ABSTRACT

Improvements in drilling, completion, and stimulation techniques have increased the productivity of many shallow shaly-sand gas reservoirs. The Bowdoin formation in Northern Montana is a good example of this reservoir type. The depositional environment for the Bowdoin formation is placed in deep water in the Upper Cretaceous time period. In the producing interval, the formation consists of a series of thin laminated sandstones, silts, bentonitic shales, and clays. The entire interval can be over 200 feet thick. Core and log indications show the clay content to be near 50 percent in the producing interval. Since the shale content is so high, and the permeability consequently low, it becomes difficult to evaluate this formation by conventional methods.

A resort to empirical methods is suggested. This paper reports the successful use of porosity-overlay methods to predict and achieve commerical-scale gas production in over fifty wells in the Bowdoin producing horizon. Sonic-neutron comparisons are used for gas detection, and sonic-density comparisons for determination of shale content. The information gained from these logs is also used to plan completions; a critical factor, since optimum stimulations are essential to profitable operations in formations like the Bowdoin.