Generation of overpressure and compaction-driven fluid flow in a Plio-Pleistocene growth-faulted basin, Eugene Island 330, offshore Louisiana

The complex pressure and porosity fields observed in the Eugene Island (EI) 330 field (offshore Louisiana) are thought to result from sediment loading of low-permeability strata. In this field, fluid pressures rise with depth from hydrostatic to nearly lithostatic, iso-pressure surfaces closely follow stratigraphic surfaces which are sharply offset by growth-faulting, and porosity declines with effective stress. A one-dimensional hydrodynamic model simulates the evolution of pressure and porosity in this system. If reversible (elastic) compaction is assumed, sediment loading is the dominant source of overpressure (94%). If irreversible (inelastic) compaction and permeability reduction due to clay diagenesis are assumed, then thermal expansion of pore fluids and clay dehydration provide a significant component of overpressure (>20%). The model is applied to wells on the upthrown and downthrown sides of the major growth fault in the EI 330 field. Assuming that sediment loading is the only pressure source and that permeability is a function of lithology and porosity, the observed pressure and porosity profiles are reproduced. Observation and theory support a conceptual model where hydrodynamic evolution is intimately tied to the structural and stratigraphic evolution of this progradational deltaic system.

     

Role of pore pressure generation in sediment transport within half-grabens

Sediment transport and overpressure generation are coupled primary through the impact of effective stress on subsidence and compaction. Here, we use mathematical modeling to explore the interactions between groundwater flow and diffusion-controlled sediment transport within alluvial basins. Because of lateral variation in permeability, proximal basin facies will have pore pressure close to hydrostatic levels while distal fine-grained facies can reach near lithostatic levels. Lateral variation in pore pressure leads to differential compaction, which deforms basins in several ways. Differential compaction reduces basin size, bends isochron surfaces across the sand–clay interface, restricts basinward progradation of sand facies, and reduces the amplitude of oscillation in the lateral position of the sand–clay interface especially in the deepest part of the section even when temporal sediment supply are held constant. Overpressure generation was found to be sensitive to change in sediment supply in permeable basins (at least 10⁻¹⁷ m² in our model). We found that during basin evolution, temporal variations in overpressure and sediment supply fluctuations are not necessarily in phase with each other, especially in tight (low permeability) basins (<10⁻¹⁷ m² in our model).     

Towards ground truthing exploration in the central Arctic Ocean: a Cenozoic compaction history from the Lomonosov Ridge

The Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), recovered the first Cenozoic sedimentary sequence from the central Arctic Ocean. ACEX provided ground truth for basin scale geophysical interpretations and for guiding future exploration targets in this largely unexplored ocean basin. Here, we present results from a series of consolidation tests used to characterize sediment compressibility and permeability and integrate these with high‐resolution measurements of bulk density, porosity and shear strength to investigate the stress history and the nature of prominent lithostratigraphic and seismostratigraphic boundaries in the ACEX record. Despite moderate sedimentation rates (10–30 m Myr^?1) and high permeability values (10^?15–10^?18 m²), consolidation and shear strength measurements both suggest an overall state of underconsolidation or overpressure. One‐dimensional compaction modelling shows that to maintain such excess pore pressures, an in situ fluid source is required that exceeds the rate of fluid expulsion generated by mechanical compaction alone. Geochemical and sedimentological evidence is presented that identifies the Opal A–C/T transformation of biosiliceous rich sediments as a potential additional in situ fluid source. However, the combined rate of chemical and mechanical compaction remain too low to fully account for the observed pore pressure gradients, implying an additional diagenetic fluid source from within or below the recovered Cenozoic sediments from ACEX. Recognition of the Opal A–C/T reaction front in the ACEX record has broad reaching regional implications on slope stability and subsurface pressure evolution, and provides an important consideration for interpreting and correlating the spatially limited seismic data from the Arctic Ocean.     

