contribution of peat compaction to relative sea level rise within holocene deltas 1 modern and forecasted flooding of deltas is accelerated by subsidence of holocene deposits subsidence caused by tectonics isostasy sediment compaction and anthropogenic processes combined with eustatic sea level rise results in drowning and increased flood risk within densely populated deltas many deltaic sedimentary successions include substantial amounts of peat which is highly compressible compared to clay silt and sand peat compaction therefore may contribute considerably to total delta subsidence existing studies are inadequate for quantifying peat compaction across deltas we present a numerical peat compaction model calibrated with an extensive field dataset the model quantifies spatial and temporal trends in peat compaction within fluvial dominated holocene flood basin sequences of different compositions subsidence due to peat compaction is highly variable in time and space with local rates of up to 15 mm yr depending on sedimentary sequence this is extremely important information for developing sound delta management strategies artificial groundwater table lowering may cause substantial additional subsidence subsidence due to peat compaction might even exceed estimates of relative sea level rise and thus may seriously increase the risk of delta drowning and human vulnerability to flooding

postorogenic denudation along the late paleozoic ouachita trend south central united states of america magnitude and timing constraints from apatite fission track data the magnitude and timing of synorogenic and postorogenic denudation along fold and thrust belts and their associated foreland basins of interest because of both tectonic and economic implications is generally poorly constrained along the late paleozoic ouachita trend a thin veneer of cretaceous strata is preserved above a low relief erosional surface that beveled the ouachita orogen and adjacent foreland areas this regional erosion surface provides a valuable constraint for interpreting new and previously published apatite fission track aft data obtained from exposed structural highs along the ouachita trend marathon llano arbuckle and benton uplifts aft data from sampled localities within the deformation belt marathon and benton uplifts exhibit younger ages and generally longer mean lengths than data from localities on the foreland side of the deformation front llano and arbuckle uplifts this observation suggests that erosion of the orogen rather than its extensional collapse was the primary mechanism responsible for flexural isostatic unloading of the foreland crust in addition all samples show evidence for mild reheating following their pre cretaceous cooling history specifically the lack of a significant population of 14 5 m tracks in all samples appears to require residence at temperatures of 55 5 c after development of the sub cretaceous erosional surface this implies that 1000 m of cretaceous paleogene strata were deposited across the entire ouachita frontal trend and subsequently removed during later tertiary time this tertiary denudation is interpreted to reflect the interplay between regional denudation and isostatic compensation in response to slow 10 m m y epeirogenic uplift of the southern midcontinent and a long term drop 200 m in eustatic sealevel during this time

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

the rate of pressure dissipation from abnormally pressured compartments an improved analytic solution is presented that describes the dissipation of abnormal pressures via darcy flow out of a compartment formed of high permeability rock capped top and bottom by lower permeability rock the solution tends to the solutions presented by previous workers in the limit of 1 a thick compartment and thin barrier and 2 a thick barrier and a thin compartment the importance of including fluid compressibility when analyzing hydrocarbon reservoirs is demonstrated previous workers in this field have only included the effect of bulk rock compressibility and have neglected fluid compressibility the solution is applied to pressure compartments on the scale of a typical hydrocarbon reservoir and overpressured regions on the basin scale it is shown that the pressure gradient in a typical hydrocarbon reservoir compartment will return to the hydrostatic gradient over timescales of the order of hours or days but that abnormal pressures are likely to dissipate over periods of tens of thousands to hundreds of thousands of years thus any reservoir compartment with a different pressure from its neighbors at discovery can be considered as an independent unit during production however abnormal pressures on the basin scale may take tens or hundreds of millions of years to dissipate there is thus no need to invoke zero permeability seals or capillary pressure seals in order to explain the existence of abnormal pressures over geological time

morphology of red creek wyoming an arid region anastomosing channel system red creek in the red desert area of the great divide basin wyoming is an arid region anastomosing stream the narrow deep and sinuous main channel is flanked by anastomosing flood channels or anabranches most anabranches are initiated at meander bends the primary mechanism of anabranch initiation is avulsion during overbank floods anabranch enlargement occurs by headward erosion anabranches act as distributary channels during floods when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches during periods of low discharges the anabranches act as tributaries to the main channel transporting runoff from the floodplain and surrounding hillslopes to the main channel of red creek aggradation is occurring in the main channel and on the floodplain throughout the study reach infilling of the main channel occurs primarily by lateral accretion while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches infilling of the main channel may cause avulsion of the main channel into an anabranch the abandoned main channel segment may then fill completely or act as an anabranch because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment periodic avulsion is the primary form of lateral mobility in the system

effects of peat on the shapes of alluvial channels examples from the cumberland marshes saskatchewan canada abstract an avulsion of the lower saskatchewan river in the 1870s inundated a large segment of peat covered floodplain that subsequently has become aggraded with a broad 500 km 2 belt of alluvium deposited by the redirected flow routing of water and sediment discharge through the avulsion affected area has been accomplished mainly by networks of sandy bedded anastomosed channels that have formed evolved and abandoned as the alluvial belt prograded down the floodplain slope these processes continue today though at a much reduced rate new channels formed by crevassing and basinward extension of distributaries are initially small and shallow with bottom elevations situated within the avulsive alluvium but above the pre avulsion peat floodplain surface subsequent enlargement and downcutting of many of these channels eventually uncovers the underlying peat layer whose resistance to erosion exerts significant influence on cross sectional shape and further channel development peat floored channels tend to have rectangular cross sections high ratios of average to maximum depth d d max and a large range of width to depth ratios if the channel continues to enlarge the peat layer eventually becomes breached commonly leading to temporarily irregular cross sections caused by localized scouring at the breach sites eventually the peat layer is completely eroded from the channel floor by undercutting and slumping after which channel shape becomes governed mainly by other perimeter characteristics channels unaffected by peat either before the peat layer is encountered during early channel development or after it is entirely removed tend to have low width depth ratios and a large range of d d max values

