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Finding New Oil and Gas Reserves in Fluvial Systems Using AVO: The Upper Frio in Wharton County, Texas


The Hutchins-Kubela Field (Kubela segment) produced over 10 million barrels of oil (MMBO) in primary production from 1936 to 1992. The field is a rollover anticline with stratigraphic trapping in upper Frio sandstones (4300 to 4700 ft depth). One significant oil reservoir (Kubela) was discovered off-structure in 1936 and abandoned by 1952. The most important reservoir (Gilcrease) was discovered in 1946 and produced 6.2 MMBO until abandonment by Howell Petroleum in 1992. Gilcrease oil (24°) occurred with a strong bottom water drive, and wells were produced to over 99% water cut before abandonment. Wells drilled in the 1980s produced gas from deeper reservoirs.

Development of the area for Yegua gas reserves in the 1980s and 1990s provided new well control, but the Frio leads were not pursued at that time. Most importantly, 3D seismic was shot across the area in 1995 for Yegua objectives. Examination of the seismic data after Yegua exploration showed previously unsuspected stratigraphic complexity, and identified several targets with anomalous amplitude and amplitude variations with offset (AVO) response. Amexco LLC leased the area and we began to re-explore this old field. Combined seismic and well log interpretation showed the Gilcrease sand to be a broad braided stream complex, thinning to the east and trapped by the western edge of the thalweg. Seismic amplitudes remained over the old field area (stronger over areas between old wells), but strong amplitude and structural highs occurred in a dip-oriented strip. Drilling proved that this was a local thick within the fluvial sandstone, forming a higher cupola with up to 7 ft of oil column in both the main sand and overlying stringers. Good production at >5% oil cut was generated from these wells.

Immediately overlying the Gilcrease sand is a separate meandering stream deposit which had been inferred from old, off-structure wells. Several strong AVO and amplitude features formed within compartments probably created by a late, mud-filled channel. One undrilled feature and one offset to 1930s production have been successful, although the strongest anomaly appears to be drained partly.

A sandstone level about 130 ft higher, the Kubela sandstone, has yielded oil and water in several wells. The seismic data showed yet another fluvial system with small anastomosing channels. Well log and production data indicated abundant compartmentalization, leading to smaller cumulative production (but more targets).

The most economic target however, turned out to be invisible on the seismic data. Re-entry of a 1980s well yielded abundant production from a thin, laminated zone, yielding oil in sidewall cores. The zone is bracketed by thick, wet, and washed-out sandstones and is internally complex. Careful integrated interpretation of seismic and stratigraphic data, as well as the use of modern drilling and completion methods, can restore production in old, abandoned Gulf Coast reservoirs.