dependence of sonic velocity on effective stress in north sea mesozoic mudstones abstract overpressure estimation methods that use sonic velocity as a proxy for porosity only account for excess pressure due to undercompaction so the influence of unloading processes in generating overpressure is ignored we have used wireline log data and pore pressure measurements in mudstones of the cromer knoll group and the heather formation in the north sea to investigate whether velocity is sensitive to the contribution of unloading processes to observed overpressures our approach was to fit the data by four alternative linear relationships with sonic velocity as the dependent variable the independent variables necessarily included porosity and vertical effective stress and optionally the natural log response and depth our preferred results are v p 3746 3744 11 1 v 4 7 in the cromer knoll mudstones and v p 4035 4269 8 6 v 5 5 in the heather mudstones for vertical effective stress over the range 10 30 mpa the sensitivity of sonic velocity to vertical effective stress is therefore around 10 m s 1 mpa 1 this sensitivity should be sufficient to improve estimates of pore pressure while drilling by using sonic density and natural logs in combination to make use of this sensitivity for pore pressure estimation robust empirical relationships with vertical effective stress as the dependent variable would have to be established for the formations of interest

geologic framework of the mississippian barnett shale barnett paleozoic total petroleum system bend arch fort worth basin texas this article describes the primary geologic characteristics and criteria of the barnett shale and barnett paleozoic total petroleum system tps of the fort worth basin used to define two geographic areas of the barnett shale for petroleum resource assessment from these two areas referred to as assessment units the u s geological survey estimated a mean volume of about 26 tcf of undiscovered technically recoverable hydrocarbon gas in the barnett shale the mississippian barnett shale is the primary source rock for oil and gas produced from paleozoic reservoir rocks in the bend arch fort worth basin area and is also one of the most significant gas producing formations in texas subsurface mapping from well logs and commercial databases and petroleum geochemistry demonstrate that the barnett shale is organic rich and thermally mature for hydrocarbon generation over most of the bend arch fort worth basin area in the northeastern and structurally deepest part of the fort worth basin adjacent to the muenster arch the formation is more than 1000 ft 305 m thick and interbedded with thick limestone units westward it thins rapidly over the mississippian chappel shelf to only a few tens of feet the barnett paleozoic tps is identified where thermally mature barnett shale has generated large volumes of hydrocarbons and is 1 contained within the barnett shale unconventional continuous accumulation and 2 expelled and distributed among numerous conventional clastic and carbonate rock reservoirs of paleozoic age vitrinite reflectance ro measurements show little correlation with present day burial depth contours of equal ro values measured from barnett shale and typing of produced hydrocarbons indicate significant uplift and erosion furthermore the thermal history of the formation was enhanced by hydrothermal events along the ouachita thrust front and mineral wells newark east fault system stratigraphy and thermal maturity define two gas producing assessment units for the barnett shale 1 a greater newark east fracture barrier continuous barnett shale gas assessment unit encompassing an area of optimal gas production where dense impermeable limestones enclose thick 300 ft 91 m barnett shale that is within the gas generation window ro 1 1 and 2 an extended continuous barnett shale gas assessment unit covering an area where the barnett shale is within the gas generation window but is less than 300 ft 91 m thick and either one or both of the overlying and underlying limestone barriers are absent

stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

global crustal stress pattern based on the world stress map database release 2008 the world stress map wsm project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators the wsm database release 2008 contains 21 750 stress data records that are quality ranked using an updated and refined quality ranking scheme almost 17 000 of these data records have a c quality and are considered to record the orientation of maximum horizontal compressional stress sh to within 25 as this is almost a triplication of data records compared with the first wsm database release in 1992 we reinvestigate the spatial wave length of the stress patterns with a statistical analysis on a global 0 5 grid the resulting smoothed global stress map displays both the mean sh orientation that follows from the maximum smoothing radius for which the standard deviation is b25 and a countour map that displays the wave length of the stress pattern this smoothed global map confirms that long wave length stress patterns n2000 km exist for example in north america and ne asia these have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region however our analysis reveals that rather short wave length of the stress pattern b200 km are quite frequent too particularly in western europe alaska and the aleutians the southern rocky mountains basin and range province scandinavia caucasus most of the himalayas and indonesia this implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern

origin of overpressure and pore pressure prediction in the baram province brunei accurate pore pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited tertiary baram delta province where all economic fields exhibit overpressures commonly of high magnitude and with narrow transition zones a pore pressure database was compiled using wireline formation interval tests drillstem tests and mud weights from 157 wells in 61 fields throughout brunei overpressures are observed in 54 fields both in the inner shelf deltaic sequences and in the underlying prodelta shales porosity vs vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner shelf deltaic sequences however the geology of brunei precludes overpressures in the inner shelf deltaics being generated by any conventional fluid expansion mechanism e g kerogen to gas maturation and we propose that these overpressures have been vertically transferred into reservoir units via faults from the prodelta shales sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer and hence different pore pressure prediction strategies need to be applied in the prodelta shales and inner shelf deltaic sequences sonic and density log data detect overpressures generated by disequilibrium compaction and pore pressures are accurately predicted using an eaton exponent of 3 0 sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly and pore pressures are reasonably predicted using an eaton exponent of 6 5