overpressure compartments in the central paleo uplift sichuan basin southwest china the sichuan basin is a prime gas producing basin in china besides the giant neoproterozoic weiyuan gas field discovered in the central paleo uplift an extra giant gas reservoir in the cambrian in this area has been confirmed in recent years although the lower paleozoic in the southwest weiyuan area is normally pressured it is overpressured in the central area moxi gaoshiti area combined with the seal distribution five pressure systems including three overpressure systems can be divided based on drill stem test mud weight and sonic transit time data overpressures appear at depth interval of 1500 to 4900 m 4921 to 16 076 ft approximately benefiting from the good sealing capacity of the gypsum in the upper and middle triassic high overpressure pressure coefficient r 2 0 has been preserved in the lower triassic and the upper triassic and cambrian are moderately overpressured 1 3 r 1 7 mechanisms for various overpressure systems are different abnormally high sonic transit time in the permian indicates disequilibrium compaction overpressure the analyses of sonic transit time effective stress suggest that disequilibrium compaction is the primary mechanism for the overpressure in the upper triassic xujiahe formation but the cambrian overpressure system is predominantly associated with fluid expansion which mainly resulted from gas generation furthermore we consider that the late thermal cracking of oil to gas is a key factor for gas and overpressure preservation in old strata the high overpressure in the lower triassic marine carbonate rocks was caused by oil cracking and gypsum dehydration mechanisms combining the origin analysis with the burial and hydrocarbon generation histories we constructed the cambrian pore pressure evolution model which is characterized by roughly normal pressure before 200 ma overpressuring from 200 to 90 ma and overpressure releasing since 90 ma

reservoir compartmentalization get it before it gets you abstract this paper examines the impact of compartmentalization on oil recovery the importance of identifying it during field appraisal and methods to evaluate it using fluid data the impact on recovery factor is highlighted using a global database of oil field recovery factors as a function of reservoir complexity and compartmentalization and emphasized in two case studies the effect of compartmentalization on oil recovery demonstrates the benefit in characterizing compartmentalization correctly during appraisal so that the field can be developed in an optimal manner early characterization of field compartmentalization requires making maximum use of available fluid data during appraisal when interpreting fluid data to identify compartmentalization it is critical to take into account the different time scales for various fluid signals pressure contacts density composition to equilibrate and to be able to extrapolate to field production time scales this is essential to avoid false negatives compartments assumed absent due to homogeneous fluid properties when in fact fluids would have equilibrated even in the presence of compartments false positives where fluid differences are interpreted as evidence of compartments when in fact there has not been sufficient time for equilibration to occur and to resolve apparently conflicting data some fluid indicators are at equilibrium others are not rigorous simulation of fluid equilibration is a complex multiphase multidimensional process and is generally reserved for specialist in depth studies however order of magnitude evaluations can be made using analytical solutions in minutes allowing many what if scenarios to be considered and uncertainty to be assessed analytical solutions for estimating the time required for spatially varying fluid properties to revert to steady state distributions are reviewed all these mixing processes are shown to be diffusive in character an effective diffusion coefficient for each process can be calculated from the reservoir rock and fluid properties for an isothermal system the different time scales and distances for each fluid property variation to attain equilibrium can be compared on a single graph where the time elapsed since fluid perturbation is known analytical solutions can be used to estimate the degree of compartmentalization e g permeability of barriers these solutions lend themselves to the development of simple practical compartment assessment tools for industry practitioners

morphology hydrology and evolution of the anastomosing ovens and king rivers victoria australia anastomosing channels form the characteristic stream pattern of the riverine plain of southeastern australia investigations of the ovens and king rivers in northeastern victoria show that the individual channels and reaches of channels vary markedly in channel morphology and age young channels have large bankfull flows low sinuosity large meander wavelength steep gradients and are unstable they are incised channels that are rapidly eroding their bed and banks old channels however have small bankfull flows high sinuosity small meander wavelength gentler gradients and are relatively stable the multiple channels develop by avulsion as individual channels become older and more sinuous they also become more hydraulically inefficient thus increasing proportions of flood discharge are displaced overbank this overbank flow concentrates in relatively straight flood plain depressions and it erodes a new channel which develops by both up and down valley incision down valley incision is initiated at outflow points and up valley incision is initiated where the overbank flow rejoins the main channel with time both channel segments join to form a new anabranch of the anastomosing channel system large anastomosing channel networks have been identified in ancient rocks by stratigraphers and sedimentologists on the basis of studies of the anastomosing ovens and king rivers it is very likely that these channel networks are composed of channels of different ages hence even at one stratigraphic level not all of the anabranches functioned simultaneously nevertheless as a result of anabranch incision which intersects older channels at depth a three dimensional network of paleochannels can form that permits fluid flow throughout the network

