Sonic Velocity Data Research Articles

Estimating pore‐space gas hydrate saturations from well log acoustic data

Relating pore‐space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment. In the present study, sonic velocities of gas hydrate–bearing sands are modeled using a three‐phase Biot‐type theory in which sand, gas hydrate, and pore fluid form three homogeneous, interwoven frameworks. This theory is developed using well log compressional and shear wave velocity data from the Mallik 5L‐38 permafrost gas hydrate research well in Canada and applied to well log data from hydrate‐bearing sands in the Alaskan permafrost, Gulf of Mexico, and northern Cascadia margin. Velocity‐based gas hydrate saturation estimates are in good agreement with Nuclear Magneto Resonance and resistivity log estimates over the complete range of observed gas hydrate saturations.

Analysis of large thermal maturity datasets: Examples from the Canadian Arctic Islands

Abstract This paper describes an approach for verifying thermal maturity data in a large historical dataset from the Canadian Arctic Islands. A compilation of more than 6000 maturity measurements (vitrinite reflectance and Rock-Eval Tmax) collected over the span of three decades involved a rigorous assessment of data quality. Some common anomalies in interpreting thermal maturity dataset include: (i) elevated thermal maturity due to Cretaceous igneous intrusion in the region, (ii) reworking of refractory material from older rocks into younger strata during the Triassic period, (iii) suppression of vitrinite reflectance and Tmax in hydrogen-rich samples, (iv) low maturity values due to cross-contamination by the younger sediments during drilling process (caving), and (v) offset maturity values obtained from different maturity measurements. The study discusses various independent checks to verify the obtained maturity parameters. The comparison between thermal maturity data with the sonic velocity of shale resulted in a satisfactory correlation. While such a correlation may vary in different sedimentary basins, it produces a useful independent assessment of thermal maturity. The results indicate that increased heat flow during the Jurassic–Early Cretaceous rifting of the Canada Basin may have caused the elevated maturity beyond the expected burial level as suggested by the discrepancy between thermal maturity and sonic velocity data. Given the fact that vitrinite reflectance records only the maximum temperature to which the enclosing rocks were exposed, deviation of the collected reflectance values from the current depth of burial serves as an indicator for the amount of geological uplift.

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Reply

Sonic velocity, density, and resistivity log data from 60 wells on the Norwegian Shelf have been used to investigate velocity-depth trends in sedimentary rocks as a function of sediment composition, porosity, pore-pressure, burial-history, and compaction processes. A first-order linear velocity-depth trend line has been estimated from published velocity data. The data analyzed in this study, however, show significant variations from this trend line, indicating that no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling. Lower Tertiary smectitic sediments from the northern North Sea and Haltenbanken are characterized by relatively low velocities compared to the overlying Pliocene and Pleistocene sediments, causing a distinct velocity inversion. A significant velocity increase at a burial depth corresponding to 70–100C was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales. Overpressured Jurassic sediments from Haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference. The reduced velocities may be a direct response to lower effective stresses and, thus, reduced elastic compaction. Low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy. The high velocity/depth ratio of Barents Sea sediments (after correcting for Tertiary exhumation) is explained by the burial history of the area, the subsequent thermal exposure of the sediments over time, and thus, the amount of quartz cementation.

Quantification of Exhumation from Sonic Velocity Data, Cooper Basin, Australia, and Implications for Hydrocarbon Exploration

Exhumation (defined as rock uplift minus surface uplift) in the Cooper Basin of South Australia and Queensland has been quantified using the compaction methodology. The sonic log, which is strongly controlled by the amount of porosity, is an appropriate indicator of compaction, and hence is used for quantifying exhumation from compaction. The traditional way of estimating exhumation based on the degree of overcompaction of a single shale unit has been modified and five units ranging in age from Permian to Triassic have been analysed. The results reveal that exhumation increases eastwards from the South Australia into the Queensland sector of the basin. The results show that exhumation in Late Triassic – Early Jurassic times, after the Cooper Basin deposition, seems to be 200–400 m higher than exhumation in Late Cretaceous – Tertiary times, after the Eromanga Basin deposition. This study has major implications for hydrocarbon exploration. Maturation of source rocks will be greater for any given geothermal history if exhumation is incorporated in maturation modelling. Exhumation values can also be used to improve porosity predictions of reservoir units in undrilled targets.

