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Publications > Expedition Publications > Logging Summaries

Logging Summaries

IODP Expedition 334:

Costa Rica Seismogenesis Project 1 (CRISP-A1)

Expedition 334 Scientific Party

Introduction

    Figure 1. Location map of CRISP program sites, IODP Expedition 334.

    Integrated Ocean Drilling Program Expedition 334, the Costa Rica Seismogenesis Project (CRISP), was designed to understand the processes that control nucleation and rupture of large earthquakes at an erosional convergent margin. The Costa Rica location was selected because of its relatively thin sediment cover, fast convergence rate, abundant seismicity, subduction erosion, and change in subducting plate relief along strike. CRISP drilling complements other deep-fault drilling (San Andreas Fault Observatory at Depth and Nankai Trough Seismogenic Zone Experiment) and investigates the first-order seismogenic processes common to most faults and to those unique to erosional margins. The primary goals of Expedition 334 were to characterize the lithological, physical, and frictional properties of upper plate material; to estimate the subduction channel thickness and the rate of subsidence caused by subduction erosion; to characterize the fluid flow system and thermal structure of the erosive margin; and to determine the change in the stress field across the updip limit of the seismogenic zone.

    The downhole logging program of Expedition 334 was designed to complement the core sample record by measuring continuous, in situ profiles of physical properties such as bulk density, porosity, resistivity, and natural gamma ray radiation. In addition to these formation properties, downhole logging provides oriented images of the borehole wall useful to determine the directions of bedding planes, fractures, and borehole breakouts. In the conventional technique of wireline logging, downhole measurements are taken by tools lowered in a previously drilled borehole. Wireline logging has had limited success in deep holes in unconsolidated clastic sequences such as those planned for Expedition 334, because these holes tend to be unstable after drilling. In logging-while-drilling (LWD), downhole measurements are taken by instrumented drill collars in the bottom-hole assembly (BHA) near the drill bit. Hence, LWD measurements are made shortly after the hole is drilled and before the adverse effects of continued drilling or coring operations. LWD has been successful in previous scientific drilling expeditions to convergent margins (Nankai Trough, Barbados, and Costa Rica), and was selected as the logging technique for Expedition 334.

     

Logging Operations

    Figure 2. Logging-while-drilling (LWD) bottom hole assembly used during IODP Expedition 334. LWD tool descriptions are available at:

    http://iodp.ldeo.columbia.edu/TOOLS

    http://slb.com/services/drilling/mwd_lwd.aspx

     

    LWD operations were carried out in Holes U1378A and U1379A (see location map in Figure 1). The Schlumberger LWD tools used during Expedition 334 were the geoVISION 675 (near-bit electrical resistivity, resistivity images, natural gamma ray), the arcVISION 675 (annular borehole pressure, resistivity, natural gamma ray), the adnVISION 675 (bulk density, neutron porosity, density and ultrasonic caliper), and the TeleScope 675 (drilling mechanics data and real-time telemetry). All these tools had a 6.75 inch (17.1 cm) diameter and were located above a 8.5 inch (21.6 cm) drill bit. Figure 2 shows the configuration of the LWD bottom hole assembly with the depth of the measuring sensors relative to the bit.

    LWD operations in Hole U1378A were stopped at a total depth of 455 mbsf and did not reach the original depth objective because drilling could not progress due to high standpipe and downhole pressures, backflow when making pipe connections, and large torques on the top drive. Similar problems were encountered later in coring Site U1378 and the nearby Site U1380 (drilled to 524 mbsf and 482 mbsf, respectively). Hole U1379A was successfully drilled and logged by LWD to a total depth of 966 mbsf, exceeding the original target depth of 950 mbsf.

    The measurements recorded by the LWD tools were downloaded and processed without difficulties and were of high quality in both holes, except for the geoVISION resistivity image data. The orientation system of the geoVISION tool malfunctioned in Hole U1378A and the tool clock did not record time properly in Hole U1379A. The geoVISION data were sent to a Schlumberger LWD data processing center in Houston, but attempts to recover useful measurements were unsuccessful. The adnVISION tool, however, measured borehole images of density and hole diameter and provided valuable information on borehole breakouts (see below).