quantification of tertiary erosion in the inner moray firth using sonic velocity data from the chalk and the kimmeridge clay abstract sonic velocities for the chalk and kimmeridge clay were analysed to determine the importance and magnitude of tertiary erosion for 26 wells in the inner moray firth apparent erosion height above maximum burial depth was derived by the displacement along the depth axis of a given velocity depth trend from the normal undisturbed trend the similarity in apparent erosion results derived from the cretaceous chalk and late jurassic kimmeridge clay successions of the inner moray firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and or diagenetic process resultant apparent erosion estimates are variable across the inner moray firth such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern inner moray firth however the actual magnitude of erosion equal to the sum of apparent erosion and post erosional tertiary burial was approximately 1 km throughout the whole inner moray firth area indicating that early tertiary erosion was regional and not simply restricted to the inverted basin margins however a well defined mechanism sufficient to have generated the regional event is still sought

barnett shale gas production fort worth basin issues and discussion newark east barnett shale field fort worth basin texas is currently the most productive gas field in texas in terms of daily production and is growing at an annual rate of more than 10 however despite the fact that the barnett play has been studied intensely by very capable geologists and engineers from several companies over a period of many years there continues to be several misunderstandings concerning fundamental factors controlling the success of the barnett play of north texas barnett gas production is poorer in areas near faults and structural flexures anticlines and synclines fractures which are most abundant in these structural settings are detrimental to barnett production open natural fractures are rare in the barnett and have little or nothing to do with barnett productivity in areas where barnett shale is thermally mature with respect to gas generation it is slightly overpressured about 0 52 psi ft 11 76 kpa m limestone beds within the barnett formation are the product of debris flows that originated on a carbonate shelf to the north of the present basin center it appears that the barnett can be used as an exploration model for other basins especially analogous basins of the ouachita trend the history of the development of the barnett reservoir in north texas provides an excellent example of how persistence can lead to success in nonconventional gas plays

post alleghanian unroofing history of the appalachian basin pennsylvania from apatite fission track analysis and thermal models results of apatite fission track analyses on 29 ordovician through permian sandstones from the appalachian basin in pennsylvania are presented ages range from 111 17 to 184 10 ma mean track lengths of 10 71 0 29 to 13 10 0 17 m with unimodal negatively skewed length distributions are indicative of slow cooling the data separate into two groups on an age versus mean length plot the younger group 111 144 ma is found in the structural depressions of the anthracite basin and the broad top basin and adjacent appalachian plateau the older group 144 184 ma is found in the structurally higher southwestern appalachian plateau and juniata culmination and adjacent central plateau fission track data suggest that the basin cooled slowly after the alleghanian orogeny with culminations cooling earlier than depressions cooling histories modeled from apatite fission track data with maximum temperatures constrained by vitrinite reflectance indicate cooling beginning soon after the alleghanian orogeny except in the juniata culmination which apparently experienced synorogenic cooling and unroofing during formation of the underlying duplex model cooling histories and available geologic information indicate that the foreland basin did not experience mesozoic reheating unroofing histories were modeled from fission track cooling histories using heat flow estimates and burial depths derived from vitrinite reflectance profiles the models suggest that the unroofing history of the appalachian basin in pennsylvania can be divided into three episodes an initial episode of relatively rapid cooling and unroofing late permian early jurassic is attributed to flexural rebound of the foreland in response to erosional removal of alleghanian topographic load initial unroofing rates are higher in eastern pennsylvania than in the west consistent with a flexural model an episode of little to no unroofing middle jurassic late oligocene began contemporaneously with the inception of drift at the atlantic continental margin at this time unloading of the orogen was replaced by subsidence and sedimentation on the new margin without flexural rebound the driving force for unroofing of the basin was removed and unroofing slowed greatly an episode of rapid unroofing over the full width of the basin occurred from the miocene to the present although the driving mechanism for unroofing at this time has not been identified it is consistent with increased sedimentation rates in the middle atlantic offshore basins for the same period

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

quantification of tertiary exhumation in the united kingdom southern north sea using sonic velocity data sonic velocities from the upper and middle chalk upper cretaceous the bunter sandstone and the bunter shale both lower triassic were used to independently quantify apparent exhumation height above maximum burial depth in the united kingdom uk southern north sea apparent exhumation is the displacement on the depth axis of a given velocity depth trend from the normal unaffected by exhumation trend apparent exhumation results derived from the upper and middle chalk the bunter sandstone and the bunter shale are statistically similar the consistency of results from carbonate and clastic units suggests that at a formational and regional scale overcompaction i e anomalously high sonic velocity in all three units analyzed reflects previously greater buria depth rather than sedimentological and or diagenetic processes and validates the use of lithologies other than shale in maximum burial depth studies the consistency of results from units of early triassic to late cretaceous age suggests that tertiary exhumation was of sufficiently great magnitude to mask any earlier mesozoic periods of exhumation and that maximum mesozoic cenozoic burial depth in the southern north sea was attained prior to tertiary exhumation the proposed magnitudes of exhumation are generally greater than those previously published for the southern north sea but they are consistent with recent estimates from apatite fission track analysis the amount of exhumation of the rock column is greatest on the recognized inversion axes i e sole pit cleveland where it reaches approximately 2 5 km however there was a regional component of 1 0 1 5 km exhumation during the tertiary that affected structurally uninverted areas and on which the more localized inversion related exhumation was superimposed cretaceous tertiary burial prior to exhumation must have been of greater magnitude and more rapid than suggested by the preserved stratigraphy the effect of this extra burial and subsequent exhumation on sedimentary rock decompaction procedure and thermal maturation modeling is illustrated for the cleethorpes 1 and 44 7 1 wells and must also be incorporated in modeling reservoir diagenesis the regional tertiary tectonic uplift associated with exhumation must have had a thick skinned origin

