temporal and spatial variations of surf zone currents and suspended sediment concentration temporal and spatial variations of surf zone currents and suspended sediment concentrations were investigated at the u s army engineer research and development center s large scale sediment transport facility lstf a longshore uniform fine sand beach 35 m alongshore 20 m cross shore and 25 cm thick was placed in the facility for these experiments two unidirectional long crested irregular wave conditions were examined one resulted in predominantly spilling breakers and one in plunging breakers waves and currents and sediment concentrations were measured at 20 and 16 hz respectively at various longshore and cross shore locations and throughout the water column both currents and sediment concentrations exhibit great temporal and spatial variations in the surf zone the variation patterns however of the longshore current cross shore current and sediment concentration are substantially different caution should be exercised when averaging these parameters over time and space for the two wave cases examined the temporal variations of longshore current including those at principal incident wave frequencies were relatively small across most of the surf zone over 70 of the variations are within approximately f60 of the mean value the wave motion with a strong peak at principal incident wave frequencies dominated the temporal variations of cross shore current temporal variations of suspended sediment concentration under the irregular waves were episodic characterized by occasional large values induced by suspension events or due to horizontal advection the variance of the concentration at the peak incident wave frequency was not significant except very near the bed time averaged longshore current profiles over the predominantly rippled sand bed were logarithmic in shape below the wave trough depth averaged longshore current excluding the portion of water column above wave trough matched well with the current measured at an elevation of 1 3 of the water depth from the bed time averaged cross shore current profiles were characterized by an onshore mass flux near the surface and a balancing offshore flow below the wave trough level undertow sediment concentration decreased very rapidly upward through the water column across most of the surf zone except at the plunging breaker line where relatively homogeneous concentration was measured throughout much of the water column above 4 cm from the bed depth averaged sediment concentration over the range from 1 cm above the bed to the bottom of wave trough roughly equaled the concentration measured at an elevation from the bed equal to 20 of the still water depth

stress state in the cuu long and nam con son basins offshore vietnam abstract the results presented in this paper are the first published estimates of the complete stress tensor in the cuu long and nam con son basins offshore vietnam we analysed in situ stress and pore pressure fields in the sedimentary formations using data from petroleum exploration and production wells to evaluate the stress state in these basins the data were obtained from the seafloor to 4300 m burial depth and include both hydrostatic and overpressured sections minimum horizontal stresses were obtained from extended leak off tests and mini fracture tests maximum horizontal stresses were estimated from drilling induced fracture parameters and borehole breakout widths in twelve wells using rock properties measured in the laboratory or estimated empirically from wireline logs seven data points are located in sediments and seventeen data points in igneous basement rocks at depths greater than 3000 m the estimated magnitudes of h are 70 110 of the v magnitudes considering the errors in the stress magnitude estimates their relative magnitudes suggest that a borderline normal strike slip stress regime presently exists in normally pressured sequences of the nam con son and cuu long basins of the twenty four data points twenty have effective stress ratios at a critical stress state for faulting on the assumption that there are faults present that are optimally oriented for failure with friction coefficients of 0 5 the results suggest that the stress state in these basins is generally critical

well planning for swf and overpressures at the kestrel well editor s note this article is excerpted from a presentation at the 2001 otc drilling in deepwater has forced a more complete understanding of and appreciation for pore pressures and fracture gradients shallow water flow swf sands perhaps exemplify the most extreme example of how narrow the window between pore pressures and fracture gradients can be as stated by bp s mark alberty at the 2000 otc paper otc11971 swf sands are overpressured unconsolidated sands encountered before pressure control is in place on the wellhead assembly they may flow into the well if their pore pressures are not balanced by the mud weights however if the mud weights are too highly overbalanced the sands may fracture resulting in lost returns and loss of well control the margins for wellbore pressures may be less than 100 psi for shallow depths below mudline in deepwater wells the kestrel well was drilled in a water depth of 2795 ft in mississippi canyon block 632 an area of well documented swf data from nearby wells were incorporated in designing the casing and mud weight program essential to the design were predictions of pore pressures and lithology efficient well design required a pore pressure prediction for the swf section and the deeper overpressured sections accurate predictions required input from a pore pressure analyst geophysicists seismic interpreters operations geologists and drilling engineers the kestrel well was designed with information from a number of wells but a great deal of the most important information came from the mississippi canyon 718 1 well approximately 7 miles wsw of the kestrel well planning for swf in this well was very well presented at the otc in 1996 identification of flowing water sand drilling hazards in the deepwater gulf of mexico by byrd et al figure 1 is a seismic line extracted from a 3d seismic data

subsurface fractures in situ stress and permeability variations in the walloon coal measures eastern surat basin queensland australia abstract in situ stress pattern and subsurface fractures control the production of coal seam gas csg reservoirs and hence comprehensive analyses are required for better development of these reservoirs the surat basin is a prolific csg province with major economic impacts in eastern australia recent studies have reported significant stress variations in this basin due to local geological structures however a systematic analysis of localised stress perturbations within the basin and their impacts on reservoir permeability have not been presented thus far in the published literature in this paper we combine different sets of data including conventional log data wellbore image logs 2d seismic lines and interpreted permeability from well test data in order to characterise the role of geological structures in the subsurface fluid flow of the walloon coal measures in eastern surat basin analysis of 7 3 km of borehole image log across 33 vertical wells revealed a regional ene wsw orientation of the maximum horizontal stress shmax in the study area however there are massive variability in the shmax orientation spatially and with depth analysis of 2d seismic lines highlight the role of geological structures in stress perturbation and the interpretation of natural fractures using image logs show that most of the open fractures are oriented sub parallel to the acute angle of local shmax orientation interpreted well test data in the study wells demonstrates that most of the high permeability data points are located where in situ stress rotation is observed this indicates the influence of local shmax orientation responding to geological structures on the subsurface fluid flow in the eastern part of the surat basin

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

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

systematic assessment of fault stability in the northern niger delta basin nigeria implication for hydrocarbon prospects and increased seismicities abstract accurate information on fault networks the full stress tensor and pore fluid pressures are required for quantifying the stability of structure bound hydrocarbon prospects carbon dioxide sequestration and drilling prolific and safe wells particularly fluid injections wells such information also provides essential data for a proper understanding of superinduced seismicities associated with areas of intensive hydrocarbon exploration and solid minerals mining activities pressure and stress data constrained from wells and seismic data in the northern niger delta basin nndb nigeria have been analysed in the framework of fault stability indices by varying the maximum horizontal stress direction from 0 to 90 evaluated at depths of 2 km 3 5 km and 4 km we have used fault dips and azimuths interpreted from high resolution 3d seismic data to calculate the predisposition of faults to failures in three faulting regimes normal pseudo strike slip and pseudo thrust the weighty decrease in the fault stability at 3 5 km depth from 1 2 mpa to 0 55 mpa demonstrates a reduction of the fault strength by high magnitude overpressures pore fluid pressures 50 mpa have tendencies to increase the risk of faults to failure in the study area statistical analysis of stability indices si indicates faults dipping 50 60 80 90 and azimuths ranging 100 110 are most favourably oriented for failure to take place and thus likely to favour migrations of fluids given appropriate pressure and stress conditions in the dominant normal faulting regime of the nndb a few of the locally assessed stability of faults show varying results across faulting regimes however the near similarities of some model based results in the faulting regimes explain the stability of subsurface structures are greatly influenced by the maximum horizontal stress shmax direction and magnitude of pore fluid pressures

