of the sandstones the other on estimated initial composition of the feldspars themselves volumetric loss of detrital feldspar on the observed scale over such a short time span has major implications for provenance interpretations and for understanding the basin wide mass balance of several major diagenetic components the wide variation of feldspar content and composition at any given depth temperature attests to the premier significance of fluid co position and flux in driving the reactions that bring about dissolution of feldspars in these rocks

chronostratigraphy and tectonostratigraphy of the columbus basin eastern offshore trinidad the columbus basin forming the easternmost part of the eastern venezuela basin is situated along the obliquely converging margins of the caribbean and south american plates the two primary structural elements that characterize the basin are 1 transpressional northeast southwest trending anticlines and 2 northwest southeast oriented down to the northeast extension normal faults the basin was filled throughout the pliocene and pleistocene by more than 40 000 ft 12 200 m of clastic sediment supplied primarily by the paleo orinoco delta system the delta prograded eastward over a storm influenced and current influenced shelf during the pliocene pleistocene depositing marine and terrestrial clastic megasequences as a series of prograding wedges atop a lower pliocene to pre pliocene mobile shale facies biostratigraphic and well log data from 41 wells were integrated with thousands of kilometers of interpreted two dimensional and three dimensional seismic data to construct a chronostratigraphic framework for the basin as a result several observations were made regarding the basin s geology that have a bearing on exploration risk and success 1 megasequences wedge bidirectionally 2 consideration of hydrocarbon system risk across any area requires looking at these sequences as complete paleofeatures 3 reservoir location is influenced by structural elements in the basin 4 the lower limit of a good quality reservoir in any megasequence deepens the closer it comes to the normal fault bounding the wedge in a proximal location 5 reservoir quality of deep marine strata is strongly influenced by both the type of shelf system developed bypass or aggradational and the location of both subaerial and submarine highs and 6 submarine surfaces of erosion partition the megasequences and influence hydrostatic pressure migration and trapping of hydrocarbons and the distribution of hydrocarbon type begin page 1906

diagenetic quartzarenite and destruction of secondary porosity an example from the middle jurassic brent sandstone of northwest europe significant amounts of feldspar have been dissolved from middle jurassic sandstone oil reservoirs in the north sea northwest europe during burial diagenesis sandstones of the middle jurassic brent group become increasingly quartzose with increasing burial depth at statfjord field 2500 m depth sandstones are arkose to subarkose at hutton field 3050 m depth they are subarkose to quartzarenite and at lyell field 3500 m depth sandstones are typically quartzarenite in spite of extensive feldspar dissolution the abundance of secondary porosity due to feldspar dissolution is similar for all fields averaging 2 9 of the total rock volume thus far more feldspar has been dissolved than is recorded as secondary porosity the limit on preservation of secondary porosity may be largely the effect of the mechanical strength of the rock an excessive number of large secondary pores lowers the rock strength below the point at which the rock can withstand overburden stress thus causing collapse of some of the secondary pores

a new approach to provenance studies electron microprobe analysis of detrital garnets from middle jurassic sandstones of the northern north sea reading provenance from heavy mineral suites is hampered by the depletion of diagnostic but unstable heavy minerals through intrastratal solution a common phenomenon in deep sedimentary basins such as the north sea this paper demonstrates the potential of electron microprobe analysis in overcoming this problem by concentrating on the compositional variations shown by detrital garnets which are relatively resistant to intrastratal solution studies of the garnets from the brent group middle jurassic of the murchison and tern oilfields in the northern north sea reveal that three distinct areas supplied detritus association 1 which characterizes the broom formation in both areas and recurs higher in the sequence in the tern field is ascribed to an orkney shetland source the location of the areas supplying the garnets of association 2 best represented in murchison and association 3 common to both fields is less clear their ultimate sources clearly include high grade metamorphics and therefore probably lie on the norwegian landmass the orkney shetland platform or conceivably the scottish landmass but the possibility of recycling makes it difficult to judge their immediate provenances nevertheless the presence of three garnet associations indicates that the most widely accepted model of brent sedimentation with sands derived from a domal uplift in the outer moray firth and channelled northward along the viking graben is untenable

primary oil migration through buoyancy driven multiple fracture propagation oil velocity and flux 1 we present a fracture mechanics based formulation to investigate primary oil migration through the propagation of an array of periodic parallel fractures in a sedimentary rock with elevated pore fluid pressure the rock is assumed to be a linearly elastic medium the fracture propagation and hence oil migration velocity are determined using a fracture mechanics criterion together with the lubrication theory of fluid mechanics we find that fracture interactions have profound effects on the primary oil migration behavior for a given fracture length the mass flux of oil migration decreases dramatically with an increase in fracture density the reduced oil flux is due to the decreased fracture propagation velocity as well as the narrowed fracture opening that result from the fracture interactions

groundwater transport and the freshwater saltwater interface below sandy beaches summary current conceptual models for groundwater flow in beaches highlight an upper saline plume which is separated from the lower salt wedge by a zone of brackish to fresh groundwater discharge there is currently limited knowledge of what conditions allow an upper saline plume to exist and what factors control its formation we used variable density saturated unsaturated transient groundwater flow models to investigate the configuration of the freshwater saltwater interface in beaches with slopes varying from 0 1 to 0 01 in the absence of waves we also varied hydraulic conductivity dispersivity tidal amplitude and inflow of fresh groundwater the simulated salinity configuration of the freshwater saltwater interfaces varied significantly no upper saline plumes formed in any beach with hydraulic conductivities less than 10 m d the slope of the beach was also a significant control steeper beach faces allowed stronger upper saline plumes to develop median sediment grain size of the beach is strongly correlated to both beach slope and permeability and therefore the development of an upper saline plume prior studies of groundwater flow and salinity in beaches have used a range of theoretical dispersivities and the appropriate values of dispersivity to be used to represent real beaches remains unclear we found the upper saline plume to weaken with the use of larger values of dispersivity our results suggest that upper saline plumes do not form in all beaches and may be less common than previously considered

