Showing papers on "Effective porosity published in 1990"

Relationship between diagenesis and reservoir quality in sandstones of the Garn Formation, Haltenbanken, mid-Norwegian continental shelf

Abstract: A quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the Middle Jurassic Garn Formation. Porosity variation is controlled by the combined effects of compaction and quartz cementation. In general, more porosity has been lost by compaction than by quartz cementation, but compactional porosity loss appears to have occurred relatively early in the burial history (before burial below 2 km below the sea floor). Thus, progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation. Permeability is a function of the abundance of intergranular macroporosity, which is interpreted to be a measure of "effective porosity." At depths less than 3.5 km below the sea fl or, intergranular macroporosity generally comprises more than 50 percent of the total porosity, but at greater depth, as total porosity decreases below about 16 percent, the ratio of effective to ineffective porosity drops below one. This depth corresponds to the onset of extensive illitization and grain dissolution, which results in major reorganization of the pore system. Generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the Garn Formation, but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis.

Effect of alteration, formation absorption, and standoff on the response of the thermal neutron porosity log in gabbros and basalts: Examples from Deep Sea Drilling Project-Ocean Drilling Program Sites

Abstract: This study focuses on the effects of hydrous alteration minerals, formation absorption, and standoff on the response of the thermal neutron porosity log in the basaltic and gabbroic rocks logged at sites 395, 418, 504, 642, and 735 during the Ocean Drilling Program. The concentration of hydrogen present in the rocks in the form of free water (pore space) and bound water (hydrous minerals) is the primary factor controlling the neutron elastic scattering process, while the presence of other elements, such as chlorine, gadolinium, boron, lithium, and samarium in the fluids and in the rock matrix can largely affect the thermal diffusion phase. These neutron absorbers cause an increase of the capture cross section, and in turn of the apparent thermal porosity. Further perturbations occur when the recording conditions depart from those under which the tool has been calibrated; a large and irregular hole diameter and a lack of eccentralization both produce erroneous porosity readings. The effect of hydrous alteration minerals on the thermal neutron porosity log has been estimated from 922 core oxide measurements using an analysis program that calculates the slowing-down length and converts it into apparent porosity. The results show that the computed apparent porosity ranges from less than 1% in fresh basalts and gabbros to about 30% in highly altered units. Depending on the alteration mineral assemblage, natural gamma ray, capture cross section, or hydrogen logs have been used to continuously predict the effect of bound hydrogen at each site. Corrected porosities generally show excellent agreement with core data for massive units, whereas they are higher for pillow basalts and fractured zones. The discrepancy is interpreted as the result of (1) difference in the volume of rock investigated (core specimens do not sample large vugs and fractures) and (2) frequent variations in the hole size and lack of tool contact with the borehole wall (standoff), not completely accounted for in the corrections. High capture cross section values measured on selected basalt samples seem to be associated with more altered basalts, suggesting a larger concentration of strong neutron absorbers in secondary minerals. The concentrations of elements with large capture cross section (Σ) measured at three of the five sites studied yield a maximum increase of 1% in the apparent neutron porosity. This is too low to explain the porosity values, some greater than 7%, observed in some of the fresh and unfractured massive units. A 1- to 2-inch standoff due to the lack of eccentralization of the tool string used in the Ocean Drilling Program is considered the most probable cause of such discrepancy. This interpretation is confirmed by the neutron porosity values recorded at site 735, where the nuclear string was for the first time fully eccentralized, and only a correction for borehole size and the presence of hydrous minerals has been applied.

Regional Trends of Sandstone Porosity Versus Vitrinite Reflectance—A Preliminary Framework

Abstract: The porosity of sandstones is commonly observed to decrease during burial. Such decrease is frequently treated as a function of burial depth, but depth is a position coordinate that may not reflect the time-temperature history by which all subsurface processes are influenced. An alternative approach that is adapted in this study is to consider porosity decrease during burial as a function of thermal maturity. Such an approach links porosity evolution in the subsurface and the maturation of kerogen and petroleum, in that the vitrinite-reflectance measurements used to define porosity trends can also be used to characterize petroleum generation and destruction. Trends of porosity versus vitrinite reflectance (Rm) of the form f=ARmB (where B is a negative number), characterizing both typical and highest sandstone porosities, are derived from 131 measurement suites incorporating 4,353 individual porosity measurements. Diverse sandstone facies and geologic settings are represented, and the data span a maturity range of Rm=0.25-1.7%. The porosity-Rm data form trends which are projected to maturities as high as Rm=7% in order to speculate upon the distribution of sandstone porosities in the very deep (mature) portions of sedimentary basins.

Fast porosity estimation by principal component analysis

Abstract: Based on experience with several wells, the author supposes that the first principal component of porosity logs (perhaps taking SP and GR logs into consideration) is related to effective porosity. This brief paper outlines the steps he followed in examining the first principal component, and presents material from one well processed by Principal Component Analysis (PCA is described briefly in Elek, 1988, and Morrison, 1979). The well and the field are well-known in Hungary (shaly sand).

Using particle density of a container medium to determine accuracy of bulk density and total porosity determinations

Abstract: The total porosity of a container medium is often inaccurate when determined from a water release curve because the use of the total porosity value and bulk density in the equation, Total porosity = (1 ‐ bulk density/particle density) x 100, gives an unrealistic value for particle density. The inaccuracy seems to be caused by incomplete saturation of the medium sample at zero suction. Total porosity should be calculated using bulk density and particle density, rather than equating total porosity with the zero suction value on the water release curve. The zero suction value is a measure of effective porosity since it does not take into account trapped air in the medium.

New approach and methodologies for characterizing the hydrogeologic properties of aquifers. Final report, August-December 1989

Abstract: In the authors' opinion, the ability of hydrologists to perform field measurements of aquifer hydraulic properties must be enhanced if they are to improve significantly the capacity to solve ground water contamination problems at Superfund and other sites. Therefore, the primary purpose of the report is to provide motivation and new methodology for measuring K(z), the distribution of horizontal hydraulic conductivity in the vertical direction in the vicinity of a test well. Measurements in nearby wells can then be used to build up three-dimensional distributions. For completeness, and to enhance the usefulness of the report as a field manual, existing methodology for the measurement of effective porosity, vertical hydraulic conductivity, storativity and hydraulic head, are presented also. It is argued that dispersion-dominated models, particularly two-dimensional, vertically-averaged (areal) models, have been pushed about as far as they can go, and that two-dimensional vertical profile or fully three-dimensional advection-dominated transport models are necessary if they are to increase significantly the ability to understand and predict contaminant transport, reaction, and degradation in the field. Such models require the measurement of hydraulic conductivity distributions, K(z), rather than vertically averaged values in the form of transmissivities.