"What is the matter with Porcupine dome?" This question is often asked by explorationists working in central Montana. Although it does not have as much closure as some central Montana structures, in areal extent it is the largest feature in the Big Snowy anticlinorium. Twenty wells drilled

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in the Judith River formation rimrock bounding the higher structural part of the dome have failed to establish production.

To determine the relation of these wells to surface structure, a reconnaissance with the aid of air photos was undertaken during the 1958 field season. An attempt was made to find mappable units in the apparently monotonous Upper Cretaceous sequence cropping out on the central part of the dome. Although lithologic changes in these formations are subtle, it is believed that substantial progress has been made in recognizing the distribution of these rocks. As a result a better understanding of the structural configuration of Porcupine dome has been reached.

From this preliminary work several important exploratory possibilities are suggested.

1. The highest structural point on the dome has not been tested.

2. The most prominent anticlinal axial trend, approximately 36 miles in length, has not been drilled for a distance of 30 miles. Several untested closures are indicated along the trend.

3. The present structure of the dome is essentially the result of Laramide orogeny. Application of hydrodynamic factors, as modified by stratigraphic controls and ensuing time, points to possible areas for oil and/or gas entrapment.

It is concluded that Porcupine dome has not been adequately explored and that its possibilities for hydrocarbon production can be determined only by the drilling of favorably positioned wells.

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