eocene hydromorphic paleosols significance for interpreting ancient floodplain processes abstract pervasive mottling iron oxide nodules and slickensides indicate that alluvial paleosols of the lower eocene willwood formation in the elk creek area of the bighorn basin wyoming were hydromorphic two basic groups of hydromorphic paleosols are present cumulative which show evidence for concurrent deposition and pedogenesis and simple noncumulative which show evidence for a single episode of deposition and pedogenesis lithologic packages consisting of vertically stacked simple paleosols and crevasse splay sandstones alternate with cumulative paleosols the simple paleosols are weakly developed indicating rapid deposition with relatively little time for pedogenesis cumulative paleosols show greater pedogenic development indicating significantly longer periods of pedogenic m dification and slower more episodic sediment accumulation differences between the simple and cumulative paleosols reflect differences in the deposition of their parent materials the packages of simple paleosols are similar to avulsion belt deposits described from the saskatchewan river and suggest that avulsion was significant in floodplain construction the upper parts of cumulative paleosols were formed on true overbank deposits which accumulated gradually after avulsion and establishment of a new channel the lower parts of many cumulative paleosols resulted from a prior episode of avulsion deposition and simple soil formation because hydromorphy is associated with texture these different depositional processes influenced lateral variations in hydromorphy of the cumulative paleosols paleosols in the elk creek area provide a simple means of identifying avulsions sediment accumulation rates suggest that avulsions occurred with a regular period of about 20 000 years and may have been induced by climatic cycles

overpressures in the anadarko basin southwestern oklahoma static or dynamic fluid pressures were estimated from mud weights and wellhead shut in pressures along a north south linear trend through the anadarko basin in southwest oklahoma fluid pressures above hydrostatic start at a depth of about 2 5 km on a normalized scale where hydrostatic pressure equals 0 and lithostatic pressure equals 1 estimates of fluid pressures were found to range from 0 4 to 0 8 over a depth range of 4 7 km using scale analyses and a simple numerical model we evaluated two end member hypotheses compaction disequilibrium and hydrocarbon generation as possible causes of overpressuring in the anadarko basin if compaction disequilibrium is the primary cause of present day overpressures in the anadarko basin the anadarko basin is required to have an average permeability of 10 23 m2 10 8 md or lower if geopressures in the anadarko basin result from hydrocarbon generation average basin permeabilities can be as high as 10 21 m2 10 6 md scaling these average permeabilities down to thicknesses of 100 m or less implies permeabilities lower than 10 25 m2 10 10 md are required in the most optimistic scenario the lowest permeabilities ever measured on sedimentary rocks are in the neighborhood of 10 22 10 23 m2 10 7 10 8 md thus it is not possible to reconcile the existence of overpressures in the anadarko basin with classic hydrodynamic theories that maintain that aquicludes do not exist either some unknown process is generating excess fluid pressures or the anadarko basin contains pressure seals geopressure models that invoke compaction disequilibrium and are commonly applied to cenozoic basins undergoing rapid sedimentation e g the gulf coast basin of the southeastern united states do not appear to apply to the anadarko basin the anadarko basin belongs to a group of cratonic basins that are tectonically quiescent and characterized by the association of abnormal pressures with natural gas begin page 146

variation of system openness and geochemical features in overpressured sandstones of the yinggehai basin offshore south china sea abstract hydrocarbon discoveries in recent years in the upper miocene of the yinggehai basin show differences in overpressure magnitude gas and mineral components overpressure close to or reaching the leakage threshold has induced high system openness for one field but the overpressure of another field remains to be safely closed a new vision to investigate the interplay between the system openness and the gas chemical and mineralogical features were conducted the gas components in the high openness system are rich in high maturity hydrocarbons and inorganic co2 which both have heavier carbon isotopes low maturity gases with lighter carbon isotopes are dominant in the low openness system the abnormal carbon isotope ratio between iso butane and n butane in the high openness system confirms that early low maturity leakage has generated abnormal compositional and isotopic fractionation although the sedimentation process was uniform in the two systems the high openness system has rare k feldspar grains and chlorite but it is rich in illite smectite mixed layers i s precipitation is characterized by ankerite and kaolinite in the high openness system whereas the low openness system primarily consists of calcite the strong dissolution of k feldspar grains and chlorite should have released abundant potassium and silicon the high abundances of i s and kaolinite which are both sensitive to potassium and the rarity of quartz overgrowths in the high openness system imply that the leakage of potassium silicon from the sandstones has induced a mass imbalance between dissolution and precipitation even the sandstone porosity was slightly increased by the mass imbalance in the high openness system the risk of hydrocarbon expulsion during brine leakage cannot be ignored

hydrodynamic aquifer or reservoir compartmentalization changes in oil water contact owc depth across a field are often a signature of reservoir compartmentalization as a result of faulting or stratigraphic barriers such as shales but may equally well be caused by an underlying hydrodynamic aquifer in particular the pressure depth data obtained from an aquifer whose flow is changing over time can look very similar to that obtained from a compartmentalized reservoir misunderstanding which of these mechanisms causes the observed changes in owc across the field may result in poor estimates of oil in place and reduced recovery to address this problem an analytic expression is presented to estimate the time taken for a steady state tilted owc to be established once an aquifer starts flowing a comparison with simulations of hydrodynamic aquifers in homogeneous compartmentalized and heterogeneous reservoir models shows that this expression can be used in combination with the one derived by m k hubbert for the steady state tilt of the owc to clarify whether a reservoir contains barriers or baffles to flow or may simply have not yet reached equilibrium