Crosswell seismic waveguide phenomenology of reservoir sands & shales at offsets >600 m, Liaohe Oil Field, NE China

SUMMARY Crosswell seismic data recorded at 620–650 m offsets in an oil-bearing sand/shale reservoir formation at the Liaohe Oil Field, northeast China, provide robust evidence for waveguide action by low-velocity reservoir layers. Crosswell-section velocity models derived from survey-well sonic logs and further constrained by observed waveguide seismic wavegroup amplitudes and phases yield plausible evidence for interwell reservoir–sand continuity and discontinuity. A pair of back-to-back Liaohe crosswell vector-seismic surveys were conducted using a source well between two sensor wells at 650 and 620 m offsets along a 200-m-thick reservoir formation dipping 7° down-to-east between depths of 2.5 and 3 km. A downhole orbital vibrator generated seismic correlation wavelets with frequency range 50–350 Hz and signal/noise ratio up to 5:1 over local downhole ambient noise. The sensor wells were instrumented with a mobile 12- to 16-level string of clamped vector-motion sensor modules at 5 m intervals. Using 5 m source depth increments, crosswell Surveys 1 and 2 cover source/sensor well intervals above and through the reservoir of, respectively, 600 m/600 m (13 000 vector traces in 9 common sensor fans) and 300 m/560 m (7000 vector traces in 7 common sensor fans). Survey 1 common sensor gathers show clear, consistent high-amplitude 20 ms waveletgroup lags behind the first-arrival traveltime envelope. Such arrivals are diagnostic of seismic low-velocity waveguides connecting the source and sensor wells. Observed Survey 1 retarded wavegroup depths tally with source and sensor depths in low-velocity layers identified in sonic well logs. Finite-difference acoustic model wavefields computed for waveguide acoustic layers constrained by well-log sonic velocity data match the observed waveguide traveltime and amplitude systematics. Model waveforms duplicate the observed m-scale and ms-scale sensitivity of waveguide spatio-temporal energy localization. Survey 2 crosswell data, in contrast, provide no comparable evidence for waveguide action despite a sensor-well sonic well log similar to that of Survey 1. Instead, acoustic wavefield modelling of Survey 2 data clearly favours an interpreted waveguide model with 10° downdip interrupted by a 75–100 m throw down-fault near the sensor well. The absence of clear waveguide arrivals is adequately explained by dispersal of waveguide energy at the fault discontinuity. Auxiliary well sonic velocity and lithologic logs confirm the model-implied 75–100 m of down-throw faulting near the sensor well.

Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada

The Mallik 2L‐38 research well was drilled to 1150 m under the Mackenzie Delta, Canada, and penetrated a subpermafrost interval where methane hydrate occupies up to 80% of the pore space. A suite of high‐quality downhole logs was acquired to measure in situ the physical properties of these hydrate‐bearing sediments. Similar to other hydrate deposits, resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework. In addition, sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed. We use monopole and dipole waveforms to estimate compressional and shear attenuation. Comparing with hydrate saturation values derived from the resistivity log, we observe a linear increase in both attenuation measurements with increasing hydrate saturation, which is not intuitive for stiffening sediments. Numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results. We also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate‐bearing sediments. We suggest that this model can be improved and extended, allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact.

Sonic velocity, submarine canyons, and burial diagenesis in Oligocene-Holocene cool-water carbonates, Gippsland Basin, southeast Australia

Burial diagenetic processes largely control sonic velocity and porosity destruction in the Oligocene-Holocene Seaspray Group carbonates of the Gippsland Basin. Extensive cementation of the carbonates begins at around 300-500 m burial depth, and most macroporosity is filled by calcite cement at a burial depth of around 1.5 km. Sonic velocity data from well logs are strongly correlated with the burial diagenetic processes operating in the carbonates. Above 300-500 m burial depth, sonic velocity increases rapidly with depth, with porosity loss being dominated by mechanical compaction. Below 500 m burial depth, the sonic velocity increases at a lesser rate, with porosity loss being dominated by pressure solution and calcite cementation. At these greater depths, sonic velocity (and therefore porosity) is a function of burial depth, carbonate content, and, to a lesser extent, sediment age. Lateral sonic velocity variations within the Seaspray Group are associated with submarine canyon facies. The submarine canyons are filled by sediments that have a higher carbonate content than the surrounding succession. During burial, these carbonate-rich canyon-fill sediments are more rapidly cemented by calcite, producing higher sonic velocities than the surrounding strata. The resultant velocity variations cause problems in time-depth conversion.

