808 The Leading Edge July 2010 SPECIAL SECTION: B r a z i l Brzail   S mall-scale changes in rock prop- erties, such as those resulting from centimeter-scale depositional layering, are usually undetectable in standard borehole logs (Murphy et al., 1984). Even high- resolution logs with a small sampling interval (e.g., 2 inches) may still have a relatively large investigation volume. Tis presents a problem when we wish to capture the full variation in physical properties for purposes such as rock physics modeling. One method that gives useful in- formation in an automated way and at a higher resolution than downhole logs is X-ray computed tomography (CT) of cores (Mees et. al., 2003). Samples of marine sediment cores are the funda- mental source of information on seabed character, depositional history, and envi- ronmental change (Rothwell and Rack, 2006). Te CT number encodes a com- bination of mass density and electron density proportional to the mean atomic number. Te environmental conditions dur- ing the sedimentation could cause pe- riodic variations of carbonate calcium concentration of the sedimentary envi- ronment, refecting periods of higher and lower ratios between pelagic carbonate plankton productivity and terrigeneous (clay, silt) sediment input. Burial diagenesis can enhance the original contrast be- tween the compositionally distinct layers. In this paper, we take advantage of a unique densely sam- pled CT and ultrasonic data set in a deep shale environment ofshore Brazil to demonstrate that these data provide valuable information on centimeter-scale heterogeneity that can be at- tributed to sedimentary cyclicity. Te Campos Basin data set Te data come from a well in Campos Basin, ofshore Bra- zil. Te interval is approximately 2800 m below sea foor, in an Oligocene-to-Miocene hemipelagic sequence of alternat- ing marls and shales that forms the overburden to the main sandstone reservoir. Te data set consists of CT images of the whole core, ultrasonic P-wave measurements on core plugs, and well logs. MARCOS HEXSEL GROCHAU and ELISABETE CAMPOS, Petrobras DARIUSH NADRI, T OBIAS M. MÜLLER , and BEN CLENNELL, CSIRO Earth Science and Resource Engineering BORIS GUREVICH, Curtin University of Technology X-ray CT images were taken every 5 cm from cores at the depth of around 2800 m. To acquire these CT images, we used a Picker-PQS scanner with X-ray voltage set at 130 kV and current at 100 mA. Te time per scan was 4 s and image resolution was 0.5 × 0.5 mm. Te standard deviation of the X-ray CT value measurements is 0.56%. Te slice thickness is 1 cm, meaning that the support volume of the measurements is smaller than the sampling interval. Tis is an important dif- ference from the high-resolution downhole density log, where the sampling interval is similar (2 inches = 5.08 cm), but the support volume is larger (≈10 cm). Ultrasonic (500 kHz) P-wave velocities were measured at diferent pressures on 23 rock samples, providing comple- mentary information about their elastic properties. Te mea- surements were done for horizontally and vertically oriented samples using a varying confning pressure from 1000 to 6000 psi. SPECIAL SECTION: B r a z i l Figure 1. X-ray CT values obtained by computing the average value over a circular area for each of the 143 CT images at every 5 cm. Alternating low and high CT values provides evidence of cyclicity. Hounsfeld units (a) and arbitrary units after conversion to an 8-bit gray scale (b). Downloaded 15 Jul 2010 to 130.116.146.70. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/