palaeomagnetic study of an allochthonous terraine the scisti silicei formation lagonegro basin southern italy summary the jurassic scisti silicei formation forms part of the lagonegro superimposed tectonic units i and ii that are thought to represent the axial and internal margins of the mesozoic lagonegro basin prior to nappe formation sampling was carried out in the lower lagonegro and upper pignola nappes in two differently oriented anticlines single and multi component magnetizations are present isothermal remanence acquisition rates show that magnetite and haematite are present which in most lagonegro specimens show the same direction of magnetization comparison of the palaeomagnetic directions with those from jurassic rocks on the stable african craton indicates a 147 anticlockwise rotation of the lower nappe which is similar to 139 previously reported for the upper nappe at vietri di potenza the same comparisons show a 44 clockwise rotation of the upper nappe at pignola these results suggest that the doubled nappe structures sampled some 50 km apart resulted from their emplacement by translation with little rotation prior to the opening of the tyrrhenian sea and that it was the opening of this sea that caused the predominantly anticlockwise rotation this work therefore indicates the way in which palaeomagnetic analyses can be used even within complex allochthonous areas as an aid to deciphering their tectonic evolution

a simple model of pressure build up caused by porosity reduction during burial summary the fluid pressure build up due to porosity reduction in sedimentary basins during burial is studied the model assumes that the void ratio decreases exponentially with depth and that the permeability is proportional to the void ratio to an arbitrary exponent simple analytical solutions are obtained for the darcy velocity and the fluid excess pressure the pressure build up during burial is studied with these solutions and it is found to be inversely proportional to the gravity number the importance of the permeability exponents on the fluid pressure is also studied gravity numbers much less than 1 are shown to yield high excess pressures during burial a reasonable approximation for the maximum darcy velocity is found to be the product of the surface void ratio and the burial rate hydrofracturing is discussed in relation to the pressure build up and cases characterized by gravity numbers much less than 1 are found to yield hydrofracturing over large depth ranges it is suggested that the average permeability of hydrofractured sediments during burial corresponds to a gravity number equal to 1

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

paleomagnetism of jurassic to miocene sediments from the apenninic carbonate platform southern apennines italy evidence for a 60 counterclockwise miocene rotation abstract the tectonic evolution of the apennine belt southern tyrrhenian sea system is addressed through a paleomagnetic study of lias to langhian sediments from the apenninic carbonate platform southern apennines italy reliable paleomagnetic data gathered from 21 sites document a regional scale post langhian 80 counterclockwise ccw rotation since previous studies of the plio pleistocene clays spread over the orogen had shown a 20 ccw rotation we conclude that the southern apennines rotated by 60 during middle late miocene our data provide evidence that the southeastward drift of calabrian block and synchronous spreading of the southern tyrrhenian sea induced saloon door like deformation of the southern apennines and sicily which underwent similar magnitude although opposite in sign orogenic rotations a paleomagnetically derived paleogeographic reconstruction shows that at 15 ma late langhian the alpine apennine belt collided with a nne oriented carbonate platform corridor surrounded by oceanic basins we speculate that both the end of the corsica sardinia rotation and the eastward jump of the locus of back arc extension from the liguro provencal to the tyrrhenian sea may have been consequences of this event

mathematical models of compaction consolidation and regional groundwater flow summary a 3 d theory of groundwater flow and consolidation that allows for compressibility of the soil or sediment matrix is that due to biot 1941 however this theory is inapplicable to virgin consolidation tests where the deformation of the soil along the critical state surface is accomplished by elastic plastic flow in fact biot s model cannot even be applied to overconsolidated soils where the deformation is elastic as it does not conserve the mass of the solid in this paper we provide a modified version of the model and show how it can be extended to allow for elastic plastic deformation and large amplitude deformations in elastic plastic compaction a particular example of the potential application of this theory is to the groundwater flow induced by the compaction of sediments by an overriding ice sheet and we show how the commonly used dupuit approximation can be employed to simplify the theory

a one dimensional analytically based approach for studying poroplastic and viscous consolidation application to woodlark basin papua new guinea 1 in the study of sedimentary basin evolution consolidation of marine sediments has historically been characterized by applying variations on poroplastic theory however recent field evidence and models of deformation by pressure solution suggest that both poroplastic and viscous modes of compaction should be investigated when evaluating basin development to study the influence of poroplastic and viscous deformation on marine sediments we constructed a generic framework that incorporates the two modes of compaction into a numerical scheme analytical solutions for pressure development and porosity change based on both poroplastic and viscous theory were incorporated into this scheme and a characteristic height was determined to indicate the depth at which viscous deformation is initiated application of our framework to a site in woodlark basin papua new guinea and comparison of the simulated results to measured porosity data suggest that poroplastic deformation is the dominant mode of consolidation at the site and that lithologic strength is controlling the observed porosity profile two simulated scenarios based on different characteristic heights showed that while a full poroplastic model may be most appropriate to characterize compaction at the site viscous deformation may be a factor in developing the low porosities 26 to 40 observed below 590 m below seafloor as such although poroplastic deformation may be predominant in basins less than 1 km deep a combined poroplastic viscous model may be useful to address sediments which exhibit markedly low porosity at shallow depths

identification and characterization of pressure seals through the use of wireline logs a multivariate statistical approach a study is currently underway to investigate the occurrence of pressure compartmentation in the st peter sandstone of the michigan basin in particular the authors are interested in identifying and characterizing the seals that bound these compartments commonly the existence of a pressure seal is inferred through the identification of vertical discontinuities in the pressure gradient often though the pressure data are neither dense enough nor extensive enough to specifically locate these seals an alternative in cases where only sparse pressure data are available is to locate zones with sufficiently low permeabilities to allow preservation of pressure differentials over geologic time scales direct measurements of permeability from in situ well tests and core plug evaluations are not readily available for the st peter however because this formation is an active gas play and most data are proprietary wireline logs provide an alternative means for estimating permeability more than 500 wells have been drilled to the st peter and the wireline logs for these wells provide an extensive data set in this paper a multivariate statistical approach is used to descretize the wireline logs into electrofacies and to correlate these electrofacies with lithofacies descriptions and hydraulic parameters from core analyses onmore the basis of these correlations the electrofacies associated with very low permeabilities i e potential seals are characterized these electrofacies characterizations are then used to identify tight zones in individual wells and to correlate between wells providing a picture of the vertical and lateral distribution of potential seal zones throughout the basin less