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

burial and exhumation history controls on shale compaction and thermal maturity along the norwegian north sea basin margin areas abstract the north sea area has been subjected to significant erosion and subsequent deposition of sediments in the basin margin and deeper basin areas respectively during the late neogene a large amount of cretaceous early quaternary sediments have been removed below the angular unconformity along the west and southwest coast of norway and deposited in the huge north sea fan at the mouth of the norwegian channel at the same time a considerable thickness of early quaternary paleocene sediments was also eroded towards the east in the central north sea and subsequently deposited in the deeper basin areas to the west this study seeks to estimate exhumation from compaction and thermal maturity based techniques by using sonic velocities of shales carbonates and vitrinite reflectance data in a large number of boreholes in the central eastern and northern north sea the results indicate no or minor exhumation in the central graben and flanking high areas whereas more than 1 km sediments are eroded in the basin margin areas towards the norwegian coast more than 500 m sediments are eroded in the egersund basin and stord basin areas a similarity of exhumation estimates from the early cretaceous early miocene shales and late cretaceous early paleocene carbonates indicates maximum burial sometime after the early miocene in most of the central and northern north sea areas however the maximum burial throughout the north sea basin may be diachronous seismostratigraphic analysis indicates maximum burial sometime during the oligocene in the sorgenfrei tornquist zone area in the eastern north sea maximum burial in the norwegian danish basin varies from miocene pliocene in eastern parts to early pleistocene in western parts whereas sediments are currently at their maximum burial in the central graben and southern viking graben areas restoration of surface elevations to their original position before the onset of erosion indicated large subaerially exposed areas in the norwegian danish basin and along the southwest coast of norway this is also supported by predominantly coastal and or deltaic environments in the norwegian danish basin area during the late neogene these subaerially exposed areas may be linked to the regional tilting and erosion of the basin margin areas to the east and progressive basinward migration of deposition centres to the west since the oligocene the exhumation had significant effects on the petroleum system in the basin margin areas by cooling down the source rock however the deeper burial of sediments may also have changed the rheological properties of sediments from more ductile to brittle due to compaction and diagenetic processes which makes them more failure prone during exhumation leading to hydrocarbon leakage or seal failure in case of co2 injection

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

quantifying cretaceous cenozoic exhumation in the otway basin southeastern australia using sonic transit time data implications for conventional and unconventional hydrocarbon prospectivity in prospective basins affected by exhumation uncertainty commonly exists regarding the maximum burial depths of source reservoir and seal horizons one such basin is the otway basin an important gas province in southeastern australia which has witnessed several exhumation events here we present estimates of net exhumation magnitudes for 110 onshore and offshore petroleum wells based on the sonic transit time analyses of lower cretaceous fluvial shales our results show significant post albian net exhumation in the eastern onshore otway basin 1500 m 4920 ft and a generally minor net exhumation 200 m 655 ft elsewhere in the otway basin consistent with estimates based on thermal history data the distribution of net exhumation magnitudes in relation to mid cretaceous and neogene compressional structures indicates that exhumation was dominantly controlled by short wavelength basin inversion driven by plate boundary forces deeper burial coupled with high geothermal gradients in the onshore eastern otway basin and along the northern basin margin during the early cretaceous have rendered lower cretaceous source rocks mostly overmature with any remaining hydrocarbons from the initial charge likely to be trapped in tightly compacted reservoirs and or secondary fracture related porosity however the embrittlement of these reservoirs during their deeper burial may present opportunities for the development of low permeability plays through hydraulic fracturing where smectite clay minerals are illitized source rocks at near maximum burial at present day are at temperatures suitable for gas generation with key controls on prospectivity in these areas including the sealing potential of faulted traps and the relationship between charge and trap development

origin of a thick redeposited carbonate bed in eocene turbidites of the hecho group south central pyrenees spain large scale carbonate beds interbedded in basin plain turbidites of the eocene hecho group northern spain are interpreted as giant turbidity current deposits because of their internal organization and lateral extent one such bed the roncal unit has been traced down the axis of the basin for 75 km and commonly reaches thicknesses in excess of 100 m internally the roncal unit displays an overall upward decrease in grain size from a basal megabreccia with large slabs of carbonate platform debris to a calcareous mudstone the roncal unit was probably deposited from a highly mobile and competent sediment gravity flow generated by an earthquake shock which led to the catastrophic collapse and subsequent transport of the upper part of a carbonate platform flanking the basin