the present day state of tectonic stress in the darling basin australia implications for exploration and production knowledge of the full present day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs the darling basin of new south wales australia is an old sedimentary basin late cambrian silurian to early carboniferous in which there was limited information about the present day stress field prior to this study in this paper we evaluate the contemporary stress field of the darling basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability fracture fault generation and reactivation our interpretations of borehole failures in borehole image logs reveal a prevailing east west orientation of the maximum horizontal stress throughout the darling basin the estimates of the magnitudes for the vertical minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike slip faulting stress regime at 600 700 m depths where the magnitude of vertical stress and minimum horizontal stress are close to each other however the presence of borehole breakouts and drilling induced tensile fractures that we observe in the image logs at greater depths 900 2100 m indicate a transition into a strike slip tectonic stress regime below a depth range of approximately 700 900 m these findings are in agreement with overcoring stress measurements east and west of the investigated wells furthermore there are several neogene to recent geological structures in the study area that indicate thrust faulting with an east west oriented maximum horizontal stress orientation around this old sedimentary basin the consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data however the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike slip stress regime sh sh similar to sv indicated by petroleum data and highlights a potential pitfall of using neotectonic structures in geomechanical models in particular careful attention and verification should be made when using neotectonic structures for input calibration or confirmation of geomechanical models especially in intraplate tectonic settings such as australia c 2016 elsevier ltd all rights reserved

seabed shear stress and bedload transport due to asymmetric and skewed waves abstract a simple conceptual formulation to compute seabed shear stress due to asymmetric and skewed waves is presented this formulation generalizes the sinusoidal wave case and uses a variable friction factor to describe the physics of the boundary layer and to parameterize the effects of wave shape predictions of bed shear stresses agree with numerical computations using a standard boundary layer model with a k e turbulence closure the bed shear stress formulation is combined with a meyer peter and muller type formula to predict sheet flow bedload transport under asymmetric and skewed waves for a horizontal or sloping bed the predictions agree with oscillatory water tunnel measurements from the literature

relationship between structural style overpressures and modern stress baram delta province northwest borneo christopher k morley mark tingay and richard hillis australian school of petroleum university of adelaide adelaide australia rosalind king

estimating pore fluid pressure stress coupling abstract fracture pressure fp prediction is a vital component in well planning for safe drilling in high pressure areas in the estimation of seal capacity and seal breach risk analysis for fluid injection and in fracture stimulation for oil gas recovery from tight reservoirs and mature source rocks coupling of pore fluid pressure pp and fp at basin scale has been recognised for several decades with coupling ratio values mostly between 0 46 and 0 87 i e for every 10 mpa of pp increase at constant depth fp increases by 4 6 8 7 mpa these values are similar to estimates for reduction of fp with reservoir depletion of an oil gas field during production poroelasticity has been suggested as the principal reason for coupling most previous coupling values were generated by examination of pressure gradients a new method to estimate pore fluid pressure stress coupling is proposed which plots the measured value of fp from a leak off test lot minus the expected fp value on the trend for normal pressures termed the fracture pressure residual fpr against overpressure op which is the magnitude of pp above normal at the same depth the coupling value is now the slope of fpr against op fpr op coupling values from 11 global basin datasets range from 0 24 to 0 43 approximately half the previous quoted basin scale coupling values a pressure depth model for rapid deposition on a continental margin with op exclusively generated by disequilibrium compaction and no change in effective stress in the overpressured section represents a base case for compaction op without poroelasticity this model helps to reveal 1 that coupling values vary with the ratio of fp to lithostatic pressure sv 2 since the 11 case study coupling values exceed the base case one or more other minor coupling mechanisms are involved potentially including poroelasticity 3 values may be independent of tectonic environment 4 one case study involving carbonate chalk is within the same range as those from siliciclastic sediments and 5 the high coupling pore pressure stress coupling values in the literature result from using fluid and fracture gradients including the shallow data where pp is normal which distort the analysis

modelling waves and currents at the lstf and other laboratory facilities the main objective of the paper is to use the detailed measurements of wave and current motions in the large scale sediment transport facility lstf at the coastal and hydraulics laboratory chl erdc vicksburg to compare with model simulations and particularly to investigate the accuracy of two wave models used for driving nearshore circulation computations the major questions asked are how accurate are the models and how do the inevitable inaccuracies in the prediction of the wave quantities influence the current predictions the lstf is one of a few large scale facilities around the world designed to accurately reproduce the 2 d horizontal flow and sediment conditions in the nearshore region of a littoral coast the measurements include all relevant wave data such as wave heights setup variations 2 d current distributions across the basin and a detailed cross shore array of measurements of the 3 d current structures the two wave model formulations used for the comparisons are versions of the kinematic wave model one using sinusoidal the other using non sinusoidal wave shapes both before and after wave breaking as wave drivers in the quasi 3 d model shorecirc sc the comparisons with the measurements are aimed at providing an overall picture of both the flow conditions and the model performance the discussions provide insight into the mechanisms behind the complex nearshore dynamics and demonstrate the strengths and weaknesses of the two methods in a comprehensive comparison of available wave and current data the comparisons also reveal that an essential requirement for a wave driver is its capability to represent the phase motion of the wave and its capability to correctly predict the phase speed of waves

identification and calculation of transfer overpressure in the northern qaidam basin northwest china the origin of the overpressure in the northern qaidam basin has not been clearly understood which has caused some difficulties in hydrocarbon exploration using a compaction study we applied a modified acoustic velocity and effective stress diagram to identify the overpressure transfer in the study area this phenomenon has not been discussed in previous studies for the present study we approximately calculated the magnitude of the transfer overpressure and analyzed the cause of the overpressured aquifer at the crest of the anticline in the study area our study indicates that the effect of overpressure transfer is very distinct and the largest contribution to the total overpressure is 57 the main media of overpressure transfer include vertical faults and lateral conducting layers the vertical faults can connect deep overpressured strata and the lateral conducting layers can connect overpressured strata at the top and wing of the anticline during anticline formation the crest fractures and then the overpressured water in the anticline wing flows into the fractured crest and forms the overpressure compartment that prevents the charging of deeper natural gas

in situ stress analysis in the yinggehai basin northwestern south china sea implication for the pore pressure stress coupling process abstract the yinggehai basin is located in the triangle between the sunda block south china block and the south china sea and is an interesting target for in situ stress investigation not only because of its specific tectonic position but also due to its abnormally high pore pressures magnitudes and orientations of in situ stress were directly constrained by measurements operated in the vertical boreholes that were drilled in the yinggehai basin gradients and magnitudes of the least horizontal stress remarkably increase with overpressure enhancement indicating that the least principal stress and pore pressure are in fact coupled the ratios of pore pressure stress coupling h pp are similar in the north and south central yinggehai basin despite the burial depths of overpressure top are different however a deeply buried strike slip faulting regime under wnw ese oriented horizontal compression was identified in the central yinggehai basin where the h pp ratios have intermediate values ranging from that for an idealized normal faulting stress regime to that for the southeastern south china a strong strike slip or even reverse faulting stress regime additionally the wnw ese orientations of maximum horizontal stress determined from drilling induced tensile fractures in the yinggehai basin are inappropriate for the present maximum horizontal stress derived from earthquake focal mechanism solutions in the northern segment of the red river faults suggesting that the dextral movement of red river faults has slightly influenced the present stress state in the yinggehai basin the present stress state of the yinggehai basin was primarily generated by the ese absolute horizontal motion of the sunda and south china blocks