buoyancy driven propagation of isolated fluid filled fractures implications for fluid transport in gulf of mexico geopressured sediments a large portion of the sediments within the northern gulf of mexico contain pore fluid pressures in excess of hydrostatic development of geopressure is generally attributed to compaction disequilibrium caused by rapid deposition of low permeability sediments in the miocene and plio pleistocene numerous studies have examined the formation of overpressures and or expulsion of geopressured fluids into overlying hydropressured strata however very little attention has been given to fluid flow within the geopressured zone itself movement of oils from cretaceous or older source rocks into plio pleistocene reservoirs in the gulf basin requires as much as 10 km of vertical migration in a few million years precipitation of cements in some geopressured sediments also implies large scale fluid flow new evidence from a deep well in the eugene island area offshore louisiana indicates that geopressured sediments are mechanically very weak with a poisson s ratio greater than 0 4 and a shear modulus or rigidity less than 1 gpa in addition large scale fluid flow either through interconnected pores or fractures is not occurring in this location at least at present an alternative hypothesis is that upward fluid transport in geopressured sediments is caused by buoyancy driven propagation of isolated fluid filled fractures using linear fracture mechanics i show that vertical fractures with lengths of a few meters can propagate at velocities of 1000 m yr mass flux rates 100 kg m2 yr are significant assuming a mechanism for formation of fluid filled fractures exists such as hydrofracturing when fluid pressures exceeded the minimum confining stress fracture propagation velocity and mass flux rate are strongly dependent on the shear modulus of geopressured sediments

storm driven groundwater flow in a salt marsh 1 storms can cause significant groundwater flow in coastal settings but prior studies of the effects of storms on groundwater flow and transport have largely focused on very large storms and used salinity as a tracer we have little information about the effects of smaller storms on coastal flow and how storm induced variability affects key tidal wetlands like salt marshes which may remain saline throughout a storm here we show that even the distant passage of a moderate storm can strongly increase groundwater flow and transport in salt marsh ecosystems and adjacent barrier islands groundwater monitoring and radium isotope tracer analyses revealed significant influx of saline creek water into the confined aquifer below the marsh platform driven by storm surge this pulse of fluids reached depths exceeding 5 m and surge enhanced tides propagated through the aquifer to affect flow in the upland 100 m from the creek bank groundwater discharge from the marsh varied significantly prior to the storm doubling during inundating tides compared to a period of noninundating neap tides storm surge then caused groundwater discharge to decline 50 compared to similar inundating tides ra and nutrient poor creek water that entered the confined aquifer below the marsh was quickly enriched in nutrients and carbon even on 12 h tidal cycles so that nutrient discharge was likely proportional to groundwater discharge storm related flow could also drive significant contaminant discharge from developed coastlines the enhanced transport and variability observed here likely affected hundreds of kilometers of the coastline impacted by the storm

stress fields a key to oil migration a coherent oil migration model based on geomechanical considerations includes both the high molecular kerogen structure and the capillary properties of source rocks oil is squeezed from kerogen by compaction following oil generation this squeezing effect should be created by the differential stress maximum compressive stress minus least compressive stress acting on the kerogen which has been chemically broken up by oil formation in sedimentary bodies whose water is at hydrostatic pressure the migration of oil seems to involve two processes 1 lateral transfer by channeling into the more coarsely microporous layers of the source rocks from the oil generation site toward the geologic structure or lower pressured zone and 2 vertical transfer from source rock to eservoir by the opening or reopening of vertical fractures in the few areas such as structural tops where the least compressive stress is slightly greater than or equal to the pore pressure and where the capillary pressure increment 2 ggr r of oil in the microporosity exceeds the tensile strength of the rock in sedimentary bodies whose water is overpressured the pore pressure should be governed by the least compressive stress and thus migration should begin by oil transfers in a system of small open fractures and eventually in larger fractures the theory demonstrates the impossibility of oil being transferred to the reservoir under true tensile conditions negative effective compressive stress and thus explains the large asphaltic veins of southeastern turkey and the well known bitumen veins of the uinta basin

finite element analysis of the stress distribution around a pressurized crack in a layered elastic medium implications for the spacing of fluid driven joints in bedded sedimentary rock bedding perpendicular joints confined to individual beds in interbedded sedimentary rocks commonly exhibit spacings which are proportional to the thickness of the jointed bed and which vary according to lithology or structural position the mechanical explanation for this relationship is well understood when the joints are driven by far field crack normal tensile stresses but poorly understood for cracks driven by elevated fluid pressures where the crack driving stress is the difference between the crack normal compression and the fluid pressure in the crack through a series of finite element numerical models we investigate how various parameters influence the driving stress distribution around pressurized cracks in layered media and thereby identify factors influencing the spacing of fluid driven joints for the situation we modeled we observe that 1 crack driving stress is reduced in the vicinity of pressurized joints and that the extent of the stress reduction depends on the contrast in elastic properties between the layers and 2 crack driving stress distribution depends on the ambient pore pressure during jointing these results indicate the spacing of fluid driven joints should depend on lithology and pore pressure

deep oxygen penetration drives nitrification in intertidal beach sands we discovered a hotspot of elevated nitrate concentration median 431 m in shallow beach pore water that extended across the entire length of two barrier islands in the southeastern united states of america we investigated this feature by surveying groundwater geochemistry measuring fluctuations in in situ dissolved oxygen do concentrations modeling groundwater flow and quantifying nitrification rates nitrification of groundwater ammonium was the only possible nitrate source with a measured potential rate of 0 84 mol m 2 h 1 however the observed nitrate concentrations were far in excess of the predicted maximum achievable by aerobic nitrification assuming a 2 1 ratio of o n and around 200 m do in air saturated seawater groundwater do concentrations within the hotspot 65 cm depth were consistently 20 50 m the nitrate hotspot was located at the top of the water table beneath dry undersaturated sand that allowed the penetration of air and the dissolution of excess oxygen into the pore fluids the total dissolved nitrogen concentration of the hotspot was higher than anywhere else on the island indicating nitrogen accumulation within the hotspot most likely via ammonium adsorption vertical dispersion was the dominant pathway for nitrate loss from the hotspot this nitrate was consumed in underlying anoxic sand coupling microbial pathways of nitrogen oxidation and reduction and removing bioavailable nitrogen from the beach ecosystem

