46. Integration of Sfl and Ild Electrical Resistivity Logs during Leg 133: an Automatic Modeling Approach

An automatic technique is developed for integrating two electrical resistivity logs obtained during Leg 133, namely, the deep induction log (ILD) and the spherically focussed log (SFL). These logs are routinely run by the Ocean Drilling Program in lower resistivity sedimentary environments, where drilling through soft, poorly consolidated lithologies often results in variable hole diameters. Examples are given to show the superior resolution of the SFL, compared to the ILD, in lithologies encountered during Leg 133, but also the degradation of the SFL response caused by variable hole conditions. A forward modeling program is used to calculate the ILD response, and a corrective scheme is developed that uses the SFL log as the starting point of an "iterative-corrective" procedure that uses, in effect, the ILD log to correct the SFL log for degradation caused by changes in the diameter of the borehole. The calculated induction log (using the corrected SFL log as the input model) is shown to be near to the ILD measured downhole. The authors consider this to be proof that the corrected SFL is a better estimate of the formation resistivity downhole (Rt) than either the SFL or ILD taken alone. Corrections are shown to be substantial in poor hole conditions, and examples are given where geological cyclicity might be confused with hole effects if uncorrected logs were to be used. Corrected SFL logs are shown to combine the high resolution of the SFL with the greater depth of investigation of the ILD. The technique is shown to improve the results of resistivity logging during Leg 133 in thin layers and to have applications when identifying paleoclimatic signals (e.g., in intervals having high sedimentation rates) and in diagenetically altered reefal carbonates where dissolution and cementation caused by changes in sea level may have produced thin layers with contrasting resistivities. INTRODUCTION A large number of resistivity logging sondes are in use for logging oil reservoirs, ODP holes, groundwater wells, and geotechnical site investigation boreholes. Electrical surveying in drill holes was invented almost 60 yr ago by Schlumberger et al. (1934). The results were applied successfully by Archie (1942) to predict the porosity and/or water saturation of oil reservoirs. Electrical resistivity logging became widely used in the oil industry as it was used as an important input for predicting of oil reserves. Major advances were made in the early 1950s by the Schlumberger Corporation with the development of the focused galvanically coupled arrays (Doll, 1951), and inductively coupled resistivity logging techniques (Doll, 1950; 1953). Today resistivity devices are available that allow assessments over a range of both depths of investigation and resolutions. The vertical resolution and depth of investigation for the resistivity devices used during Leg 133 are shown in Figure 1. The "micro" resistivity imaging device (FMS) has a depth of investigation of less than 1 in. and a resolution of 0.2 in The "macro" resistivity devices (SFL and ILD) have depths of investigation ranging from 25 in. (SFL) to 150 in. (ILD) and resolutions of 30 in. (SFL) and 60 in. (ILD). The depth of investigation of the deep induction log (ILD) is controlled by the resistivity of the formation, being lower in low resistivities. Figure 1 shows that a gap exists in both resolution and depth of investigation between the FMS and SFL (factors of 100 and 20 approx. respectively), indicating there may be difficulty in relating FMS images to SFL and ILD resistivity logs in environments other than those with layering normal to the wellbore. McKenzie, J.A., Davies, PJ., Palmer-Julson, A., et al., 1993. Proc. ODP, Sci. Results, 133: College Station, TX (Ocean Drilling Program). 2 British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom. 3 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, U.S.A. 1000