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

fluvial aeolian interactions part i modern systems two modern fluvial aeolian depositional systems great sand dunes national monument colorado and the mojave river wash california are remarkably similar in spite of different climates sizes fluvial sediment textures and relative directions of aeolian and fluvial transport dune growth and migration and deflation of blowouts create 8 10 m of local relief in unflooded aeolian landscapes there are six prominent fluvial aeolian interactions 1 fluvial flow extends into the aeolian system until it is dammed by aeolian landforms 2 interdune areas overbank interdunes upstream of aeolian dams and alongside channels are flooded 3 water erodes dunes alongside channels and interdunes 4 flood waters deposit sediment in interdune areas 5 fluvially derived groundwater floods interdunes interdune playas 6 wind erodes fluvial sediment and redeposits it in the aeolian system unique and characteristic sediments are deposited in overbank interdunes and in interdune playas reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions these fluvial aeolian interdune deposits are characterized by irregular concave up bases and flat upper surfaces containing mudcracks or evaporite cement characteristic low relief surfaces form in aeolian systems as an effect of flooding fluvial deposits are resistant to aeolian deflation aeolian sand is preserved when flood sediments are deposited around the bases of the dunes thus repetitive fluvial and aeolian aggradation tends to be stepwise as interdune floors are suddenly raised during floods the effects of flooding should be easy to recognize in ancient aeolianites even beyond the area covered with overbank muds

avulsion deposits in lower eocene alluvial rocks bighorn basin wyoming abstract eocene alluvial rocks in the northern bighorn basin can be subdivided into three major kinds fine grained deposits on which cumulative paleosols developed lithologically heterogeneous packages associated with immature paleosols and major sheet sandstone bodies the heterogeneous packages which alternate vertically with the cumulative paleosols are dominated by fine grained deposits and also contain ribbon sandstone bodies and thin 1 m sheet sandstone bodies most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages cut and fill deposits with fine grained fills are locally important because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine grain d facies they are referred to as simple paleosol packages the major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits contrary to classical models of meandering streams only a portion of the fine grained rocks in the study area were deposited by overbank flooding of a trunk channel on the basis of their stratigraphic position and few ribbon sandstones the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel farther from the trunk channel overbank floods deposited alluvium on which the cumulative paleosols formed the greater parts of the simple paleosol packages including their fine grained deposits were deposited rapidly during channel avulsion ribbon sandstone bodies represent ancient splay channels on the avulsion belt simple paleosol packages in the willwood formation support the contention of smith et al 1989 that n many meandering river systems a significant portion of the fine grained deposits were deposited as the trunk channel avulsed deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences

mechanisms of anabranch avulsion within gravel bed braided rivers observations from a scaled physical model the physical modelling of gravel bed braided rivers using a 1 20 froude scale model permits analysis of the types and relative occurrence of different avulsion mechanisms anabranch avulsion within braided rivers involves three main mechanisms choking avulsion caused by blockage of one channel by a sediment lobe constriction avulsion produced by deflection confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends each avulsion mechanism is described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers three factors are found to control anabranch avulsions namely flow discharge sediment flux and floodplain topography these are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium

natural levee deposition during the 2005 flood of the saskatchewan river abstract in summer 2005 a controlled flood of the saskatchewan river east central saskatchewan canada resulted in general floodplain inundation and extensive natural levee deposition along a 60 km reach extending from 40 km below the e b campbell dam to cumberland lake levee crests along channel banks were inundated for up to 7 weeks in some areas of the floodplain new deposits on levee crests varied from 0 to 70 cm in thickness displaying large variations both along reach and in opposing sites across channels mean grain sizes mainly silt and very fine sand likewise varied considerably among sample sites pre and post flood surveys of channel cross sections along the flooded reach permitted assessments of relationships between channel area changes and patterns of levee sedimentation in this system in which virtually all new flood sediment was derived by channel scour results show that both net deposition and net erosion occurred within the channel cross sections but that on average net channel enlargement of 4 2 prevailed over the entire survey reach when weighted by cross section size over the 60 km flooded reach zones of thick levee deposition occur at or just downstream of two areas of major channel enlargement and an intermediate zone of thin levee deposits is associated with an intermediate area of net channel aggradation this bimodal distribution of flood deposit thickness is inferred to have resulted from differences in sediment supply produced locally by the different extents of channel perimeter erosion two other factors 1 position of interfacial zones between clear floodbasin water and turbid channel water and 2 difference in pre flood levee heights contributed to the poor correspondence in thickness and grain size between opposing levees at some sites additional features of the new levee deposits including increases in transverse slopes abrupt basinward fining and paucity of deposition in distal areas due to clear floodbasin waters are characteristic of strong front loading that results when suspended sediment production is restricted to channel erosion processes