    LWD logs were acquired at the beginning of Expedition 334 in the first hole drilled at Sites U1378 and U1379. As these holes were drilled without coring, the LWD data had to be monitored to detect gas entering the wellbore. This LWD hydrocarbon monitoring procedure substitutes the IODP standard of using gas ratio measurements made on cores. The LWD monitoring protocol used during Expedition 334 was similar to protocols used in previous IODP expeditions where LWD holes were drilled before coring, Expedition 308 (Gulf of Mexico hydrogeology) and 311 (Cascadia margin gas hydrates). The primary measurement used for gas monitoring was annular pressure while drilling (APWD), measured downhole by the arcVISION LWD tool and transmitted to the surface in real time. As free gas in the borehole lowers the borehole fluid density and decreases the pressure, the monitoring procedure consisted primarily in monitoring variations of APWD over a baseline hydrostatic pressure trend. A sustained drop in pressure greater than a specified threshold required stopping drilling and circulating a full volume of the borehole annulus while monitoring pressure. If the pressure remained static and equal to the hydrostatic pressure trend, drilling could be resumed. If the pressure was lower than hydrostatic, the protocol required killing the hole with weighted mud and abandoning the hole. The specific threshold pressure drops requiring attention were chosen to ensure that gas flow in the well could be killed with weighted mud. During LWD operations in Expedition 334, no sustained pressure drops that exceeded the threshold set in the monitoring protocol were observed, and no drilling interruptions were necessary.

     

Logging Results

    Figure 3. Summary of LWD measurements in Hole U1378A.

    Hole U1378A

    Figure 3 shows a summary of the LWD data acquired in Hole U1378A. The LWD measurements are of high quality, except for anomalously low densities measured in intervals where the borehole was enlarged to 10-12 inch (25-30 cm). These enlarged borehole intervals with low measured densities are visible in the borehole radius image of Figure 3 in the depth intervals 290-310 and 340-370 mbsf (see also the comparison to core data below).

    Two logging units were defined on the basis of the LWD measurements. Logging Unit 1 (0-82 mbsf) corresponds to a compacting sequence where density and resistivity increase and porosity decreases with depth. The top of Logging Unit 2 (82-455 mbsf) is marked by a step increase in density and resistivity, which then increase slowly with depth. Porosity shows a matching small decrease with depth in Logging Unit 2.

     

    Figure 4. Summary of LWD measurements in Hole U1379A.

Hole U1379A

Figure 4 shows a summary of the LWD data acquired in Hole U1379A. The LWD measurements are of high quality, except for anomalously low densities measured in intervals where the borehole was enlarged to 10-13 inch (25-33 cm). These enlarged borehole intervals with low measured densities are visible in the borehole radius image of Figure 4 in the depth intervals 340-360 and 600-890 mbsf (see also the comparison to core data below).

Four logging units were defined on the basis of the LWD measurements. Logging Unit 1 (0-492 mbsf) corresponds to a compacting sequence where density and resistivity increase and porosity decreases with depth, reaching nearly constant values at the base of the unit. The top of logging Unit 2 (492-600 mbsf) is marked by a small step increase in density and resistivity. The distinguishing feature of Logging Unit 3 (600-892 mbsf) is the presence of many borehole enlargements, which are likely to correspond to unconsolidated sand layers or fractured intervals. Logging Unit 4 (892-955 mbsf) corresponds to the basement rocks of the sedimentary sequence, and is clearly identified by a sharp shift in the baseline of natural gamma ray, density, and resistivity logs. Compared to the sediments above, the basement unit shows a marked increase in the average density and resistivity and a corresponding decrease in porosity.

 

Scientific Highlights

    Borehole Breakouts

    Borehole breakouts are sub-vertical hole enlargements that form on opposite sides of the borehole wall by local failure due to non-uniform stress. In a vertical borehole, the breakout direction is parallel to the minimum principal horizontal stress orientation and perpendicular to the maximum principal horizontal stress orientation. Therefore, borehole breakouts are key indicators of the state of stress in the subsurface.