evidence for overpressure generation by kerogen to gas maturation in the northern malay basin gas generation is a commonly hypothesized mechanism for the development of high magnitude overpressure however overpressures developed by gas generation have been rarely measured in situ with the main evidence for such overpressures coming from source rock microfractures the physical necessity of overpressures for primary migration laboratory experiments and numerical modeling indeed previous in situ observations suggest that gas generation only creates highly localized overpressures within rich source rocks pore fluid pressure data and sonic velocity vertical effective stress plots from 30 wells reveal that overpressures in the northern malay basin are primarily generated by fluid expansion and are located basinwide within the miocene 2a 2b and 2c source rock formations the overpressures are predominantly associated with gas sampled in more than 83 of overpressure measurements and have a sonic density response consistent with gas generation the association of fluid expansion overpressures with gas combined with the sonic density response to overpressure and a regional geology that precludes other overpressuring mechanisms provides convincing in situ evidence for basinwide gas generation overpressuring overpressure magnitude analysis suggests that gas generation accounts for approximately one half to two thirds of the measured excess pore pressure in the region with the remainder being generated by coincident disequilibrium compaction thus the data herein suggest that gas generation if acting in isolation is producing a maximum pressure gradient of 15 3 mpa km 0 676 psi ft and not lithostatic magnitudes as commonly hypothesized the gas generation overpressures in this article are not associated with a significant porosity anomaly and represent a major drilling hazard with traditional pore pressure prediction techniques underestimating pressure gradients by 2 3 1 5 mpa km 0 1 0 07 psi ft

present day stress of the central persian gulf implications for drilling and well performance abstract the present day state of stress in the persian gulf is poorly understood but has significant impacts on well drilling and performance the upper permian to lower triassic formation of kangan dalan persian gulf exhibits a complex structural context in the neighborhood of the oman mountains and the zagros orogenies this formation is divided into four reservoir layers k1 to k4 where three main lithologies limestone dolomite and anhydrite are alternating we conduct an analysis of the present day stress and natural fractures at the wellbore using full bore fmi logs leak off test and density logs for this purpose borehole breakout and tensile fracture data are used to determine orientation of s h furthermore density log leak off test and kirsch equation for tensile fracture formation in the wellbores are used to calculate the magnitude of s v s h and s h respectively vertical stress s v gradient at 3100 m depth approximates 20 mpa km 2 9 psi m indicating a bulk density of 2 04 g cm 3 a total of 131 drilling induced tensile fractures and 21 breakouts with an overall length of 262 m are observed in two wells indicating a mean maximum horizontal stress s h orientation of n53 18 45 for drilling induced tensile fracture ditf data and n50 10 79 for breakout data the mean orientation of s h rotates counterclockwise with depth from k2 n70 4 2 to k4 n40 5 1 reservoirs noticed correlation between these data and stress orientations from earthquake focal mechanism solution first of all indicates that the stresses are linked to the resistance forces generated by the arabia eurasia collision at the zagros orogeny and secondly confirms the reliability of focal mechanism solution data near continental collision zones in the kangan dalan formation the nw se main open fracture direction is found as a common regional direction which is sub perpendicular to the present day maximum horizontal stress minimum horizontal stress s h gradient in reservoir sections is estimated to be equal to 17 mpa km 2 5 psi m the concluded strike slip stress regime s h s v s h in the study area is consistent with the compressive regime in the zagros thrust fold belt the present day stress in the kangan dalan formation has implications for wellbore stability lost circulation and well inflow performance relationship ipr wells are more unstable if deviated toward the s v direction whereas well productivity and mud loss increase in wells deviated toward s h which conveys the idea of a strike slip faulting effect tends to keep the natural fractures open in that direction

3d spatial variation in vertical stress in on and offshore netherlands integration of density log measurements and basin modeling results abstract the principal vertical stress at a certain depth is equivalent to the weight of the overburden rocks and pore fluids at that depth vertical stress trends will be similar across areas with a simple layer cake lithostratigraphic sequence and similar burial history these conditions are not fulfilled for the lithostratigraphy and burial history of the onshore and offshore netherlands burial histories vary over short distances and resulted in highs platforms and inverted basins with different lithostratigraphic sequences and porosity distributions density log measurements and basin modeling results were used to assess the spatial variation of vertical stress in the dutch subsurface this paper presents the approach and assessed spatial variation in vertical stress the 3d basin modeling proved to be a good tool for correcting and extending density log derived vertical stress depth relations results show that the vertical stress varies spatially and that the vertical stress gradient is less than the standard gradient in large part of the area the largest local deviations from the standard gradient occur in areas with a thick package of low density cenozoic sediments and large low density salt structures i e in the northern offshore relatively old and more dense stratigraphic units occur at relatively shallow depth on the highs and in strongly inverted parts of the mesozoic basins in these areas the vertical stress reaches and follows the standard vertical stress gradient at shallower depths than in other areas

transverse drilling induced tensile fractures in the west tuna area gippsland basin australia implications for the in situ stress regime abstract drilling induced tensile fractures ditfs have been interpreted on image logs from vertical wells in the gippsland basin offshore southeastern australia interpreted axial vertical ditfs have previously been well described worldwide we also interpret transverse horizontal ditfs which are horizontal fractures that are electrically conductive non planar bimodal and constrained to the tensile region of the wellbore elasticity theory predicts formation of both transverse and axial drilling induced tensile fractures ditfs in vertical wells depending on the magnitude of the principal in situ stresses pore pressure and mudweight drilling induced tensile fractures initiate in very specific stress environments axial ditfs can closely constrain a lower bound to the maximum horizontal stress s h max magnitude where the minimum horizontal s h min stress is known if transverse ditfs are observed they can constrain a lower bound to maximum and minimum horizontal stress magnitudes the observation of transverse ditfs on image logs can constrain the stress field to one on the border of strike slip and reverse faulting s h max s h min s v without requiring knowledge of the s h min or s h max magnitude the observation of transverse ditfs in the west tuna area combined with wireline log data leak off tests and pore pressure data are used to constrain the in situ stress tensor the interpreted in situ stress tensor lies on the border of a strike slip and reverse faulting regime s h max 40 5 mpa km s h min s v 21 mpa km interpreted data from leak off tests in the west tuna area confirm that s h min s v

longshore current forcing by irregular waves 1 the terms in the wave averaged depth integrated alongshore momentum balance are populated with detailed measurements from a laboratory basin experiment obliquely incident irregular waves were generated in a large wave basin equipped with instruments to collect free surface elevation and velocity measurements over a sand beach pumps matched the wave generated currents and experimental evidence indicates that the equations can justifiably be simplified by assuming alongshore uniformity while the use of data measured below the trough level is straightforward the velocity field in the region near the surface is extrapolated from the measured positions and constrained by conservation of cross shore mass flux the role of the dispersive mixing mechanism due to depth dependent currents is shown to be small but acts to shift the peak of the longshore velocity shoreward slightly radiation stresses are the primary forcing mechanism and computed stresses based on measured wave induced hydrodynamics are considerably smaller than the widely used linear representation indicating that the small amplitude assumption of linear theory is not appropriate in the surf zone the time averaged bottom shear stress remains as the primary balance to the gradient in alongshore momentum and the inferred shear stress compares favorably with the values computed using the quadratic drag law and a spatially constant friction coefficient