testing the basin centered gas accumulation model using fluid inclusion observations southern piceance basin colorado the upper cretaceous mesaverde group in the piceance basin colorado is considered a continuous basin centered gas accumulation in which gas charge of the low permeability sandstone occurs under high pore fluid pressure in response to gas generation high gas pressure favors formation of pervasive systems of opening mode fractures this view contrasts with thatofothermodelsoflow permeabilitygasreservoirsinwhich gas migrates by buoyant drive and accumulates in conventional traps with fractures an incidental attribute of these reservoirs we tested the aspects of the basin centered gas accumulation model as it applies to the piceance basin by determining the timing of fracturegrowth and associated temperature pressure and fluid composition conditions using microthermometry and raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth trapping temperatures of methane saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185 c and then decrease to approximately 158 c over time which indicates fracture growth during maximum burial conditions calculated pore fluid pressures for methanerich aqueous inclusions of 55 to 110 mpa 7977 15 954 psi indicate fracture growth under near lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge lack of systematic pore fluid pressure trends

velocity depth trends in mesozoic and cenozoic sediments from the norwegian shelf sonic velocity density and resistivity log data from 60 wells on the norwegian shelf have been used to investigate velocity depth trends in sedimentary rocks as a function of sediment composition porosity pore pressure burial history and compaction processes a first order linear velocity depth trend line has been estimated from published velocity data the data analyzed in this study however show significant variations from this trend line indicating that no general velocity depth function can be used when performing more accurate analyses like depth conversion of seismic data pore pressure prediction or basin modeling lower tertiary smectitic sediments from the northern north sea and haltenbanken are characterized by relatively low velocities compared to the overlying pliocene and pleistocene sediments causing a distinct velocity inversion a significant velocity increase at a burial depth corresponding to 70 100c was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales overpressured jurassic sediments from haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference the reduced velocities may be a direct response to lower effective stresses and thus reduced elastic compaction low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy the high velocity depth ratio of barents sea sediments after correcting for tertiary exhumation is explained by the burial history of the area the subsequent thermal exposure of the sediments over time and thus the amount of quartz cementation

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

the influence of a reverse reactivated normal fault on natural fracture geometries and relative chronologies at castle cove otway basin abstract despite the ubiquity of normal faults that have undergone compressional inversion documentation of the structural history of natural fractures around these structures is limited in this paper we investigate the geometries and relative chronologies of natural fractures adjacent to a reverse reactivated normal fault the castle cove fault in the otway basin southeast australia local variations in strain resulted in greater deformation within the fault damage zone closer to the fault structural mapping within the damage zone reveals a complex tectonic history recording both regional and local perturbations in stress and a total of 11 fracture sets were identified with three sets geometrically related to the castle cove fault the remaining fracture sets formed in response to local stresses at castle cove rifting in the late cretaceous resulted in normal movement of the castle cove fault and associated rollover folding and the formation of the largest fracture set reverse reactivation of the fault and associated anticlinal folding occurred during late miocene to pliocene compression rollover folding may have provided structural traps if seals were not breached by fractures however anticlinal folding likely post dated the main episodes of hydrocarbon generation and migration in the region this study highlights the need to conduct careful reconstruction of the structural histories of fault zones that experienced complex reactivation histories when attempting to define off fault fluid flow properties

variation of vertical stress in the carnarvon basin nw shelf australia abstract variations in vertical stress magnitudes can be attributed to lateral facies variations diagenesis localized uplift and disequilibrium compaction overpressure vertical stress magnitudes have been calculated using density and checkshot data from 47 wells in the carnarvon basin on the nw shelf of australia the vertical stress ranges from 20 2 mpa km 1 to 23 8 mpa km 1 below seabed demonstrating a significant variation across the basin the largest vertical stress magnitudes were calculated for the dampier sub basin the eastern barrow sub basin and on the peedamullah shelf the smallest vertical stress magnitudes were calculated in the kangaroo syncline the western barrow sub basin and offshore across the exmouth plateau high rock densities thus high vertical stress magnitudes can be due to uplifted rock that is over compacted relative to its current depth in the carnarvon basin areas of elevated vertical stress magnitudes correspond to areas of identified uplift uplift at the bambra anticline has previously been estimated to be approximately 900 m calculations within the bambra 4 well demonstrate that a 900 m increase in burial depth of a regionally consistent lithostratigraphic unit the muderong shale equates to a 0 22 g cm 3 increase in rock density thus the difference between uplifted over compacted rock and normally compacted rock no uplift is equivalent to a difference in the vertical stress magnitude of 2 1 mpa km 1 at the bambra anticline this value totalling the difference between over compacted uplifted and normally compacted sedimentary rock 2 1 mpa km 1 accounts for some part of the 3 6 mpa km 1 variation in vertical stress magnitude observed across the carnarvon basin therefore recent tectonic uplift has had a significant impact on the present day magnitude of the vertical stress and is interpreted to be responsible for the majority of observed variation of vertical stress in the carnarvon basin

