Portions of thisDrilling problems included hole deviation and hole instability that prevented the timely completion of this borehole. Drilling methods used include rotary tri-cone and rotary hammer drilling with conventional and reverse circulation using aidwater, aidfoam (Davis mix), and bentonite mud.Geologic cuttings and geophysical logs were obtained from the well. The rocks penetrated by the ER-12-1 drillhole are a complex assemblage of Silurian, Devonian, and Mississippian sedimentary rocks that are bounded by numerous faults that show substantial stratigraphic offset.The final 7.3 m (24 ft) of this hole penetrated an unusual intrusive rock of Cretaceous age. The geology of this borehole was substantially different from that expected, with the Tongue Wash Fault encountered at a much shallower depth, paleozoic rocks shuffled out of stratigraphic sequence, and the presence of an altered biotite-rich microporphyritic igneous rock at the bottom of the borehole.Conodont CAI analyses and rock pyrolysis analyses indicate that the carbonate rocks in ER-12-1, as well as the intervening sheets of Eleana siltstone, have been thermally overprinted following movement on the faults that separate them. The probable source of heat for this thermal disturbance is the microporphyritic intrusion encountered at the bottom of the hole, and its age establishes that the major fault activity must have occurred prior to 102.3+0.5 Ma (middle Cretaceous).Geophysical logs run in the saturated and unsaturated sections of the borehole were invaluable for interpretation of stratigraphy and structure. Problems encountered during logging were lack of service tables for stacked logs, lack of calibration tables for the ER-12-1 hole size, and lack of written procedures for running these logs in the field. Hydrologic investigations consisted of water level monitoring, flow logging, aquifer tests, and drill-stem tests. The results indicate that the static composite fluid level in well ER-12-1 was 469 m ( 1540 ft) below land surface. Drill-stem tests and flow logs determined that the lower two intervals in the well are underpressured relative to the upper zones by approximately 396 m (1 300 ft). Aquifer tests, drill-stem tests, and flow logs determined that the transmissivity of the well ranged from 7.5 x m2/s, with the most transmissive zone being 518 to 555 m (1700 to 1820 ft) below land surface followed by the 9 14 to 963 m (3000 to 3 160 ft). The pressure differential between these zones allowed for substantial crossflow to occur while the well was open.to 4 x Two types of geochemical samples were acquired from this well. Water quality samples taken during drilling and testing indicated very few problems associated with the well. Those identified consisted of elevated quantities of volatile and semivolatile organics (sample 10006) and metals (sample 10007) associated with the drilling process. Geochemical characterization samples were taken only from the uppermost zone, 518 to 555 m (1700 to 1820 ft). The results from this sample vii indica...