8. log evaluation of leg 1 of the deep ... - deep sea drilling

8. LOG EVALUATION OF LEG 1 OF THE DEEP SEA DRILLING PROJECT1

K. F. Kennedy, H. A. Shillibeer & C. E. KonenGulf Research & Development Company, Houston, Texas

Introduction

The objective of this chapter is to evaluate the well logsthat have been run on Leg 1 of the Deep Sea DrillingProject. Since, however, only one complete set of re-sistivity, gamma-ray, neutron, and qualitative densitylogs is available, this section will consist of an evalua-tion of the logs from Site 1.

The use of well logs for evaluating the physical andchemical composition of subsurface formations is acommon practice in the petroleum and mining indus-tries. The devices normally utilized for these procedureswere designed for larger diameter openings than thoseused in this project. The size limitation imposed heremade it necessary to use a qualitative density measure-ment rather than a quantitative log.

Evaluation of Log Responses

Prior to an evaluation of the logs, the curves weredigitized and re-recorded as a composite log. The com-posite log consisted of gamma-ray, resistivity, and den-sity curves.

An attempt was made to establish a density scale interms of grams per cubic centimeter by using the re-ported GRAPE (Gamma Ray Attenuation Porosity Eval-uator) measurements of bulk density, however, thiswas not successful. This does not preclude the estab-lishment of a quantitative scale at a later date; if a suf-ficient number of bulk density measurements becomeavailable. It may then be possible to establish a rela-tionship between bulk density and counting rate.

It should be noted that variations in the diameter of theborehole may cause large excursions in the densitycounting rate. The effects of variation in hole diam-eter are not restricted to the density measurement

Prepared at the request of the JOIDES Advisory Panel onWell Logging in the absence of a specialist in the evaluation ofwell logs on the permanent staff of the Project.

alone. If hole size variations occur, the recordedvalues of resistivity and natural gamma rays will vary,too. Resistivity and gamma-ray counting rate will alsovary with mud resistivity and mud weight, respectively.

The neutron curve which was recorded inside the drillpipe reflects little information about the formationspenetrated. The lithological information, if any, iscompletely obscured by the drill pipe, collars, etc.

The composite log shows a very good correlation be-tween the gamma-ray, resistivity, and density curves.Whether the log responses are only reflecting lithologicalvariations is questionable. The general increase of re-sistivity and the decrease of density counting ratewith depth indicate increasing consolidation. A numberof zones, not cored, denote the existance of silt andquartz laminae. These intervals are indicated by low re-sistivity, high density counting rates and low gamma-raydeflection. The separation of the 16 inch and 64 inchnormal curves signifies that invasion of drilling fluidinto the formations has occurred. This would suggestthat the formations are permeable and have a largergrain size than the surrounding beds.

The gamma-ray curve also shows an increase in naturalradioactivity with depth. This could indicate that holeenlargement has occurred in the upper portions of theborehole with a gradual reduction with depth.

Conclusions

1. The gamma-ray, resistivity, and density logs show de-finite correlative features.

2. The large excursions of the density log may or maynot reflect lithological variations depending uponthe constancy of hole diameter. The large variationsin counting rate do not necessarily reflect largechanges in formation density since a logarithmicfunction probably relates bulk density and countingrate.

3. When more bulk density data becomes available, itshould be possible to calibrate the density log interms of bulk density.

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