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

natural fractures in the barnett shale and their importance for hydraulic fracture treatments gas production from the barnett shale relies on hydraulic fracture stimulation natural opening mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone knowledge of both the present day maximum horizontal stress which controls the direction of hydraulic fracture propagation and the geometry of the natural fracture system which we discuss here is therefore necessary for effective hydraulic fracture treatment design we characterized natural fractures in four barnett shale cores in terms of orientation size and sealing properties we measured a mechanical rock property the subcritical crack index which governs fracture pattern development natural fractures are common narrow 0 05 mm 0 002 in sealed with calcite and present in en echelon arrays individual fractures have high length width aspect ratios 1000 1 they are steep 75 and the dominant trend is west northwest other sets trend north south the narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability but the population may follow a power law size distribution where the largest fractures are open the subcritical crack index for the barnett shale is high indicating fracture clustering and we suggest that large open fractures exist in clusters spaced several hundred feet apart these fracture clusters may enhance permeability locally but they may be problematic for hydraulic fracture treatments the smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments because the maximum horizontal stress trends northeast southwest and is nearly normal to the dominant natural fractures reactivation widens the treatment zone along multiple strands

the characteristics and significance of conventional and unconventional sinian silurian gas systems in the sichuan basin central china abstract oil and gas discoveries within proterozoic cambrian strata and the majority of shale gas production occurs in relatively stable tectonic regions and characterised by moderate thermal maturity r o o 2 5 the system comprises two components namely a conventional sinian cambrian dolomite hosted gas resource and an unconventional cambrian silurian shale gas deposit both represent trillion cubic meter accumulations the migration accumulation type sinian cambrian gas system is a result of numerous processes including 1 paleo high trapping within a relatively stable tectonic setting 2 appropriate source materials located in a paleo rift trough 3 large scale dolomite grainstone reservoirs located near the paleo high 4 adequate gas produced from in situ crude oil cracking and 5 gypsum salt and shale cap rock preservation the self contained source reservoir cambrian silurian shale gas system is localized by deepwater organic rich shelf facies characterized by high silicon and calcium contents the recent discovery of these two massive gas fields in old and over matured sedimentary strata has effectively extended the upper maturity limit of r o over 3 0 which may indicate the efficacy of broadening current natural gas exploration areas thus potentially increasing global natural gas resources significantly furthermore the discovery may spark interest in natural gas exploration in widely distributed old strata thus encouraging co development of conventional and unconventional petroleum accumulations

cenozoic exhumation on the southwestern barents shelf estimates and uncertainties constrained from compaction and thermal maturity analyses abstract the barents sea is believed to have been influenced in most parts by cenozoic uplift and erosion episodes the rocks in the area are not currently at their maximum burial depth the exhumation of the sedimentary rocks has had large effects on rock physical properties and hydrocarbon maturation and migration the current study seeks to estimate exhumation from shale compaction and thermal maturity techniques and discuss its implications for hydrocarbon exploration in the uplifted barents sea area this study uses well logs and thermal maturity data together with widely distributed shot gather data along long offset seismic reflection lines the use of shale compaction techniques to estimate exhumation was focused particularly on the regionally preserved aptian albian kolmule formation and paleogene torsk formation shales normal compaction reference curves were established for these units in areas currently at their maximum burial depth e g sorvestsnaget basin and vestbakken volcanic province the results suggest widespread cenozoic exhumation throughout the southwestern barents sea the exhumation magnitudes increase towards east and northeast the average exhumation estimates from the three data sources range from 800 to 1400 m within the hammerfest basin 1150 1950 m on the loppa high 1200 1400 m on the finmark platform and 1250 2400 m on the bjarmeland platform the marked differences in glacial erosion from mass balance and average erosion estimates from the current study suggest a significant pre glacial uplift and erosion in the southwestern barents sea area the observed stratigraphy and presence of significant volumes of late oligocene middle miocene sediments in basins at the outer margin and increased erosion rates at the same time in source areas suggest that maximum burial in the southwestern barents sea may have occurred sometime during the oligocene or even earlier in the eocene the results from this study are useful input for modelling of source rock maturation generation migration and trapping of hydrocarbons in the area these results are also an important input for the prediction of more precise reservoir and seal rock properties in frontier areas away from the exploration wells and provide valuable knowledge for the use of interval velocities in the uplifted areas

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential discussion montgomery et al 2005 have written a very useful information filled review article on the state of knowledge of the barnett shale play in north texas a topic of great current interest and importance one error exists however the burial history that they present shows no uplift during the early and middle mesozoic and strong uplift after the cretaceous whereas the geologic record indicates major pre cretaceous uplift this error substantially affects the discussion of the maturation history of the barnett and should be corrected in the literature i will also briefly discuss the implications of pre cretaceous erosion and ouachita thrusting to barnett maturity in the deep fort worth basin in montgomery et al s 2005 figure 7 they show a time depth burial history diagram for eastland county that is contrary to what is known about the area in that figure and in the text they indicate that the barnett was rapidly buried during the pennsylvanian and early permian remained at depth with no uplift or subsidence except for minor subsidence in the early cretaceous then was uplifted some 1 8 km 6000 ft between the middle cretaceous and the eocene with very slight subsidence to the present the surface geology of eastland county and surrounding areas barnes 1972 shows that flat lying lower cretaceous strata antlers sand and overlying edwards group marine carbonates lie unconformably above units ranging from the strawn group mingus formation desmoinesian regional age in the southeast to the lower cisco group graham formation virgilian regional age in the northwest the west northwest dipping pennsylvanian strata form part

geodynamic process and metallogeny history and present research trend with a special discussion on continental accretion and related metallogeny throughout geological history in south china geodynamic process and metallogeny constitute an important research field all over the world studying the tectonic evolution process we can get to know why some groups of ore deposits occur in some specific geological settings and why large amounts of metals are accumulated in some specific places of the metallogenic belts or regions in this paper a review is given concerning the research work conducted in this field since 1960 s four important periods of researches have been proposed i e the observation of the submarine exhalative ore system and the study of the similarities in the geological history in the present continents the metallogeny along the mobile continental margins the intra continental orogenic and extensional processes and the continent convergence or even terrain accessory and related metallogeny throughout the geological history exemplified by south china comprising cathynian and yangtze blocks a world class nonferrous metallic province the authors propose three important lateral or vertical continental accessories and ore forming processes throughout the geological history they include neoproterozoic rodinia assembly and breaking up and related sn cu ni mineralization emeishan mantle plume and related cu ni pt pd v ti mineralization and late triassic assembly of indo china south china and north china cratons and related late jurassic early cretaceous large scale non metallic mineralization