New method for conversion of gas sonic velocities to density second virial coefficients

AbstractA new, numerical method was developed to calculate density second virial coefficients B(T) from sonic velocity measurements in gases at low pressures. Unlike existing methods, this procedure requires no model assumption as to the form of the temperature variation of B(T). Rather, it differences the measured accoustic second virial coefficient according to a new mathematical identity. While two higher‐ordered terms in the identity must be ignored to initiate the numerical calculations, the magnitude of these terms can later be found from the initial determination of B(T). This article describes new equations developed, numerical procedures, calculations with Redlich‐Kwong and Peng‐Robinson gases and, with a Lennard‐Jones model gas, to show the accuracy of the method and the magnitude of the two error terms, and the method applied to existing sets of sonic velocity data.

Rock Physics Characterization for Gas Hydrate Reservoirs: Elastic Properties

Abstract: We offer a first‐principle‐based, effective medium model for elastic‐wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate or free gas. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame and (b) hydrate becomes a component of the solid phase, reducing porosity and modifying the elasticity of the frame. The goal of the model is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) that are consistent with estimates obtained from resistivity, chlorinity, and evolved gas data.

Elastic‐wave velocity in marine sediments with gas hydrates: Effective medium modeling

We offer a first‐principle‐based effective medium model for elastic‐wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame; (b) hydrate becomes a component of the solid phase, modifying the elasticity of the frame. The goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) consistent with estimates obtained from resistivity, chlorinity and evolved gas data.

Top-seal leakage through faults and fractures: the role of mudrock properties

Abstract Mudrocks are effective top seals for hydrocarbon accumulations because they possess very low permeabilities, high capillary entry pressures, and are often laterally continuous basin-wide. For leakage through the seal to take place, an additional mechanism must provide enhanced permeability in mudrocks. Tectonically induced, dilatant faulting and fracturing in brittle rocks is such a mechanism. The effectiveness of mudrocks as seals may be compromised by a number of other factors, such as tectonic fault displacements in excess of the seal thickness; tensile fracturing under extreme fluid pressure conditions; and leakage via a network of juxtaposed thin leaky beds across sub-seismic faults within the seal. Before hydrocarbon trap integrity analysis, seismic interpretations should honour the fundamental geometry of the trap as closely as possible because top seals can be reliably appraised only when basic geometries are accurately determined. Fluid pressure is a proven risk in many exploration provinces, in terms of mudrock top-seal leakage via opening mode fractures. This natural hydrofracturing can take place if buoyancy pressures, combined with fluid overpressures, exceed the minimum in situ horizontal stress, plus the tensile strenght of the seal, an occurrence that leads to mechanical failure of the seal rock. The rheology of a seal is another important factor as it determines the failure mode, i.e. whether the rocks are ductile and remain sealing after deformation or whether they deform in a brittle manner to create permeable leak paths. Several techniques have been developed to predict rock rheology; these rely on uplift, sonic velocity and clay content data. Burial curves can be used to determine overconsolidation of mudrocks. Sonic velocity data can be used to estimate unconfined compressive strength in mudrocks. The most direct method utilizes relationships between mudrock friction angle, swelling clay content and mudrock surface area to determine ductility. The effect on trap integrity of sub-seismic faults is also quantifiable because advances in structural geological technology now permit predictions of the numbers of sub-seismic faults in a trap. This information, combined with detailed top-seal stratigraphic data, provides the power to screen traps for the risk of top-seal leakage via sub-seismic fault juxtapositions within the seal. A simple strategy incorporating these factors and linking them to other structural and stratigraphic information can contribute to reducing uncertainty about top-seal leakage caused by tectonic deformation.