Origin of early overpressure in the Upper Devonian Catskill Delta Complex, western New York state

The Upper Devonian Rhinestreet black shale of the western New York state region of the Appalachian Basin has experienced multiple episodes of overpressure generation manifested by at least two sets of natural hydraulic fractures. These overpressure events were thermal in origin and induced by the generation of hydrocarbons during the Alleghanian orogeny close to or at the Rhinestreet's ～3.1 km maximum burial depth. Analysis of differential gravitational compaction strain of the organic‐rich shale around embedded carbonate concretions that formed within a metre or so of the seafloor indicates that the Rhinestreet shale was compacted ～58%. Compaction strain was recalculated to a palaeoporosity of 37.8%, in excess of that expected for burial >3 km. The palaeoporosity of the Rhinestreet shale suggests that porosity reduction caused by normal gravitational compaction of the low‐permeability carbonaceous sediment was arrested at some depth shy of its maximum burial depth by pore pressure in excess of hydrostatic. The depth at which the Rhinestreet shale became overpressured, the palaeo‐fluid retention depth, was estimated by use of published normal compaction curves and empirical porosity‐depth algorithms to fall between 850 and 1380 m. Early and relatively shallow overpressuring of the Rhinestreet shale likely originated by disequilibrium compaction induced by a marked increase in sedimentation rate in the latter half of the Famennian stage (Late Devonian) as the Catskill Delta Complex prograded westward across the Appalachian Basin in response to Acadian tectonics. The regional Upper Devonian stratigraphy of western New York state indicates that the onset of overpressure occurred at a depth of ～1100 m, well in advance of the Rhinestreet shale's entry into the oil window during the Alleghanian orogeny.     

Modelling of pore pressure evolution associated with sedimentation and uplift in sedimentary basins

Unconformities, which represent either periods of interruption of sedimentation or, in most cases events characterized by deposition and subsequent erosion, are commonplace geological phenomena in sedimentary basins, and will affect the pore pressure evolution of the basin fill. The effect of unconformities on pore pressure, as well as on sediment compaction and on burial processes is studied using a numerical basin model. For coarse sediments, which are permeable so that their pore pressure always remains nearly hydrostatic, the effects of both pure deposition interruption (hiatus) and deposition-erosion events are negligible for pore pressure evolution. However, for fine-grained sediments, unconformities can modify the pore pressure and the stress state to varying degrees. The results show that the rate of removal of overlying sediments, the permeability of sediments and time play important roles in the pore pressure evolution. In the East Slope of the Ordos Basin (China), in which overpressure has not been detected in deep wells, the modelling results suggest that the large-scale erosion occurring in the Late Cretaceous and in the Tertiary may have removed high overpressure existing in the basin before the erosion.     

Overpressures and hydrocarbon generation in the Sable sub-basin, offshore Nova Scotia

One of several interconnected depocentres lying offshore eastern Canada, the Sable sub-basin preserves a thick sequence of Mesozoic-Cenozoic clastic sediments, significant gas accumulations and an extensive development of abnormal pressures. In order to understand the basin's hydrocarbon generation, migration and accumulation history it is necessary to quantify the interplay between its burial, thermal, and maturation history, and to determine the influence on these of the basin's excess pressure history. Simple, one-dimensional reconstructions of maturity and pore pressure histories are derived for exploration well and pseudo-well locations on a seismic line running from the basin's structural high to its depocentre. Calibrated, where possible, by reference to measured maturity, temperature and pressures, these models provide a basic dynamic framework within which it is possible to consider the generation history of the basin's source rocks. Late Jurassic to Early Cretaceous sediments underwent an initial rapid, rift-related subsidence. The thermal/maturation models suggest that source rocks lying within these intervals quickly matured and began generating gas and condensates. Similarly, this rapid burial was translated, through sediment compaction disequilibrium processes, into an early expression of abnormal pressures. The pore pressure/time reconstructions in the modelling assume that sediment compaction disequilibrium and gas generation are the principal causal mechanisms. Matching pore pressure reconstructions with present-day pressure-depth profiles is particularly sensitive to assumed seal permeability profiles. Although the seal permeabilities used as model input are based on actual measured permeabilities at the present day, this does not mean that the permeability-time curves derived through the model's decompaction assumptions accurately reflect seal permeability evolution.     

Fractal Approach,Analysis of Images and Diagenesis in Pore Space Evaluation

Simple net model constructed by authors, facies analysis and compaction models, were applied to evaluate reservoir properties of sandstone facies of the Carpathian Flysch (the Istebna sandstones). The applied net model was built on the base of fractal approach proposed by Don Turcotte in 1977 and computer analysis of images. Laboratory measurements include porosity, density, permeability to nitrogen, mercury injection capillary pressure tests, and microscopic analysis of thin sections. D.W. Houseknecht's theory, proposed in 1987, was applied for compaction and cementation modeling. The residual saturation data were used to validate obtained results. Net model allows an evaluation of filtration properties of analyzed sandstones and to distinguish the classes of similarity of pore space. The extracted parameters of classes of similarity were correlated with facies scheme of the investigated geological structure. Influence of compaction and cementation on pore space parameters was discussed.     