overpressure dissipation mechanisms in sedimentary sections consisting of alternating mud sand layers abstract in stable subaqueous environments sedimentary deposition often results in thick mudstone rich successions with mainly argillaceous sediments containing some intercalated sandstone beds and siltstones laminae such depositional sequences are commonly hosting abnormally pressured fluids in some part of their structure abnormally pressured compartments generally overpressured ones are indicative of effective sealing conditions along their boundaries in order to prevent the dissipation of hydraulic pressure this work focuses on the various mechanisms of overpressure dissipation in such overpressured mudstone dominated systems the study is based on the mathematical and numerical simulation of hydraulic phenomena occurring in various geometries of sandstone mudstone series using 1d models it is shown that once overpressure is developed in such a compartment it can be kept inside for a very long time even if no other pressure process is acting a basic mechanism is that the mudstone sandstone succession reacts actively to pressure dissipation when overpressure is released outward of the compartment subsequent compaction tends to occur in the neighbouring sediments this in turn results in a continued pressure generation and tends to maintain the overpressure in the compartment at geological time scale natural hydraulic fracturing tends to dissipate overpressure but it does not release the entire overpressure because fracturing is a threshold mechanism which keeps the stress level in the seals at their hydraulic fracturing limit the opening of faults may modify the pressure pattern through the rapid generation of efficient fluid flow paths however its effect is a quasi instantaneous release of overpressures in sandstones but not in mudstones 2d numerical simulation suggests that once the fault is closed the overpressures remaining in mudstones are rapidly transmitted into adjacent sandstones which then tend to recover their previous high pressure the most efficient mechanisms of overpressure dissipation in mudstone overpressured compartments are large scale and long term tectonic uplift and resulting erosion of overburdens studies on the conditions and mechanisms of overpressure dissipation in mudstone rich successions provide insight into processes of hydrocarbon migration and accumulation processes and the efficiency of cap rock in basins that have experienced multiple episodes of tectonic disturbance

hydrology and hydraulic properties of a bedded evaporite formation the permian salado formation in the delaware basin of new mexico is an extensively studied evaporite deposit because it is the host formation for the waste isolation pilot plant a repository for transuranic wastes geologic and hydrologic studies of the salado conducted since the mid 1970 s have led to the development of a conceptual model of the hydrogeology of the formation that involves far field permeability in anhydrite layers and at least some impure halite layers pure halite layers and some impure halite layers may not possess an interconnected pore network adequate to provide permeability pore pressures are probably very close to lithostatic pressure in the near field around an excavation dilation creep and shear have created and or enhanced permeability and decreased pore pressure whether flow occurs in the far field under natural gradients or only after some threshold gradient is reached is unknown if far field flow does occur mean pore velocities are probably on the order of a meter per hundreds of thousands to tens of millions of years flow dimensions inferred from most hydraulic test responses are subradial which is believed to reflect channeling of flow through fracture networks or portions of fractures that occupy a diminishing proportion of the radially available space or through percolation networks that are not saturated fully interconnected this is probably related to the directional nature of the permeability created or enhanced by excavation effects inferred values of permeability cannot be separated from their associated flow dimensions therefore numerical models of flow and transport should include heterogeneity that is structured to provide the same flow dimensions as are observed in hydraulic tests modeling of the salado formation around the wipp repository should also include coupling between hydraulic properties and the evolving stress field because hydraulic properties change as the stress field changes

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

middle and late holocene avulsion history of the river rhine rhine meuse delta netherlands at present little evidence is available on avulsion rates avulsion frequencies and interavulsion periods of aggrading fluvial systems over time scales of millenia the large number of high quality 14c ages from the holocene rhine meuse delta central netherlands enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency 14c histogram analysis indicates that instantaneous avulsion coexistence of successive channel belts for less than a few centuries has dominated in this area interavulsion periods were typically 1000 yr although values of 500 or 5000 yr have also been found avulsion frequency was especially large prior to 4300 cal yr b p indicating a strong control by aggradation rate in turn controlled by sea level rise in addition periods with high avulsion frequency appear to correlate with time intervals characterized by large scale transgressive tendencies the time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style

post orogenic exhumation of an auriferous terrane the paleoplacer potential of the early carboniferous st marys basin canadian appalachians the st marys basin of mainland nova scotia canada consists of late devonian early carboniferous clastic rocks of the horton group deposited in the waning stages of the ca 420 360 ma acadian orogeny in the canadian appalachians clast lithologies and lithogeochemical analyses indicate that the detritus was predominantly derived from the meguma terrane that occurs to the south of the basin the meguma terrane contains abundant mesothermal gold deposits that are coeval with peak magmatic activity from ca 380 to 370 ma and underwent rapid uplift and erosion between ca 370 and 360 ma within the st marys basin the contact between the lacustrine little stewiacke river formation and the fluviatile barrens hills formation is interpreted to represent a shoreline and a potentially favorable environment for depositing paleoplacer gold geochemical analyses of lithologies adjacent to this contact indicate that the siltstones are predominantly derived from meguma terrane metasedimentary rocks whereas the sandstones and conglomerates are predominantly derived from meguma terrane granitoids geochemical and mineralogical analysis indicate the accumulation of heavy minerals including zircon and gold micron scale 150 m gold identified in the matrix of the conglomerates has a nuggety appearance and flakey microtexture indicative of a detrital origin this observation indicates that the mesothermal deposits were exhumed by the latest devonian consistent with regional tectonic syntheses that invoke rapid uplift of the meguma terrane following peak orogenic activity this study suggests that favorable depositional environments for paleoplacer deposits may occur in horton group rocks that are derived from the meguma terrane