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential the mississippian barnett shale serves as source seal and reservoir to a world class unconventional natural gas accumulation in the fort worth basin of north central texas the formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce at present production is mainly confined to a limited portion of the northern basin where the barnett shale is relatively thick 300 ft 92 m organic rich presentday total organic carbon 3 0 thermally mature vitrinite reflectance 1 1 and enclosed by dense limestone units able to contain induced fractures the most actively drilled area is newark east field currently the largest gas field in texas newark east is 400 mi 2 1036 km 2 in extent with more than 2340 producing wells and about 2 7 tcf of booked gas reserves cumulative gas production from barnett shale wells through 2003 was about 0 8 tcf wells in newark east field typically produce from depths of 7500 ft 2285 m at rates ranging from 0 5 to more than 4 mmcf day estimated ultimate recoveries per well range from 0 75 to as high as 7 0 bcf efforts to extend the current barnett play beyond the field limits have encountered several challenges including westward and northward increases in oil saturation and the absence of lithologic barriers to induced fracture growth patterns of oil and gas occurrence in the barnett in conjunction with maturation and burial history data indicate a complex multiphased thermal evolution with episodic expulsion of hydrocarbons and secondary cracking of primary oils to gas

variation in vertical stress in the baram basin brunei tectonic and geomechanical implications the vertical or lithostatic stress is an important factor in tectonic and geomechanical studies and is commonly used in the prediction of pore pressures and fracture gradients however the vertical stress is not always calculated in situ and the approximation of 1 0 psi ft 22 63 mpa km is often used for the vertical stress gradient vertical stress has been determined in 24 fields in the baram basin brunei using density log and checkshot velocity survey data the baram basin shows a variation in vertical stress gradient between 18 3 and 24 3 mpa km at 1500 m depth below the surface this variation has a significant effect on in situ stress related issues in field development such as wellbore stability and fracture stimulation the variation is caused by a bulk rock density change of 2 48 2 07 g cm 3 from the hinterland of the delta to its front differential uplift and erosion of the delta hinterland and undercompaction associated with overpressure are the interpreted causes of the density and hence vertical stress variation q 2004 elsevier ltd all rights reserved

cenozoic exhumation of the southern british isles rocks that crop out across southern britain were exhumed from depths of as much as 2 5 km during cenozoic time this has been widely attributed to paleocene regional uplift resulting from igneous underplating related to the iceland mantle plume our compilation of paleothermal and compaction data reveals spatial and temporal patterns of exhumation showing little correspondence with the postulated influence of underplating instead being dominated by kilometer scale variations across cenozoic compressional structures which in several basins are demonstrably of neogene age we propose that crustal compression due to plate boundary forces transmitted into the plate interior was the major cause of cenozoic uplift in southern britain witnessing a high strength crust in western europe

loading configurations and driving mechanisms for joints based on the griffith energy balance concept abstract using the griffith energy balance concept to model joint propagation in the brittle crust two laboratory loading configurations serve as appropriate analogs for in situ conditions the dead weight load and the fixed grips load the distinction between these loading configurations is based largely on whether or not a loaded boundary moves as a joint grows during displacement of a loaded boundary the energy necessary for joint propagation comes from work by the dead weight i e a remote stress when the loaded boundary remains stationary as if held by rigid grips the energy for joint propagation develops upon release of elastic strain energy within the rock mass these two generic loading configurations serve as models for four common natural loading configurations a joint normal load a thermoelastic load a fluid load and an axial load each loading configuration triggers a different joint driving mechanism each of which is the release of energy through elastic strain and or work the four mechanisms for energy release are joint normal stretching elastic contraction poroelastic contraction under either a constant fluid drive or fluid decompression and axial shortening respectively geological circumstances favoring each of the joint driving mechanisms are as follows the release of work under joint normal stretching occurs whenever layer parallel extension keeps pace with slow or subcritical joint propagation under fixed grips a substantial crack normal tensile stress can accumulate by thermoelastic contraction until joint propagation is driven by the release of elastic strain energy within the earth the rate of joint propagation dictates which of these two driving mechanisms operates with faster propagation driven by release of strain energy like a dead weight load acting to separate the joint walls pore fluid exerts a traction on the interior of some joints joint propagation under fluid loading may be driven by a release of elastic strain energy and or work by the pore fluid during poroelastic contraction again propagation velocity dictates the driving mechanism with critical to supercritical joint propagation accompanying fluid decompression and subcritical crack propagation taking place under a constant fluid drive during axial shortening a jointing process called axial splitting may occur following wing crack growth from an initial crack tilted relative to the maximum compressive stress here the joint driving mechanism may be either the work of a remote stress i e a dead weight or the release of elastic strain energy i e fixed grips

