early wells produced oil and gas from the Arbuckle dolomite, a formation that does not usu- ally have significant porosity or permeability in this region. Late in 1991 a well targeting a small feature
Large granite fragments
in the central depression penetrated an unusu- ally thick shale section before finding oil in what drillers called a glassy formation, at 8,800 ft [2,680 m].17
Large vugs that store oil and enable flow
Small and very fine fragments of granite
The borehole then encountered more
than 320 ft [98 m] of what turned out to be oil- bearing brecciated Precambrian granodiorite base- ment (left). The well became a prolific producer. Subsequent drilling, however, had mixed results. By 1992, geologists put together the apparent
ring-like distribution of productive Arbuckle wells, the extra thickness of shale and the clues of glass and brecciated basement rock to propose an impact origin for the Ames structure.18
To test
> Core sample from the Ames structure. Some of the Ames oil production comes from brecciated granodiorite basement rock. This sample contains large holes and vugs amid fragments of granite.
the impact hypothesis, scientists from several organizations conducted petrographic, mineral- ogic and geochemical studies of drill cuttings and core samples from Ames boreholes. Additional investigations tested whether gravity and mag- netic fields in the vicinity of the structure are consistent with an impact origin (below left).19
In
the hope of understanding the distribution of reservoir-quality rock, Continental Resources commissioned 2D and 3D seismic surveys over the structure.20 Microscopic scrutiny of cuttings and core
Bouguer Gravity
samples revealed mineralogic changes to quartz that could be explained only by forces one to two orders of magnitude greater and 10 orders of magnitude shorter in duration than anything cre- ated internally in the Earth, such as volcanism and seismicity. The shock-impact features and the presence of rare impact glass confirmed the impact origin of the Ames crater.21 The scenario for the creation of the Ames
–0.6 –1.0 –1.4 –1.8 –2.2
Oilfield Review Autumn 09 Impact Fig. 10
ORAUT09-Impact Fig. 10
> Gravity signature of the Ames crater. Bouguer gravity anomalies indicate lateral subsurface density variations. In sedimentary rocks impact craters that have diameters similar to that of the Ames structure typically produce a negative gravity anomaly. A negative anomaly signifies material of low density. (Image courtesy of Judson Ahern.)
structure can be described in steps (next page). Approximately 470 million years ago, 1,700 ft [520 m] of platform carbonates in a shallow sea overlaid granitic Precambrian basement rock. A 1,000-ft [300-m] diameter asteroid traveling at 20 mi/s [32 km/s] created a bowl-shaped depres- sion, excavating target rock to a depth of 2,000 ft [600 m] and compressing and fracturing the basement rock. Pressures exceeded 50 GPa. The basement rock at the center of the crater subsequently rebounded. Around this central high, raised basement and carbonate rock formed a ring 3 mi [5 km] in diameter, 1 mi [1.6 km] thick and 1,600 ft [490 m] higher than the crater floor. Postimpact collapse of the crater’s inner
22
Oilfield Review
Distance north, km
16 20
18
14 12
10 8
6 4 2
0 0 2 4 Distance east, km
–1.0 –2.0
mGal
6
8
10
12
14
16
18
20
mGal
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