ABSTRACT

During the past two years, concessions have been granted for oil exploration in many offshore areas where no geological or geophysical information is available. In many such areas, reconnaissance surveys are required to determine whether or not a sedimentary basin exists, and if it does to delineate its size and shape and to estimate its depth. A wide choice of geophysical techniques is available for obtaining such information but the selection of the most suitable approach requires familiarity with the type of geological information that each method yields as well as with the economic factors that are involved.

The techniques generally used for such reconnaissance surveys are 1) gravity, employing both shipborne and bottom meters, 2) magnetic, both with airborne and towed marine magnetometers and gradiometers, 3) seismic refraction, both with dynamite and with non-explosive sources, 4) reflection profiling with various non-explosive sources such as sparker or air gun and 5) conventional reflection. Many of these techniques involve types of equipment which are so new that their capabilities may not be generally known to those planning exploration programs. The characteristics of the various available systems will be summarized with emphasis on those most recently introduced.

It is the purpose of this paper to review the characteristics of the geophysical data and the nature of the geological information which can be derived from the various techniques used in reconnaissance. For greatest effectiveness the different methods should be integrated into a coherent program that will yield the most information at the lowest cost. Information available from adjacent areas must be drawn upon in evaluating the desirability of the various approaches.

In general reconnaissance packages involving combinations of several tools such as shipborne gravity, towed magnetometer, single-ship refraction and seismic profiler will give the desired information most rapidly and economically. Cost factors for this type of survey will be compared with costs involved in more conventional reflection surveys; there is often a trade off between cost and information which must be taken into account in deciding upon the best program.

INTRODUCTION

Although the search for oil under the continental shelves has been going on for nearly a quarter of a century, only a small proportion of, the total shelf area of the world has been covered up to this time. There are still vast offshore areas, potentially petroliferous, that have never been explored for oil or gas at all. In some of these the governments of the adjoining countries have not granted concessions. In other areas, as in the Arctic regions, geographic and logistic factors are thus far unfavorable. In still others, the geology so far as it is now known makes the oil prospects look unpromising, so that the incentives to carry out exploration work are limited for the present.

During the past year or two, an unusually large number of offshore regions have been opened to oil and gas exploration for the first time. These include the shelves around the Indonesian islands, the Gulf of Thailand, the Argentine shelf and the coastal waters surrounding the southern part of Africa.

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