characterization of gas shale pore systems by porosimetry pycnometry surface area and field emission scanning electron microscopy transmission electron microscopy image analyses examples from the barnett woodford haynesville marcellus and doig units the nanometer scaled pore systems of gas shale reservoirs were investigated from the barnett marcellus woodford and haynesville gas shales in the united states and the doig formation of northeastern british columbia canada the purpose of this article is to provide awareness of the nature and variability in pore structures within gas shales and not to provide a representative evaluation on the previously mentioned north american reservoirs to understand the pore system of these rocks the total porosity pore size distribution surface area organic geochemistry mineralogy and image analyses by electron microscopy were performed total porosity from helium pycnometry ranges between 2 5 and 6 6 total organic carbon content ranges between 0 7 and 6 8 wt and vitrinite reflectance measured between 1 45 and 2 37 the gas shales in the united states are clay and quartz rich with the doig formation samples being quartz and carbonate rich and clay poor higher porosity samples have higher values because of a greater abundance of mesopores compared with lower porosity samples with decreasing total porosity micropore volumes relatively increase whereas the sum of mesopores and macropore volumes decrease focused ion beam milling field emission scanning electron microscopy and transmission electron microscopy provide high resolution 5 nm images of pore distribution and geometries image analysis provides a visual appreciation of pore systems in gas shale reservoirs but is not a statistically valid method to evaluate gas shale reservoirs macropores and mesopores are observed as either intergranular porosity or are confined to kerogen rich aggregates and show no preferred orientation or align parallel with the laminae of the shale networks of mesopores are observed to connect with the larger macropores within the kerogen rich aggregates

constraints on velocity depth trends from rock physics models estimates of depth overpressure and amount of exhumation based on sonic data for a sedimentary formation rely on identification of a normal velocity depth trend for the formation such trends describe how sonic velocity increases with depth in relatively homogeneous brine saturated sedimentary formations as porosity is reduced during normal compaction mechanical and chemical compaction is normal when the fluid pressure is hydrostatic and the thickness of the overburden has not been reduced by exhumation we suggest that normal porosity at the surface for a given lithology should be constrained by its critical porosity i e the porosity limit above which a particular sediment exists only as a suspension consequently normal velocity at the surface of unconsolidated sediments saturated with brine approaches the velocity of the sediment in suspension furthermore porosity must approach zero at infinite depth so the velocity approaches the matrix velocity of the rock and the velocity depth gradient approaches zero for sediments with initially good grain contact when porosity is just below the critical porosity the velocity gradient decreases with depth by contrast initially compliant sediments may have a maximum velocity gradient at some depth if we assume that porosity decreases exponentially with depth we have used published velocity porosity depth relationships to formulate normal velocity depth trends for consolidated sandstone with varying clay content and for marine shale dominated by smectite illite the first relationship is based on a modified voigt trend porosity scaled by critical porosity and the second is based on a modified time average equation baselines for sandstone and shale in the north sea agree with the established constraints and the shale trend can be applied to predict overpressure a normal velocity depth trend for a formation cannot be expressed from an arbitrary choice of mathematical functions and regression parameters but should be considered as a physical model linked to the velocity porosity transforms developed in rock physics

quantification of the magnitude of net erosion in the southwest barents sea using sonic velocities and compaction trends in shales and sandstones abstract during specific intervals within mesozoic and cenozoic times several areas of the southwestern barents sea were subjected to uplift and erosion areas with missing shallow stratigraphic interval sections and major erosion can be seen at several places along interpreted regional profiles in the southwestern barents sea a new normal compaction trend nct for two selected shale and sandstone dominated lithologies has been constructed based on sonic logs in the southwestern barents sea the shale dominated nct is calibrated to the cretaceous shales in the northern north sea and norwegian sea and applied to the cretaceous shales of the barents sea the sandstone dominated nct is calibrated to the lower jurassic are formation of the norwegian sea and applied to the lower jurassic upper triassic coastal plain section in the barents sea by utilising the nct model the study estimates net apparent erosion in 28 selected barents sea wells based on comparison of sonic log velocities a net apparent erosion map of the study area was constructed by gridding of the well values the accuracy of the map is limited in areas with little well control such as in the northwest where the east west transition into the southwestern barents sea region is poorly constrained with that in mind the map clearly shows two regional trends which dominate the erosion pattern in the study area an increasing amount of erosion towards the north and a sharp decrease of erosion westwards of the hinge zone into the southwestern barents sea the highest erosion estimates are observed towards svalbard with values up to 2500 m the results of this study can be further utilized in petroleum system studies in the eroded areas

