The Williston Basin
The Williston Basin contained some of the largest oil
deposits in North America.
Consequently, it has been explored extensively by oil geologists. Exploration is typically done by
drilling, and retrieving Òdrill cores,Ó some 8 inches in diameter, extending
from the surface to the depth of the exploratory drilling. By use of micro-fossils contained in
the drill core, oil geologists can assign different sections of the core to
different geological eras. Because
some of the rock layers, or Òrock unitsÓ are exposed on the surface in various
locations in Montana, the Dakotas, and Saskatchewan, it is possible to identify
particular layers that have been named.
Cores from North Dakota contain the entire geological
column, from the present to the Precambrian. At least, this is the interpretation that geologists have
used, based on the fossils. One of
the things we really want to know is the actual ages of these rocks.
Igneous rocks like granite and basalt are particularly good
for age determination. Any
radioactive potassium (K40) that they contain will decay at the
standard half-life of this radioactive element. K40 decays to non-radioactive Ar40. Argon is a gas, and escapes into the
atmosphere as soon as it is formed, unless
it is trapped in solid rock.
Igneous rock formed from magma; argon escapes from magma. Therefore, any
Ar40 trapped in such rocks has accumulated since the rock
solidified. By careful measurement
of the amounts of K40 and Ar40 in igneous rock,
therefore, it is possible to determine how long it has been since that rock
formed.
The particular core that we will examine is from a drill
hole of about 10,000 feet. The
core, if reassembled into a single unit, would look something like this:
Surface
- 10,000 feet
Different types of rocks have different appearances, in
terms of color, texture, chemical composition, etc. These different characteristics allow us to identify
specific rock units. The alignment
of the core with known rock units is shown here:
The arrows indicate the locations of samples that were taken
for analysis. Below is a table
that provides the raw data (the ratio of argon to potassium) and the age that
has been calculated from this ratio (feel free to check the accuracy of the
calculations). Measurement of
several samples allows the estimation of the error of measurement. All of the ages are presented in MYA,
Òmillions of years ago.Ó
|
Rock Unit |
Ratio of Ar40/K40 |
Calculated Age
(MYA) |
|
Deadwood Formation |
0.035876 |
533 ± 50 |
|
Flaxville Formation |
0.000643 |
11 ± 0.5 |
|
Hell Creek formation |
0.003732 |
63 ± 2 |
|
Junction of Lobo Shale and Tullock Members of Fort Union Formation |
0.003491 |
59 ± 3 |
|
Junction of the Mowry Shale and the Newcastle Sandstone |
0.005798 |
97 ± 3 |
|
Junction of the Winnipegosis Formation and the Ashern Formation |
0.025595 |
395 ± 5 |
|
Minnelusa Formation |
0.018057 |
287 ± 9 |
|
Nesson Formation |
0.010299 |
169 ± 5 |
|
Niobrara Formation |
0.004943 |
83 ± 3 |
|
Stony Mountain Formation |
0.029750 |
452 ± 38 |
|
Top of the Opeche Formation |
0.014975 |
241 ± 8 |
These are the data. What should we make of them?