fluid flow in the south eugene island area offshore louisiana results of numerical simulations abstract numerical simulations of heat and fluid transport in the south eugene island sei area offshore louisiana suggest fluids migrate from deep abnormally pressured sediments along fault zones and into overlying oil and gas reservoirs in the simulations fluid flow along the fault produces a narrow thermal anomaly around the fault a negative thermal anomaly forms adjacent to the thermal maximum when permeable sediments dip away from the fault zone in the simulations this negative thermal anomaly dissipates 100 200 years after it forms the presence of a similar negative thermal anomaly in the sei area suggests that fluid movement in the area is a recent event the observation that the thermal maximum in the study area is not centered about the fault zone further suggests that the primary vertical conduit for fluid flow in the sei area is not a major fault but minor splay faults in the hanging wall

modelling petroleum migration through microcrack propagation in transversely isotropic source rocks summary to investigate primary petroleum migration through microfracturing of source rocks we develop a theoretical multiphysics model incorporating simultaneous generation of oil and gas from kerogen elastic anisotropy of the source rock and propagation of microcracks filled with oil and gas the variations of excess fluid pressure in the crack and crack propagation distance with time are determined a detailed parametric analysis is performed to study the sensitivity of petroleum migration behaviour to changes in the input parameters including kerogen type chemistry of kerogen to oil gas conversion elastic anisotropy of source rocks geothermal gradient and burial rate numerical results show that a microcrack in type iii kerogen bearing source rocks can attain greater length than in rocks containing type i and ii kerogen which have higher oil potentials and transform to oil gas much faster than type iii kerogen elastic anisotropy of source rocks has a profound influence on the crack propagation distance but only a marginal effect on the duration of crack propagation and the excess fluid pressure the simulation also shows that higher geothermal gradients and fast burial reduce the crack propagation duration significantly but excess pressure and final crack length are not sensitive to the variations of geothermal gradient and burial rate

major and trace elements in pyrope almandine garnets as sediment provenance indicators of the lower carboniferous culm sediments drahany uplands bohemian massif abstract the lower carboniferous culm basin is part of the european variscan foreland basin garnet is an abundant heavy mineral at all stratigraphic levels of the culm sediments the protivanov rozstani and myslejovice formation from base to top this paper compares the major element electron microprobe and trace element la icp ms compositions of garnet for the purpose of studying the provenance of the culm sediments the polymict garnet assemblages ranging from the protivanov to lower part of the myslejovice formation goniatite zones pe go pass into oligomict ones dominated by pyrope almandine garnets uppermost part of the myslejovice formation goniatite zones go go as the predominant low grossular pyrope almandines from oligomict garnet assemblages are very uniform and homogeneous in major element composition we studied their trace element composition and zoning pyrope almandine garnets are poor in lree and show enrichment in hree the chondrite normalized patterns are almost flat from dy to lu and show a significant negative eu anomaly typical for granulite grade garnets major element compositions of detrital low grossular pyrope almandines can only be matched with some granulites and garnetiferous felsic gneisses of the bohemian massif garnets from granulites cropping out at the present day erosion level of the bohemian massif have usually higher ca and or lower mg contents the major and trace element compositions and zoning patterns of pyrope almandine garnets from the upper part of the myslejovice formation compare well with garnets from the miroslav granulites and small moldanubian granulite bodies west of the t ebic massif the moldanubian granulites are commonly associated with mantle derived peridotites and eclogites the presence of detrital cr spinels with high cr and low mg single grains of cr spinels and fine grained kelyphitic intergrowths of cr spinel with chlorite comparable with spinel types from moldanubian peridotites could indicate the joint occurrence of peridotites serpentinized peridotites and granulites at the erosion level of the bohemian massif during the deposition of the uppermost culm sediments 325 ma while the older culm sediments polymict garnet assemblages represent the accumulation of clastic material from the northern and north western areas moravo silesian and lugian zone the uppermost culm sediments oligomict garnet assemblages contain material from western and south western areas moldanubian zone and moravian nappe the general influx of clastic material changed as a result of clockwise rotation of the eastern part of the variscan belt during this time span 340 325 ma

scaled physical model of secondary oil migration a scaled physical model was constructed to simulate gravity and capillary controlled flow of oil into a water saturated sand during secondary hydrocarbon migration the model provided both visualization of the flow patterns and estimates of hydrocarbon transport rates and efficiencies the dimensions and physical properties of the model were designed so the balance of gravity capillary and viscous forces was the same in the model as in the geologic system the physical model was a sand pack between glass plates the pack was 52 cm high 100 cm long 2 5 cm thick and inclined at a 5 dip its porosity was 42 and its permeability was 7 0 10 10 m2 710 d it modeled a geologic carrier bed 27 m thick with 20 porosity and 9 9 10 14 m2 100 md permeability overlying a source rock a dyed oil was injected into the lowest corner at a rate of 1 cm3 day and it exited the highest corner at atmospheric pressure oil movement was followed both visually and by ultrasonic velocity measurements the behavior of fluids in the model led us to the following conclusions for oil transport through water wet homogeneous carrier beds the rate limiting step in charging a trap is not secondary migration but rather the rate of oil release from the source rock high hydrocarbon loss may occur during vertical migration when the carrier bed lies above the source because of extensive dispersion losses during lateral migration are probably minimal because flow is concentrated below the top seal

