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Natural fracturing and petrophysical properties of the Palisades dolerite sill

By
D. Goldberg
D. Goldberg
Lamont-Doherty Earth Observatory, Rte 9W, Palisades, NY 10964, USAgoldberg@ldeo.columbia.edu
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K. Burgdorff
K. Burgdorff
GeoMechanics International, Inc., Parmelia House, 191 St George';s Terrace, Perth, WA 6000, Australiaburgdorff@geomi.com
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Published:
January 01, 2005

Abstract

This investigation of naturally occurring fractures in the mafic rocks of the Palisades dolerite sill characterizes the porosity of this crystalline rock sequence, and yields a method of determining the in situ porosity when complete down-hole information is not available. Two holes, 229 m and 305 m deep, were drilled 450 m apart through the sill and into the underlying Triassic sediments of the Newark Basin. Both holes were logged with geophysical tools, including the acoustic borehole televiewer (BHTV), to identify intervals of high porosity, fracturing, and potential zones of active fluid flow. Using the BHTV data, 96 and 203 fractures were digitally mapped within the sill in Well 2 and Well 3, respectively. Most fractures dip steeply (76–78°). There is a shift in fracture orientation between Well 2 and Well 3, although the lithology of the sill is continuous. The dolerite penetrated in both holes is fresh and unaltered, and intersects a 7-m thick olivine-rich layer about 15 m above the bottom of the sill. Several fractures identified in the sill have large apparent aperture (>6 cm) that correspond to high-porosity zones (6–14%), measured from both resistivity and neutron logs in Well 2. We use a relationship between porosity and apparent fracture aperture in Well 2 to infer the porosity in Well 3. This correlative method for estimating porosity may be applicable between holes in other crystalline rock environments where down-hole log data are incomplete. Changes in the temperature gradient log also indicate active fluid flow, although flow appears to be most active in fractured and high-porosity zones in the sediments.

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Contents

Geological Society, London, Special Publications

Petrophysical Properties of Crystalline Rocks

P. K. Harvey
P. K. Harvey
University of Leicester, UK
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T. S. Brewer
T. S. Brewer
University of Leicester, UK
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P. A. Pezard
P. A. Pezard
Université de Montpellier II, France
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V. A. Petrov
V. A. Petrov
IGEM, Russian Academy of Sciences, Russia
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Geological Society of London
Volume
240
ISBN electronic:
9781862394889
Publication date:
January 01, 2005

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