Investigation of Neutron-Porosity Log Uncertainties: Ocean Drilling Program Hole 642E

Lysne, P. C.

doi:10.2973/odp.proc.sr.104.194.1989

Cited by 8 publications

(8 citation statements)

References 7 publications

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“…The above corrections partly accounted for the 12% difference observed by Lysne [1989] 6, 8, 9, 11, 12, 13. The estimated increase in • is too low to yield a significant increase in the thermal porosity.…”

Section: Porosity Correctionmentioning

confidence: 88%

“…of boron, the absorption effect reaches its peak around 15% (about 4%) and then decreases as porosity increases to 30%. In basaltic rocks, the presence of strong neutron absorbers has been recently proposed by some authors [Broglia and Moos, 1988;Lysne, 1989;Moos, 1989] to explain the discrepancies between log and core porosity measurements in fresh, unfractured massive basalts. The selection of such intervals for the log-core calibration is based on the assumption that, in spite of the different volume investigated, due to the absence of fracturing and alteration, both logs and cores should provide the same porosity measurement (i.e.…”

Section: Porosity Correctionmentioning

confidence: 99%

See 1 more Smart Citation

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

Broglia¹,

Ellis²

1990

J. Geophys. Res.

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DARWIN ELLIS Schlumberger-Doll Research, Ridgefield, ConnecticutThis 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 (E) 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. and Chiaromonte, 1983; Gilchrist et al.scientif...

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Section: Porosity Correctionmentioning

confidence: 88%

Section: Porosity Correctionmentioning

confidence: 99%

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

Broglia¹,

Ellis²

1990

J. Geophys. Res.

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“…In clay-free sediments saturated with water this interaction depends mainly on hydrogen atoms in pore fluids. In igneous and metamorphic rocks, the formation -tool response can be influenced by the presence of hydrous minerals (particularly secondary phases), neutron absorbers such as chlorine or gadolinium in the rock or the pore fluid/drilling mud (e.g., seawater), and the higher matrix density of the rocks [e.g., Lysne, 1989;Broglia and Ellis, 1990;Harvey and Brewer, 2005]. Although sophisticated logging tools minimize for some of these effects and provide neutron porosity values closer to porosities measured on core samples, neutron porosity values still overestimate porosity in the upper oceanic crust.…”

Section: Comparison Between Physical Properties From Core Samples Andmentioning

confidence: 99%

Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

Bartetzko

Fisher

2008

J. Geophys. Res.

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[1] We compile and compare petrophysical data from the upper oceanic crust in Hole U1301B, on 3.5 Ma seafloor on the eastern flank of the Juan de Fuca Ridge. Measurements include core-scale (lab-based) and in situ (wireline) measurements of formation porosity, electrical resistivity, bulk density, P wave velocity, and gamma ray emission. A comparison between Hole U1301B data and those from other sites, ranging in age from 5.9 to 167 Ma, demonstrates some important differences. Hole U1301B samples tend to have lower bulk density values for a given porosity than do other young crustal sites. Hole U1301B samples also tend to have higher values of formation factor (the ratio of fluid conductivity to saturated rock conductivity), which may indicate development of tortuous microporosity that is more consistent with samples from older seafloor. Evaluation of global trends of petrophysical properties versus crustal age indicates that Hole U1301B in situ data is consistent with global trends, but core-scale P wave velocity and bulk density values fall below global trend values predicted for young crust. This last observation is likely related to basement topography and the regional exposure, until relatively recently, of basement outcrops around Site U1301. Until most of the exposed basement in this area was buried, hydrothermal circulation extracted much of the lithospheric heat, kept basement fluid temperatures low, and limited the rate and extent of alteration; a subsequent period of higher-temperature fluid circulation with more restricted and less oxidative conditions resulted in enhanced and pervasive alteration of crustal rocks.

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“…ODP neutron porosity logs consistently overestimate the porosity of basalts [ Anderson et al , 1985; Broglia and Moos , 1988; Lysne , 1989; Moos , 1990]. Broglia and Ellis [1990] utilized logs from five sites to analyze the relative contributions of several factors to this systematic bias.…”

Section: Datamentioning

confidence: 99%

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

et al. 2003

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[1] The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties (porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity-dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large-scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age-dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular-scale crustal alteration occurs after the first 10-15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.

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Investigation of Neutron-Porosity Log Uncertainties: Ocean Drilling Program Hole 642E

Cited by 8 publications

References 7 publications

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

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

Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

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