From Bally to Basement: using outcrops to unlock buried rocks

By: Stacey Evans

For the last several years, Oklahoma has been experiencing an increased number of earthquakes. The most widely accepted hypothesis for the increase is the disposal of produced waste water from oil and gas wells into the Arbuckle Group, a porous geologic formation composed primarily of limestone and dolomite. I’m one of many researchers at the Oklahoma Geological Survey working to identify relationships between waste water injection into the Arbuckle Group and induced seismicity in the basement rocks. One of the ways I’m addressing this problem is using paleomagnetic analysis.

Paleomagnetism, or pmag for short, is the study of Earth’s magnetic field that was acquired by rocks at the time of their formation. The magnetism held by the rock can be re-set when the rock undergoes certain diagenetic processes. Most of my past pmag research has been determining if fluids have moved through rocks and caused the magnetism to re-set. Using “Paleo-magic!” (Fig 1), paleomagnetists determine the initial magnetization acquired when the rock became a rock and any remagnetizations that may have occurred during the lifetime of the rock.

Fig. 1: Zijderveld diagrams are used to plot the vector magnetization projected onto two orthogonal planes (meaning, if you went back in time and stood with a compass at the time the magnetization was acquired, would the needle be pointing towards the North Pole or the South Pole? Would it be dipping slightly towards the ground, or pointing slightly up towards the sky? Those are the two different planes that are represented on a zijderveld). From these diagrams, the paleomagician can determine the different magnetizations contained within a rock. Like I said – MAGIC.

I’m using my experience with fluid flow and paleomagnetism to investigate fluid flow between the Arbuckle Group and the basement rocks in Oklahoma. There aren’t many subsurface cores with both the Arbuckle Group and basement available, so my collaborators and I decided to use Arbuckle rocks that are exposed at the surface of Bally Mountain in southwestern Oklahoma as an analog. By better understanding how fluids have moved through rocks that are at the surface, we will be able to better identify and understand areas where fluids have moved through rocks in the subsurface. Here are the steps for paleomagnetic field work:

Step 1: Identify Sampling Location

Fig 2: Sampling the Alamo Impact Breccia at Mt. Irish in Nevada. The rocks we needed to sample were in a hole about 5′ above the ground. Some creative teamwork was necessary to get the samples.

The ease of getting samples for paleomagnetic analysis is highly dependent on location. I’ve had to drill in some rather precarious situations (Fig 2), so the sampling at Bally Mountain was pretty straightforward. After getting permission to drill from the landowner, we headed to the field on a brisk November morning. In the field we checked our geologic and topographic maps and did a quick scouting mission to find suitable outcrops to drill.

Step 2: Carry ALL the Water

Samples are drilled with a modified chainsaw. The saw has been replaced with a 1-inch diameter diamond-tipped drill bit and is hooked up to a water pump to provide lubrication while drilling. Sometimes you’ll luck out and have a source of water near the outcrop you want to drill. More likely, you won’t. Most of the time you have to carry equipment AND anywhere from 5 to 20 gallons (approximately 40-160 lbs) of water to the outcrop. There weren’t any water sources nearby on this outing, but we were able to park relatively close to sampling locations.

Step 3: Drill, Baby, Drill!

Drilling is usually at least a two person job: one person to pump the water, one person to operate the drill. The drill bit is hollow so water can flow through, cooling the bit and adding lubrication between the bit and the rock (Fig 3).

Fig 3: Drilling for pmag samples is a messy job. Water sprays out of the end of the drill bit to provide cooling and lubrication. A mix of water and pulverized rock pours out of the hole and creates a mud puddle.

The person pumping the water can use the airtight container to build up pressure so they don’t have to constantly be pumping, but they do have to pay attention. If the pressure runs out and the water quits flowing, the bit can become stuck or ruined.

Fig 4: My first time drilling. I was a natural.

The first time handling the drill can be a little awkward (Fig 4). The drill bit is spinning so quickly that it’s easy for it to just skip right across the surface of the rock instead of drilling in. Counterintuitively, it works best when you rev the engine on the drill, really get the bit spinning, and steadily put the bit to the rock. Rocks are hard, some more than others, making it easy to get tired while drilling. What is the best position to hold the drill so you’re comfortable while still getting enough torque to actually drill the rock? This depends on the outcrop, but my favorite drilling trick is to let gravity do some of the work for me (see Fig 5). I’m not going to get very far if I try to push the drill into rock with just my upper body strength, so any time that I can find an outcrop that will allow me to let my body weight help push the drill, I do it! We usually drill 6-10 samples per site; at Bally Mountain, there were seven sites with a total of 47 individual samples (Fig 6).

Fig 5: Using gravity and body weight to more effectively drill.
Fig 6: One of the sites from Bally Mountain. This site has six individual samples drilled. A couple of the samples were drilled partially overlapping a shallowly drilled reference circle. A reference circle is used to orient the core sample in case it breaks loose from the rock before its orientation can be measured.

Step 4: Tag and Bag

The orientation of the samples must be measured before they are removed from the rock.   An inclinometer (Fig 7) is slid around the sample and used to measure declination and inclination (Fig 8).

Fig 7: Tools of the trade (from left to right): inclinometer, screwdriver, copper wire, permanent marker, and straight edge.

But wait! Don’t pull that inclinometer off yet!  There is a thin slit on the tube of the inclinometer.  Sliding a copper wire through this slit will leave a small copper mark on the sample. Measurements have to be precise and the copper marks the exact orientation of the sample that corresponds with the measured declination and inclination.  Now, if all has gone according to plan, the sample will still be attached to the outcrop and you can simply wedge a screwdriver in the space between the sample and the outcrop, apply a little pressure, and snap the sample loose at the base.  Once the sample is out, use a straight edge to permanently mark the orientation line, note which end of the sample was exposed at the surface, write the sample number on it, and throw it in a bag with the rest of the samples from that site.

Fig 8: Using the inclinometer to measure the declination and inclination of samples before extracting them from the outcrop.

Step 5: Pack It In, Pack It Out

If you have extra water left, dump it out.  Carry everything else back to the vehicle.

Step 6: Back to the Lab

After returning from the field, samples have to be prepped for the magnetometer, which is used to measure the magnetism present in the rock. Prepping includes cutting the samples, weighing them, and creating electronic files with the samples’ orientation measurements.

For this project, we will be measuring the magnetism of the samples collected at Bally Mountain as well as samples from subsurface Arbuckle Group and basement rocks from northern Oklahoma. If we can find magnetizations associated with fluid flow in the past, we can use that information to better understand how fluids are moving through Arbuckle Group rocks and basement rocks now.

I’ll keep you updated on our progress!

Thanks to Dr. Doug Elmore and Matt Hamilton at the OU School of Geology and Geophysics, my collaborators on this field trip.

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