Geostatistical Rock Physics Inversion for Predicting the Spatial Distribution of Porosity and Saturation in the Critical Zone

Mathematical Geosciences - Understanding the subsurface structure and function in the near-surface groundwater system, including fluid flow, geomechanical, and weathering processes, requires...     

Geometry and pressure-temperature significance of the kyanite-sillimanite isograd in the Mica Creek area,British Columbia

Late Precambrian and Lower Paleozoic metapelitic rocks near Mica Creek, British Columbia, range in metamorphic grade from biotite zone to sillimanite zone. The kyanite-sillimanite isograd was established after phase 2 folding but was deformed by phase 3 folding. Topographic relief of about 2 km, combined with phase 3 folding, permits reasonably precise determination of the geometry of the isogradic surfaces. The effects of phase 3 folding have been accounted for and the isobaric surfaces at the time of metamorphism are inferred to have been dipping gently. Using the kyanite-sillimanite experimental phase diagram, intersection of the isobars and the isograd permits estimates of differences in temperature and pressure along a cross section. These estimates are about 400 bars and 20° C. Mineral geothermometry (garnet-biotite) and geobarometry (garnetplagioclase-Al ₂SiO₅-quartz) does not provide a fine enough resolution to detect these predicted differences.     

Correlation coefficient patterns and their interpretation in three basaltic suites

Single stage magmatic fractionation processes result in igneous rock suites in which correlation coefficients between pairs of elements are generally high. The Tertiary lavas of E. Iceland are used as an illustration of this type of behaviour. When a second process such as fractionation at a different pressure, wall-rock assimilation, or mixing with a different magma, occurs, correlation coefficients are altered in a predictable way if the process and the chemical characteristics of the participating rocks, minerals and liquids are known. A computer program has been devised to simulate complete correlation coefficient matrices for any desired two- or multi-stage process e.g. fractional crystallisation coupled with contamination. Basaltic suites from Mahabaleshwar, India and the Rooi Rand, southern Africa show correlation coefficient patterns characteristic of multi-stage evolution, and these are interpreted with the aid of computer simulation. The Mahabaleshwar suite shows clear evidence of contamination by a granitic component. The Rooi Rand rocks however remain problematic.     

Effects of non-conserved denominators on pearce element ratio diagrams

Pearce element ratios (PER's) have conserved denominators which have not participated in the material transfer processes that cause chemical variations in rocks. Theoretically, there is no truly conserved element (constituent) which can be used as a PER denominator because in every material transfer process all constituents have non-zero concentrations in the phases that are being transferred. Thus, constituents used as denominators of PERs may have undergone at least a small amount of material transfer. This communication investigates the degree to which a non-conserved PER denominator changes the trend of data produced by a material transfer process from that produced by the same process but plotted on a PER diagram with a truly conserved denominator. An equation is developed that utilizes the partition coefficient as the measure of the degree of involvement of the denominator constituent in the phase undergoing transfer. This equation is examined to determine how the magnitude and direction of a PER diagram data trend change with increasing involvement of the denominator constituent in the transferring phase. A set of plagioclase fractionation examples are presented which use different elements as PER denominators and consider the effects that small amounts of these elements in the plagioclase structure will have on the data trend, as a function of the element partition coefficient between crystal and melt. Results demonstrate that the direction of change in slope of a material transfer data trend is a function of the initial relative magnitudes of the numerator constituents on the PER diagram. Additionally, if the amount of involvement of a PER denominator in a separating phase is very small relative to the amount of the numerator constituents in the separating phase, there is no significant change in the data trend caused by material transfer on a PER diagram. Moreover, if the denominator constituent substitutes for a numerator constituent in the phase undergoing transfer, the intercept of the trend of the data may not converge to zero when there is a large partition coefficient, as would be expected from theory. Thus, statistical tests to determine if a PER denominator is conserved, which evaluate whether the intercept is significantly different from zero, may not be very powerful because a large amount of denominator variation is necessary before the intercept of a data trend is forced through the origin, if at all.     