tracking the burial and tectonic history of devonian shale of the appalachian basin by analysis of joint intersection style the most pervasive systematic joints hosted by devonian black shale of the appalachian plateau include east northeast joints and younger northwest striking cross fold joints both sets were driven exclusively by fluid pressure generated as a consequence of hydrocarbon related maturation supplemented by subsequent tectonic compaction during the alleghanian tectonic cycle in the more deeply buried proximal region of the catskill delta joints of both sets cross cut however in stratigraphically equivalent black shale of the distal shallower region of the delta to the west 25 of the joint intersections are crosscutting whereas 75 of the intersections are defined by east northeast joints curving into and abutting cross fold joints east northeast joints in the distal shale propagated early but were neither as long nor as pervasive as similarly oriented joints in the more deeply buried proximal delta deposits however these same joints are much better developed more closely spaced stratigraphically lower in the distal shale succession where essentially all intersections are crosscutting it is likely that relatively high contact stress an effective stress on east northeast joints in the more deeply buried parts of the delta enabled the unimpeded transmission of elastic stress concentration at the tip of cross fold joints across the older joints in the distal delta exhumation of the devonian shale sequence following establishment of the contemporary stress field in middle tertiary time to within 1 km of the earth s surface resulted in renewed propagation of east northeast joints the propagation path of the reactivated joints curved into and terminated against cross fold joints at angles of 60 90 the curving to termination intersection style is a manifestation of crack tip stress field blunting as a consequence of slippage along the interface of cross fold joints enabled by exhumation induced relaxation of contact stress on these latter joints it is noteworthy that the orientation of the densely formed east northeast joints parallel to the maximum horizontal stress direction of the contemporary lithospheric stress field likely imparts a meaningful permeability anisotropy to these hydrocarbon source rocks

modeling of gas generation from the barnett shale fort worth basin texas the generative gas potential of the mississippian barnett shale in the fort worth basin texas was quantitatively evaluated by sealed gold tube pyrolysis kinetic parameters for gas generation and vitrinite reflectance ro changes were calculated from pyrolysis data and the results used to estimate the amount of gas generated from the barnett shale at geologic heating rates using derived kinetics for ro evolution and gas generation quantities of hydrocarbon gas generated at ro 1 1 are about 230 l t 7 4 scf t and increase to more that 5800 l t 186 scf t at ro 2 0 for a sample with an initial total organic carbon content of 5 5 and ro 0 44 the volume of shale gas generated will depend on the organic richness thickness and thermal maturity of the shale and also the amount of petroleum that is retained in the shale during migration gas that is reservoired in shales appears to be generated from the cracking of kerogen and petroleum that is retained in shales and that cracking of the retained petroleum starts by ro 1 1 this result suggests that the cracking of petroleum retained in source rocks occurs at rates that are faster than what is predicted for conventional siliciclastic and carbonate reservoirs and that contact of retained petroleum with kerogen and shale mineralogy may be a critical factor in shale gas generation shale gas systems together with overburden can be considered complete petroleum systems although the processes of petroleum migration accumulation and trap formation are different from what is defined for conventional petroleum systems

interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling four sets of thin section scale mode i open mode cemented microfractures are present in sandstone from the eocene misoa formation maracaibo basin venezuela the first set of microfractures is intragranular f1 formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 8c the second set of microfractures f2 is cemented by bituminite pyrite formed at temperatures between 60 and 100 8c and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks the third set of microfractures f3 is fully cemented by either quartz cement or calcite cement the former has fluid inclusion homogenization temperatures between 149 and 175 8c these temperatures are mostly higher than maximum burial temperatures 160 8c suggesting that upward flow caused by a pressure gradient transported silica vertically which crystallized into the fractures upward decompression may have also caused a pco2 drop which at constant temperature allowed simultaneous carbonate precipitation into the third microfracture set the fourth set of thin section scale microfractures f4 is open or partially cemented by siderite hematite and other iron oxides the presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the misoa formation in order to time and place constraints on the depth of formation of the fourth set of microfractures we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if in fact they could be associated with the uplift of the formation our results suggest that thermoelastic contraction caused by the formation s uplift erosion and consequent cooling is a feasible mechanism for the origin of the last fracture set hence we infer that meteoric water invasion into the fractures at the end of the uplift cause the precipitation of oxides and the transformation of siderite to hematite d 2005 elsevier b v all rights reserved

topo oz insights into the various modes of intraplate deformation in the australian continent abstract as the fastest lowest flattest and amongst the most arid of continents australia preserves a unique geomorphic record of intraplate tectonic activity evidencing at least three distinct modes of surface deformation since its rapid northward drift commenced around 43 million years ago at long wavelengths several 1000s km systematic variations in the extent of neogene marine inundation imply the continent has tilted north down southwest up at intermediate wavelengths several 100s km several undulations of 100 200 m amplitude have developed on the 1 10 myr timescale at still shorter wavelengths several 10s km fault related motion has produced local relief at rates of up to 100 m myr over several million years the long wavelength north down tilting can be related to a dynamic topographic effect associated with australia s northward drift from the geoid low dynamic topography low now south of the continent to the geoid high dynamic topography low centred above the south east asian and melanesian subduction zones the short wavelength fault related deformation is attributed in time to plate wide increases in compressional stress levels as the result of distant plate boundary interactions and in space in part to variations in the thermal structure of the australian lithosphere at the intermediate wavelengths transient low amplitude undulations can be ascribed to either lithospheric buckling or the development of instabilities in the thermal boundary layer beneath the lithosphere in the latter case topographic asymmetries suggest the australian lithosphere is moving north with respect to the mantle beneath providing a unique attribution to the progressive alignment of seismic anisotropy and absolute plate motion observed near the base of the australian lithosphere