geology and diagenetic history of overpressured sandstone reservoirs venture gas field offshore nova scotia canada deep exploratory wells in the scotian basin offshore nova scotia canada have encountered overpressured formations with pressures 1 9 x the normal hydrostatic gradient the overpressures occur over an area of approximately 10 000 sq km in the venture field the abnormal pressures are confined below a depth of 4500 m and are associated with upper jurassic lower cretaceous gas and condensate bearing sandstone reservoirs the overpressures occur within normally compacted shales containing numerous overpressured sandstone reservoir beds the development of overpressures seals and secondary reservoirs are all diagenetically driven the volumetric increase achieved by kerogen to petroleum gas conversion and hydrocarbon expulsion are believed to be the main driving forces or venture overpressures three secondary porosity depth levels which top at 2500 m 65 degrees c 3700 m 95 degrees c and 4600 m 130 degrees c correlate with major steps in the organic matter maturation in the basin secondary porosity is initially achieved by alumosilicate dissolution with ferroan sparry calcite cement dissolution dominating below 4000 m porosity enhancement and preservation is not the result of a single diagenetic event but instead the result of a series of diagenetic events that overlapped in time formation of dynamic diagenetic barriers within the zone of peak gas generation helps retard the diffusive migration of hydrocarbons and other fluids expelled during shale diagenesis resulting in pressure build up the preservation of up to 32 percent porosity under 500 1000 atm of pressure could not be achieved without simultaneous pressuring of developing voids thus the expansion of secondary reservoirs in the overpressured zone also had to be late 50 ma or younger and about synchronous with pressurizing of the reservoirs significant for hydrocarbon exploration is that venture type diagenetic overpressures are not associated with undercompacted sediments and hence they cannot be predicted from compaction trends during drilling petrographic diagenetic and lithofacies studies can be instrumental in predicting potential areas of deep subsurface secondary reservoirs development

changes in quartz solubility and porosity due to effective stress an experimental investigation of pressure solution experimental compaction of quartz sand was conducted in semistatic flow through systems at 150c to examine the chemical and physical processes associated with pressure solution pore fluid chemistry and porosity were monitored through time to investigate the role of effective stress on silica solubility and compaction rate the concentration of silica in the pore fluid varied directly with effective stress at constant pore fluid pressure but not in a linear fashion increases in silica were not transient and could be partially reversed by removal of the effective stress porosity decreased steadily under constant nonzero effective stress at 150c but remained essentially constant under identical loading conditions at room temperature initial compaction rates increased linearly with effective stress deformed samples show indented contacts with fractures and dissolution features healed microfractures and a decrease in average grain size multiple mechanisms pressure solution subcritical crack growth crack healing appear to operate at grain contacts during compaction the authors interpret long term time dependent compaction accompanied by stress induced changes in fluid chemistry as evidence for pressure solution

lateral fluid flow in a compacting sand shale sequence south caspian basin the article lateral fluid flow in a compacting sand shale sequence south caspian basin by john d bredehoeft rashid d djevanshir and kenneth r belitz aapg bulletin v 72 no 4 april 1988 p 416 424 needs the following corrections there are several typographical errors that were the author s error equation 2 should be equation 2 equation 3 should be equation 3 although the equations were incorrect in the article the associated discussion and equations 4 and 5 are based upon the correct expressions as shown above table 3 the calculated regional sand permeabilities is also incorrect the hydraulic conductivity which in our case has the units of meters per second was incorrectly converted to millidarcys values in meters per second were multiplied by the standard conversion 1 m sec equals 1 04 108 md in retrospect this conversion was a conceptual error table 3 should read table 3 calculated regional sand permeabilities acknowledgment we appreciate the comments of stan lohman who brought our attention to these errors end of article last page 1525

a palaeomagnetic study of apennine thrusts italy monte maiella and monte raparo abstract three separate structural blocks within the southern apennines have been sampled for palaeomagnetic investigation to constrain their original separation and movement during mid tertiary deformation the mesozoic limestones are weakly magnetic and the nrm intensity of all samples from upper and lower cretaceous limestone from the alburni platform and from upper cretaceous limestone at monte maiella were too low to yield results lower cretaceous limestone at monte maiella contained a mean magnetisation after structural correction of d 326 i 42 k 44 n 9 57 n 263 e and cretaceous limestone at monte raparo a mean of d 132 i 61 k 50 n 19 54 n 306 e the monte maiella results near the central part of the apennine thrust front are compatible with a local clockwise block rotation during deformation while monte raparo results may bear evidence of the major east west thrust motion during shortening in addition to anticlockwise block rotations already reported from the southernmost apennines

a portable electrical based percussion coring system for use in deep water this article describes a sediment coring system that can be used in deep water 30 to 60 m with no aid from divers the system is light and compact enough to be operated by three persons and transported in a small boat and or in a small airplane a core tube penetrates into sediments with the force caused by percussion of a water proof electric hammer driven by a generator on the boat with this device four sand cores were retrieved from a lagoon floor at depths ranging from 40 m to 58 m a flexible vinyl tube is used as a core liner and it provides an exact measure of core recovery

smectite dehydration its relation to structural development and hydrocarbon accumulation in northern gulf of mexico basin a comparison of clay diagenesis data obtained from a study of tertiary shales from the brazos colorado river system of texas the mississippi river system of louisiana and the niger river system of nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs the threshold temperature required to initiate diagenesis ranges from about 160 f 71 c in mississippi river sediments to more than 300 f 150 c in the niger delta water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time in either situation the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported observations from the northern gulf of mexico basin indicate a close relation between buildup of high fluid pressure and the smectite illite transformation process abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed the depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting dips of some faults in texas have been observed to change abruptly within the interval of smectite diagenesis and some faults formed in the overpressured miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum although these faults may be important for fluid redistribution in shallow sandstone shale sections they are a minor factor in moving hydrocarbons out of shale below the faults in much of the texas offshore area fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern gulf of mexico basin this flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present shale tectonism is also the primary mechanism for structural development in the tertiary section of the niger delta however seismic data indicate that the rate of dip change of seaward dipping listric growth faults is commonly less than that observed in texas where dips as low as 10 15 can occur at depths as shallow as 10 000 15 000 ft 3 048 4 572 m syndepositional faults in nigeria are formed in sandstone shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section subtle differences in structural styles in the tertiary sections of texas and nigeria are probably the result of differences in clay composition of the shaly sections being deformed

