Formation Evaluation and Gas Detection in Shallow, Low-Permeability, Shaly Sands of the Northern Great Plains Province
Authors: Kukal Gerald C. 
Published: December 2007

Summary
Montana log, core, and production data are combined with geology in a systematic approach to improve log analysis of shallow Upper Cretaceous gas sands having a high silt/clay content. Qualitative (overlay) techniques are reviewed with emphasis on the normalized At-compensated-neutron overlay for gas detection.Log interpretation of the Bowdoin member of the Carlile shale and Eagle sandstone in the area peripheral to the Bowdoin Dome field is discussed in detail. Porosity-tool responses are examined with respect to observed lithology and mineralogy. Lithology crossplots, when compared with predicted log responses, may aid in detecting gas and minerals that could be related to natural fracture systems (e.g., gypsum and pyrite).Quantitative techniques for estimation of effective porosity, volume clay, and water saturation are presented. The total shale relation appears to give useful water saturation values. Log parameters relating to this equation are generalized and refined.

Introduction
With increasing gas prices, exploration for shallow, low-deliverability resources in the northern Great Plains province is becoming more attractive. Log interpretation always has been a problem in the Upper Cretaceous sands of north-central Montana due to the extreme shaliness of potential reservoirs, very fresh formation water, high water saturations, complex lithology, and indefinite logging assumptions. This paper suggests techniques to improve gas detection and formation evaluation, particularly in marginally economical areas peripheral or adjacent to production in the Bowdoin Dome field (Fig. 1).This report uses a synergetic approach to improve log interpretation. The lithology and mineralogy of the Bowdoin member of the Carlile shale and the Eagle sandstone are described in detail. This information is useful for making predictions of log response and in refining logging assumptions. Suggestions made for crossplot methods on the basis of these log responses may be useful to detect gas and/or to identify mineral components of the complex lithology. Finally, suggestions are presented for improving quantitative log interpretations.

Lithology and Mineralogy
The Upper Cretaceous clastic section in the Bowdoin Dome area is dominated by shales and clayey siltstones. The rock may carry various amounts of gypsum, pyrite, carbonates, and micas. Reservoir rocks may contain up to 50% clay minerals.The Bowdoin sand (Fig. 2) is a poor-quality reservoir rock except where fractured. Most of the gas storage is in silty laminae less than 1 cm thick. Permeability is low, and artificial stimulation methods are used to make the marginal wells more economical.Table 1 is a summary of the recent X-ray mineralogical analysis of 30 sidewall cores taken from two wells. The mineral composition for each well is expressed as weight percent and is averaged into] a tool analysis and a clay size fraction analysis.The sieve size distribution of Well 2962 cores (Figs. 1) consists of 60% silt (2 to 62 m), 35% clay (greater than 2 m), and 5% sand (greater than 62 m). The analysis shows the rock to be made up dominantly of quartz (52%), with the major clays being illite and kaolinite. Pyrite and gypsum are present in significant amounts.Well 0370 cores (Fig. 1) comprise a similar size distribution with 62% silt, 35% clay, and 30% sand. The mineralogy is nearly identical to Well 2962.
JPT
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