origin of abnormal pressure in the upper paleozoic shale of the ordos basin china abstract overpressure in the upper paleozoic shale of the ordos basin upsob in china occurred mainly in the early cretaceous and is modeled using the petromod simulation method in this paper the origin of paleo overpressure was analyzed based on the multiple logging combination method bowers method and the velocity density cross plot method using logging data and shale porosity the causes of the underpressure and normal pressure and the history of their evolution were investigated based on a large amount of drilling and pressure data for the basin the results indicate that paleo overpressure in the upsob produced in the early cretaceous was not the cause of disequilibrium compaction but this occurred owing to increased hydrocarbon generation since the late cretaceous during the overall uplift of the ordos basin the decrease in temperature has led to a reduction in pore pressure of 23 93 34 35 mpa accounting for 56 69 69 47 of total pressure loss a combination of the expansion and diffusion of natural gas has led to a decrease in pore pressure of 9 52 22 60 mpa accounting for 27 63 41 83 of total pressure loss the effects of pore rebound on pressure loss were weak therefore temperature decrease alone could not have led to the current underpressure and normal pressure and the expulsion of gas owing to tectonic uplifting might have been the most important factor the upsob likely had a relatively high gas expulsion efficiency especially during late stage tectonic uplifting which may be a major risk for shale gas exploration

plate boundary forces are not enough second and third order stress patterns highlighted in the world stress map database 1 the world stress map project compiles a global database of contemporary tectonic stress information of the earth s crust early releases of the world stress map project demonstrated the existence of first order plate scale stress fields controlled by plate boundary forces and second order regional stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound the 2005 release of the world stress map project database provides for some areas high data density that enables us to investigate third order local stress field variations and the forces controlling them such as active faults local inclusions detachment horizons and density contrasts these forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic we present and discuss examples for venezuela australia romania brunei western europe and southern italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns

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

wave setup and setdown generated by obliquely incident waves an analytical theory is developed for the wave setup and setdown induced by obliquely incident waves on an impermeable swell built beach profile the wave setup and setdown are found to decrease as wave obliquity increases the incorporation of wave obliquity in wave setup and setdown formulation offers the physical reality in engineering applications the general solutions presented in this paper yield the limiting case of normal wave incidence and the result is consistent with the classical theories published the present theory is primarily applicable to the spilling and plunging breaker across the surf zone within which wave amplitude is assumed to be linearly related to the local water depth experiments were conducted in a large scale wave basin to compare with theoretical results and especially to investigate the applicability of this assumption to the case of obliquely incident waves the dimensionless setup versus the distance offshore within the surf zone is found to depend on wave breaking angle and the shape of the beach profile and it has a non zero value at the original shoreline position this implies that the original shoreline will advance landwards and that the extent of this movement can be related to wave angle at breaking and the beach profile under consideration the results of the present theory are in good agreement with experimental data and field measurements available

numerical simulation of the effects of meteoroid ablation and solar euv x ray radiation in the dayside ionosphere of mars mgs mex observations we have developed a model which produces three plasma layers simultaneously due to the impact of the meteoroids solar x ray 0 5 9 nm and euv 9 102 6 nm radiation at altitude range 75 85 km 100 115 km and 135 140 km respectively in the dayside ionosphere of mars the calculated results are compared with the radio occultation measurements made by mars express mex and mars global surveyor mgs on 18 april 2004 and 11 may 2005 when comets p 2003 wc7 linear catalina and 10p tempel 2 intersected the orbit of mars respectively the densities of 21 ions co2 o2 co o no n2 mg fe si mgo feo sio mgco2 mgo2 feco2 feo2 sico2 sio2 mgn2 fen2 and sin2 and 10 neutral species mg fe si mgo feo sio mgco3 feco3 mgo2 and feo2 have been computed self consistently for both days the metallic ions are formed in the middle ionosphere due to the ablation of meteoroids it is found that the middle ionosphere of mars strongly depends on incoming mass and flux of the meteoroids the meteoroids of fluxes 4 0 10 15 and 8 3 10 16 cm 2 s 1 for masses 4 5 10 7 g and 1 0 10 3 g have produced third layers which are good in agreement with the observations made by mex and mgs on 18 april 2004 and 11 may 2005 respectively

a sporadic third layer in the ionosphere of mars the daytime martian ionosphere has been observed as a two layer structure with electron densities that peak at altitudes between about 110 and 130 kilometers the mars express orbiter radio science experiment on the european mars express spacecraft observed in 10 out of 120 electron density profiles a third ionospheric layer at altitude ranges of 65 to 110 kilometers where electron densities on average peaked at 0 8 10 10 per cubic meter such a layer has been predicted to be permanent and continuous its origin has been attributed to ablation of meteors and charge exchange of magnesium and iron our observations imply that this layer is present sporadically and locally

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

nighttime ionosphere caused by meteoroid ablation and solar wind electron proton hydrogen impact on mars mex observation and modeling 1 we report that radio science rs experiment onboard mars express mex has observed three plasma layers in the nighttime ionosphere of mars at altitudes 80 100 km 120 km and 160 km which are reproduced by model calculation due to impact of meteoroid solar wind proton and electron respectively the densities of 21 ions mg fe si mgo mgco2 mgo2 mgn2 feo feo2 fen2 feco2 sio sico2 sin2 sio2 co2 n2 o o2 co and no have been computed between altitude 50 km and 200 km the model shows that all atmospheric ions co2 n2 o co o2 and no are produced above 100 km due to solar wind electron and proton impact ionizations the metallic ions are formed between 50 km and 100 km due to ablation of micrometeoroids it is found that mass 3 0 10 4 g of incoming meteoroid is sufficient for meteor ablation and its characteristic flux 4 0 10 15 cm 2s 1 could produce the nighttime metallic layer observed by mex the calculated electron densities are also compared with the occultation measurements made by mars 4 5 in the nighttime ionosphere of mars

review of pore pressure prediction challenges in high temperature areas pore pressure prediction relies heavily on interpretation of seismic and well attributes such as velocity resistivity and density which capture porosity changes during shale compaction under vertical loading relationships such as eaton 1975 were developed in the gulf of mexico using a relatively simple lithological mix of geologically young sandstones and shale mudrocks at relatively low temperatures an alternative approach using data from the same region was more deterministic also with vertical stress e g hottman and johnson 1965 bowers 1995 another approach using mean stress developed by harrold et al 2000 used similar sand and shale sequences at relatively low temperatures from data in southeast asia basins all the above approaches can be shown to provide acceptable prediction of pore pressure in shale mudrocks in young rapidly deposited siliciclastic sequences such as the baram delta brunei tingay et al 2009 and along the west african margin swarbrick et al 2011 the re

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

vertical flow structure during sandy duck observations and modeling abstract observations of the vertical distribution of time averaged cross shore and alongshore flows during the sandy duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour the measurements were obtained with a vertical stack of eight two component current meters with the lowest and highest sensor at respectively o 0 1 and o 2 7 m above the bed observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column consistent with laboratory observations under non breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface again in line with laboratory results analogous to previous observations the measured longshore current velocity profiles are logarithmic under non breaking conditions and become more depth uniform under breaking conditions the model description of the vertical structure of the flow includes the presence of wind stresses wave stresses pressure gradients turbulent eddy viscosity and a wave boundary layer the model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer eddy viscosity is enhanced in regions where turbulence is produced i e near the surface in the case of breaking waves and within the bottom boundary layer estimates of the wave breaking induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model measurement discrepancies predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results

estimating hinterland exhumation from late orogenic basin volume nw borneo a new approach for restoring sediment volumes onto the sediment source area to estimate uplift timing and magnitude is discussed and used in the context of late tertiary basin and topographic development in nw borneo sediment volumes for the baram deltaic province were estimated for four time periods latest early middle and late miocene and pliocene recent using 2d and 3d seismic horizons wells and outcrops volume restoration onto the palaeo sediment source area determined exhumation amount at the drainage divide c 5 km from 17 ma to recent and provided a reasonable match with other denudation estimation methods cooling ages and an older regional sediment restoration study using 1979 vintage sediment isopach maps restoration took into account 1 changing sediment densities during the erosion deposition cycle 2 changing area of the sediment source province with time e g changing shoreline location as a result of eustacy uplift and delta progradation tectonic shortening 3 uplift and partial erosion of baram deltaic province sediment denudation at the drainage divide for the middle miocene late miocene and pliocene recent has proceeded at a similar rate for each period initial uplift of central borneo has been attributed to buoyancy of thinned continental crust that jammed the subduction zone under nw borneo in the early miocene however the absence of decay in the erosion rates with time from the middle miocene to recent suggests operation of an additional uplift mechanism that may be related to delamination of mantle lithosphere slab detachment is the favoured explanation

