Lone Pine Field; exercise in structural geology
Lone Pine Field; exercise in structural geology (in Rocky Mountain Section AAPG, 23rd Annual Meeting)
AAPG Bulletin (1974) 58 (5): 906
Lone Pine field, in the northwest part of North Park basin, Colorado, lies between the Park Range on the west and the Sheep Mountain-Delaney Butte thrust on the east. The oil field underlies a surface anticline mapped by Hale in 1965. Seismic work by a major oil company condemned this prospect, and it was farmed out to Burton-Hawks Exploration Company, who drilled the discovery well in November 1971. In general, the structural configuration and drilling depths conformed closely with Hale's cross section interpretation. The discovery well had a gas blowout from the Dakota Sandstone while coming out of the hole for a drill-stem test. After the well was controlled the Lakota Formation was drilled and completed as an oil producer with an I.P.F. of 515 BOPD. Subsequent drilling has found nine more oil wells and one gas well, all south of the discovery. The northern productive limit appears to have been found, with two dry holes just north of the discovery; however, the southern extent of the field remains to be determined. Present oil production extends 1.25 mi along both flanks of the structure, with a gas cap separating the two flanks. Some minor oil production is obtained from the Dakota Sandstone oil ring in two wells. The Lone Pine structure is a tightly folded, complexly faulted anticline, with 25-45 degrees dips on the east flank and 45-60 degrees dips on the west flank in the oil column. An oil column of approximately 300 ft has been established. Stratigraphic thickness of the Lakota is 70-75 ft, with 60-65 ft of effective pay. Average porosity of the Lakota is 18 percent. Drilling depths range from 2,400 to over 3,000 ft depending on topography and hole deviation. Wells are spaced about 800 ft apart along the east flank; optimum spacing on the west flank is yet to be determined. Each east-flank well proves up an estimated 300,000 bbl of recoverable oil, and west flank wells approximately 150,000-200,000 bbl of oil. A strong water drive hopefully will assure flowing wells throughout the life of the field. Detailed structural analysis has been essential in guiding development drilling. Large scale, precise structural cross sections are built through each well. Logs are examined closely to determine even the smallest faults. All possible surface data are utilized. Concentric fold form, constant bed thickness and bed length, and a detailed analysis of fault behavior are combined to map as closely as possible the structural attitude in development wells while drilling. The target area for each well is a bank of production (oil ring) only 300-500 ft wide, and attempts are made to penetrate the oil ring near its midpoint to avoid water fingering from below and gas fingering from above. In addition, because of strong tendency for hole deviation in the steeply dipping strata, and the desire to penetrate the Lakota pay "broadside" where possible, the wells must be located at the surface (away from the axis) about 400 ft from the desired location at total depth. Frequent directional surveys while drilling have been necessary, particularly in the early development where unpredictable structural complexities were found. Despite the problems and additional expense of this type of development drilling, Lone Pine field should prove very profitable. Some of the wells are capable of flowing over 1,000 BOPD; however, all wells are choked back to 100-150 BOPD to prevent premature water fingering, or possible gas fingering.