Use of velocity of sound in predicting thermodynamic properties of dense fluids from cubic equations of state

AbstractIn this paper it has been shown that velocity of sound data may be used to obtain constants of a cubic equation of state and then thermodynamic properties can be accurately calculated through the constants extracted from sonic velocity data. Results are presented for estimation of heat capacities, enthalpies, vapor pressures and compressibility factors of some pure vapor and liquid compounds as well as some binary mixtures for pressures up to 100 MPa and temperatures up to 600 K for hydrocarbons from methane to hexadecane. The method will be useful for those mixtures containing heavy compounds for which accurate data on critical constants are not available, but for which the sonic velocity can be measured.

Pore Pressure Estimation From Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompaction

Summary A new method for estimating pore pressure from formation sonic velocity data is presented. Unlike previous techniques, this method accounts for excess pressure generated by both undercompaction, and fluid expansion mechanisms such as aquathermal pressuring, hydrocarbon maturation, clay diagenesis, and charging from other zones. The method is an effective stress approach; the effective stress is computed from the velocity, and the result is subtracted from the overburden stress to obtain pore pressure. To include multiple sources of overpressure, a pair of velocity-vs.-effective-stress relations are introduced. One relation accounts for normal pressure and overpressure caused by undercompaction. The second is applied inside velocity reversal zones caused by fluid expansion mechanisms. Example applications of the method are presented from the U.S. gulf coast, the Gulf of Mexico, and the Central North Sea. Some other pore pressure estimation approaches are also examined to demonstrate how these techniques have unknowingly accounted for overpressure mechanisms other than undercompaction. It is also explained how velocity-vs.-effective-stress data can be used to identify the general cause of overpressure in an area. For instance, the empirical correlation of Hottman and Johnson indicates that overpressure along the U.S. gulf coast cannot be due only to undercompaction.

Quantification of Tertiary Exhumation in the United Kingdom Southern North Sea Using Sonic Velocity Data

Sonic velocities from the Upper and Middle Chalk (Upper Cretaceous), the Bunter Sandstone, and the Bunter Shale (both Lower Triassic) were used to independently quantify apparent exhumation (height above maximum burial depth) in the United Kingdom (UK) southern North Sea. Apparent exhumation is the displacement, on the depth axis, of a given velocity/depth trend from the normal (unaffected by exhumation) trend. Apparent exhumation results derived from the Upper and Middle Chalk, the Bunter Sandstone, and the Bunter Shale are statistically similar. The consistency of results from carbonate and clastic units suggests that, at a formational and regional scale, overcompaction (i.e., anomalously high sonic velocity) in all three units analyzed reflects previously greater buria depth, rather than sedimentological and/or diagenetic processes, and validates the use of lithologies other than shale in maximum burial depth studies. The consistency of results from units of Early Triassic to Late Cretaceous age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that maximum Mesozoic-Cenozoic burial depth in the southern North Sea was attained prior to Tertiary exhumation. The proposed magnitudes of exhumation are generally greater than those previously published for the southern North Sea, but they are consistent with recent estimates from apatite fission track analysis. The amount of exhumation of the rock column is greatest on the recognized inversion axes (i.e., Sole Pit-Cleveland), where it reaches approximately 2.5 km. However, there was a regional component of 1.0-1.5 km exhumation during the Tertiary that affected structurally uninverted areas, and on which the more localized inversion-related exhumation was superimposed. Cretaceous-Tertiary burial prior to exhumation must have been of greater magnitude and more rapid than suggested by the preserved stratigraphy. The effect of this extra burial and subsequent exhumation on sedimentary rock decompaction procedure and thermal maturation modeling is illustrated for the Cleethorpes-1 and 44/7-1 wells, and must also be incorporated in modeling reservoir diagenesis. The regional, Tertiary tectonic uplift associated with exhumation must have had a thick-skinned origin.