lower tertiary and upper cretaceous source rocks in louisiana and mississippi implications to gulf of mexico crude oil the lower tertiary sparta formation wilcox group and the midway group in southern louisiana include organic rich source facies that generate crude oil at relatively high thermal maturities the wilcox group is an important source of wilcox crude oil and regional kerogen variations explain two crude oil subfamilies wilcox crude oils in downdip areas of southern louisiana migrated short distances but long range lateral migration about 150 km best explains wilcox crude oils far updip from mature source rocks crude oils in oligocene and younger reservoirs in southern louisiana migrated vertically from deep lower tertiary source rocks some crude oils in upper cretaceous tuscaloosa reservoirs were emplaced by long range lateral migration from tuscaloosa source rocks g ven little evidence of upper tertiary source rocks and the overmaturity problems of mesozoic source rocks most crude oils in upper tertiary and pleistocene reservoirs of the gulf continental shelf are best explained by vertical migration from deep lower tertiary source rocks even so it is simplistic to assume an exclusive lower tertiary origin many tertiary and pleistocene crude oils of this study probably include an overprint of high maturity hydrocarbons from mesozoic sources

ice melting and downward transport of meltwater by two phase flow in europa s ice shell with its young surface very few impact craters and the abundance of tectonic and cryovolcanic features europa has likely been subjected to relatively recent endogenic activity morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface which may result from enhanced tidal heating a major issue concerns the thermal gravitational stability of these water reservoirs here we investigate the conditions under which water can be generated and transported through europa s ice shell we address particularly the downward two phase flow by solving the equations for a two phase mixture of water ice and liquid water in one dimensional geometry in the case of purely temperate ice we show that water is transported downward very efficiently in the form of successive porosity waves the time needed to transport the water from the subsurface region to the underlying ocean varies between 1 and 100 kyr depending mostly on the ice permeability we further show that water produced in the head of tidally heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell within less than a few hundred kiloyears our calculations indicate that liquid water will be largely absent in the near subsurface with the possible exception of cold conductive regions subjected to strong tidal friction recently active double ridges subjected to large tidally driven strike slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on europa

temperature dependent viscoelastic compaction and compartmentalization in sedimentary basins the near surface compaction regime of most sedimentary basins is characterized by hydrostatic fluid pressures and is therefore determined entirely by sediment matrix rheology within this regime compaction is initially well described by a pseudoelastic rheological model with increasing depth precipitation dissolution processes lead to thermally activated viscous deformation the steady state porosity profile of the viscous regime is a function of two length scales the viscous e fold length related to the compaction activation energy and a scale determined by the remaining parameters of the sedimentary process overpressure development is weakly dependent on the second scale for activation energies 20 kj mol application of the steady state model to pannonian basin shales and sandstones indicates a dominant role for viscous compaction in these lithologies at porosities below 10 and 25 respectively activation energies and shear viscosities derived from the profiles are 20 40 kj mol and 1020 1021 pa s at 3 km depth the analytical formulation of the compaction model provides a simple method of predicting both the depth at which permeability limits compaction resulting in top seal formation and the amount of fluid trapped beneath the top seal fluid flow during hydraulically limited compaction is unstable such that sedimentation rate perturbations or devolatilization cause nucleation of porosity waves on the viscous e fold length scale 0 5 1 5 km the porosity waves are characterized by fluid overpressure with a hydrostatic fluid pressure gradient and propagate through creation of secondary porosity in response to the mean stress gradient the waves are a mechanism of episodic fluid expulsion that can be significantly more eycient than uniform darcyian fluid flow but upward wave propagation is constrained by the compaction front so that the waves evolve into essentially static domains of high porosity following cessation of sedimentation yielding mechanisms do not appreciably alter the time and length scale of episodic fluid flow because fluid expulsion is ultimately controlled by compaction the flow instabilities inherent in viscous compaction are similar to and a possible explanation for fluid compartments 2000 elsevier science b v all rights reserved

diagenesis compaction and fluid chemistry modeling of a sandstone near a pressure seal lower tuscaloosa formation gulf coast petrographic isotopic and fluid inclusion evidence from normally and overpressured sand stones of the lower tuscaloosa formation upper cretaceous in the gulf coast documents quartz overgrowth precipitation at 90 c or less calcite cement precipitation at approximately 100 and 135 c and prismatic quartz cement precipitation at about 125 c textural evidence suggests that carbonate cement dissolution occurred before the second phases of calcite and quartz precipitation and was followed by precipitation of grain rimming chlorite and pore filling kaolinite geochemical calculations demonstrate that present day lower tuscaloosa formation water from 5500 m depth could either dissolve or precipitate calcite cements in model simulations of upward water low calcite dissolution or precipitation depends on pco2 variability with depth i e whether there is one or two phase flow or on the rate of generation of co2 with depth calculations suggest that 105 106 rock volumes of water are required to flow through the section to precipitate 1 10 calcite cement compaction analysis suggests that late stage compaction occurred in normally pressured sandstones after dissolution of carbonate cements but was hindered in overpressured sandstones despite the presence of high porosity these results document the inhibition of compaction by overpressured fluids and constrain the timing of pressure seal formation modeling results demonstrate that the proposed paragenesis used to constrain timing of pressure seal formation is feasible and that most of the cement diagenesis occurred before the pressure seal became effective as a permeability barrier

a continuum model of multi phase reactive transport in igneous systems multiphase reactive transport processes are ubiquitous in igneous systems a challenging aspect of modelling igneous phenomena is that they range from solid dominated porous to liquid dominated suspension flows and therefore entail a wide spectrum of rheological conditions flow speeds and length scales most previous models have been restricted to the two phase limits of porous melt transport in deforming partially molten rock and crystal settling in convecting magma bodies the goal of this paper is to develop a framework that can capture igneous system from source to surface at all phase proportions including not only rock and melt but also an exsolved volatile phase here we derive an n phase reactive transport model building on the concepts of mixture theory along with principles of rational thermodynamics and procedures of non equilibrium thermodynamics our model operates at the macroscopic system scale and requires constitutive relations for fluxes within and transfers between phases which are the processes that together give rise to reactive transport phenomena we introduce a phase and process wise symmetrical formulation for fluxes and transfers of entropy mass momentum and volume and propose phenomenological coefficient closures that determine how fluxes and transfers respond to mechanical and thermodynamic forces finally we demonstrate that the known limits of two phase porous and suspension flow emerge as special cases of our general model and discuss some ramifications for modelling pertinent two and three phase flow problems in igneous systems

paleomagnetic evidence for a pleistocene counterclockwise rotation of the sant arcangelo basin southern italy new paleomagnetic results from 182 cores sampled at 15 sites in pliocene and pleistocene clayey units in the sant arcangelo basin southern italy and its surroundings are presented they indicate that this area underwent a counterclockwise rotation of 22 after the deposition of the lower pleistocene clays

