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

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...

Geophysical Properties of Oceanic Crust at Sites 768 and 770

“…Thus we conclude that the neutron logs at Sites 768 and 770 overestimate porosity by about 10%-20%. This discrepancy is near the upper limit of the range of biases detected by Broglia and Ellis (1990) in other ODP crustal sites.…”

“…Several recent studies of DSDP and ODP neutron-porosity logs (Anderson et al, 1985;Broglia and Moos, 1988;Lysne, 1989;Moos, 1990) have found that these logs consistently overestimated the porosity of basalts. Broglia and Ellis (1990) utilized logs from five sites to analyze the relative contributions of several factors to this systematic bias. They concluded that the most important factor is the ODP technique of running the neutron-porosity tool without appropriate eccentralization.…”

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt

“…L s is the root-mean-squared distance that a fast neutron must travel before it is thermalized, i.e., before reaching the thermal energy range of < 0.1 eV (Schlumberger Wireline and Testing, 1989). Slowingdown length is a function of the bulk density, the concentration of neutron moderators, and the scattering and capture cross sections of those moderators (Broglia and Ellis, 1990). When hydrogen is present, its L s will dominate all others due to its very large scattering and capture cross sections.…”

Expedition 350 methods

“…Upon reaching thermal energies (0.025 eV), the neutrons are captured by the nuclei of Cl, Si, B, and other elements, resulting in a gamma ray emission. This neutron capture cross section (Σ f ) is also measured by the tool and can be used to identify such elements (Broglia and Ellis, 1990;Brewer et al, 1996).…”