Trend analysis of a thin sedimentary sequence within the Upper Carboniferous of Ayrshire, Scotland

Trend analysis was made of selected variables from a unit representing two seatearth- to- seatearth cycles within the Upper Carboniferous coal measures of Ayrshire, Scotland. A positive correlation is shown between structural level and thickness of the unit at local, but not regional level, indicating that the broad structure was initiated in late or post-Carboniferous time, but that local structural features were active during deposition. Trend analysis of clastic proportions of the unit suggest a derivation of coarse material from the northeast in a postulated model of sand lobes and sheets extending down paleoslope across a platform" of mud deposition. Individual coal-thickness variations seemingly are complex and probably subject to a mixture of controls at local and regional scales.     

H<Subscript>2</Subscript>-rich and Hydrocarbon Gas Recovered in a Deep Precambrian Well in Northeastern Kansas

In late 2005 and early 2006, the WTW Operating, LLC (W.T.W. Oil Co., Inc.) #1 Wilson well (T.D. = 5772 ft; 1759.3 m) was drilled for 1826 ft (556.6 m) into Precambrian basement underlying the Forest City Basin in northeastern Kansas. Approximately 4500 of the 380,000 wells drilled in Kansas penetrate Precambrian basement. Except for two previous wells drilled into the arkoses and basalts of the 1.1-Ga Midcontinent Rift and another well drilled in 1929 in basement on the Nemaha Uplift east of the Midcontinent Rift, this well represents the deepest penetration into basement rocks in the state to date. Granite is the typical lithology observed in wells that penetrate the Precambrian in the northern Midcontinent. Although no cores were taken to definitively identify lithologies, well cuttings and petrophysical logs indicate that this well encountered basement metamorphic rocks consisting of schist, gneiss, and amphibolitic gneiss, all cut by aplite dikes. The well was cased and perforated in the Precambrian, and then acidized. After several days of swabbing operations, the well produced shows of low-Btu gas, dominated by the non-flammable component gases of nitrogen (20%), carbon dioxide (43%), and helium (1%). Combustible components include methane (26%), hydrogen (10%), and higher molecular-weight hydrocarbons (1%). Although Coveney and others [Am. Assoc. Petroleum Geologists Bull., v. 71, no, 1, p. 39–48, 1987] identified H₂-rich gas in two wells located close to the Midcontinent Rift in eastern Kansas, this study indicates that high levels of H₂ may be a more widespread phenomenon than previously thought. Unlike previous results, the gases in this study have a significant component of hydrocarbon gas, as well as H₂, N₂, and CO₂. Although redox reactions between iron-bearing minerals and groundwater are a possible source of H₂ in the Precambrian basement rocks, the hydrocarbon gas does not exhibit the characteristics typically associated with proposed abiogenic hydrocarbon gases from Precambrian Shield sites in Canada, Finland, and South Africa. Compositional and isotopic signatures for gas from the #1 Wilson well are consistent with a predominantly thermogenic origin, with possible mixing with a component of microbial gas. Given the geologic history of uplift and rifting this region, and the major fracture systems present in the basement, this hydrocarbon gas likely migrated from source rocks and reservoirs in the overlying Paleozoic sediments and is not evidence for abiogenic hydrocarbons generated in situ in the Precambrian basement.