magnetostratigraphy of the jurassic cretaceous boundary in the maiolica limestone umbria italy the magnetic stratigraphy of a 110 m section of the early cretaceous maiolica pelagic limestone in the bosso valley umbria italy shows clearly defined magnetozones that are correlated with the geomagnetic polarity record derived from the m sequence marine magnetic anomalies m19 to m14 and possibly m13 using an established paleontological zonation of this section based on calpionellids the position of the jurassic cretaceous boundary is located close to the base of the negative magnetozone corresponding to anomaly m17

problem of secondary porosity frio formation oligocene texas gulf coast secondary porosity formed by the dissolution of both carbonate and silicate minerals especially k feldspars is widely developed in sandstones of the frio formation oligocene in the texas gulf coast co 2 produced by decarboxylation of organic matter is commonly suggested as the acid required for dissolution material balance calculations indicate that co 2 produced by decarboxylation of organic matter in frio formation shales can account for a regional average of only 1 or 2 secondary porosity in frio formation sandstones yet point count data indicate an average of 10 secondary porosity long distance fluid transport many kilometres and or other mechanisms of acid generation should therefore be considered carbon isotopic data on dissolved inorganic carbon in formation water co 2 in produced natural gas and carbonate cements indicate that co 2 produced by decarboxylation had a minor impact on these carbon reservoirs the reaction kerogen water methane carbon dioxide can explain the isotopic data but alone it is insufficient to account for all the secondary porosity unless long distance material transport is involved

modeling porosity evolution and cementation of sandstones abstract recent models of quartz cementation of sandstones attribute the source of silica to dissolution of quartz at stylolites bjorkum 1993 walderhaug 1994 aase et al 1996 oelkers et al 1996 quartz dissolution is believed to be catalyzed by the presence of mica illitic clayswhich are not dissolved or destroyed during the quartz dissolution process bjorkum1993 a supersaturation of silica is created at the stylolites which leads to a diffusive transport of silica into the interstylolite regionwhere the silica precipitates as cement interstylolite cementation is observed to be smooth which has led to the conclusion that the precipitation step is the slowest step in the entire quartz cementation process this is because the silica supersaturationcreated by dissolution of quartz at the stylolites is almost uniform throughout the interstylolite region the supersaturation is estimated to be 1 above the silica equilibrium concentration aase et al 1996 oelkers et al 1996 this estimate is based on the increase in the silica equilibrium concentration needed to grow microcrystalline quartz grains of a size 1 m petrographic studies of deeply buried jurassic north sea sandstones show that sandstones with microcrystalline quartz coatings have well preserved porosities microcrystalline quartz coatings are able to inhibit quartz cementation because the supersaturation created by dissolution of quartz at the stylolites is insufficient to grow the microcrystalline quarts aase et al 1996 the idea that stylolites are the source for silica cement was already suggested by heald et al 1955

pressure seals their diagenetic banding patterns abstract pressure seals are economically significant geological phenomena since they play an important role in deep natural gas entrapment identified in basins worldwide they offer a new frontier for exploration of natural gas reservoirs below 10 000 ft 3000 m pressure seals are low permeability envelopes which enclose abnormally pressured reservoirs there are three different types of seals basal lateral and top basal seals which define the bottom of abnormal pressure compartments usually follow a stratigraphic horizon lateral seals are generally associated with faults top planar seals may parallel or cut across time stratigraphic boundaries they may be developed in any lithology the southeastern portion of the anadarko basin displays a layered sequence of abnormally pressured fluid compartments at depths ranging from 3 000 to 13 500 ft 900 to 4150 m these compartments are separated from each other by pressure seals in mcclain county a top pressure seal separating two abnormally pressured compartments is located between 10 000 ft 3000 m to 12 000 ft 3650 m within the simpson group ordovician the seal zone is characterized by alternating cemented and porous intervals silica and carbonate cements comprise a diagenetic laminated and banded pattern the compositional and textural heterogeneity within this structure evolved through diagenetic processes active during the seal evolution

porosity reduction by pressure solution a theoretical model for quartz arenites pressure solution and the subsequent precipitation of the dissolved material may play an important role in the lithification of sandstones and limestones this paper presents a one dimensional model for a well sorted quartz arenite sequence undergoing diagenesis the sedimentary sequence is treated as a saturated porous medium in which pressure solution occurs at individual grain contacts quartz dissolved at grain contacts precipitates on free surfaces of adjacent grains this solution and precipitation process reduces porosity and forces the migration of pore fluid permeability decreases with porosity hindering fluid migration and excess fluid pressures are generated distributions of porosity pore pressure and fluid flow rates are given for several sedimentation rates and thermal gradients complete reduction of the primary intergranular porosity was accomplished in all cases examined by a depth of 3 5 km

coupled fluid flow and sediment deformation in margin scale salt tectonic systems 2 layered sediment models and application to the northwestern gulf of mexico 1 in paper 1 we described a methodology to model coupled fluid flow and deformation in composite salt and siliciclastic tectonic systems and investigated their compaction and overpressuring behavior prior to and during continental margin scale gravitational spreading compaction driven darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional plastic yield and mechanical deformation viscous flow of the underlying salt is independent of fluid pressure paper 1 presented prototype models that are limited to single uniform sediment lithologies either sandstone type or shale type that undergo mechanical and volumetric viscous compaction in this paper we present models with layered sandstone type and shale type lithologies designed to better approximate the more complex stratigraphy of the gulf of mexico our natural example a first set of models demonstrates that layered lithologies can produce fluid pressure regimes similar to those observed in sedimentary basins we then introduce an improved formulation of viscous compaction that includes a stronger dependence on porosity and depth used as proxy for temperature thereby more effectively self limiting viscous compaction a second set of models with the improved viscous compaction formulation demonstrates that the onset of gravity spreading is mainly controlled by overpressuring in the landward end of the salt basin and that resulting shortening in the distal part is partly accommodated by horizontal compaction models with moderately high fluid pressure best reproduce conditions considered to have been necessary for large scale gravitational spreading in the northwestern gulf of mexico which led to the formation of the perdido fold belt