Fluid flow in sedimentary basins: model of pore water flow in a vertical fracture

Open fractures provide high-permeability pathways for fluid flow in sedimentary basins. The potential for flow along permeable or open fractures and faults depends on the continuity of flow all the way to the surface except in the case of convective flow. Upward flowing fluid cools and may cause cementation due to the prograde solubility of quartz, but in the case of carbonates such flow may cause dissolution. The rate and duration of these processes depend on the mechanisms for sustaining fluid flow into the fracture, the geometries of fracture and sedimentary beds intersected, permeability, pressure and temperature gradients. Heat loss to the adjacent sediments causes sloping isotherms which can induce non-Rayleigh convection. To analyse these problems we have used a simple model in which a single fracture acts as a pathway for vertically moving fluid and there is no fluid transport across the walls of the fracture except near its inlet and outlet. Four mechanisms for fluid flow into the lower part of the fracture are considered: decompression of pore water; compaction of intersected overpressared sediments; focusing of compaction water derived from sediments beneath the fracture; and finally focusing of pore water moving through an aquifer. Water derived from the basement is not considered here. We find that sustained flow is unlikely to have velocities much higher than 1–100 m/yr, and the flow is laminar. The temperature of the fluid expelled at the top of the fracture increases by less than 1% and the vertical temperature gradient in the fracture remains close to the geothermal gradient. Where hot water is introduced from basement fractures (hydrothermal water) during tectonic deformation, much higher velocities may be sustained in the overlying sediments, but here also this depends on the permeability near the surface. Most of the cooling of water with (ore) mineral precipitation will then occur near the surface. In most cases, pore water decompression and sediment compaction will yield only very limited pore water flux with no significant potential for cementation or heating of the sediments adjacent to the fracture. Focusing of compaction water from sediments beneath the fracture or from an intersected aquifer can yield fluxes high enough to cement an open fracture significantly but the flow must be sustained for a very long time. For velocities of 1–100 m/yr, it takes typically 0.3–30 Myr to cement a fracture by 50%. The highest velocities may be obtained when a fracture extends all the way to the surface or sea floor. When a fracture does not reach the sediment surface, the flow velocity is reduced by the displacement of water in the sediments near the top of the fracture. The flow into the fracture from the sediments may often be rate limiting rather than the flow on the fracture. Sedimentary rocks only a few metres from the fracture will receive a much lower flux than the fracture. The fracture will therefore close due to cementation before significant amounts of silica can be introduced into adjacent sandstones. The isotherm slope in the adjacent sediments will in most cases be less than 10–20°. Non-Rayleigh convection velocities in the sediments adjacent to the fracture are too small to cause any significant diagenetic reactions such as quartz cementation. These quantifications of fluid flow in fractures in sedimentary basins are important in terms of constraining models for diagenesis, heat transport and formation of ore minerals in a compaction-driven system.     

Geological constraints of pore pressure detection in shales from seismic data

Methods for detection of pore fluid overpressures in shales from seismic data have become widespread in the oil industry. Such methods are largely based on the identification of anomalous seismic velocities, and on subsequent determination of pore pressures through relationships between seismic velocities and the vertical effective stress (VES). Although it is well known that lithology variations and compaction mechanisms should be accounted for in pore pressure evaluation, a systematic approach to evaluation of these factors in seismic pore pressure prediction seems to be absent. We have investigated the influence of lithology variations and compaction mechanism on shale velocities from acoustic logs. This was performed by analyses of 80 wells from the northern North Sea and 24 wells from the Haltenbanken area. The analyses involved identification of large‐scale density and velocity variations that were unrelated to overpressure variations, which served as a basis for the analyses of the resolution of overpressure variations from well log data. The analyses demonstrated that the overpressures in neither area were associated with compaction disequilibrium. A significant correlation between acoustic velocity and fluid overpressure nevertheless exists in the Haltenbanken data, whereas the correlation between these two parameters is weak to non‐existing in the North Sea shales. We do not presently know why acoustic velocities in the two areas respond differently to fluid overpressuring. Smectitic rocks often have low permeabilities, and define the top of overpressures in the northern North Sea when they are buried below 2 km. As smectitic rocks are characterized by low densities and low acoustic velocities, their presence may be identified from seismic data. Smectite identification from seismic data may thus serve as an indirect overpressure indicator in some areas. Our investigations demonstrate the importance of including geological work and process understanding in pore pressure evaluation work. As a response to the lack of documented practice within this area, we suggest a workflow for geological analyses that should be performed and integrated with seismic pore pressure prediction.     