sample dilation and fracture in response to high pore fluid pressure and strain rate in quartz rich sandstone and siltstone 1 natural hydraulic fractures nhfs are inferred to form where pore pressure exceeds the least compressive stress by an amount equal to the tensile strength of the rock we improved upon an experimental protocol that meets the nhf criterion within cylindrical samples with the most tensile effective stress parallel to the sample axis the effective tensile stresses achieved during these experiments ranged from 17 47 mpa the pore fluids used had higher viscosities than water and the axial strain rate was rapid 10 3s 1 to delay dissipation of fluid pressure by flow four experiments on st peter sandstone samples and two on an abo formation siltstone sample were performed under these conditions and under drained conditions none of the drained experiments resulted in failure but all of the sandstone and one of the siltstone samples fractured in response to elevated pore pressures consistent with field and theoretical studies mechanical heterogeneity was a first order control on fracture location in the absence of mesoscopic heterogeneity fracture location coincided with the maximum pore pressure samples responded to elevated pore pressures and differential stresses by dilating the magnitude of which was sufficient to achieve atmospheric pore pressure samples failed 2 to 250 s after experiencing the greatest pore pressures when the effective stresses were no longer tensile thus the high pore pressures and effective tensile stresses experienced early in the experiments were sufficient to fracture the rocks even though they were not sustained until failure these results provide insight into processes of fluid driven fracture formation

an analysis of horizontal microcracking during catagenesis example from the catskill delta complex horizontal bitumen filled microcracks are common within clay laminae of the finely laminated organic carbon rich shale in the lower half of the heavily jointed upper devonian dunkirk shale western new york state such cracks are not found higher in the dunkirk shale where moderate bioturbation resulted in a relatively porous and permeable microfabric horizontal microcracks in a hydrocarbon source rock that carries regional vertical joints indicating a horizontal least principal stress owe their presence to material properties of the fractured shale and the magnitude and orientation of the crack driving stress during kerogen maturation three material properties favored the horizontal initiation of microcracks in the dunkirk shale 1 the abundance of flat kerogen grains oriented parallel to layering 2 a marked strength anisotropy in large part caused by the laminated nature of the rock and 3 the tight strongly oriented planar clay grain fabric produced by gravitational compaction of flocculated clay at shallow burial depth the latter was especially important to sustaining elevated pore pressure the crack driving stress which was generated by the conversion of kerogen to bitumen poroelastic deformation of the low permeability laminated shale pressurized by catagenesis perhaps enhanced by compaction disequilibrium prior to kerogen conversion elevated the in situ horizontal stress in excess of the vertical stress which remained constant during pore pressure buildup thereby favoring the propagation of microcracks in the horizontal plane

mechanisms controlling rupture shape during subcritical growth of joints in layered rocks during joint propagation the orientation of plume lines on joint faces allows for mapping the path taken by the crack tip line during rupture growth rupture shapes in layered clastic sediments of the devonian catskill delta finger lakes district new york indicate joint growth through three stages where velocity of the tip line v tl varies as a function of the crack tip stress intensity k i the initial stage of growth is characterized by a rupture of approximately circular or elliptical shape that expands from an initiation fl aw with a velocity v tl in some cases primary growth involves self correction where an elliptical rupture redistributes the crack tip stress so that the rupture returns to a more stable circular shape the crack tip line eventually propagates to a bedding interface where properties of either a plastic or a noncohesive bedding interface cause v tl 0 along a portion of the tip line the onset of secondary growth occurs when the rupture tip line intersects both bedding interfaces and splits into two discontinuous segments that propagate synchronously as a single coherent rupture nonuniform nonsystematic v tl eventually interferes with the rupture s ability to grow coherently and thus leads to the transition to tertiary growth characterized by the detachment of the coherent rupture into one or more independently propagating noncoherent tip lines throughout all three stages of rupture growth the k i dependent v tl points to subcritical propagation variation from a smooth to rough plume morphology is consistent with propagation through region i of the subcritical regime with a transition at v tl 10 5 m sec rupture velocity may enter region ii of the subcritical regime but such a broad range of k i must be crossed to reach region iii and quasistatic behavior at the critical stress intensity k ic that region ii acts as a barrier through which few joints pass thus greatly limiting the number of postcritical joints in the catskill delta if not in the crust of the earth

isotopic provenance of sandstones from the eocene tyee formation oregon coast range the tyee formation of eocene age in the oregon coast range has been studied by a variety of isotopic techniques in order to determine its provenance traditional basin analyses including paleocurrent measurements lithofacies mapping and study of sandstone compositions made previously suggest derivation from the klamath mountains which lie to the south in contrast the isotopic compositions of whole rock sandstone samples white mica and potassium feldspar separates preclude derivation solely from this local source area nd sm rb sr k ar 18 o 16 o and d h analyses of sandstones from the tyee and related formations yield the following information about their source areas 1 whole rock nd values between 7 1 and 7 3 at the time of deposition indicate that an old crustal component 700 ma was incorporated in the source rocks 2 whole rock rb sr systematics implies an older age than those of sandstones clearly derived from the klamath mountains these rb sr values are similar to those of modern sands of the columbia river that were derived from eastern source areas 3 either apparent rb sr ages of potassium feldspars are too old or their initial 87 sr 86 sr ratios are too high to have been derived from plutonic rocks of the klamath mountains or sierra nevada 4 white micas have a fairly consistent late jurassic rb sr isochron but have k ar ages of 68 ma an overprint not recognized in the klamath terranes 5 white micas have 18 o values of 9 5 a value typical of s type granges such as are found in the idaho batholith but too high for normal i type granites such as in the northern sierra nevada and too low for metamorphic rocks in the klamath mountains 6 white micas have d values consistent with those observed for plutonic white mica in the idaho batholith but markedly lower than those of white mica from schist in the klamath mountains 7 potassium feldspars have 18 o values that vary widely and that mainly are not in oxygen isotope equilibrium with coexisting white mica suggesting that these minerals were not derived from the same source area these results indicate that the provenance of these sandstones included s type two mica granites that formed in late jurassic time from sources that included an old crustal component minerals in the granites underwent subsequent thermotectonic age resetting in late cretaceous time rocks in the klamath mountains and northern sierra nevada do not possess these features and consequently are precluded from being major source areas for the tyee formation the sandstones most likely were derived from the idaho batholith abundant detritus from that source area is consistent with a model in which the oregon coast range basin lay much farther east closer to idaho during deposition and subsequently moved westward to its present position such major displacement is compatible with the tectonic rotation history documented for the oregon coast range that began during the time of deposition of the tyee formation

