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10 Interpretation of Logs Downhole logging devices provide virtually continuous infor- mation on rock properties from top to bottom of a well that can be used to infer rock types, fracture densities, and fluid parame- ters. Calibration of logs has become quite sophisticated in the oil field environment, but experience is limited (Daniels et a/., 1983), to say the least, in crystalline terranes such as the southern Ap- palachians. Core samples can provide information with which to calibrate logs. Borehole logging and core analysis acquire comple- mentary pieces of information, and each produces information not available from the other. This is the fundamental reason that the Ocean Drilling Program requires both continuous core and logging in all holes more than 400 m deep. Because one never returns 100 percent of the cored section to the surface, logs are useful for locating where the core came from and for providing geophysical and geochemical measurements of the missing interval. But in order to know which sections are missing, core measurements must be used to match log results. This circular reasoning can be circumvented by spotting side- wall core at exact depths in the well. These then serve to locate not only the logs, but the cores as well. Sidewall diamond-drill coring technology has developed to the point at present where both continuous and discrete interval coring from a wireline can 38
39 be done economically. Both will be required by a scientifically rigorous investigation of deep continental drill holes. Logging tools measure over larger volumes around the hole than are represented in the recovered core. Laboratory measure- ments may be made on the core to observe small scale features such as crack morphologies or fluid inclusion chemistry that are not observable by logging tools. Drill hole measurements deter- mine the in situ physical and chemical environmental conditions, but laboratory pressure vessels can measure core properties in a wide variety of physical and chemical environments. Such labo- ratory measurements are especially important when information must be extrapolated deeper than the hole was drilled. Borehole measurements may be obtained in areas where core recovery is poor, but only core provides continuous data. Even though drill hole logs look continuous, there are usually only a few measure- ments per meter of holeâthin beds and fault planes are sometimes missed. The same information cannot be acquired from cuttings as from core. Cuttings frequently exhibit changes in clay mineralogy due to the high temperature of the drilling process. They are contaminated by pieces of the drill bit and drilling fluids. They are mixed over long depth intervals, making location of lithologic contacts uncertain. Finally, much textural information concerning bulk density, porosity, pore morphology, hydraulic conductivity, and so forth is lost. MAGNETIC PROPERTIES If oriented samples are obtained (Table 2), the tools of pa- leomagnetics may be applied (Van der Voo and Channell, 1980). They may yield information about rock history through the alter- ation of magnetic minerals, magnetostratigraphy, and magnetic anistotropy; ages may possibly be obtained from paleopole posi- tions and reversals. At a minimum, without oriented core, the results may be useful in determining paleotemperatures or for mapping stratigraphic horizons from changes in magnetic prop- erties (Strangway, 1981) such as alteration of magnetic minerals, magnetic susceptibility, and field intensity. Both kinds of infor- mation will be useful in interpreting surface magnetic anomaly signatures. Magnetic field intensity and magnetic susceptibility drill hole logs are available, but the remaining measurements are
40 only available through core studies. In addition to all the usual problems and uncertainties in working with cuttings, pieces of the drill or core bit appear in the cuttings, destroying their utility for magnetic measurements. At intervals throughout the hole (deter- mined from the continuous core and logs), gyro-oriented sidewall samples should be acquired for paleomagnetics, strain, and other measurements requiring oriented core. ELECTRICAL PROPERTIES In order to measure the electrical properties of rock sam- ples, the core must be sealed to isolate it from the atmosphere to prevent changes in water content and chemistry. Electrical measurements will yield information related to texture, porosity, permeability, chemical reactivity, alteration, clay mineralogy, and so forth. Some of these may be affected by rock anisotropy so that oriented samples will be needed. Of particular interest are the electrochemical measurements that make possible investigation of active processes, such as oxidation-reduction and ion exchange reactions, as well as the diffusion and kinetic coefficients of the reactions (Olhoeft, 1981). By putting core samples in laboratory environmental cham- bers, electrical properties can be used as monitors of changing mineralogy and chemistry as the sample is subjected to different conditions of pressure, temperature, and environmental chemistry. This is a powerful technique for extrapolating conditions in the crust away from the original hole in depth, lateral extent, and time. Electrical properties are sensitive to small amounts of clay minerals and their distribution within the rock. Different elec- trical responses and hydraulic conductivities are obtained from pore-bridging, pore-lining, and pore-blocking clay morphologies. Electron microscopy of core samples is required to distinguish these morphologies and calibrate the interpretation of the elec- trical (and other) bore hole logging measurements. In addition to calibration of logging results, knowledge of electrical proper- ties of rocks encountered in drilling will aid in interpretation of geophysical studies, such as magnetotelluric soundings that might be carried out far from the drill hole but in geologically similar terrane (Stanley et al., 1977).
41 HYDRAULIC PROPERTIES Both these measurements are best done in situ, because the area of investigation for core is too small to be representative of the general geological environment. Core measurements, however, are crucial to calibration and proper interpretation of in situ mea- surements. Porosities from a variety of logging techniques give values representative of microcrack, grain-boundary, fracture, and closed-structure geometries. Core measurements are necessary to calibrate the small-scale porosity measures so that larger-scale porosities determined from logs are themselves accurate. Perme- ability measurements made in borehole are effective bulk values applicable only over the interval tested. That is, packers can be spaced so that either horizontal bulk permeability or effective ver- tical permeability can be measured. Permeability measurements on core give a grain-size lower-limit to these in situ measurements. The difference between permeability measured in the laboratory on core and that at various scales measured in situ provides essential information about structural control of hydrological parameters not available by any other means. Core sample size requirements are similar to those for other physical property measurements (Table 2). ELASTIC AND DEFORMATIONAL PROPERTIES Elastic constants are used in modeling rock deformation. Dy- namic elastic constants can be obtained from measurements of density and of compression al and shear wave velocities as func- tions of pressure on core samples of most rocks. Static conditions, in which a sample is loaded by an externally applied stress, are more appropriate for measurement of elastic constants for mate- rials that deform by creep. Fracture and frictional strengths also must be known in order to accurately model rock deformation, es- pecially faulting. Such data are useful in interpretation of in situ stress measurements. Preservation of core material in a condition that is representative of rock properties at depth is essential. Sam- ple requirements are similar for all these types of determinations. Core samples about 2.5 cm in diameter and 7.5 cm long are suffi- cient (Table 2). The small improvement in quality of data yielded by oriented cores probably does not outweigh the additional effort and expense that may be necessary to achieve it.
