Log Interpretation In Bolivia

Bolivia, one of the largest countries in South America, offers many interesting and challenging interpretation problems to the Log Analyst. The complex sub-surface geology in addition to the wide range of porosities, permeabilities and formation water salinities found throughout the sedimentary basin requires the use not only of logs but also the frequent assistance of wire-line formation tests, sidewall cores, dipmeters and the continuous gas detector. The great majority of the wells in Bolivia have been drilled with fresh-water mud. In view of this fact and the problems mentioned above, almost all commercially available logging services have found useful application in one part or another of the country. The 6FF40 Induction-Electrical log, Microlog, Sonic and Density logs are standard surveys. In many fields caving is a major problem and cement jobs are not always satisfactory; hence the regular use of Cement Bond logs. The Dipmeter has been instrumental in locating a number of subsurface anomalies such as faults and unconformities, some of them hydrocarbon traps. The wire-line Formation Tester has found wide application in the evaluation of the commercial possibilities of the sands traversed by the borehole. The most frequently used interpretation method is the Rwa technique. Among its advantages, besides its time-saving features, are its applicability in shaly sands and the possibility of determining the value of Rw which is often unknown. When Sonic and Density logs are taken over the same intervals it is customary to make q-plots to evaluate the relative amount of dispersed shale in the sands of interest. Qualitative and quantitative interpretation of Formation Tester results is standard procedure and the local experience gained from the many tests taken in the past usually allows for accurate production predictions. Examples of logs taken under typical conditions in the main oil provinces of Bolivia will be found enclosed in the text. Whenever thought necessary, a short reference is made to local geography and geology.