numerical simulation of hydrocarbon migration in tight reservoir based on artificial immune ant colony algorithm a case of the chang 81 reservoir of the triassic yanchang formation in the huaqing area ordos basin china abstract the hydrocarbon migration in tight reservoirs is a complex process the fluid flow patterns of which are notably different from those of conventional reservoirs therefore specific mathematical models are needed to simulate the secondary hydrocarbon migrations this study presents a numerical simulation method based on artificial immune ant colony algorithm aiaca to simulate the secondary hydrocarbon migrations in tight reservoirs it consists of three core parts 1 the release modes of artificial ants based on the intensity of hydrocarbon generation 2 the wandering patterns of artificial ants under the control of the dynamic field and the distribution of pheromones 3 the updating modes of pheromones based on the changes in reservoir wettability the simulation of secondary migration can be realized by the observing the dynamic movements and accumulations of the artificial ants the method has been tested in the chang 8 1 tight sandstone reservoir which is part of the triassic yanchang formation in the huaqing area ordos basin in china and proved to be successful in matching the current data in exploration and development

alteration of detrital fe ti oxides in sedimentary rocks scanning electron microscope study has revealed that in proterozoic sandstones shales and carbonate rocks of the visingso group in southern sweden detrital fe ti oxides have undergone alteration by dissolution and or replacement mainly by titanium oxides hematite reddish rocks or pyrite greenish sandstones shales and carbonate rocks the titanium oxides occur as either poorly crystalline masses cryptocrystalline aggregates or discrete euhedral crystals of anatase brookite and rutile that attain a variety of crystal habits microprobe analyses have shown that the alteration of ilmenite occurs through several intermediate phases each successively enriched in titanium and depleted in iron to an almost pure form of tio 2 some of the iron and titanium which is released during alteration of the fe ti oxides is incorporated in associated clay minerals

stability of detrital heavy minerals on the norwegian continental shelf as a function of depth and temperature abstract petrographic analysis of 1931 thin sections from sandstones of pliocene to devonian age on the norwegian continental shelf indicate that the heavy minerals zircon tourmaline rutile apatite and spinel survive to depths exceeding 5000 m and to temperatures of around 200 c without showing signs of intrastratal dissolution on the other hand the maximum depth and temperature of occurrence for amphibole are 841 m and 40 c around 1500 m and 55 c for sphene when 0 5 km of probable uplift is taken into account 2231 m and 92 c for kyanite 2559 m and 95 c for epidote 2810 m and 109 c for staurolite and 4589 m and 175 c for garnet initial signs of garnet dissolution occur at around 2000 m depth and all garnets are partially dissolved at depths greater than 3500 m monazite is present down to 4858 m and 182 c and chloritoid to 3811 m and 149 c it is however uncertain whether this is due to the low number of samples deeper than 4858 m and to the rare occurrence of chloritoid or to intrastratal dissolution the same sequence of heavy mineral stability is found when volume of quartz cement is used as a proxy for time temperature exposure quartz cement is not found in samples containing amphibole or detrital sphene a maximum of a fraction of a percent quartz cement is found in samples containing kyanite up to 1 together with epidote up to 6 with staurolite up to 24 with chloritoid up to 25 with garnet whereas maximum values of 25 35 are present together with zircon tourmaline rutile patite spinel and monazite the sequence of persistence of heavy minerals versus depth found in this study is very similar to results from several previous studies although unstable heavy minerals survive to greater depths in very young and rapidly buried sandstones and there are some differences regarding the stability of sphene and epidote heavy mineral dissolution has created secondary pores and possibly has contributed to precipitation of kaolinite illite quartz and carbonates in the studied sandstones but these reactions are of very limited volumetric importance

carbon nutrient and trace metal cycling in sandy sediments a comparison of high energy beaches and backbarrier tidal flats abstract in order to evaluate the importance of coastal sandy sediments and their contribution to carbon nutrient and metal cycling we investigated two beach sites on spiekeroog island southern north sea germany and a tidal flat margin located in spiekeroog s backbarrier area we also analyzed seawater and fresh groundwater on spiekeroog island to better define endmember concentrations which influence our study sites intertidal sandy flats and beaches are characterized by pore water advection seawater enters the sediment during flood and pore water drains out during ebb and at low tide this pore water circulation leads to continuous supply of fresh organic substrate to the sediments remineralization products of microbial degradation processes i e nutrients and dissolved trace metals from the reduction of particulate metal oxides are enriched in the pore water compared to open seawater concentrations the spatial distribution of dissolved organic carbon doc nutrients po43 no3 no2 nh4 si oh 4 and total alkalinity trace metals dissolved fe and mn as well as sulfate suggests that the exposed beach sites are subject to relatively fast pore water advection which leads to organic matter and oxygen replenishment frequent pore water exchange further leads to comparatively low nutrient concentrations sulfate reduction does not appear to play a major role during organic matter degradation high nitrate concentrations indicate that redox conditions are oxic within the duneward freshwater influenced section while ammonification denitrification manganese and iron reduction seem to prevail in the ammonium dominated seawater circulation zone in contrast the sheltered tidal flat margin site exhibits a different sedimentology coarser beach sands versus finer tidal flat sands and nutrients dissolved manganese and doc accumulate in the pore water ammonium is the dominant pore water nitrogen species and intense sulfate reduction leads to the formation of sulfide which precipitates dissolved iron as iron sulfide these findings are due to slower advective pore water exchange in the tidal flat sediments this study illustrates how different energy regimes affect biogeochemical cycling in intertidal permeable sediments

