The present facilities for isotope age measurements at the Shell Development Company, Exploration and Production Research Division Laboratory, permit measurement by all of the well established age methods--potassium-argon, rubidium-strontium, uranium-lead, and thorium-lead. This gives us the advantage of a wide choice of minerals, and at least among the igneous and metamorphic rocks some age data can usually be obtained by at least one of the methods mentioned. Age measurements are often used to determine the age of igneous rocks encountered in wells. This enables the geologist to judge whether additional sedimentary reservoirs could be expected. In some places the age might suggest that "true igneous basement" has been reached and that no further sedimentary section can e reasonably expected. In other places, the presence of igneous sills and dikes, or volcanic layers is suggested, below which additional reservoirs could be expected. Samples of bottom-hole rocks from wells drilled prior to the general use of age determination methods have now been examined with the same objective. In general the results of these measurements on wells in different parts of the United States can be shown to fall in line with the suggested basement periods (G. R. Tilton and S. R. Hart, Science, vol. 140, p. 357, 1963).

Frequently age determinations are used to determine time and influence of igneous activities in new exploration areas. In this case surface samples are used. This type of information helps to unravel the structural evolution of an area. This is important background for any study of the hydrocarbon potential of a new exploration province.

The use of glauconite for potassium-argon and rubidium-strontium dating of sediments has been common for many years in oil exploration for obvious reasons. The use of the minerals in bentonite to date sediments is the subject of another paper in this symposium (R. E. Folinsbee, H. Baadsgaard, G. L. Cumming, and J. Nascimbene, Radiometric Dating of the Bearpaw Sea). The first dates on both biotite and zircon in bentonites were those measured at the Shell laboratory.

Recently we have been measuring the age of detrital minerals in sediments as an aid in determining the source of conglomerates and micaceous sandstones, and this work is continuing.

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