Modeling Permeability Changes Caused by Hydrothermal Circulation

A new model for simulating porosity and permeability changes caused by sedimentary reservoirsdiagenesis is presented. Permeability is computed from changes in the mineral volume fractionsresulting from precipitation and dissolution of the rock-forming mineral as fluid flows throughvariable salinity and temperature fields. Its evolution is controlled by a power—law relationship,in which a weighting coefficient is assigned to clay minerals. This approach allows theincorporation of the widely observed influence of clay content on the porosity—permeabilityrelationship. A synthetic example is set up to analyze the sensitivity of the results to a set offour controlling parameters: the effect of the clay-weighting coefficient compared to the effectof the salinity gradient, temperature gradient, and exponent coefficient of the permeabilityevolution law. Using a large range of values for these parameters, the results show that theirinfluence is of equivalent magnitude in terms of permeability evolution rate. It also seemsthat the value of the clay-weighting coefficient affects the evolution trend: permeability mayincrease when the porosity decreases (and vice versa). The model is compared to the classicalapproach for which permeability is a function of porosity change only. Results display thestrong influence of even low values of the clay-weighting coefficient on the permeabilitychange. Consequently, the specific influence of mineral transfers on pore structure changes isa key parameter for modeling permeability changes and cannot be bypassed by the use ofsimple porosity—permeability evolution law.     

A disequilibrium compaction model constrained by seismic data and application to overpressure generation in The Eastern Black Sea Basin

Locating and quantifying overpressures are essential to understand basin evolution and hydrocarbon migration in deep basins and thickly sedimented continental margins. Overpressures influence sediment cohesion and hence fault slip in seismically active areas or failure on steep slopes, and may drive catastrophic fluid expulsion. They also represent a significant drilling hazard. Here, we present a method to calculate the pore pressure due to disequilibrium compaction. Our method provides an estimate of the compaction factor, surface porosity and sedimentation rate of each layer in a sediment column using a decompaction model and the constraints imposed by seismic data and geological observations. For a range of surface porosities, an ad hoc iterative equation determines the compaction factor that gives a calculated layer thickness that matches the observed thickness within a tolerance. The surface porosity and compaction factor are then used to obtain a density profile and a corresponding estimate of P‐wave velocity (V_p). The selected parameters are those that give a good match with both the observed and calculated layer thicknesses and V_p profiles. We apply our method to the centre of the Eastern Black Sea Basin (EBSB), where overpressures have been linked to a low‐velocity zone (LVZ) at ca. 5500–8500 m depth. These overpressures were generated by the relatively high sedimentation rate of ca. 0.28 m ka^?1 of the low permeability organic‐rich Maikop formation at 33.9–20.5 Ma and an even higher sedimentation rate of ca. 0.85 m ka^?1 at 13–11 Ma. We estimate a maximum pore pressure of ca. 138 MPa at ca. 8285 m depth, associated with a ratio of overpressure to vertical effective stress in hydrostatic conditions () of ca. 0.7. These values are lower than those presented in a previous study for the same area.     

Tectonic compaction shortening in toe region of isolated listric normal fault,North Taranaki Basin,New Zealand

Industry 2D and 3D seismic data across the North Taranaki Basin displays two listric normal faults that formed during Pliocene shelf edge clinoform progradation. The faults die out in the down‐transport direction with no evidence for contractional structures, except for two small thrust faults in one narrow zone. When active, the detachments lay at depths of about 1000 m below the seafloor. The overlying section had high initial porosities (30–60%). It is estimated that loss of about 17–20% pore volume by lateral compaction, and fluid expulsion over a distance of about 4–6 km in the transport direction occurred in place of folding and thrusting. Seismic and well evidence for abnormally highly compacted shales suggests there is about 6% less porosity than expected for in the prekinematic section, which possibly represents a residual of the porosity anomaly caused by lateral compaction. The observations indicate significant shortening (~20%) by lateral compaction and probably some layer parallel thickening are important deformation mechanisms in near‐surface deepwater sediments that needs to be incorporated into shortening estimates and ‘balanced’ cross‐sections. A key factor in listric fault initiation near the base of slope is inferred to be transient, increased pore fluid pressure due to lateral expulsion of fluids from beneath the prograding Giant Foresets Formation.     

