Oil Migration into Reservoir Rock
hydrocarbon source rock and reservoir seal.23
The
impact structure was buried under 10,000 ft [3,000 m] of sediments, ensuring its preserva- tion. Analysis of the burial history and of source rock and petroleum geochemistry indicates most of the hydrocarbons were generated in the Triassic, at approximately 225 million years ago (Ma).24
The hydrocarbons migrated into three
types of reservoir rocks: Arbuckle dolomite with intercrystalline porosity, Arbuckle dolomite with leached porosity, and granite breccias (left). Ultimate recoverable reserves are estimated
at 25 million bbl oil [4 million m3] and 100 Bcf gas [2.8 billion m3]. As of 2009, the Ames impact structure has produced 17 million bbl [2.7 mil- lion m3] of oil and 80 Bcf [2.3 billion m3] of gas.25
Oil Source rock Seal rock
> Ames petroleum system cross section. Organic-rich shales that filled the Ames crater became the source rock for reservoirs that formed in the fractured and brecciated granites and dolomites beneath the crater floor. Petroleum migrated into upthrown blocks in the central ring and the outer rim. Additional shales acted as seals. The section of Devonian and younger layers overlying the structure is about 7,000 ft thick (not shown to scale).
Red Wing Creek Field The Red Wing Creek structure, in western North Dakota, USA, is one of several producing impact structures in the Williston basin.26
Early in its
exploration history, the feature was a seismic anomaly that fit no known pattern.27
Shell drilled NW
0 0
km mi 1 1 Triassic Pennsylvanian-Permian Mississippian
Devonian Silurian
SE
Cretaceous Jurassic
two wells in the area in the 1960s; both pene- trated unusually thick Mississippian and Pennsylvanian sections but were dry. In 1972 True Oil LLC drilled a well about 1 mi [1.6 km] away and discovered a 2,700-ft [820-m] gross oil column in highly fractured and brecciated car- bonates of the Mission Canyon Formation. Net pay of 1,600 ft [490 m] was in extreme contrast to the 20- to 40-ft [6- to 12-m] pay sections in the surrounding area.28
Subsequent wells helped
delineate the field. Data from seismic surveys and well logs dem-
Red Wing Creek structure
Oilfield Review Autumn 09 Impact Fig. 13
North Dakota
ORAUT09-Impact Fig. 13
onstrated the existence of a circular central uplift as much as 1 mi across and comprising Mississippian carbonates, evaporites and interca- lated siliciclastics at about 3,300 ft [1,000 m] above their normal stratigraphic position for the region (left). Surrounding the uplift is a depres- sion more than 1 mi wide. An uplifted ring 5 mi [8 km] across encloses the complex structure, which is buried under almost 7,000 ft [2,100 m] of sediments. The discovery of shatter-cone fragments in
drill cuttings was the first indication that the oil was trapped in a complex meteorite impact struc- ture.29
More recent work confirmed there were
PDFs in quartz grains in cuttings from one of the True Oil wells.30
Analysis of the PDF orientations
> Impact crater oil trap in North Dakota. The Red Wing Creek field produces from a 2,700-ft section of fractured carbonate breccia in the crater’s central uplift (blue). This complex impact crater is about 6 mi [10 km] wide, but the productive zone is concentrated in a 1-mi2 [2.6-km2] area in the center.
indicates shock pressures reached 12 to 20 GPa [1.7 million to 2.9 million psi]. Estimates from stratigraphy place the age of the structure at 220 to 200 Ma.
24
Oilfield Review
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