diagenesis of sandstone reservoirs of pattani basin gulf of thailand miocene sandstones from gas fields in the gulf of thailand s pattani basin provide an example of rapid decline in porosity and permeability with increasing burial depth this decline results from rapid burial diagenesis that is related to very high geothermal gradients in the basin porosity loss with depth in these subquartzose sandstones results from mechanical compaction and from progressive cementation by quartz overgrowths kaolinite and illite quartz overgrowths increase with depth indicating continuous or episodic silica cementation kaolinite occurs as a pore filling cement and is abundant between the depths of 1 980 and 3 050 m 6 500 and 10 000 ft minor cements include calcite ankerite mixed layer illite smectite siderite pyrite chlorite and barite the best porosity and permeability in pattani basin reservoirs are generally associated with large intergranular pores in sandstones between 915 and 1 980 m 3 000 and 6 500 ft at greater depths most interparticle pores have been occluded and porosity is mainly secondary in origin skeletonized feldspars indicate progressive dissolution with increasing burial depth in low permeability sandstones from deeper zones 2 285 3 050 m or 7 500 10 000 ft porosity is mainly restricted to dissolution voids within detrital feldspars these secondary pores are usually partly filled by authigenic kaolinite and illite and their pore apertures are generally smaller 1 15 m than intergranular pore apertures 10 75 m however favorable reservoir properties may occur locally at de th where large feldspars have been leached from coarse grained sandstones

a palaeomagnetic study of the upper mesozoic succession in northern tunisia summary the upper mesozoic section from northern tunisia provided an upper jurassic palaeomagnetic pole of 65 2 s 20 3 e ag5 6 1 calculated from the means of normal and reversely magnetized samples from the uppermost callovian oxfordian kimmeridgian and portlandian rocks in general the only cretaceous rocks to yield acceptable results were the few samples collected from fresh outcrops a polarity sequence can be established for the upper jurassic which can be correlated with the oceanic keathley anomaly sequence one consequence of the proposed correlation of the oceanic anomaly with the terrestrial palaeomagnetic sequence is to suggest a slightly different age for the oxfordiankimmeridgian boundary one interpretation of the frequent intermediate directions of magnetization in the cretaceous sequence is that there may be a number of unrecognized short period reversals within the cretaceous and more particularly during the so called cretaceous normal period

contemporaneous thrusting and large scale rotations in the western sicilian fold and thrust belt in the western sicilian fold and thrust belt large clockwise rotations of allochthons occurred during late cenozoic contraction of part of the southern tethyan margin the magnitude of rotation decreases stepwise from over 120 in the upper sheets lying on the north coast of sicily to no appreciable rotation in the frontal portion of the belt flanking the southern coast of the island the allochthons are composed of imbricate thrust sheets derived primarily from individual basin and platform assemblages of the old tethyan margin paleomagnetic and structural data indicate that the rotation of the allochthons was accommodated by coherent torsional displacements on relatively low angle detachment surfaces timing relations for the imbrication history of the sicilian fold and thrust belt are derived from stratigraphic overlap and local involvement of sediments deposited in a series of foreland and piggyback basins the locus of deposition within successive foreland basins first migrated easterly then southerly during progressive deformation in the orogen imbrication began in the early miocene burdigalian langhian and continued at least through the early pleistocene and appears to be continuing today rotation is related to thrusting and accompanies a 70 change in the tectonic transport direction from easterly to southerly easterly striking right oblique transpressional faults and associated northeasterly trending folds postdate thrust sheet rotation in the interior of the thrust belt and were active contemporaneously with south directed thrusting in the foreland region pleistocene and possibly older late pliocene extension strongly modified the older thrust morphology along the tyrrhenian coast of northwestern sicily with the development of down to the north listric normal faults the extensional structures apparently are related to the opening and subsequent deformation of the tyrrhenian sea to the north

the significance of stylolitization and intergranular pressure solution in the formation of pressure compartment seals in the st peter sandstone ordovician michigan basin the middle to late ordovician st peter sandstone of the michigan basin is a fine to medium grained quartz sandstone extensive stylolitization and intergranular pressure solution have been major factors in reducing the porosity of certain horizons within the st peter resulting in pressure compartmentation of the reservoir pressure versus depth data for various michigan basin wells indicate that the basin contains compartments that are overpressured by as much as 500 psi horizons bounding these compartments are often affected by intense stylolitization or intergranular pressure solution and quartz cementation and have been correlated with zones of low porosity and permeability phi 0 3 k 50 mu d these tight zones can be correlated within single gas fields and some may extend across the michigan basin the st peter sandstone has been buried to depths of about 3 500 m in the central part of the basin and 1 500 m at the margins intensely stylolitized zones are found at all depths throughout the basin and do not appear to change in abundance or style with depths factors that influence the formation morphology and abundance of stylolites in the st peter include 1 clay intraclasts 2 intergranular clay and 3 fine grained feldspar rich sand stylolitesmore also occur at contacts between quartz cemented and carbonate cemented zones and within well cemented sands intergranular pressure solution and stylolites may be responsible for the formation of a compartment seal understanding their genesis can allow prediction of variations in porosity in michigan basin well cores less

continental margin shale tectonics preliminary results from coupled fluid mechanical models of large scale delta instability abstract we investigate the evolution of rifted continental margin shale tectonics using 2d finite element models that couple sediment deformation to compaction driven darcy fluid flow via the effective stress fluid overpressures are generated in the models by a combination of mechanical compaction grain rearrangement and viscous compaction grain dissolution and local reprecipitation and lead to failure and flow of visco plastic bingham shale model results indicate that pore fluid pressures must be 90 95 of the lithostatic pressure to cause shale failure and delta destabilization mechanical compaction alone is insufficient to generate fluid overpressures required for failure in the models and viscous compaction is the primary source the numerical models include delta progradation lateral lithology variation and flexural isostatic compensation seaward shale flow and associated overburden deformation results in the formation of landward regional and counter regional fault bounded extensional basins a transitional domain of thickened and folded shale beneath the continental slope and a seaward fold and thrust belt at the delta toe the structural styles generated by the preliminary numerical models are compatible with features observed in unstable cenozoic deltas e g the niger delta and provide additional insight into the fundamental relationships between deltaic sedimentation fluid pressure generation and margin scale gravity spreading

mechanical behaviour of fine grained sediments experimental compaction and three dimensional constitutive model abstract the usual undimensional description of deformations of sediments based on a phenomenological relationship between porosity and the vertical effective stress does not give information about the horizontal stress and moreover is irrelevant for sedimentary structures in which deformation process is not only vertical modelling three dimensional deformation processes in sedimentary basins requires a three dimensional mechanical constitutive model for sediments a special dometric cell was conceived in g 3s this cell allows the measurement of the lateral stress during dometric compaction and so enables us to elaborate a three dimensional model of mechanical behaviour four varieties of fine grained sediments were studied by this cell in a large scale of strain and stresses the modified cam clay model was used for fitting the experimental data and it was shown that this model can reproduce the mechanical compaction of sediments up to 50 mpa of vertical effective stress about 3 km burial depth if its parameters are deduced from dometric zero lateral strain data