reprint of comparison of modern fluid distribution pressure and flow in sediments associated with anticlines growing in deepwater brunei and continental environments iran abstract differences in fluids origin creation of overpressure and migration are compared for end member neogene fold and thrust environments the deepwater region offshore brunei shale detachment and the onshore arid central basin of iran salt detachment variations in overpressure mechanism arise from a the availability of water trapped in pore space during early burial deepwater marine environment vs arid continental environment and b the depth temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development chemical reactions associated with smectite rich mud rocks in iran occur shallow 1900 m smectite to illite transformation causing load transfer related moderate overpressures whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore brunei the basal detachment in deepwater brunei generally lies below temperatures of about 150 c where chemical processes and metagenesis are inferred to drive overpressure development overall the deepwater brunei system is very water rich and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines the result is a wide variety of fluid migration pathways and structures from deep to shallow levels particularly mud dykes sills laccoliths volcanoes and pipes fluid escape pipes crestal normal faults thrust faults and widespread inflationary type overpressure in the central basin the near surface environment is water limited mechanical and chemical compaction led to moderate overpressure development above the upper red formation evaporites only below thick early miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms fluid leakage episodes across the evaporites have either been very few or absent in most areas locations where leakage can episodically occur e g detaching thrusts deep normal faults salt welds are sparse however in both iran and brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds

present day stress orientation in brunei a snapshot of prograding tectonics in a tertiary delta the baram delta province of nw borneo is unusual when compared with most other tertiary deltas as it has built up upon an active margin hence structures observed in the baram delta province are the result of both margin parallel gravity driven deltaic tectonics and approximately margin normal transpressive tectonics associated with the active margin image and dipmeter logs have been examined for breakouts and drilling induced tensile fractures ditfs in 47 wells throughout brunei breakouts and ditfs observed in 19 wells suggest that the maximum horizontal stress is oriented margin normal nw se in the proximal parts of the basin and margin parallel ne sw in the outer shelf region the margin parallel outer shelf stress field is interpreted as a local deltaic stress field caused by the shape of the clastic wedge the margin normal maximum horizontal stress in the inner shelf is interpreted to reflect basement stresses associated with the active margin however the maximum horizontal stress in the inner shelf is approximately perpendicular to the strike of miocene pliocene normal growth faults suggesting that maximum horizontal stress in the inner shelf has rotated from margin parallel deltaic to margin normal basement associated over time hence approximately the same stress rotation has occurred over time in the inner shelf as is currently observed spatially from the outer to inner shelf the spatial and temporal stress rotations in brunei are thus interpreted to be the result of deltaic and basement associated tectonic regimes that are prograding basin wards the proximity of the active margin has resulted in progressive uplift and inversion of the hinterland that has forced the delta system to prograde rapidly the zone of active deltaic growth faulting and margin parallel maximum horizontal stress has shifted basin wards prograded as the delta system has rapidly prograded across the shelf after uplift and delta progradation the old growth faults of the inner shelf ceased being active and have then been successively reactivated by a similarly prograding margin normal inversion front

time average equation revisited expressions that relate velocity to porosity and to pore fluid compressibility are among the most important deliverables of rock physics such relations are used often as additional controls for inferring porosity from well logs as well as in situ indicators of pore fluid type the oldest and most popular is the wyllie et al 1956 equation p s f where p is the measured traveltime of a p wave s is the traveltime expected in the solid phase material and f is the traveltime expected in the pore fluid it follows from equation 1 that 1vp 1 vps vpf where is porosity vp is the measured p wave velocity and vps and vpf are the p wave velocities in the solid and in the pore fluid phases respectively

influence of basement structures on in situ stresses over the surat basin southeast queensland the jurassic to cretaceous sedimentary rocks of the surat basin in southeast queensland host a significant volume of coal seam gas resources consequently knowledge of the in situ stress is important for coal permeability enhancement and wellbore stability using wireline log data and direct stress measurements we have calculated stress orientations from 36 wells and stress magnitudes from 7 wells across the surat basin our results reveal a relationship between high tectonic stress and proximity to structures within the underlying basement rocks the influence of tectonic stresses is diminished with depth in areas with thicker sedimentary cover that are relatively far from the basement structures we suggest that this relationship is due to the redistribution of in situ stresses around areas where basement is shallower and where basement structures such as the leichhardt burunga fault system are present this behavior is explained by a lower rigidity in the thickest basin cover which reduces the ability to maintain higher tectonic stress over the entire surat basin a significant amount of variability in in situ stress orientation is observed the authors attribute this stress variability to complex plate boundary interactions on the northern and eastern margins of the indo australian plate

effects of cross shore boundary condition errors in nearshore circulation modeling the paper analyzes the effect which prescribed errors in the cross shore boundary conditions for a computational domain along a beach have on the flow field predicted inside the domain this problem is relevant because errors in boundary conditions are unavoidable when modeling limited domains of a nearshore region for simplicity we consider a longshore uniform plane beach with monochromatic obliquely incident waves and assume depth uniform currents it is then studied analytically and numerically how small perturbations of the boundary conditions along both upstream and downstream cross shore boundaries spread inside the computational domain it is found that the errors at the upstream cross shore boundary tend to spread over a long distance downstream of the boundary while the influence of the errors in the downstream boundary condition is limited to the adjacent upstream area of the computational domain both the numerical and analytical solutions show that the errors introduced at the upstream boundary decay exponentially in the surf zone at a rate proportional to the bottom friction a simple formula is developed to estimate the influence distance of the upstream errors if we consider the mismatch in the volume flux at the upstream boundary the error merely redistributes in the cross shore direction to conserve volume in the case of excessive flux or velocity specified at the cross shore boundaries a circulation cell tends to appear in the offshore region where the errors caused by the boundary mismatch increase with the cross shore width of the model domain

paleostress pattern and salt tectonics within a developing foreland basin north western subhercynian basin northern germany analysing the paleostress field in sedimentary basins is important for understanding tectonic processes and the planning of drilling campaigns the subhercynian basin of northern germany is a perfect natural laboratory to study the paleostress field in a developing foreland basin the simple layer cake geometry of the basin fill is dominated by several piercing and non piercing salt structures we derived the paleostress field from the orientation of fracture sets faults slickensides and stylolites on a regional scale the basin fill is characterized by a horizontal compressional paleostress vector that is mainly nne ssw oriented which reflects the late cretaceous inversion phase in central europe we show that the local paleostress field is distinctly perturbated due to the salt structures along the edge of the salt pillows the maximum horizontal paleostress vector is deflected by up to 90 from the regional trend in the case of the elm salt pillow it forms a radial pattern restoration of balanced cross sections demonstrates at least 9 of the shortening of the north western part of the subhercynian basin was achieved by folding the salt structures in the north western subhercynian basin are the result of varying stress conditions initial extension in the triassic caused first salt movements that prevailed during the jurassic and early cretaceous most important is the late cretaceous contractional phase that shortened the diapirs and led to the formation of the salt pillows between diapirs due to detachment folding we derive four main controlling factors for such salt dominated contractional basins 1 the wedge shape basin fill is the product of the dynamic load at the southern margin of the basin 2 a basal salt layer fed the diapirs and acted as a detachment horizon during the later shortening 3 detachment folding was the dominating deformation mechanism during contraction and 4 the pre existing diapirs controlled the position of the detachment folds

