Lithology

Natural gamma ray serves as an indicator of lithology when substantiated with nearby core or outcrops. Since not all gamma-ray logs in the study area were scaled in API units, the gamma-ray logs were normalized to vary from 0 (salt) to 100 (shale). The minimum and maximum gamma values used in the normalization were obtained from an interval extending from 20 ft above the top 3-finger dolomite to 100 ft below the top Hutchinson salt. Mean normalized gamma values were extracted for 9 intervals from D1 to S3 and are displayed in map view in Figure 5. Mean gamma was not extracted for stratigraphic intervals above D1, because of varying depths to the casing shoe above this marker. Steel casing generally reduces the gamma ray response of the logging tool and casing collars further reduce the signal, thus limiting the use of gamma ray data in cased intervals for quantitative purposes. .

The mean normalized gamma ray maps of the S2/M6–S3/M7 (Figure 6A) and S1/M5-S2/M6 (Figure 6B) intervals show fairly uniform low gamma over most of the mapped area, indicating that these intervals are primarily comprised of halite. Increased gamma values along the eastern mapped area are related to facies changes in the halite beds, believed to reflect the depositional limit of the halite along the margins of the Hutchinson Salt evaporite basin (Watney et al., 1988). For the S2/M6-S3/M7 interval, there is also an increase in the mean gamma associated with the thinning of the isopach below 6 ft, most likely an artifact of the tool resolution.

Within the upper Wellington Shale above the top of Hutchinson salt (S1), normalized gamma ray shows that the shaliest mapped interval is between the base of the 3-finger dolomite and M1. The cleanest interval (lowest gamma) is M4-S1/M5, just above the top of Hutchinson salt.

All of the intervals between S1 and D1 appear to show a northwest-southeast-trending band of lower gamma ray which follows the trend of gas-producing vent wells in the middle of the survey area. This feature is most pronounced for M1A-M2 (Figure 6F), where the trend is also seen in the isopach map. For M4-S1/M5 (Figure 6C), a north-south-trending band of low gamma is superimposed on the northwest-southeast trend.

Elevated gamma radiation of a thinner interval may reflect the reduction of cleaner evaporite through localized dissolution and relative increase in shale content, e.g., along the northwest-southeast thinning at the center of the S1-S2 isopach. Areas of lower gamma ray and interval thickening may suggest greater preserved evaporite or thicker carbonate, e.g., the S1-S2 thick centered on Hutchinson. Higher gamma ray and a thicker interval may relate to a greater proportion of clastics in place of evaporites or carbonate strata, e.g., the southwestern edge of the M3-M4 maps and the western and northeast portions of the top 3-finger-M1 maps.

The 3-Finger Dolomite Interval

An isopach of the 3-finger dolomite (Figure 7A) shows it to be between 17 and 20 ft thick throughout most of the study area, with no clear thickening or thinning trend.

Variations in lithology of the 3-finger dolomite interval were investigated by looking at maps of mean normalized gamma ray (Figure 7B) and minimum normalized gamma ray (Figure 7C). Because dolomite is brittle and fracture-prone in contrast to shale and evaporites, which are in general more ductile, dolomite is likely to undergo induced fracturing and be able to maintain open fractures when subjected to high-pressure gas that exceeds the fracture pore pressure of the dolomite. An increase in the amount of gypsum and shale, both less brittle than dolomite, would tend to inhibit fracturing and prevent migration of gas, thus, the focus of gas flow through the 3-finger dolomite as indicated by the gas bearing vent wells.

Mean gamma (Figure 7B), which shows average shaliness of the interval, indicates that, in general, the 3-finger dolomite is cleanest along a northwest-southeast trending corridor from Yaggy to Hutchinson, including the city proper, with considerable increase in shaliness to the southwest and minor increase in shaliness to the northeast. Minimum gamma, which identifies the shaliness of the cleanest dolomite within the interval (the top dolomite in all but a few wells), shows an even more dramatic decrease beneath Hutchinson and along the corridor northwest of the city toward Yaggy. The areas of lowest gamma ray in the 3-finger dolomite within and near the city are deemed more susceptible to fracturing and possibly have extant natural fractures that could serve as gas conduits, as the parting pressures of the extant fractures could be considerably lower than the pressure needed to create new fractures. Based on patterns of inferred evaporite dissolution and stream drainage patterns, joints and fractures appear to be related to both the northwesterly lineament of the Arkansas River and the north-northeasterly trending lineaments related to the Voshell Anticline. These lineaments underwent organized episodic movement as evidenced by the extended continuity (miles in length) of mapped trends of evaporite dissolution, facies change, and drainage patterns suggesting oriented, fracture clusters. Gas could migrate along these preferred fracture sets if parting pressures of the fractures were exceeded.