loading and unloading of sedimentary basins the effect of rheological hysteresis 1 we present a model for the study of the effect of rheological hysteresis on compaction in sedimentary basins when surface loading and unloading occurs when compaction is slow sedimentation rate is large and or permeability is small the hysteresis has little effect on the basal compaction layer but in the more realistic case where compaction is fast i e sedimentation is slow and or permeability is large surface unloading leads to downward propagation of a decompaction front across which the vertical porosity gradient jumps while subsequent surface reloading leads to downward propagation of a discontinuity in porosity despite the fact that the porosity is governed by a diffusive equation of richards type

generation of overpressure by cementation of pore space in sedimentary rocks summary overpressure build up is studied when the main cause for porosity reduction is cementation of the pore space sourced locally the average porosity reduction for siliclastic sediments is modelled with nth order kinetics it is shown that the overpressure in one layer at a constant depth will decrease exponentially with time in the case of first order kinetics for the porosity reduction the overpressure is studied in one layer subjected to cementation during constant burial along a thermal gradient a small overpressure build up is shown above the window for cementation with a steep rise in overpressure in the upper part of the window the overpressure build up is then seen to decrease rapidly towards the end of the window for cementation overpressure build up is also studied when cementation is the main cause for porosity reduction in the entire column of sediments during deposition and burial the overpressure regime characterized by gravity numbers larger than one is studied this regime corresponds to low or moderate overpressures in the case of mechanical compaction cementation is shown to imply a steep pressure build up in the window of cementation which will easily exceed the lithostatic pressure the porosity loss due to cementation is seen to have a strong impact on the permeability which leads to the formation of a pressure seal although most of the potential for fluid expulsion is exhausted below the seal because most of the porosity is cemented up the permeability of the seal is sufficiently low for hydrofracturing to take place this scenario is consistent with overpressure observations in many wells

a unified approach to geopressuring low permeability zone formation and secondary porosity generation in sedimentary basins geopressuring secondary porosity and low permeability zones occur in many of the world s sedimentary basins and are often associated with major petroleum reservoirs we present a mechanical model which simultaneously reproduces these phenomena assuming pressure solution to be the dominant rock deformation mechanism a solid state rock skeleton viscosity is introduced which relates the deformation of the rock skeleton to the effective stress numerical solutions for compaction indicate that a soft sedimentary layer preferentially compacts to form a low permeability zone there is a large liquid pressure gradient through the low permeability zone in the more slowly compacting region beneath the low permeability zone the pressure gradient remains hydrostatic but the absolute pressure can exceed the lithostatic pressure in the absence of hydraulic fracturing this results in the formation of secondary porosity

pore pressure variation within the tuscaloosa trend morganza and moore sams fields louisiana gulf coast repeat formation tester rft mark of schlumberger pore pressure data from the lower tuscaloosa formation upper cretaceous are analyzed across a pressure transition zone in two gas fields of the tuscaloosa trend louisiana documenting a pressure seal within a sandstone dominated interval that maintains a pressure anomaly of 20 mpa pressure measurements within the formation span a depth interval of 5500 6060 m and are limited to the crests of rollover structures associated with growth faults the onset of overpressure occurs at a depth between 5620 m and 5690 m in two fault blocks while the strata are displaced across the fault by 100 to 120 m suggesting that the pressure seal may be near horizontal crosscutting stratigraphic boundaries the pressure seal zone is as thin as 38 m and occurs within interbedded sandstones and shales overpressures in all wells of the moore sams field and in two wells of the morganza field follow local hydrostatic gradients indicating that portions of the overpressured zone are in fluid communication though isolated from shallower normally pressured fluids in other wells overpressures increase with depth in a stair step manner to 117 mpa at depths of 5 9 km the occurrence of the pressure seal in interbedded sandstones and shales where higher permeability is expected suggests that the sandstones of the seal zone are unusually tight a companion petrographic study of sandstones near the pressure seal suggests that extreme compaction of the sandstones after dissolution of grain supporting cements may have contributed to the low permeability of the seal zone

pressures and hydrogeology in petroleum basins abstract in most deep sedimentary basins in the world there is a layered arrangement of at least two superimposed hydrogeological systems the shallowest hydrogeological system can extend to great depths however in many basins it extends from the surface down to only about 10 000 ft greatest historical depth of burial the shallow hydrogeological systems generally are basin wide in extent and exhibit normal pressures the pore water apparently is free to migrate approximately in accordance with the theory of fluid migration proposed by munn 1909 and quantified by hubbert 1940 and 1953 the deeper hydrogeological systems usually are not basin wide in extent and exhibit abnormal pressures they generally consist of a layer of individual fluid compartments which are sealed off from each other and from the overlying system in some basins mainly in the onshore u s there is an even deeper near normally pressured noncompartmented section the compartmented layer in those basins generally is in the sequence of rocks which were deposited during the period of most rapid deposition the underlying noncompartmented layer where present usually is in prebasin shelf deposits and basement rock the uppermost noncompartmented layer usually is in rocks which were deposited during the last 10 000 ft in basin filling correlations of petroleum in reservoirs with the rocks from which the petroleum was generated has imposed a system of mainly upwards paths for the first several thousand feet of migration of petroleum from source rocks largely within the compartmented layer to pools generally in the shallow hydrogeological system the migration paths of petroleum and water may not be coincident

modeling subsurface flow in sedimentary basins groundwater flows that arise in sedimentary basins from the effects of topographic relief buoyant convection sediment compaction erosional unloading and combinations of these driving forces can be described using quantitative modeling techniques models can be constructed to consider the effects of heat and solute transport petroleum migration and the chemical interaction of water and rocks the accuracy of model predictions however is limited by the difficulty of predicting hydrologic properties of sediments on regional dimensions estimating past conditions such as topographic relief and knowledge of how physical and chemical processes interact over gelogic time scales progress in basin modeling will accelerate as hydrologic research efforts are better integrated with those of other specialities such as sedimentology rock mechanics and geochemistry