a laboratory study of longshore currents over barred and non barred beaches abstract a detailed description is given of the results of laboratory experiments on wave driven longshore currents on both barred and non barred beaches the objective is to examine the cross shore distribution of the longshore current velocities for purely wave driven currents with emphasis on the position of maximum current velocity with respect to areas where wave energy is dissipated unidirectional obliquely incident waves both regular and random were used the measurements were performed in a large wave basin with a pump recirculation system to create spatially homogeneous longshore currents the experiments yielded information on wave transformation set up of the mean water level and the cross shore distribution of wave driven longshore current velocity a number of cases are presented and compared with each other the measurements show that in the case of purely wave driven longshore currents the maximum current velocities occur close to areas where wave breaking is most intense the effect of mixing bottom friction and wave rollers on the longshore current velocity profile are examined in more detail with help of numerical modelling existing model equations based on the assumption of alongshore uniformity are used the results for the mean longshore current profile on a barred beach are in close agreement with the measurements

nearshore placement of a sand dredged mound abstract as a part of the dredging operations and environmental research doer program movable bed physical model experiments were performed at the u s army engineer research and development centre s erdc large scale sediment transport facility lstf to investigate the fate and evaluate the benefits of nearshore placed dredged material the resulting bathymetry was measured with detailed surveys the migration of the mound was quantified and comprehensive observations of hydrodynamics were obtained the potential suitability of dredged material placement in the nearshore surf zone was demonstrated after 10 h of oblique wave attacks it has been shown that as the mound was located at the edge of the surf zone very likely wave breaking induced horizontal circulation may be dominant a downdrift accretion of the submerged beach was observed which is due to the transport of part of the sediment suspended by breakers at the mound and captured by the longshore currents the experiments provided useful validation data for numerical morphological models

laboratory measurements of uniform longshore currents abstract experiments on uniform longshore currents in a wave basin are described the measurements were performed in a basin with a pumped recirculation through openings in the wave guides minimal return flows in the offshore region of the basin are found to be accompanied with longshore currents which are virtually uniform alongshore three dimensional longshore current velocity distributions were measured with much attention to quality control detailed experimental results are presented for different wave fields two beach slopes and two beach roughnesses

physical characteristics and occurrence rates of meteoric plasma layers detected in the martian ionosphere by the mars global surveyor radio science experiment 1 low altitude plasma layers are present in 71 of 5600 electron density profiles from the martian ionosphere obtained by the mars global surveyor radio science experiment these layers are produced by the ablation of meteoroids and subsequent ionization of meteoric atoms the mean altitude of the meteoric layer is 91 7 4 8 km the mean peak electron density in the meteoric layer is 1 33 0 25 1010 m 3 the mean width of the meteoric layer is 10 3 5 2 km the occurrence rate of meteoric layers varies with season solar zenith angle and latitude seasonal variations in occurrence rate are particularly strong often exceeding an order of magnitude meteoric layer altitude peak electron density and width are all positively correlated with correlation coefficients of 0 3 0 4 other correlation coefficients between the physical characteristics of meteoric layers and atmospheric or observational properties such as scale height solar zenith angle and solar flux have absolute values that are significantly smaller indicating lack of correlation the photochemical lifetime of plasma in meteoric layers is 12 days and depends on altitude

laboratory validation of swash longshore current modelling abstract in this paper the ability of the numerical phase resolving model swash simulating waves till shore to hindcast wave induced longshore currents is evaluated using default settings for all processes modelled highly accurate results are found for wave heights mean water levels and longshore currents while wave current interaction is intrinsically modelled insights into the spatial variation of wave driven longshore currents are found additionally vertical variations of modelled longshore currents have been compared depth uniform profiles of longshore current within surf zone are noted on plane beaches under regular waves except for minor deviations near the shoreline the apparent validity of a depth uniform longshore current encourages the use of a depth averaged moment balance equation to compute the longshore current a simpler model is shown to also be able to predict a proper magnitude of longshore current although the cross shore distribution in contrast with swash needs tuning for the eddy viscosity and the bottom friction coefficient since the distribution of the wave induced longshore current heavily depends on lateral mixing

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

balancing deformation in nw borneo quantifying plate scale vs gravitational tectonics in a delta and deepwater fold thrust belt system abstract recent gps measurements demonstrate that nw borneo undergoes 4 6 mm of plate scale shortening a year which is not accommodated by plate scale structures the only geological structure in nw borneo described to accommodate on going shortening is the baram delta system located on the outer shelf to basin floor delta toe fold thrust belts are commonly thought to be caused by margin normal compressional stresses generated by margin parallel upslope gravitational extension the baram delta system is divided into three neotectonic provinces 1 an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province 2 an outer shelf region of present day deltaic extension and 3 a compressional delta toe however it is uncertain whether compression in the delta toe area is purely driven by gravity i e shortening is less than upslope extension or if there is a component of regional shortening involved which can only be demonstrated if shortening is greater than upslope extension in order to quantify the balance between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic framework in the baram delta system we have used a geomechanical code based on the finite element method and conservation of mass and momentum dynel 2d this enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that the total shortening observed in the delta toe does not balance against the active extension in the delta top with the system exhibiting 1 8 shortening overall or 2 0 km this additional shortening is therefore attributed to plate scale shortening across nw borneo produced by far field compression using a convergence rate of 4 mm y 1 demonstrated by gps measurements the delta toe has accommodated far field compression only in the last 0 5 my

effect of alongshore nonuniformities on longshore current predictions longshore currents have been primarily modeled by assuming alongshore uniformity in this paper we revisit the problem of nonuniform longshore currents to refocus attention on the importance of accounting for alongshore nonuniformity in longshore current predictions and to provide an analytical estimate of the importance of this nonuniformity we derive a semianalytical solution for the longshore currents that allows for weak alongshore variations in the bottom topography the solution shows that the alongshore pressure gradient induced by alongshore variations in the bottom topography could contribute substantially to the forcing for the longshore currents an example calculation shows that the longshore current could deviate by up to 30 from the mean for a 10 deviation of the bottom topography thus we suggest that in many practical cases it is important to include the alongshore pressure gradient to accurately model longshore currents

mars ionosphere a review of experimental results and modeling studies 1 in this paper we review results from atmospheric and ionospheric experiments on the early planetary missions like the mariners mars and viking 1 and 2 orbiters landers we then discuss the new results obtained from the two latest missions namely the mars global surveyor mgs and mars express mex the mgs had three ionospheric and atmospheric related experiments namely 1 the radio science experiment which generated 5600 electron density profiles covering a major portion of sunspot cycle 23 2 the magnetometer electron reflectometer experiment which very clearly answered the question about the presence or absence of martian intrinsic magnetic field and 3 the accelerometer experiment which provided a large database of atmospheric density at various martian locations during the aerobraking phases the topside sounder on the mex provided electron density profiles for altitudes above the primary ionospheric peak with a very high time resolution thereby providing opportunity for exploring ionospheric conditions during events of rapid changes like solar flares unlike venus where simultaneous electron density ion density and magnetic field measurements were made mars lacks this kind of information consequently most of our current understanding of mars plasma environment is based on theoretical models we therefore review the various atmospheric and ionospheric models for mars which have been generated during the last 4 decades