Quantification of Tertiary exhumation from sonic velocity data, Celtic Sea/Southwestern Approaches

Quantification of Tertiary exhumation from sonic velocity data, Celtic Sea/Southwestern Approaches

Regional Tertiary Exhumation in and around the United Kingdom

Abstract Sonic velocities in the Upper and Middle Chalk, the Bunter Sandstone and the Bunter Shale were used independently to quantify apparent exhumation (amount of missing section) in the UK Southern North Sea (SNS). Apparent exhumation results derived from these units are statistically similar. The consistency of results from chalks, shales, and sandstones in the UK SNS, and in other areas (Inner Moray Firth and Celtic Sea/South-Western Approaches), suggests that, at a formation and regional scale, burial-depth is the primary control on velocity (and porosity) in these units, and validates the use of lithologies other than shale in maximum burial-depth determination. The consistency of apparent exhumation results from units of Early Triassic to Late Cretaceous age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that maximum Mesozoic-Cenozoic burial-depth was attained prior to Tertiary exhumation. The analysis of sonic velocities, apatite fission tracks, vitrinite reflectance, normalized drilling rate and clay mineral diagenesis all suggest that there was a regional component of approximately 1 km of exhumation during Tertiary times, which affected structurally uninverted areas in and around the UK. The widely-recognized inversion-related exhumation is superimposed on regional Tertiary exhumation. Regional Tertiary exhumation need not have been contemporaneous with structural inversion. Further refinement beyond Tertiary age is not possible from the sonic velocity data. Regional exhumation, however, is most likely to be associated with regional Palaeocene or Oligocene/Miocene unconformities. The evidence for regional exhumation implies that there was a pre-exhumational phase of burial (during which the eroded sedimentary rocks were deposited). Exhumation and prior burial associated with the regional Tertiary event need to be incorporated in aspects of basin analysis such as sediment decompaction and diagenetic and maturation modelling, and can, for example, account for the maturity of the Carboniferous of the onshore East Midlands Shelf, and the Lias exposed on the north coast of Somerset. The regional, Tertiary tectonic uplift associated with exhumation must have had a thick-skinned origin, and, in areas where there is no evidence for significant Tertiary igneous activity, a decoupled, two-layer model of lithospheric compression is invoked to account for the subsidence/uplift patterns of both inverted and uninverted areas.

UPLIFT AND EROSION ON THE ASHMORE PLATFORM, NORTH WEST SHELF: CONFLICTING EVIDENCE FROM MATURATION INDICATORS.

The Ashmore Platform is situated to the north of the Browse Basin, on the North West Shelf, off the coast of Western Australia. Apatite fission track analysis (AFTA™), vitrinite reflectance and fluo­rescence alteration of multiple macerals (FAMM) measurements were undertaken on drill cuttings material recovered from the Late Triassic sequence of the oil exploration well, Ashmore Reef-1.Vitrinite reflectance measurements indicate that the Late Triassic sequence is currently experienc­ing maximum temperature. However, reflectance methods were suspected of being unreliable due to suppression of the reflectance, a common problem when dealing with marine influenced sediments. The FAMM technique was used to provide an alter­native maturity estimate using the same speci­mens. The FAMM results suggested that vitrinite reflectance is suppressed and that the true matu­rity is higher than conventional reflectance mea­surements predict.The results also suggest that some of the cuttings material from the sampled level is contaminated by material from higher in the Late Triassic sequence. Both the AFTA™ and FAMM data show bi-modal populations from some depths. It was possible to distinguish between the two populations and esti­mate the maturity of the caved material. FAMM and AFTA™ results together imply that maximum palaeotemperature was reached in the Mid-Creta­ceous, corresponding to a major unconformity in the well.The FAMM results do not agree with published maturity estimates based on conodont alteration indices (CAI), which suggest that temperatures have only recently and rapidly reached current levels. The AFTA™ results can also be interpreted to support this model. Furthermore, sonic velocity data in Miocene limestone suggests post-Miocene erosion, which would be expected to be associated with a temperature drop.