dissipation of anomalous pressures in the subsurface 1 zones of anomalous pressure higher and lower than hydrostatic pressure have been observed in many sedimentary basins around the world these normally consist of groups of pressure compartments volumes of higher permeability rock surrounded on all sides by lower permeability barriers knowledge of the timescales over which these abnormal pressures are maintained and the mechanisms by which they dissipate is critical for understanding how fluids such as oil and gas move in the subsurface existing analytic solutions investigate pressure dissipation through low permeability barriers on top of or underneath an isolated pressure compartment there are no analytic solutions describing pressure dissipation through lateral barriers such as faults or investigating the impact of groups of pressure compartments on the rate of pressure dissipation this paper presents simple analytic models to investigate pressure dissipation through barriers such as faults forming the sides of pressure compartments the timescales are compared with a solution for pressure dissipation through barriers on top of and underneath the compartment it also investigates analytically the rate of pressure dissipation from groups of pressure compartments lateral seal permeabilities of 10 19 m2 may delay pressure equilibration for millions of years provided the compartment has a sufficiently high fluid storage capacity factors contributing toward a high fluid storage capacity include a high fluid compressibility as is the case in hydrocarbon reservoirs and a high porosity the grouping of abnormally pressured compartments into megacompartment complexes may delay pressure dissipation for hundreds of millions of years

a numerical study of sediment transport and event bed genesis during tropical storm delia some event beds tempestites are thought to be emplaced on shallow marine shelves by the combined action of strong coastal currents and high waves during fairly short lived storms to test this hypothesis a storm sedimentation system has been constructed from six numerical models describing a cyclonic wind field three dimensional coastal circulation wind waves generated over the continental shelf the combined effect of steady and oscillatory currents within the benthic boundary layer suspended and bed load transport of sediment and conservation of the seafloor this model system is used to hindcast winds currents waves and resulting sedimentation during tropical storm delia which passed over the texas louisiana shelf on september 3 4 1973 sensitivity to the initial substrate is investigated in four experiments using uniform silt uniform sand a mud line at the 20 m isobath and a simplified modern sediment distribution modeled coastal currents are vertically uniform and do not reveal the structure predicted by the mid latitude geostrophic storm circulation model because the predicted depth of the wind mixed layer is greater than the water depth over the shelf shelf currents in excess of 2 m s flow predominantly along the coast to the southwest during most of the storm driven by the wind stress and the trapped coastal wave which peaks at 180 cm near galveston significant wave heights reach 8 m on the outer shelf but are less than 4 m over the inner shelf these waves combine with steady currents to produce bed shear velocities which locally exceed 20 cm s the region of highest stresses always lies to the right of the storm track viewed down the path and moves across the shelf with the eye of the storm three general sediment transport paths are evident 1 onshore transport of finer sediment over the outer shelf to the right of the storm track 2 westward directed along shelf transport of predominantly fine sediment between approximately 40 m and 20 m water depths and 3 minor offshore transport of sand from the shoreface to depths less than 30m the resulting event bed has a ragged appearance with a maximum thickness of about 20 cm in region 1 and covers an area of approximately 3 104 km2 to the right of the storm track aside from local transport associated with finer sediments these results are relatively insensitive to initial sediment type comparison of model results to observed data from buccaneer platform shows that the different models performed adequately during the peak of the storm except for a significant underprediction of the significant wave height by the wind sea model the estimated uncertainty in the calculated combined shear stresses u based on errors produced by the individual model components is most dependent on the wave bottom orbital amplitude the total uncertainty in u is estimated to be approximately 7

avulsion and anastomosis in the panhandle region of the okavango fan botswana abstract gps water surface elevation data from the panhandle region of the okavango fan northwestern botswana show that a prominent anastomosed reach occurs within a relative valley gradient depression inferred to represent a small graben structure the okavango river undergoes an abrupt change in pattern as it approaches the upstream fault zone of the graben developing small meander loops and higher sinuosity in response to increased valley slope induced by faulting four channels split from and ultimately rejoin the okavango trunk channel to define the anastomosed reach the oldest filipo channel diverted from the okavango on the upstream horst block rejoining the trunk channel 26 km downstream within the graben the younger channels b c and d all developed subsequent to the filipo avulsion by forming linkages between the okavango trunk and floodplain tributaries that had evolved by headward extension and incorporation of older abandoned channel segments diversion of the three younger anastomosed channels was likely abetted by increased aggradation of the trunk channel following down faulting neotectonic movement although involving only slight displacement is seen to be the underlying cause of avulsion anastomosis and certain channel pattern changes in the okavango panhandle this contrasts with other parts of the okavango fan system where avulsion appears to result more from autogenic in channel sedimentation conditioned by various roles of vegetation