stress determination in active thrust belts an alternative leak off pressure interpretation abstract in thrust belts fluid flow through critically stressed fractures will occur at pressures less than the overburden stress which is the minimum stress we propose that low leak off pressures obtained in active thrust belts may result from this mechanism leading workers to infer that apparent minimum stresses are 30 60 less than the overburden stress in some compressional settings traditionally leak off pressure data have been used to constrain the magnitude of minimum stress assuming that the rock is dilating against the minimum stress during a leak off test in our new interpretation we constrain the stress state by assuming that the leak off test causes shear failure along pre existing weaknesses rather than tensile opening while this mechanism has been discussed in a small number of borehole stability and hydraulic fracture papers it has not been directly applied to leak off tests we considered this interpretation because we observed that some leak off tests imply an apparent contradiction between the stress states from the standard interpretation of leak off tests versus the stress state inferred from geologic and geophysical evidence in tectonically active thrust belts we present two examples with one in an onshore fold thrust belt and one in a deepwater fold thrust belt our new interpretation of stresses based on shear failure resolves the contradiction and also provides a constraint on the maximum horizontal stress in the fold thrust belts

stress magnitudes across uk regions new analysis and legacy data across potentially prospective unconventional resource areas abstract stress magnitude data across the uk is limited spatially and stratigraphically with information available for only 21 sites in the latest release of the world stress map this information is largely derived from geothermal resource exploration and radioactive waste storage site assessment active exploration of unconventional resources in the uk has highlighted a lack of information to adequately characterise the stress field in particular in regions underlain by potentially prospective shale formations understanding the in situ stress conditions is critical to the planning of sub surface operations and the potential extraction of unconventional resources legacy stress magnitude data from 75 sites is combined with new analysis of wireline data to re characterise the stress field across two regions which are underlain by the bowland shale formation which has resource potential for unconventional hydrocarbons these regions are east yorkshire and north nottinghamshire and cheshire and lancashire vertical stress gradients vary between 23 and 26 mpakm 1 for the regions studied pore pressure is similar for both regions and is hydrostatic with a gradient of 10 19 mpakm 1 lower bounds for the minimum horizontal stress have been estimated from the available data and show that the magnitude of the minimum horizontal stress is 2 6 mpakm 1 higher to the east of the pennines the compiled legacy data show that the maximum horizontal stress is consistently greater than the vertical stress which in turn is greater than the minimum horizontal stress indicating that at depth within the two regions the faulting regime is predominantly strike slip

incompatible stress regimes from geological and geomechanical datasets can they be reconciled an example from the carnarvon basin western australia abstract knowledge of the in situ stress field is essential in petroleum basins as it exerts a prime control over seal integrity fracture stimulation wellbore stability and fluid flow previous geomechanical analysis of australia s carnarvon basin the continent s premier hydrocarbon region highlights a normal to strike slip fault stress regime however neotectonic evidence and contemporary seismicity suggest the possibility of a strike slip to reverse fault stress regime we attempt to reconcile these conflicting datasets through new analysis of the in situ stresses principal stress magnitudes and orientations were defined with data from 76 previously unanalysed petroleum wells wellbore image logs yield a maximum horizontal stress orientation of 113 n consistent with modelling of the stress field throughout the indo australian plate vertical stress magnitudes are estimated to range from 20 mpa km 1 to 22 4 mpa km 1 leak off tests lot from 42 wells are analysed using two methodologies one assuming tensile failure as traditionally accepted the other assuming shear failure as outlined by couzens schultz and chan 2010 and result in two different sets of horizontal stress magnitude estimates traditional interpretation of lots results in a minimum horizontal stress gradient of 16 8 mpa km 1 and a maximum horizontal stress gradient of 21 8 mpa km 1 the new method for lot interpretation results in a minimum horizontal stress gradient of 18 1 mpa km 1 and a maximum horizontal stress gradient of 25 4 mpa km 1 inclined natural fractures observed at lot depths supports application of the new method of lot interpretation traditional interpretation of stress magnitudes implies a normal to strike slip fault stress regime while the new method implies a strike slip fault stress regime this latter interpretation is favoured by the authors as it allows for a reconciliation of the geological and geomechanical datasets the assessment undertaken herein allows for reinterpretation of stress magnitudes in seismically active basins hosting differing geomechanical and neotectonic regimes potentially altering existing understanding of seal integrity fracture stimulation wellbore stability and fluid flow

establishing uniform longshore currents in a large scale sediment transport facility a large scale laboratory facility for conducting research on surf zone sediment transport processes has been constructed at the u s army engineer research and development center successful execution of sediment transport experiments which attempt to replicate some of the important coastal processes found on long straight beaches requires a method for establishing the proper longshore current an active pumping and recirculation system comprised of 20 independent pumps and pipelines is used to control the cross shore distribution of the mean longshore current pumping rates are adjusted in an iterative manner to converge toward the proper settings based on measurements along the beach two recirculation criteria proposed by visser coastal eng 15 1991 563 were also used and they provided additional evidence that the proper total longshore flow rate in the surf zone was obtained the success of the external recirculation system and its operational procedure and the degree of longshore uniformity achieved along the beach are the subjects of this paper to evaluate the performance of the recirculation system and as a precursor to sediment transport experiments two comprehensive test series were conducted on a concrete beach with straight and parallel contours 1 30 slope one using regular waves and the other using irregular waves in the regular wave case the wave period was 2 5 s and the average wave height at breaking was approximately 0 25 m in the irregular wave case the peak wave period was 2 5 s and the significant breaking wave height was approximately 0 21 m the longshore current recirculation system proved to be very effective in establishing uniform mean longshore currents along the beach in both cases this facility and the data presented here are unique for the following reasons 1 the high cross shore resolution of the recirculation system and the ease with which changes can be made to the longshore current distribution 2 the degree of longshore uniformity achieved as a percentage of the length of the basin even near the downdrift boundary 3 the scale of the wave conditions generated and 4 the relatively gentle beach slope used in the experiments compared to previous laboratory studies of the longshore current measured data are provided in an appendix for use in theoretical studies and numerical model development and validation

influence of poroelastic behavior on the magnitude of minimum horizontal stress sh in overpressured parts of sedimentary basins in many sedimentary basins of the world the minimum hori zontal stress s h is greater in overpressured zones than in normally pressured zones at equivalent depths a common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress s v and s h within certain bounds and the difference is smaller under lower effective stress i e higher pore pressure p p however in the overpressured parts of the central north sea graben united kingdom and the sable subbasin of the scotian shelf canada conventional friction envelopes underestimate the magnitude of s h these data instead indicate that s h increases at a rate proportional to but less than the rate of increase of p p a condition consistent with a p p induced deformation of the rock called poroelastic behavior this paper argues that whereas friction may govern s h in normally pressured basins poroelastic behavior is responsible for the unusually high s h in the overpres sured parts of these same basins data on the p p and s h gradients from these basins suggest that s h p p 0 7

the interdisciplinary use of overpressure abstract overpressure is a polysemic word that has a variety of meanings within and across different disciplines this is likely to be a particular problem in analysis of geothermal resources where reservoir engineers volcanologists and structural geologists may each confidently use overpressure but mean different things we suggest that to avoid confusion the term should be carefully and accurately defined whenever used and ideally only used to mean fluid pressure in excess of hydrostatic pressure

