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The Powder River basin in northeastern Wyoming has long been a productive oil province. Abrupt lithology changes of the Upper Pennsylvanian-Permian Minnelusa Formation have provided a variety of hydrocarbon traps. However, these same abrupt changes have also yielded many surprises to the hopeful explorationist.
The upper Minnelusa is composed mainly of sands, dolomites, and anhydrites, and was deposited in sabkha environment. Unconformably overlying the Minnelusa is the supratidal Opeche shale. Hydrocarbon traps in the upper Minnelusa sands are usually stratigraphic in nature, and are of two common types. In one, a porous sand is trapped by an updip facies change; in the other, thick Opeche deposits in interdunal areas provide an updip seal for the porous sands.
The C-H field area, located in Campbell County, Wyoming, was chosen for initial study. Abrupt updip termination of the thick productive upper Minnelusa sand appeared to provide an ideal situation for a stratigraphic-seismic study. Sonic logs used to construct a geologic cross section showed a significant difference in the sonic response of porous upper Minnelusa sand versus no sand. Synthetic seismograms were then produced from these wells and, when organized in cross-section form, they again showed an obvious difference in response from sand to no sand conditions.
A "pilot" seismic line tying these wells was acquired. From this data, a good correlation between synthetic and actual seismic data was achieved. The updip termination of the productive Minnelusa sand was clearly identifiable on the seismic data.
The drilling phase of our exploratory program yielded mixed results. Some excellent development wells were drilled, one being completed for 625 BOPD. Although we were 70 to 80% successful in predicting the presence or absence of porous upper Minnelusa sand, only 20% of our extension or wildcat wells were productive.
Two major problems were soon discovered: (1) the seismic response from a thick, relatively low velocity Opeche shale is very difficult to distinguish from an upper Minnelusa sand; and (2)thin, porous Minnelusa sands are difficult to identify seismically, thus updip trap limits are not easily defined.
A variety of seismic trace attributes were examined in hopes that subtle amplitude and frequency differences would help distinguish thick Opeche shale from Minnelusa sand. This approach produced very limited success. Better results were achieved on the second problem, that of thin bed resolution. Accentuating the upper portion of the seismic frequency spectrum (40 to 80 Hz) did allow better mapping of thin Minnelusa sands.
In conclusion, through a closely coordinated geologic-geophysical effort, a useful methodology was developed which can be applied to a variety of stratigraphic-seismic exploration projects. The basic steps involved are as follows. First, determine if known lithologic changes can be seen on sonic or density logs. If successful, can the changes on logs be seen on synthetic seismograms? If successful, can the change be seen on a pilot seismic line? Finally, prepare for some complications and failures.
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