The size frequency and abundance of subtidal bloodworms (Glycera dibranchiata,ehlers) in Montsweag Bay,Woolwich-Wiscasset,Maine

A study was conducted in the subtidal portion of Montsweag Bay (Woolwich-Wiscasset, Maine) between the mean low water mark and 1.5 m depth, to determine the size composition, abundance, and monetary value of bloodworms (Glycera dibranchiata). Eight 1-m² subplots were sampled in each of three regions of equal size with the aid of a compressed air lift apparatus. Bloodworms collected varied in length between 1 and 31 cm; the majority were smaller than commercial size (<12 cm). A mean length of 4.6±0.03 cm (95% confidence level) was reported for noncommercial size worms and 19.6±2.0 cm, (95% confidence level) for commercial worms. Although the numbers of bloodworms of all sizes collected in region 1 were significantly greater than the numbers collected from region 3, no significant differences among the numbers of commercial-sized bloodworms were found in subplots within the three regions. Estimates of the standing crop and value of commercial bloodworms present were derived from overall estimates of the numbers m^?2 and the standard error at 95% confidence (1.04±0.58 worms), the subtidal area, and the 1971 and 1983 value of commercial worms. During 1971, the 6,059,040±3,379,080 commercial-sized bloodworms present were valued at $242,362.00±$135,163.00. At 1983 prices, the value of these worms would be from $363,542.00±$202,745.00 to $605,904.00±$337,908.00. Construction of a causeway, which created subtidal conditions where intertidal conditions formerly existed, apparently had no adverse effect upon the bloodworm population present.     

Proto-solid bitumen in petroleum altered by thermochemical sulfate reduction

Thermochemical sulfate reduction (TSR) involves a complex series of redox reactions whereby petroleum is oxidized by sulfate forming H₂S and CO_2. A highly aromatic, sulfur rich carbonaceous residue, which we define as TSR-solid bitumen, is commonly seen in reservoir rocks where TSR has occurred. Using atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, we find that TSR altered oils and condensates contain highly condensed polynuclear aromatic and naphthenoaromatic species with 0–3 S atoms. These chemical species are not present in petroleum fluids of equivalent maturity that have not experienced TSR and must have been formed by the TSR process. We call these species proto-solid bitumen as they represent the type of organic compounds that could easily precipitate from the TSR altered oils with slight chemical alteration or changes in reservoir conditions.     

Estimation of regional-scale groundwater flow properties in the Bengal Basin of India and Bangladesh

Quantitative evaluation of management strategies for long-term supply of safe groundwater for drinking from the Bengal Basin aquifer (India and Bangladesh) requires estimation of the large-scale hydrogeologic properties that control flow. The Basin consists of a stratified, heterogeneous sequence of sediments with aquitards that may separate aquifers locally, but evidence does not support existence of regional confining units. Considered at a large scale, the Basin may be aptly described as a single aquifer with higher horizontal than vertical hydraulic conductivity. Though data are sparse, estimation of regional-scale aquifer properties is possible from three existing data types: hydraulic heads, ¹⁴C concentrations, and driller logs. Estimation is carried out with inverse groundwater modeling using measured heads, by model calibration using estimated water ages based on ¹⁴C, and by statistical analysis of driller logs. Similar estimates of hydraulic conductivities result from all three data types; a resulting typical value of vertical anisotropy (ratio of horizontal to vertical conductivity) is 10⁴. The vertical anisotropy estimate is supported by simulation of flow through geostatistical fields consistent with driller log data. The high estimated value of vertical anisotropy in hydraulic conductivity indicates that even disconnected aquitards, if numerous, can strongly control the equivalent hydraulic parameters of an aquifer system.     

An evaluation of the engineering properties of some Nigerian limestones as construction materials for highway pavements

Several hundred limestone samples representing eight different deposits within Nigeria have been evaluated and results indicate that some of the deposi highway pavement construction materials. The evaluation included tests on index properties, strength, chemical, petrographic and resistance-to-attritio

The suitability or adverse features of the eight limestone deposits have been evaluated and the various engineering characteristics are discussed. base recommendations are made for the use of selected limestones as pavement materials in Nigeria.