present day stress orientations and tectonic provinces of the nw borneo collisional margin received 23 september 2009 revised 17 may 2010 accepted 9 june 2010 published 16 october 2010 1 borehole failure observed on image and dipmeter logs from 55 petroleum wells across the nw borneo collisional margin were used to determine maximum horizontal stress sh orientations combined with seismic and outcrop data they define seven tectonic provinces the baram delta deepwater fold thrust belt exhibits three tectonic provinces its inner shelf inverted province sh is nw se margin normal its outer shelf extension province sh is ne sw margin parallel and its slope to basin floor compression province sh is nw se margin normal in the inverted province sh reflects inversion of deltaic normal faults the sh orientations in the extension and compression provinces reflect deltaic gravitational tectonics the shale and minibasin provinces have been recognized in offshore sabah in the shale province sh is n010 e which aligns around the boundary of a massif of mobile shale currently no data are available to determine sh in the minibasin province in the balingian province sh is ese wnw reflecting ese absolute sunda plate motions due to the absence of a thick detachment seen elsewhere in nw borneo the central luconia province demonstrates poorly constrained and variable sh orientations these seven provinces result from the heterogeneous structural and stratigraphic development of the nw borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages

effect of fluid pressure distribution on the structural evolution of accretionary wedges numerical experiments on evolving accretionary wedges usually implement predefined weak basal decollements and constant strength parameters for overlying compressed sequences although fluid pressure ratio and therefore brittle strength can vary strongly in sedimentary basins a two dimensional finite difference model with a visco elasto plastic rheology is used to investigate the influence of different simplified fluid pressure ratio distributions on the structural evolution of accretionary wedge systems results show that a linear increase in fluid pressure ratio towards the base leads to toeward verging thrust sheets and underplating of strata while simulations with a predefined decollement form conjugate shear zones supporting box fold type frontal accretion surface tapers are in agreement with the critical wedge theory which here is modified for cases of varying fluid pressure ratio furthermore the numerical results resemble findings from natural examples of accretionary wedges

examination of the influence of several factors on longshore current computation with random waves abstract influence of various factors affecting the longshore currents induced by obliquely incident random waves is examined through numerical calculation seven numerical models for random wave breaking process are found to yield large differences in the wave heights in the surf zone and longshore current velocities the turbulent eddy viscosity formulation by larson and kraus larson m and kraus n c 1991 numerical model of longshore current for bar and trough beaches j waterway port coastal and ocean eng asce 117 4 pp 326 347 functions almost equal to that by battjes battjes j a 1975 modeling of turbulence in the surf zone proc symp modeling techniques pp 1050 1061 but the formulation by longuet higgins longuet higgins m s 1970 longshore current generated by obliquely incident sea waves 1 and 2 j geophys res 75 33 pp 6779 6801 produces excessive diffusion of longshore currents into the offshore zone the generation and decay process of the surface roller is indispensable in the longshore current analysis the random wave transformation model called pegbis parabolic equation with gradational breaker index for spectral waves by goda goda y 2004 a 2 d random wave transformation model with gradational breaker index coastal engineering journal jsce and world scientific 46 1 pp 1 38 produced good agreement with several laboratory and field data of longshore currents

vertically transferred overpressures in brunei evidence for a new mechanism for the formation of high magnitude overpressure overpressures in sedimentary basins are commonly assumed to be the result of two distinct and separate mechanisms disequilibrium compaction and fluid expansion however the potential for overpressures to be redistributed or transferred to other pressure compartments over time has been seldom considered and rarely demonstrated pore pressure data and velocity effective stress plots from 61 fields across the baram delta province of brunei northwest borneo reveal that two different types of overpressure occur in stratigraphically defined sections the basal pro delta shales contain overpressures generated by disequilibrium compaction whereas the overlying sand shale deltaic sequence contains overpressures that appear to be generated by fluid expansion however the geology of the deltaic sequences and high magnitude of the pore pressures precludes the overpressures in the deltaic sequences being generated by any conventional fluid expansion mechanism such as kerogen to gas maturation or clay diagenesis the fluid expansion overpressures are located in fields that were inverted during the pliocene an event that resulted in large scale fluid migration from the pro delta shales into the deltaic sequences including charging of the numerous oil fields in the inner shelf hence we propose that the overpressures in brunei provide the first evidence for a new overpressuring mechanism whereby overpressured fluids have been vertically transferred from the pro delta shales into the deltaic sequences during basin inversion furthermore vertical transfer may be a mechanism for explaining overpressures observed in basins that have been recently uplifted or inverted

a numerical model of beach morphological evolution due to waves and currents in the vicinity of coastal structures a numerical model was developed of beach morphological evolution in the vicinity of coastal structures the model includes five sub models for random wave transformation surface roller development nearshore wave induced currents sediment transport and morphological evolution the model was validated using high quality data sets obtained during experiments with a t head groin and a detached breakwater in the basin of the large scale sediment transport facility at the coastal and hydraulics laboratory in vicksburg miss usa the simulations showed that the model reproduced well the wave conditions wave induced currents and beach morphological evolution in the vicinity of coastal structures both salient and tombolo formation behind a t head groin and a detached breakwater were simulated with good agreement compared to the measurements c 2011 elsevier b v all rights reserved

evolution of shear bands and cracks in multi stage triaxial tests with water saturated sandstone a study of micro tectonics with a fractal perspective abstract a series of multi stage triaxial compression tests revealed quite chaotic reactions of water saturated permotriassic sandstones on changing pore water pressures not before focussing the various test stages with respect to occurring critical phenomena which strike the fabric structures due to their fractal nature during any triaxial testing eventually an approach became tractable pore systems with cracks and shear bands can be widened in a quasi static way but only up to a stability limit thereafter the fabric collapses with a seismogenic contraction of the pore system causing a rise of the pore water pressure changes of structural patterns occur as formation either of fissures up to collapse with acoustic emission or of a rather fractal pattern of crossing shear bands generated without audible noise but with fine grit due to comminution dilation and contraction of the pore system are visible from overall volume changes both modes of pattern formation can be understood as critical phenomena of the mineral solid scale independent features of these changes of fabric patterns can be transferred to the meso and macro scales in the field and in the lithosphere respectively however questions of scale dependent features related to cohesion should be further investigated

a numerical model of nearshore waves currents and sediment transport a two dimensional numerical model of nearshore waves currents and sediment transport was developed the multi directional random wave transformation model formulated by mase mase h 2001 multidirectional random wave transformation model based on energy balance equation coastal engineering jounnal 43 4 317 337 based on an energy balance equation was employed with an improved description of the energy dissipation due to breaking in order to describe surface roller effects on the momentum transport an energy balance equation for the roller was included following dally and brown dally w r brown c a 1995 a modeling investigation of the breaking wave roller with application to cross shore currents journal of geophysical research 100 c12 24873 24883 nearshore currents and mean water elevation were modeled using the continuity equation together with the depth averaged momentum equations sediment transport rates in the offshore and surf zone were computed using the sediment transport formulation proposed by camenen and larson camenen b larson m 2005 a general formula for non cohesive bed load sediment transport estuarine coastal and shelf science 63 249 260 camenen b larson m 2007 a unified sediment transport formulation for coastal inlet application technical report erdc chl cr 07 1 us army engineer research and development center vicksburg ms camenen b larson m 2008 a general formula for non cohesive suspended sediment transport journal of coastal research 24 3 615 627 together with the advection diffusion equation whereas the swash zone transport rate was obtained from the formulas derived by larson and wamsley larson m wamsley ty 2007 a formula for longshore sediment transport in the swash proceedings coastal sediments 07 asce new orleans pp 1924 1937 three high quality data sets from the lstf experimental facility at the coastal and hydraulics laboratory in vicksburg usa were used to evaluate the predictive capability of the model good agreement between computations and measurements was obtained with regard to the cross shore variation in waves currents mean water elevation and sediment transport in the nearshore and swash zone the present model will form the basis for predicting morphological evolution in the nearshore due to waves and currents with special focus on coastal structures c 2009 elsevier b v all rights reserved less