Effects of biogenic silica on sediment compaction and slope stability on the Pacific margin of the Antarctic Peninsula

Analysis of physical properties measured on cores and on discrete samples collected by the Ocean Drilling Programme (ODP) Leg 178 on the Pacific margin of the Antarctic Peninsula reveals anomalous down‐hole curves of porosity, density, water content, and P‐wave velocity. These indicate an overall trend of increasing porosity with depth and suggest that the drifts are mostly undercompacted. In one of the two boreholes analysed, a sharp decrease in porosity, matching increasing bulk sediment density and increasing compressional velocity occurs towards the base of the hole, which corresponds to a bottom‐simulating reflector in the seismic section. Analysis of seismic reflection, down‐hole logging, geotechnical and mineralogical data from two drilling sites indicates that the observed anomalous consolidation trends are a consequence of the presence of biogenic silica (diatom and radiolarian skeletons) even with a small to moderate amount. Above the bottom‐simulating reflector, intergranular contacts among whole or broken siliceous microfossils prevent normal sediment consolidation. Diagenetic alteration of biogenic opal‐A to opal‐CT causes a dramatic reduction of intra‐ and interskeletal porosity allowing sediments to consolidate at depth. This results in overpressuring and a decrease in the effective stress. Excess fluids are expelled towards the sediment surface through near vertical, small throw normal faults extending from the diagenetic front to the seafloor and affecting the stability of the submarine slope in the form of gravitational creep along a weakened surface. This work shows how physical properties of shallow fine‐grained marine sediments can be analysed as basin‐wide indicators of biogenic silica abundance. The diagenetic alteration of siliceous microfossils is a possible cause of slope instability along world continental margins where bottom‐simulating reflectors related to silica diagenesis are present at a regional scale.     

Seal capacity estimation from subsurface pore pressures

A cap rock's capacity to seal hydrocarbons depends on its wettability and the sizes of the pore throats within the interconnected pore system that the leaking hydrocarbons must penetrate. These critical pore throat sizes are often poorly constrained in hydrocarbon exploration, partly because measurements of pore throat sizes have not been performed, and partly because pore throat measurements on a few individual samples in the cap rock may not be representative for the seal capacity of the top seal as a whole. To the contrary, the presence of formation overpressure can normally be estimated in drilled exploration targets. The presence of overpressure in reservoirs testifies to small pore throats in the cap rocks, as large pore throats will result in sufficiently high cap rock permeability to bleed off the overpressure. We suggest a stepwise procedure that enables quantification of top seal capacities of overpressured traps, based on subsurface pressure information. This procedure starts with the estimation of cap rock permeabilities, which are consistent with observed overpressure gradients across the top seals. Knowledge of burial histories is essential for these estimations. Relationships between pore throat size and permeability from laboratory experiments are then applied to estimate critical pore throat diameters in cap rocks. These critical pore throat diameters, combined with information of the physical properties of the pore fluids, are then used to calculate membrane seal capacity of cap rocks. Estimates of top seal capacity based on this procedure are rather sensitive to the vertical fluid velocity, but they are also to some extent sensitive to inaccuracies of the pore throat/permeability relationship, overpressure gradient, interfacial tensions between pore fluids, hydrocarbon density and water viscosity values. Despite these uncertainties, applications of the above‐mentioned procedure demonstrated that the mere presence of reservoir overpressures testifies to sufficient membrane seal capacity of cap rocks for most geological histories. Exempt from this statement are basins with rapid and substantial sediment compaction in the recent past.     