1 d modelling rock compaction in sedimentary basins using a visco elastic rheology abstract recent experimental studies on unlithified sediments suggest that the compaction is in part viscous at the basin scale and on a geological time scale processes such as pressure solution can be approached by creep deformation subjected to a viscous rheology in the present work we have studied porosity reduction and fluid pressure development resulting from the visco elastic compaction of a sedimentary basin during its formation model equations include continuity equations darcy s law and a visco elastic rheology law which relates the strain rate to the effective stress and to the rate of change of this effective stress under the assumption that permeability is a power law function of porosity the equations become essentially non linear model results illustrate how the decrease of porosity starts at the base of the basin and spreads upward with increasing time in a wide range of input parameter values calculations indicate a zone of almost linear increasing of pore pressure just below the basin surface a transition zone of rapidly increasing fluid pressure with a large pressure gradient and a zone of lithostatic fluid pressure below this is consistent with the general features of zoning of fluid pressure distribution in overpressured areas but zones with high pressure also correspond to low porosity at depth the relative thickness of the zones depends on time subsidence velocity and the physical parameters of sediments which can be combined in order to define a characteristic compaction length and a characteristic compaction time in the upper zone the decrease of porosity results in a boundary layer within this layer the porosity decreases from its initial value down to its minimum value the deeper zone appears when the time of formation of the given depth basin exceeds the characteristic compaction time and the thickness of the basin is in order of compaction length zones of fluid overpressure may also develop due to the spatial variations of the physical properties of the sediments

an inexpensive diver assisted percussion coring device abstract mountain lake giles county virginia contains soft sediments from which undisturbed cores are logistically difficult to collect by conventional gravity coring whereas other more expensive types of corers might successfully sample the sediments in this or similar aquatic systems our new diver assisted percussion coring device assembled with hardware costing about 50 00 enabled flawless collection of 7 cores 100 of tries we describe here this percussion coring device its operation and probable application

pore pressure generation in sedimentary basins overloading versus aquathermal the principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade among the various mechanisms proposed mechanical overloading and aquathermal pressuring have probably received the greatest attention most previous discussions however have been based upon qualitative conceptual models here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and second by comparing the relative importance of the overloading versus the aquathermal mechanisms the coupling between pore pressure and compaction as well as the nonlinear and path dependent material properties are found to be critical in the present analysis we find that although changes in stress and temperature both contribute to pressure generation mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles

holocene lake level fluctuations in lakes keilambete and gnotuk southwestern victoria australia reconstructed holocene lake level curves from two saline hydrologically closed maar crater lakes in southwestern victoria australia show near synchronous lake level changes throughout the holocene we show that lake levels reconstructed from sediment particle size and ostracod valve chemistry 18o and sr ca have undergone rapid 10 m fluctuations throughout the holocene finer sampling resolution shows a more sensitive response to holocene climate than was previously presented for lake keilambete both maar crater lakes show a short lived maximum in holocene lake levels around 7 2 ka the period of lake filling leading to peak lake levels matches the phase of most effective precipitation 7 4 7 0 ka reconstructed from a high resolution speleothem record from northern tasmania water levels declined in both lakes during the mid holocene with a more substantive decline after 5 ka which coincides with the end of the southern hemisphere hypsithermal water levels continued to oscil

coupled fluid flow and sediment deformation in margin scale salt tectonic systems 1 development and application of simple single lithology models 1 a methodology is presented to model coupled fluid flow and deformation in rifted continental margin composite salt and siliciclastic tectonic systems and we investigate their compaction and overpressuring behavior associated with continental margin scale gravitational spreading compaction driven darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional plastic yielding and mechanical deformation viscous flow of underlying salt is independent of fluid pressure numerical models are adapted to the oligo miocene phase of large scale gravitational failure in the northwestern gulf of mexico and represent the first study of this system that includes dynamically evolving fluid pressure here we present the methodology and prototype models with single uniform sediment lithologies and simple parameterizations of their properties the models serve to illustrate the interactions among compaction generation of fluid overpressure and gravitational failure and spreading mechanical and viscous compaction behavior of sandstone type and shale type sediments are investigated results demonstrate that mechanical compaction can generate moderate overpressure in thick shale type material whereas high overpressure requires viscous compaction in sandstone type material only viscous compaction can generate significant overpressure though this requires tens of millions of years changes in the stress regime during gravitational driven deformation enhance compaction and overpressure although illustrative of the methodology and basic processes none of the prototype single lithology models satisfactorily reproduces oligo miocene fluid pressure and deformational regimes of the gulf of mexico numerical models of layered sediments together with an improved formulation of viscous compaction presented in part 2 of this set of companion papers are more successful

hydrothermal processes at milos island greek cyclades and the mechanisms of compaction induced phreatic eruptions abstract the few hundred meters thick rhyolitic tuff layers that cover the volcanic island of milos host intense hydrothermal alteration which completely transforms the tuffs into clays and locally into microcrystalline quartz aggregates at the top of one of these quartz layers hundreds of 15 20 m large and 3 4 m deep craters are found randomly scattered over a 1 2 km 2 area in another area craters with smaller diameters 1 2 m and depths 0 5 1 m are found geochemical data reveal that the quartz results from the leaching of the tuff or clay layers by a high temperature 100 200 c flow of steam and gas because of their micrometer size the quartz grains have a viscous rheology due to solution transfer creep the effective viscosity is as low as 10 14 pa s as a result the steam or water saturated porous quartz layers deform by compaction the compaction length is about 3 and 15 m when the fluid is superheated steam and water respectively when the upwelling fluid flow encounters a region with a reduced porosity compaction generates pocket like domains of enhanced porosity these pockets propagate toward the surface by deforming the solid framework of the porous layer one dimensional simulations of the porosity waves show that the steam or water concentration in the pockets likely exceeds 70 the fluid compressibility and viscosity drastically influence the development of the waves for steam like water like fluids the size of the pockets is about 15 20 m 3 4 m and their velocity is about 50 60 m yr 2 4 m yr the fluid overpressure carried by the waves is about 3 4 bar and 0 3 0 5 bar when the fluid is steam like and water like respectively for steam like fluids the 2 4 bar overpressures at the top of the waves are able to re open healed fissures at a depth of about 5 10 m then the steam catastrophically escapes from the rocks through these fissures and the eruption proceeds as a caldera collapse this leads to the suggestion that the arrival of steam pockets can trigger phreatic eruptions that form the 15 20 m large 3 4 m deep craters when the fluid is water like the overpressures are too small to generate an eruption the fluids percolate through the quartz aggregate leading to the formation of 2 3 m wide 0 5 1 m deep bowl like cavities this model is consistent with field observations at milos

modeling the occurrence of overpressures on the scotian shelf offshore eastern canada many wells on the scotian shelf offshore from nova scotia have drilled into overpressured zones in particular the wells comprising the venture gas field have all encountered overpressures at depths below 4 5 km the characteristics of the overpressure zones in the venture field vary in some important details from overpressured zones on the u s gulf coast in the venture field the shales in the overpressured zones are observed to be normally compacted and interbedded with layers of overpressured sandstones this is in contrast to the gulf coast where the shales in the overpressure zone are undercompacted resistivity logs sonic logs and bulk density logs all provide some indications of overpressures in the venture field but are not as unequivocal indicators of overpressure as they are on the gulf coast theoretical investigations of the causes of overpressures in an environment such as the venture field show that overpressures are unlikely to be caused by loading of sediments the most probable cause of overpressures is recent and continuing fluid generation which is related to the transformation of kerogen to hydrocarbons limits are placed on the average permeabilities and fluid generation rates which are required if overpressures are to be produced by fluid generation