controls of hydrocarbon generation on the development of expulsion fractures in organic rich shale based on the paleogene shahejie formation in the jiyang depression bohai bay basin east china abstract the development of expulsion fractures in organic rich shale is closely related to hydrocarbon generation and expulsion from kerogen organic rich shales from the upper part of the fourth member and the lower part of the third member of the paleogene shahejie formation in the jiyang depression bohai bay basin east china are used as an example based on thin sections sem and thermal simulation experiments the characteristics of hydrocarbon generation and the conditions supporting the development of expulsion fractures were explored the key factors influencing these fractures include the presence of kerogens their distribution along laminae and around particle boundaries their exposure to heat and the build up in pressure due to confinement by low permeability the development of excess pore fluid pressures and intrinsic low rock fracture strength are the main influencing factors pressurization by rapid generation of hydrocarbon provides impetus for fracture initiation and cause bitumen to migrate quickly the shale laminae results in distinctly lower fracture strength laminae parallel than laminae normal and this directs the formation of new fractures in the direction of weakness when pore fluid pressure increases maximum and minimum principal effective stresses decrease by different proportions with a larger reduction in the maximum principal effective stress this increases the deviatoric stress and reduces the mean stress thus driving the rock towards failure moreover the tabular shape of the kerogen aids the generation of hydrocarbon and the initiation of expulsion fractures from the tip and edge the resulting fractures extend along the laminae when the tensile strength is lower in the vertical direction than in the horizontal direction particle contact boundaries are weak and allow fractures to expand around particles and to curve as the stress strength regime changes when pore fluid pressure fields at different fracture tips overlap fractures will propagate and interconnect forming a network this paper could provide us more detailed understanding of the forming processes of expulsion fractures and better comprehension about hydrocarbon expulsion primary migration in source rocks

nutrient biogeochemistry in a gulf of mexico subterranean estuary and groundwater derived fluxes to the coastal ocean we hypothesize that biogeochemical processes in a subterranean estuary in the northeastern gulf of mexico significantly alter nutrient concentrations that are discharged into the ocean via submarine groundwater discharge sgd this subterranean estuary is a site of steep gradients over small spatial scales so selecting the appropriate groundwater endmember for flux calculations is difficult two endmember mixing models typically applied to investigate biogeochemical processes in surface estuaries are not useful in this case because of aquifer heterogeneity and complex solid fluid interactions groundwater residence times redox potential source strength and aquifer matrix appear to be the most important drivers for dissolved organic carbon doc and nutrient concentrations in this pristine coastal plain aquifer the distribution of dissolved species was consistent with a sequence of reactions in a narrow 2 m surface layer in which nitrate is initially exhausted likely because of denitrification organic nitrogen is remineralized releasing ammonium and some doc remains on the basis of a 222rn derived non steady state box model total terrestrial marine sgd rates were estimated to oscillate around 11 cm d21 the sgd is composed almost entirely of recirculated seawater accounting for 95 of total sgd fluxes conservative estimates of groundwater derived nitrogen inputs into the coastal ocean were 8 2 6 1 2 mmol m22 d21 consisting of ammonium 58 organic nitrogen 28 and nitrate 14 we suggest that these mainly represent recycled rather than new nutrient additions when extrapolated to the florida gulf coast the nitrogen and doc fluxes were comparable to the main regional rivers supporting the proposition that sgd plays a major role in coastal biogeochemistry the biogeochemical implications of submarine groundwater discharge sgd into coastal environments are becoming increasingly recognized globally groundwater discharge has been estimated to be only a few percent of the total freshwater flux to the oceans burnett et al 2006 in spite of that small fraction the input of nutrients to coastal environments via groundwater pathways could be globally more important because nutrient concentrations in groundwater are often much higher than those in river water in addition high dissolved n p ratios in groundwater relative to surface waters might drive the coastal ocean toward p limitation within the coming decades perhaps changing the present n limited coastal primary production slomp and van cappellen 2004

crack propagation sequence and pore fluid conditions during fault bend folding in the appalachian valley and ridge central pennsylvania bedding parallel bpv strike sv and cross fold cfv veins represent a sequence of polyphase fracturing during the development of a fault bend fold in lower paleozoic carbonate beds in the appalachian valley and ridge province pennsylvania brecciation and layer parallel shearing played important roles in the development of the earliest vein set bpv which propagated prior to folding fluid inclusions in calcite bpv are highly saline brines 23 4 wt naci trapped at conditions close to lithostatic p 180 mpa t 267 c while passing through the lower kink plane of a fault bend fold strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp renewed extension upon passing through a second kink plane led to propagation of antitaxial sv along the stylolitized joints by the crack seal process slightly less saline fluids 22 4 wt naci were trapped in sv at fluid pressures 144 mpa and temperatures 217 c with the concurrent formation of lateral ramps the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial cfv due to further mixing of fresh waters the salinity of fluids forming cfv decreased 20 5 wt nacl with trapping conditions at p 116 mpa and t 179 c the fluid evolution path from bpv to cfv through sv shows a modest decrease in salinity with a sharp decrease in possible trapping pressures

nacacl saline formation waters frio formation oligocene south texas usa products of diagenesis abstract saline formation waters from reservoirs in oligocene deltaic clastic sedimentary rocks frio formation south texas are derived from three sources mixtures characterize most samples accounting for the wide range in chemistry low salinity water is derived from cenozoic shales in contrast saline water having variable br cl ratios is allochthonous and is derived from at least two sources in mesozoic strata which underly the cenozoic units saline water in this area regardless of br cl ratio contains variable high concentrations of ca calcium is derived primarily from dissolution of plagioclase as demonstrated by petrography 87 sr 86 sr analyses and chemical modelling the mccl 2 or carpenter function of saline formation waters in this area thus can be derived from water rock interaction and cannot be used to identify the ultimate source of solutes reverse osmosis is an unimportant processes in this area and 18 o and d values converge on rock buffered values of approximately 8 o and 30 o respectively