Subsidence, sedimentation and sea-level changes in the Eromanga Basin, Australia

Abstract The Jurassic-Cretaceous subsidence history of the Eromanga Basin, a large intracratonic sedimentary basin in central eastern Australia, has been examined using standard backstripping techniques, allowing for porosity reduction by compaction and cementation. Interpretation of the results suggests that during the Jurassic the basin was subsiding in a manner consistent with the exponentially decreasing form predicted by simple thermally based tectonic models. By the Early Cretaceous, the rate of subsidence was considerably higher than that expected from such models and nearly half of the total sediment thickness was deposited over the final 20 Myr of the basin's 95 Myr Mesozoic depositional history. The Early Cretaceous also marks the first marine incursion into the basin, consistent with global sea-level curves. Subsequently, however, the sediments alternate between marine and non-marine, with up to 1200 m of fluvial sediments being deposited, and this was followed by a depositional hiatus of about 50 Myr in the Late Cretaceous. This occurred at a time when global sea-level was rising to its peak. A model is presented which is consistent with the rapid increase in tectonic subsidence rate and the transgressive-regressive nature of the sediments. The model incorporates a sediment influx which is greater than that predicted by the thermally based tectonic models implied by the Jurassic subsidence history. The excess sedimentation results in the basin region attaining an elevation which exceeds that of the contemporary sea-level, and thereby giving the appearance of a regression. The present day elevation of the region predicted by the model is about 100–200 m above that observed. This discrepancy may arise because the primary tectonic subsidence is better represented by a linear function of time rather than an exponentially decreasing form.     

Simulation of diagenesis and permeability variation in two-dimensional rock structure

A model is suggested to simulate the physical aspect of diagenesis in porous rocks. A bidisperse ballistic deposition model with relaxation of deposited grains is used to generate the porous structure. Sedimentation and erosion are allowed to restructure the pore space as a fluid flows through the rock. The effect of this restructuring of the pore space on permeability is studied. The Navier–Stokes equation is solved numerically by the finite difference method to determine the pressure and velocity distributions in the pore space. We find that though deposition is the dominant process in our model of diagenesis, reducing the porosity, the permeability may increase dramatically in some cases. These are when the erosion takes place at a single narrow constriction in the pore channel.     

A simple model of pressure build-up caused by porosity reduction during burial

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

A new theoretical treatment of compaction and the advective-diffusive processes in sediments: a reviewed basis for radiometric dating models

Classical treatment of mass conservation of solids in growing sediments states an advection-diffusion equation for the bulk sediment density. Thus, the diffusion coefficient has to account for elemental processes of exchange of solid particles by pore water or reciprocally. Nevertheless, in a gravity field, these exchanges are forced and cannot be treated as diffusion but as mass flow. A compaction potential energy is defined so that its spatial gradients force a mass flow involving a conductivity function. This leads to a more consistent definition of mass sedimentation rates and to a writing of the continuity equation for density involving only an advection term. Typical bulk density profiles show an asymptotic increase with depth. With the present formulation, this can be obtained as a steady-state solution under constant sedimentation rate and constant conductivity, while the classical formulation fails to do it. Alternatively, it can be found that under these conditions, the compaction potential is a linear function of the bulk density. The mass flow due to compaction and the compaction potential are found for several sediment cores from literature data. From this basis, the advection-diffusion equation for a particle-associated tracer is rewritten. In particular, when the mass flow term due to compaction is considered, the resulting sub-grid scale processes lead to a different formulation of the diffusive fluxes: they are proportional to the gradients of specific concentration in solids instead of the concentration per unit bulk volume. This new formulation is most suitable to find out analytical solutions for radiometric dating models involving mixing and compaction. Numerical solutions are found for the new and the classical treatment for some particular cases to illustrate differences.     

黄河口不同粒度泥沙沉积与扩散分析

本文采集和收集大量黄河三角洲沉积物剖面和钻孔泥沙粒度资料以及黄河口来沙粒度组成数据,定量研究了黄河口泥沙的沉积与扩散特征.结果 显示黄河口来沙以粉砂为主,黏土次之,砂最少,年均中值粒径无长期变化趋势.黄河三角洲平原相泥沙与来沙的黏土、粉砂和砂含量无显著差异;前缘相泥沙比来沙的黏土含量较低,砂含量较高;前三角洲相泥沙比来...     

The acquisition of post-depositional detrital remanent magnetization in a variety of natural sediments

Summary. Post-depositional detrital remanent magnetization (pDRM) is the primary means whereby many sediments acquire their palaeomagnetic signal. We have studied the acquisition of this magnetization in a variety of natural sediments. Our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field. Most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments. Grain sizes were measured by the settling tube method which led to the determination of the clay, silt and sand fractions in each sediment. Isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite. In the pDRM acquisition studies two distinct modes of behaviour were found. For sediments with a sand content less than 60 per cent, the original direction of magnetization was preserved regardless of the water content. Such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid-filled voids and hence are able to realign along a new magnetic field direction. For sediments with a sand content in excess of 60%, remagnetization along a new magnetic field direction occurred as expected, provided the sediments were sufficiently wet. Studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older, desiccated sediments.