DSDP Hole 418A, drilled through 324 m of sediments and 544 m of basalts during DSDP Legs 51-53 at the southern end of the Bermuda Rise, was reoccupied during ODP Leg 102 for the purpose of obtaining a comprehensive suite of geophysical logs. Although a hole restriction at 469 m below seafloor (145 m into basement) prevented the recording of the complete log suite throughout the open-hole interval, two passes of the first Schlumberger tool combination, consisting of resistivity-sonic-gamma-ray-caliper, were recorded from just below the sediment/basalt interface at 324 to 788 mbsf. The remaining 80 m was not logged owing to the suspected presence of a logging tool and cable lost during DSDP Leg 53. The second Schlumberger lowering, consisting of compensated neutron porosity, natural gamma spectroscopy, and lithodensity, and the third lowering, dual resistivity laterolog and gamma ray, were both recorded in open hole from 464 to 788 mbsf. The second combination was also recorded through the pipe to the mud line. Careful calibration of these measurements in fresh flow basalts, corrections for known borehole effects, and comparison of gamma-ray results to core mineralogy result in an excellent set of physical-property measurements throughout the logged interval.Differences in gamma-ray activity, mostly due to the potassium-40 decay series, separate the basement interval into three zones on the basis of smectite content. The uppermost zone 1 consists of a series of relatively unaltered, low-porosity (<15%), high \ p (>5 km/s) massive basalts and more porous lower velocity pillows capped by a 9-m-thick, more highly altered pillow unit, the total extending from 324 to 388 mbsf (log lithologic Units 1 to 4). In zone 2, from 388 to 514 mbsf (log lithologic Unit 5 and Subunit 6A), the pillow basalts contain large amounts of smectite within fractures and voids; porosities generally range from 15% to 25%, and velocities vary inversely with smectite content and porosity from 3.5 to 5 km/s. The bottom of the Subunit 6A breccia at 514 mbsf marks a geophysical boundary separating these altered pillows from the much fresher pillows and flows of Subunit 6B to Subunit 13C in the remainder of the logged interval, which we refer to as zone 3. In this zone velocities are generally above 4 km/s and reach 6 km/s in the massive flows. Porosities are generally below 20%; smectite is generally less than 5%. The density log is more sensitive to well-bore rugosity than the other logs, but aside from this effect, density ranges from 2.4 to 2.95 g/cm 3 . After correction for the presence of smectite, density is generally inversely proportional to porosity, as expected in a two-phase (seawater-basalt) system.Although velocities within the altered section (zone 2) are somewhat lower than in the remainder of the hole, nowhere are they as low as in seismic Layer 2A. In the uppermost 190 m of the basement where smectite is replaced by void space, the average velocity can be reduced to less than 3.5 km/s. Thus, alteration and smectite infilling over ...
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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