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

four storm event beds and the tropical cyclones that produced them a numerical hindcast a three dimensional numerical prediction system for storm sedimentation has been constructed to compute a cyclonic wind field coastal circulation storm waves generated over the continental shelf the combined effects of steady currents and waves on the benthic boundary layer both suspended and bed load transport of sediment and conservation of the sea floor it has been used to hindcast the oceanographic and sedimentological responses of the western gulf of mexico to four historical tropical cyclones in order to investigate the effects of coastal configuration and storm variability on event bed genesis the simulations reveal several common responses to these storms 1 onshore flow to the fight of the storm track generally transports fine sediment landward 2 off shore flow to the left transports coarser sediment seaward and 3 for an observer facing the coast right to left along shelf flow transports finer sediment in deep water and coarser sediment in shallow water the main source of sandy sediment on the inner shelf is the shoreface which is eroded by currents driven by wind stress geostrophy and offshore flows associated with collapse of the coastal setup coastal geometry is the dominant factor in determining sedimentation patterns along the coast in front of each storm the volume of sediment transported obliquely onshore or offshore is a function of shelf gradient and coastal configuration steeper gradients constrain flow to a more longshore pattern concave coastlines promote greater shoreface erosion because of increased coastal setup none of the simulations reveals the vertical current structure predicted by the geostrophic model for continental shelf circulation because the calculated depth of the wind mixed layer exceeded local water depth everywhere over the shelf thus in these experiments most of the continental shelf fell within the friction dominated zone in which the upper and lower boundary layers overlap resulting in vertically uniform along shelf flow

factors controlling peat compaction in alluvial floodplains a case study in the cold temperate cumberland marshes canada abstract subsidence due to peat compaction may have important implications in alluvial floodplains because it leads to wetland loss land inundation and damage to buildings and infrastructure furthermore it potentially influences spatial and temporal river sedimentation patterns and hence the evolution of alluvial floodplains as the vast majority of peatlands occur in cold temperate regions alluvial plains situated in such areas are most susceptible to these implications to determine which locations within alluvial floodplains are most vulnerable to high amounts of compaction induced subsidence it is necessary to quantify compaction and to identify which factors influence the amount and rate of peat compaction for this we carried out field work in the cumberland marshes east central saskatchewan canada guided by the following two questions 1 how much peat compaction has occurred over the past decades to centuries and 2 which factors control the amount and rate of peat compaction at the same timescale to address these questions we used methods involving construction of stratigraphic cross sections organic matter content and bulk density measurements and radiocarbon dating a new sampling device was devised to sample uncompacted peat in the field results show that peat layers have compacted up to 43 within a few centuries with compaction rates of up to 6 08 mm yr the dominant factors influencing peat compaction are 1 organic matter content 2 stress imposed on a peat layer and to a lesser extent 3 plant species composition in an alluvial setting crevasse splays and natural levees are sites that are most susceptible to high amounts of peat compaction at short timescales 100 102 years sheet like splay deposits initially compact underlying peat uniformly whereas differential compaction commonly occurs beneath natural levees due to lateral variations in sediment thickness subsidence due to peat compaction creates additional accommodation space and hence locally enhances floodplain sedimentation rates

paleohydrology of the delaware basin western texas overpressure development hydrocarbon migration and ore genesis this study integrates quantitative modeling techniques with field observations to establish a paleohydrologic framework of the delaware basin western texas the reconstructed paleohydrologic models allow for a better understanding of the development and maintenance of anomalous overpressures hydro carbon generation and migration and ore genesis in the basin results of numerical modeling show that disequilibrium compaction and oil generation might generate excess fluid pressures during the late permian in response to the rapid deposition of evaporite beds the preservation of this overpressure to the present however requires the presence of an extremely low permeability 10 11 d top seal most shaly sediments with permeability ranging from 10 4 to 10 8 d thus may be too permeable by several orders of magnitude to preserve overpressure for more than 250 m y the predicted present day gas window is located within the overpressure zone suggesting that the volume increase associated with the oil to gas conversion may be attributed to present overpressures the native sulfur deposits likely formed in a fluid mixing zone resulting from the laramide uplift of the western basin during the tertiary in our model meteoric water recharged along the basin s uplifted western margin and discharged basinward hydrocarbons migrated landward by pressure gradients and buoyancy and discharged upward along faults in the western basin where they mixed with meteoric water many oil and mineral reservoirs may have formed in the fluid mixing zone where extensive chemical reactions take place in the culberson sulfur ore district for example fluids including hydrocarbons and meteoric water migrated upward through faults from underlying carrier beds into the permian salado limestone there the mixture of fluid drives biochemical reactions that precipitate native sulfur

fine grained splay deposition in the avulsion belt of the lower saskatchewan river canada abstract in or about 1883 a northward avulsion of the meandering lower saskatchewan river at the cumberland marshes initiated a broad belt of alluvial sedimentation that has continued to evolve to the present and now covers some 500 km2 the avulsion belt comprises a complex alluvial terrain dominated by individual and coalesced splays and associated wetlands connected by networks of active and abandoned channels of various sizes the windy lake splay a predominantly fine grained feature typical of large parts of the avulsion belt was examined to investigate the relationships between splay evolution and facies development the splay now largely inactive mostly formed over a 35 year period depositing nearly 2 m of sediment over a 7 2 km2 area the splay surface is ch racterized by stable well defined anastomosed distributary channels that separate interchannel wetland basins heights of channel levees diminish downstream and the levees merge with channel mouth bars whose progradation forms the locus of lakeward channel extension and the base for subsequent subaerial levee deposition the anastomosed channel pattern was created when the mouth bars of separate distributary channels merged during progradation vibracore and soil probe profiles show the splay to be dominated by upward coarsening lacustrine prodeltaic mouth bar facies sequences capped by levees and organic rich interchannel wetland deposits channels now containing sand only sporadically in narrowed or slightly migrated reaches will eventually fill with fine sediment when abandonment s complete the windy lake splay is typical of stage iii splays smith et al 1989 whose deposits and associated lacustrine fills form at least half of the avulsion belt thin widespread predominantly upward coarsening sequences capped by either nondepositional horizons or true overbank fines may be characteristic of many avulsive suites in the ancient alluvial record