    Despite their limited azimuthal resolution (image data are sampled in 16 azimuthal sectors, i.e., every 22.5°), the LWD borehole images acquired in Expedition 334 clearly display borehole breakouts as two parallel, vertical bands of low density or large radius 180° apart. Hole U1378A shows an interval of well-developed breakouts at 220-438 mbsf (Figure 3). The average azimuth of the breakouts is roughly NE-SW to ENE-WSW, indicating that the maximum horizontal stress is oriented NW-SE to NNW-SSE. Breakouts are evident in Hole U1379A in the interval 292-885 mbsf (Figure 4). In Hole U1379A, the average azimuth of the breakouts is roughly N-S to NNW-SSE, indicating that the maximum horizontal stress is oriented E-W to ENE-WSW.

       

      Figure 5. Comparison of LWD log data in Hole U1378A (colored curves) and core measurements in Hole U1378B
      Figure 6. Comparison of LWD log data in Hole U1379A (colored curves) and core measurements in Hole U1379C

    Core-log integration: natural gamma ray and density

    Figure 5 and Figure 6 show a comparison of natural gamma ray radioactivity, bulk density, and porosity measured by LWD and in core samples at Sites U1378 and U1379. This comparison is useful to correlate depths in the LWD logs and depths of core samples and to integrate information from log and core measurements.

    Natural gamma ray log measurements are calibrated to a degree API (gAPI) scale by comparison to a standard artificial formation built to simulate about twice the radioactivity of a typical shale and conventionally set to 200 gAPI. In contrast, the natural gamma ray (NGR) measurement made on whole core sections on the JOIDES Resolution is in units of counts per second. The comparison of log and core natural gamma ray measurements in Figure 5 and Figure 6 shows that their curves overlap if 1 count/second equals about 2 gAPI. Most patterns in the log and core natural gamma ray records match closely, with only a few depth shifts likely due to the data being collected in different holes and to uncertainties in the depth measurement. The general agreement between log and core measurements of natural gamma ray radioactivity indicates a close correlation in the depths of the log and core records.

    Figure 5 and Figure 6 also compare image-derived bulk density logs (IDRO) to densities measured on whole core sections by gamma-ray attenuation (GRA) and on discrete core samples by moisture and density (MAD) analysis. The bulk density values are generally consistent, with the exception of several intervals where the log densities are clearly lower than the core densities and the corresponding logged porosities are unrealistically high (e.g., the interval 350-375 mbsf of Site U1378). As noted above, these extremely low logged values of density are likely to be due to borehole enlargements.

    There are intervals in both sites where the core densities are systematically lower than the logged densities (110-200 mbsf at Site U1378 and 110-500 mbsf at Site U1379). The differences are 3-11% of the overall bulk density value. A contributing factor to this difference may be core expansion by elastic rebound, as many cores in these depth intervals showed more than 100% recovery. The MAD porosities are density porosities calculated from the measured bulk and grain densities in each core sample. As the MAD densities in these intervals are slightly lower than the log densities, the MAD porosities are slightly higher than the porosities computed from the density log.

     

Summary

Logging-while-drilling of Holes U1378A and U1379A of IODP Expedition 334 measured profiles of natural gamma-ray radioactivity, density, neutron porosity, and electrical resistivity together with images of the borehole wall. While technical difficulties prevented us from acquiring resistivity images, the LWD tools successfully collected 360-degree coverage images of bulk density and borehole radius. The borehole radius images show clear evidence of borehole breakouts, which form when there are differences in the principal horizontal stresses. Analysis of this unique borehole image data set will provide estimates of the state of stress in the subsurface of the Costa Rica convergent erosive margin.

 

    Alberto Malinverno: Logging Staff Scientist, Borehole Research Group Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA
    Email: Alberto Malinverno

    Saneatsu Saito: Logging Scientist, Institute for Frontier Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061 Japan
    Email: Saneatsu Saito

     


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