secondary compaction after secondary porosity can it form a pressure seal petrography analysis of sandstones from the vicinity of a pressure seal transition from normal to overpressure at 5 5 km depth in the lower tuscaloosa formation in louisiana documents local high porosity above and below the seal packing analysis shows that compaction is greater in normally pressured high porosity sandstones than in overpressured high porosity sandstones compaction in overpressured high porosity sandstones is similar to that in normally pressured well cemented sandstones we propose that focused corrosive fluids created a zone of high secondary porosity allowing further compaction that we call secondary compaction secondary compaction is greater above the seal than below suggesting that high pressure fluid below the seal has preserved porosity and that the pressure seal became effective soon after dissolution of cement cuttings from the pressure seal zone reveal an unusual texture of fragmented pressure solved grains and matrix which may be a result of extensive secondary compaction

solution transfer compaction of quartzites progress toward a rate law data on dissolution diffusion and growth in the quartz water system show that diffusion through grain boundary films is rate limiting for solution transfer and too slow to permit significant compaction in geologic time but that rates are more reasonable if the rough texture of natural grain boundaries is considered on the basis of scanning electron microscope observations a model is proposed that uses a hierarchical plumbing system in which grain boundaries consist of islands capped by grain boundary films separated by microcapillaries and connected to intergranular porosity a rate law based on such a model has an inverse grain size dependence up to a critical grain size that is large 10 cm under typical compaction conditions

a model for sedimentary compaction of a viscous medium and its application to inner core growth summary a detailed study of the physics of a 1 d sedimentary compaction of a viscous medium was carried out both numerically and analytically for columnar and self gravitating spherical cases in view of applying it to the inner core growth process of the earth the effects of sedimentation rate and surface porosity upon the porosity profile were investigated it was found that the porosity profile differs depending on whether or not the sedimentation rate is larger than the darcy velocity velocity of the solid matrix when the fluid flows by buoyancy alone when the sedimentation rate is larger than the darcy velocity a thick constant porosity layer develops at the surface and below it the porosity decreases gradually towards the bottom when the sedimentation rate is smaller than the darcy velocity the porosity profile is characterized by a mushy layer at the top where the fluid is expelled by the deformation of the solid underlain by a thick layer of constant porosity termed the residual porosity such a porosity profile can be understood as the propagation of a half sided solitary wave the study was extended further for the self gravitating spherical case formation of an unstable porosity structure and the appearance of solitary waves were discovered for the case of monotonically decreasing sedimentation rate given the size of the sphere formed by sedimentary compaction according to the magnitude of the ratio of sedimentation rate to darcy velocity three types of porosity structure which differ in force balance and the typical length scale required for porosity decrease were discovered one such structure is where a low porosity layer forms at the top accompanied by solitary waves beneath it indicating that a crust like region can develop at the surface of the inner core

paleomagnetically determined rotations in the western sicilian fold and thrust belt new paleomagnetic data from the west sicilian fold and thrust belt indicate large scale clockwise rotations during tectonic transport for the platform and seamount paleogeographic units of the fold and thrust belt panormide and trapanese units there is a general consistency in the magnitude of rotation along strike suggesting the existence of large coherent structural units for the panormide platform clockwise rotations in the range of 90 to 140 relative to the underlying iblean parautochthon are recorded along 125 km of outcrop strike in the case of the trapanese unit clockwise rotations in the range of 47 to 70 relative to iblei are recorded along about 90 km of outcrop strike intervening basinal paleogeographic units imerese and sicanian units show clockwise rotations of similar magnitude although few reliable data are available from these units palinspastic reconstruction of the thrust belt incorporating the rotations and arguments for the present orientation of the margins of the paleogeographic units implies that the panormide platform and trapanese seamounts were originally elongated more or less north south relative to the present orientation of iblei the two units apparently coalesced to the south and were discontinuous to the north they and their intervening basins were part of a passive continental margin which occupied a large part of the area that was subsequently rifted to form the tyrrhenian sea

a unified approach to mechanical compaction pressure solution mineral reactions and the temperature distribution in hydrocarbon basins abstract in modelling sediment compaction and mineral reactions the rheological behaviour of sediments is typically considered as poroelastic or purely viscous in fact compaction due to pressure solution and mechanical processes in porous media is far more complicated a generalised model of viscoelastic compaction and the smectite to illite mineral reaction in hydrocarbon basins is presented a one step dehydration model of the mineral reaction is assumed the obtained non linear governing equations are solved numerically and different combinations of physical parameters are used to simulate realistic situations in typical sedimentary basins comparison of numerical simulations with real data has shown very good agreement with respect to both the porosity profile and the mineral reaction

counter current convection in a volcanic conduit abstract volcanoes of strombolian type are able to maintain their semi permanent eruptive states through the constant convective recycling of magma within the conduit leading from the magma chamber in this paper we study the form of this convection using an analytic model of degassing two phase flow in a vertical channel we provide solutions for the flow at small grashof and large prandtl numbers and we suggest that permanent steady state counter current convection is only possible if an initial bubbly counter current flow undergoes a regime transition to a churn turbulent flow we also suggest that the magma in the chamber must be under pressured in order for the flow to be maintained and that this compromises the assumed form of the flow

the compaction of igneous and sedimentary rocks in many problems of interest to geology and soil mechanics a fluid moves through a solid matrix which is also being deformed in most cases of interest the effective viscosity of the matrix is many orders of magnitude greater than that of the percolating fluid and both form interconnected networks in three dimensions the partial differential equations governing such behaviour can now be obtained and a variety of simple model problems have been investigated some of the more surprising solutions contain solitary waves and compaction fronts some of the less exotic solutions are presently of more geological interest the mobility of volatile rich melts at melt fractions as small as 0 1 has important consequences for trace element geochemistry the geometry of layered intrusives appears to result from differential compaction of the crystal mush and the rates required can be used to estimate a viscosity of 3 x 10 18 pa s for an olivine matrix this value agrees excellently with laboratory experiments when proper account is taken of the grain size dependence compaction in sedimentary rocks results in overpressure when the porosity becomes sufficiently small and can also lead to the development of secondary porosity