origin of fracture porosity example from altamont field utah the occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial diagenesis uplift and erosional unloading variations in the state of stress within rock arise for example from changes in temperature pore pressure weight of overburden or tectonic loading hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress and this information used to compute a stress history from this stress history predictions may be made as to when in the burial cycle to expect fracture joint formation what type of fractures extension or shear may occur and which geologic factors are most favorable to development of fractures a stress history is computed for strata of the naturally fractured altamont oil field in utah s uinta basin calculations suggest that fractures formed in extension that the well cemented rocks are those most likely to be fractured that fractures began to develop only after strata were buried to great depth and that the fracture system continued to develop as strata were uplifted and denuded of overburden geologic evidence on fracture genesis and development is in accord with the stress history prediction stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays

discharge of nitrate containing groundwater into a coastal marine environment the discharge of nitrate containing fresh groundwater from a sandy coastal aquifer and into the adjacent shallow marine environment was investigated near esbjerg at the northern end of the wadden sea in denmark the geological structure of the coastal site was determined from drilling data and geoelectrical methods such as multi electrode profiling mep the distribution of shoreface sediment containing freshwater was mapped using mep underwater multi electrode profiling umep and the electrical conductivity measured on pore waters freshwater discharge fluxes were measured using seepage meters and estimated from observed head gradients and measured hydraulic conductivities in the aquifer sediments the nitrate distribution in the coastal sediments was obtained from water samples a groundwater flow model was established to quantify the groundwater flow and travel times and the distribution of freshwater discharge along the coastline results show that on the watershed scale the fresh groundwater discharge is controlled by sand filled buried channels that connect the aquifer with the shoreface sediments the freshwater discharge along the coastline is at this scale probably best estimated by the groundwater flow model however at a more detailed scale the distribution of freshwater discharge in the intertidal zone is controlled by small scale geological heterogeneity and models are unable to predict what happens at a small scale on the beach and offshore for that purpose umep seepage meters pore water sampling and local hydraulic gradients are more useful these measurements indicate that the freshwater discharge occurs in distinct zones and that the highest discharge is near the high tide line decreasing rapidly in a seaward direction nitrate is abundant in the shallow groundwater of the coastal aquifer and is also present in the discharging fresh groundwater at certain patches along the coast however on average very little nitrate is observed in the freshwater discharging at the coast the maximum travel time of groundwater through the aquifer until it discharges at the coast is 100 years and since nitrate leaching from soils has only taken place during the last 40 50 years part of the fresh groundwater discharging at the coast must be free of nitrate

measuring pore water oxygen of a high energy beach using buried probes abstract we conducted six field campaigns to investigate the spatial and temporal evolution of pore water oxygen content on a high energy sandy beach aquifer during several tidal cycles at different seasons we buried autonomous probes in intertidal sands to record dissolved oxygen saturation salinity temperature and water head at a 2 10 min frequency oxygen concentrations display significant changes both with time tide and seasons and space cross shore and vertical variations seawater circulation in tidal sands forms a saline pore water circulation cell with oxygen saturated pore water in the zone of seawater recharge oxygen depleted pore water in the lower beach is the result of in situ respiration processes that occur during seawater circulation oxygen depletion varies throughout the year and anoxic conditions are reached at the end of spring as planktonic organic matter becomes abundant in seawater and more organic matter is therefore supplied to pore water on a shorter time scale weeks to minutes oxygen variations are driven directly by physical forcing tidal amplitude affects the extent of the saltwater circulation cell and the associated location of the recharging and discharging zones of the beach the evolution of the water table level during the tidal cycle influences the circulation of pore water in the sand and ultimately the timing of oxygen variations during flood and ebb this first in situ study in a high energy sandy beach shows that the dynamics of pore water oxygen are governed by biogeochemical processes at the seasonal scale and by physical forcing at the time scale of minutes to a few days

provenance history of a carboniferous gondwana margin forearc basin new england fold belt eastern australia modal and geochemical constraints abstract the visean stephanian myall trough eastern australia formed part of a forearc basin to the east of a magmatic arc situated along the palaeo pacific margin of gondwana petrographic modal whole rock geochemistry and mineral phase chemistry of sandstones siltstones conglomerates and associated igneous rocks provide a consistent and coherent data set which indicate sediment derivation from a magmatic arc of predominantly silicic volcanic composition the low quartz modes of the sandstones the largely volcanic origin of lithic fragments and the overall high sio 2 content of the siliciclastic sediments and igneous rocks require the arc source to be largely dacitic to rhyolitic in composition the predominance of vitric type highly siliceous volcanic rocks lithic fragments suggests a source composed largely of silicic pyroclastics the presence of potassium feldspar as well as the presence of biotite as the principal detrital ferromagnesian phase in the sandstones suggest a source of medium to high k tholeiite composition the high trace element abundances of relatively immobile elements such as zr la ce v nb y and th and corresponding high th u zr y la y la sm ratios and low zr th zr nb la th and ti zr ratios for both the igneous rocks and derived siliciclastic sediments in the myall trough also require a high k and or strongly medium k arc source a lack of temporal or spatial trends in the data indicate that the magmatic arc remained of uniform composition throughout the 25 ma interval of strata preserved within the trough the provenance history of the myall trough requires accumulation along an andean type continental margin rather than an intra oceanic arc outboard of the gondwana margin