Quantification of Tertiary exhumation from sonic velocity data, Celtic Sea/South-Western Approaches

Abstract Sonic velocities from the Danian Chalk, the Upper Cretaceous Chalk, the Lower Cretaceous Greensand/Gault Clay, and the Triassic Mercia Mudstone were used to quantify apparent exhumation (i.e. amount of missing section) in the Celtic Sea/South-Western Approaches (southern Irish, south-western United Kingdom, and north-western French continental shelves). Analysis of all four of these stratigraphic units yield similar results. Since it is unlikely that sedimentological and/or diagenetic processes are responsible for similar amounts of overcompaction in the chalks, shaly sandstones and shales analysed, burial at depth greater than that currently observed is the most likely cause of overcompaction (anomalously high velocities). The use of lithologies other than shales in maximum burial-depth/exhumation studies is thus validated. The consistency of results from units of Triassic to Danian age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that the maximum Mesozoic-Cenozoic burial-depth in the Celtic Sea/South-Western Approaches was attained prior to Tertiary exhumation. The recognized inversion axes of the Brittany Basin, South-West Channel Basin and North Celtic Sea Basin are highs in apparent exhumation, however highs in apparent exhumation of approximately 1 km also occur in the uninverted St. Mary’s and Plymouth Bay Basins, and on the margins of the Armorican and Cornubian Platforms and the Pembroke Ridge. Tertiary exhumation was regional and not restricted to inversion axes, and thus had a thick-skinned origin. Heterogeneous lithospheric compression is the geodynamic model preferred to account for the regional exhumation of the Celtic Sea/South-Western Approaches. The identification of regional exhumation of approximately 1 km has important implications for modelling thermal maturation in the Celtic Sea/South-Western Approaches, with source rocks previously believed to be immature shown to be potentially mature for oil generation. When corrected for exhumation, the post-rift sequence in the Celtic Sea/South-Western Approaches area shows relatively little localization over the syn-rift Jurassic-Lower Cretaceous sequence, suggesting that during basin formation, the lower lithosphere, thermal re-equilibration of which controls post-rift subsidence, did not experience localized thinning on the scale of the syn-rift basins.

Random and systematic errors in thermophysical property measurements

AbstractGeneral procedures are outlined for the simulation and propagation of random and systematic errors in thermophysical property experiments. Density second virial coefficients B(T) from sonic velocity and Joule‐Thomson (J‐T) experiments are examined for error propagation where the connecting thermodynamic identity is a differential equation with missing boundary conditions. A recent controversy is addressed concerning B(T) at subcritical temperatures for pure hydrocarbon gases from direct density measurements vs. new sonic velocity data. Sonic velocity results are more likely correct with adsorption errors causing the problem in the density measurements.Two new model consistency tests are developed for checking assumed temperature models in the reduction of sonic velocity and J‐T data to B(T). Excellent values of B(T) are then obtained from either type of data when the original experiments are free of errors. Random errors propagate systematically when the connecting equation is a differential equation. Sonic data must be of high precision (± 10 ppm) to generate B(T) to ±1 cm3/mol due to complications in data reduction arising from the temperature model/random error interaction. Except perhaps for adsorption errors, systematic errors in the sonic velocities are unimportant to B(T). J‐T data provide propagation factors near unity with errors in B(T) higher at higher temperature, unlike sonic velocities.

Quantification of Tertiary erosion in the Inner Moray Firth using sonic velocity data from the Chalk and the Kimmeridge Clay

Sonic velocities for the Chalk and Kimmeridge Clay were analysed to determine the importance and magnitude of Tertiary erosion for 26 wells in the Inner Moray Firth. Apparent erosion (height above maximum burial depth) was derived by the displacement, along the depth axis, of a given velocity-depth trend from the normal (undisturbed) trend. The similarity in apparent erosion results derived from the Cretaceous Chalk and Late Jurassic Kimmeridge Clay successions of the Inner Moray Firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and/or diagenetic process. Resultant apparent erosion estimates are variable across the Inner Moray Firth. Such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern Inner Moray Firth. However, the actual magnitude of erosion (equal to the sum of apparent erosion and post-erosional Tertiary burial) was approximately 1 km throughout the whole Inner Moray Firth area, indicating that Early Tertiary erosion was regional and not simply restricted to the inverted basin margins. However, a well defined mechanism sufficient to have generated the regional event is still sought.