the 1870s avulsion of the saskatchewan river several major avulsions of the saskatchewan river have occurred in the cumberland marshes east central saskatchewan during the past few thousand years the most recent avulsion occurred in about the 1870s converting over 500 km2 of floodplain into a belt of anastomosing channels splay complexes and small lakes a region that is still evolving today the avulsion began near the tip of a large meander bend by following a small outflowing creek sturgeon river which in turn followed an abandoned former channel of the saskatchewan river flow began to permanently divert out of the saskatchewan when a narrow strip of floodplain separating the sturgeon river from the nearby torch river became breached diversion into the connected sturgeon torch began to increase sometime in the 1870s and probably culminated around 1882 the triggering event for the avulsion may have been a chute cutoff of the meander bend shown by numerical modeling experiments to have significantly raised water surface elevations at th

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

late quaternary palaeolakes rivers and wetlands on the bolivian altiplano and their palaeoclimatic implications drill cores of sediments from the rio desaguadero valley bolivia provide new information about the climate of tropical south america over the past 50 000 years the modern rio desaguadero is fed by lake titicaca overflow and by local tributaries in the wetter northern altiplano and discharges into lake poopo in the more arid central altiplano during the late quaternary the rio desaguadero valley was the site of several generations of palaeolakes and wetlands that formed during periods of increased precipitation and local runoff augmented by increased overflow from lake titicaca sediments recovered by drilling in eight localities along the 390 km long valley of the rio desaguadero yield a regional history of lacustrine sedimentation and effective precipitation lacustrine strata in the drill cores record 12 distinct wet periods in the past 50 000 years four of these wet periods resulted in the formation of major palaeolakes in the rio desaguadero valley during the last glacial maximum from before 20 000 to 16 000 cal yr bp during the late glacial from about 14 000 to 12 000 cal yr bp in the early holocene from about 10 000 to 7900 cal yr bp and in the late holocene from 4500 cal yr bp to present the period that appears to have been most arid was between 7900 and 4500 cal yr bp the altiplano wet periods were generally synchronous with north atlantic cold events respectively the last glacial maximum the younger dryas the 8200 cal yr bp event and the neoglacial implying a relationship between past precipitation variability in tropical south america and north atlantic sea surface temperature copyright 2005 john wiley sons ltd

disequilibrium compaction as the cause for cretaceous paleogene overpressures in the danish north sea cretaceous paleogene overpressure distribution in the danish central graben shows a remarkable coincidence with the thickness of the rapidly deposited middle miocene to holocene succession slow deposition of smectite dominated clays in a deep marine environment occurred from the late paleocene until the middle miocene and the resultant mudstone succession constitutes the main barrier that delays pressure dissipation between the late miocene and the holocene the upper cretaceous paleogene succession became overpressured probably because of accelerated depositional rates quantification of this disequilibrium compaction mechanism relies mainly on a determination of permeability and effective compressibility of the paleogene shales this article shows that realistic permeabilities can be assumed provided that compressibilities describing the plastic process of compaction are used in the pressure equation instead of the elastic compressibilites that for instance can be derived from log data one dimensional 1 d modeling is applied in two cases a well from the dan chalk field where accelerated deposition since the tortonian 11 2 ma produced a present day overpressure of 7 97 mpa 1156 psi and a well from the south arne chalk field where accelerated deposition since the early serravallian 14 6 ma produced a present day overpressure of 13 9 mpa 2016 psi this is based on an identical set of parameters and compares with the observed 7 7 and 14 8 mpa 1117 and 2147 psi overpressure at the two locations the modeled development of the pressure profiles shows that an effective stress minimum occurred in the upper part of the paleogene succession this is consistent with the observed ubiquitous intraformational faulting at that level about 80 of the added neogene load is estimated to have been converted to overpressure

sealing efficiency of shales the sealing efficiency of shale layers is studied through the hydraulic parameters which are required for sustaining overpressure during geologically significant periods assuming a 1d sedimentary complex composed of a sealing layer overlying a permeable one we give approximate solutions for the dissipation of a given initial overpressure the time constant for relaxation involves the thickness hydraulic conductivity and specific storage of the seal and also those of the permeable layer the values of the various hydraulic parameters are discussed it is argued first that the specific storage corresponding to plastic deformation during burial compaction is larger than the one which would correspond to elastic deformation when taking into account i plausible values for specific storage of the upper shale layer and ii the storage effect of the lower permeable layer it is found that a shale layer of several tens of m with an hydraulic conductivity of the order of 10 15 ms 1 maintains overpressure for 1 myr for such hydraulic parameter values in the absence of on going pressuring forces initial overpressures would decay with a time constant corresponding to a decrease by a factor e 2 7 of 1 myr this is interpreted as supporting a dynamic origin for observed abnormal pressures