geochemical and detrital mode evidence for two sources of early proterozoic sedimentary rocks from the tonto basin supergroup central arizona the tonto basin supergroup includes up to 6 5 km of early proterozoic sedimentary and volcanic rocks that were deposited in a relatively short period of time at about 1 7 ga in central arizona moderate correlations of rare earth elements ree and ti with al2o3 and ree distributions in detrital sediments of this supergroup suggest that these elements are contained chiefly in clay mica and or zircon fractions ree distributions including negative eu anomalies in most tonto basin sediments are similar to those in phanerozoic shales weak to moderate correlations of fe sc ni and co to al2o3 also suggest a clay mica control of these elements detrital modes and geochemical characteristics of sediments indicate two dominant sources for sedimentary rocks of the tonto basin supergroup a granitoid source and a volcanic source the granitoid source was important during deposition of the upper part of the succession the mazatzal group as shown by increases in k2o al2o3 and th in pelites with stratigraphic height and increases in zr and hf and decreases in eu eu cr and ni in in pelites of the maverick shale sediment provenance characteristics and paleocurrent indicators are consistent with deposition of the supergroup in a continental margin back arc basin the granitoid sediment source appears to have been the north american craton on the north and the volcanic source a more local source from an arc on the south

evolution of petrophysical properties of oil shales during high temperature compaction tests implications for petroleum expulsion abstract the transport properties of permian to miocene oil shales torbanite posidonia messel himmetoglu and condor were studied using petrophysical and geochemical techniques the aims of this study were to assess permeability of oil shales evaluate the evolution of porosity specific surface area and intergranular permeability during high temperature compaction tests and to verify the suitability of intergranular permeability for petroleum expulsion measured permeability coefficients for two samples were 0 72 10 21 m for the eocene messel shale and 2 63 10 21 m for the lower jurassic posidonia shale from s germany respectively bet specific surface areas of the original samples ranged from 0 7 to 10 6 m g and decreased after compaction to values from 0 3 to 3 7 m g initial porosity values ranged from 7 6 to 20 1 for pre deformation and from 9 99 to 20 7 for post deformation samples porosity increased during the high temperature compaction experiments due to petroleum generation and expulsion permeability coefficients estimated using the kozeny carman equation varied from 6 97 10 24 m to 5 22 10 21 m for pre deformation and from 0 2 10 21 m to 4 8 10 21 m for post deformation samples reflecting the evolution of their porosity and bet specific surface areas measured and calculated permeability were similar for the messel shale whereas calculated permeability was two orders of magnitude lower for the posidonia shale from s germany petroleum expulsion efficiencies under the experimental conditions ranged from 38 6 for the torbanite to 96 2 for the posidonia shale from s germany they showed strong positive correlation with the petroleum generation index r 0 91 and poor correlations with porosity r 0 46 average pore throat diameters r 0 22 and compaction r 0 02 estimated minimum pore system saturations for petroleum expulsion during the experiments were 12 for the torbanite and 30 for the posidonia shale from n germany pore system saturation determines whether expulsion occurs mainly through matrix or fracture permeability for samples with saturation levels above 20 fracture permeability dominated during the experiments evidence based on the measured permeability coefficients expulsion flow rates consideration of capillary displacement during generation related pore invasion and the existence of transport porosity suggests that fracture permeability is the principal avenue of petroleum expulsion from source rocks this conclusion is supported by microscopic observations

methanotrophy controls groundwater methane export from a barrier island abstract methane concentrations can be high in coastal groundwater resulting in methane export driven by submarine groundwater discharge however the magnitude of this methane flux depends significantly on the rate of methanotrophy the often overlooked process of microbial methane consumption that occurs within coastal aquifer sediments here we describe a zone of methanogenesis within the freshwater lens of a barrier island aquifer and investigate the methane source sink behavior of the barrier island system as a whole the median concentration of methane dissolved in fresh groundwater beneath the center of the island was 0 6 mm supported by high rates of potential methanogenesis 22 mmol m 2 day 1 however rates of microbial methane consumption were also elevated in surrounding sediments 18 mmol m 2 day 1 groundwater flowing from the zone of methanogenesis to the point of discharge into the ocean had a long residence time within methanotrophic sediments 195 days such that the majority of the methane produced within the barrier island aquifer was likely consumed there

influences of provenance and diagenesis on detrital garnet suites in the paleocene forties sandstone central north sea abstract the geochemistry of detrital garnets in the upper paleocene forties formation has been studied by electron microprobe techniques in this sand body variations in garnet geochemistry can be related to provenance and diagenetic factors two separate sand fans can be mapped using the technique the dominant forties montrose fan being supplied from the metamorphic terrain of the northern highlands of scotland and the subsidiary gannet fan supplied from the mainly granitic terrain of the grampian highlands intrastratal solution caused by high temperature pore fluids circulating at depth has removed some of the garnets from the assemblages garnets with the highest calcium contents are the least stable and are dissolved at comparatively shallow depths the dependence of garnet stability on calcium content in deep burial must be taken into account when assessing provenance by this technique but nevertheless garnet is far less sensitive to diagenetic and hydraulic controls than is the total heavy mineral suite

magnetic carriers and remanence mechanisms in magnetite poor sediments of pleistocene age southern north sea margin pleistocene sedimentary sequences in the east anglian region of britain record both major and minor climatic oscillations and the impact of isostatic and eustatic variations intensively studied in terms of their lithology and biostratigraphy the sequences have been difficult to place in an absolute timeframe dating and correlation by magnetostratigraphy has been attempted over a num ber of years however these sediments are difficult to date by palaeomagnetic means because they are poor in detrital magnetite are subject to post depositional deformation and diagenesis and have unknown rates of sedimentation determining whether their natural remanence magnetisation nrm directions are reliable thus requires information on the mode and timing of remanence acquisition here we apply palaeomagnetic rock magnetic and mineralogical analyses to identify the nrm carriers in these sediments and hence their palaeomagnetic reliability within oxidised fluvial sediments the kesgrave formation the magnetic carriers appear to be relict magnetic minerals fer rian ilmenites chromites haematite and goethite which sometimes carry a reliable primary deposi tional remanence drm but often an overprinting viscous time varying remanence vrm within some reduced marine and intertidal sediments within the crag basin the iron sulphide greigite has been found to carry a reliable syn depositional chemical remanence crm in all the sediments magnetic inclusions within silicates are abundant are significant for the mineral magnetic signal but contribute little to any recoverable palaeomagnetic information copyright 2005 john wiley sons ltd