Oil was discovered in a shallow water well fifteen miles northwest of Toyah Texas by J.D. Leatherman in 1903. The oil was thick and viscous and was marketed as grease for windmills at the time, but it was the first documented oil discovery in the Delaware Basin.
Attempts to duplicate the discovery were made in the following year by drilling 1.25 miles to the southwest of the discovery well ended in failure. The exact location of the Leatherman well is not recorded in the state’s well database, but county records indicate that Mr. Leatherman leased school board lands in Block 59 of Reeves County. The reservoir is only briefly described as oil bearing sand within blue clay along with sandstone containing streaks of limestone in the drilling log of a 1904 well proximal to the Leatherman well. The field was revisited in 1980, and while several wells were drilled, only two clusters of wells produced a limited amount of oil. These clusters of wells encountering shallow oil at 170 to 300 feet below the surface were only a half mile apart, while multiple dry holes were drilled in the surrounding area. Factors leading to the success or failure of these wells are not published, but recent publications reveal the existence of dissolution collapse structures that border the field to the north and south.
Full Tensor (Gravity) Gradiometry (FTG) data acquired by Bell Geospace over a significant portion of the Delaware Basin in Reeves County Texas reveal complex near surface density contrast that is attributed to dissolution collapse structures within the Ochoan evaporite series. The FTG data reveal not only the dissolution collapse structures bordering the field, but also anomalies between the structures which is believed to be attributed to compartmentalization of the reservoir containing the shallow oil.
The Ochoan Series of the Permian in the Delaware Basin includes the Castile, Salado and Rustler formations. This series is unconformably overlain by Cretaceous limestones along with clastic sands and gravels. The density contrast between the Ochoan evaporites and overlying clastic sediments is up to 0.7g/cc, which is easily detectable by Full Tensor Gradiometry. Areas of low density are associated with accumulation of clastic infill and or karsted evaporites. Both lower density clastic sediments and karsted evaporites are excellent reservoirs for hydrocarbon accumulation.
The oil discovery northwest of Toyah Texas was accidental through drilling for water. Surface geology was roughly mapped at the time of discovery, and nothing more than a barely discernible swale was noted in the observations. Geophysical methods such as gravity and seismic reflection profiling were not developed at the time of discovery, and 3D seismic data were not acquired until the late 1990s. Regardless of data availability, near surface imagery is plagued with problems including surface roll and lack of coherent reflections until reaching the Delaware Mountain Group.
Method and/or Theory
Full Tensor Gradiometry data are sensitive to the significant density contrast observed in outcrop and known to exist between the collapse breccia/sedimentary infill and surrounding evaporites (Morgan et al, 2020b). Qualitative methods can map contacts between the dense evaporites and the lower density sedimentary fill.
Results of wells encountering hydrocarbons are compared to qualitative enhancements of the FTG data to identify local fairways. Successful wells and dry holes are compared to the FTG anomaly data. Wells encountering hydrocarbons follow the flanks of local anomaly minima. Similar anomaly patterns are numerous in the proximity of the field that have not been drilled.
FTG data are extremely sensitive to the density contrast associated with evaporite karst and adjoining clastic sedimentary infill. Compartmentalization of the field are interpreted from the FTG data, noting similar anomaly patterns proximal to the field that are yet to be exploited. Additional data acquisition and or reprocessing of shallow seismic data are recommended for follow-up.
Morgan, A., Brewster, J., Paddock, D., Sudhakar, V., and Fang, J. 2020. Identification and assessment of depressions within the Ochoan evaporites through the integrated interpretation of 3D seismic and full tensor gravity gradiometry in the Delaware Basin of Texas., SEG Technical Program Expanded Abstracts 2020.
Richardson, George Burr 1904. Report of a Reconnaissance in Trans-Pecos Texas: North of the Texas and Pacific Railroad: Texas Univ. Bulletin 23 (Mineral Survey Service Bulletin 9) 119p.
Alan Morgan will be participating in the Permian Basin Geophysical Society event in Midland. May 12th.
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