US oil production has been booming the past few years, due in large part to North Dakota’s Bakken formation, a rock layer tapped through fracking. Each well travels down about two miles, then turns horizontally and snakes through the rock formation for another two miles. There were 8,406 of these Bakken wells, as of North Dakota’s latest count. If you lined them all up—including their vertical and horizontal parts—they’d loop all the way around the Earth.

As a journalist digging into the long-term potential for shale oil—how much oil it might supply, and at what economic and environmental costs—I wanted to create a map showing the extent of this drilling boom to help me look for trends. In this post, I’ll explain how I did that, but first I want to say why this matters.

If there is a lot less oil and natural gas available at affordable prices, this could be good news or bad news, depending on your values, and how the country reacts. If we have a realistic sense of the size of the resource available, and plan for the long term, we could make a smooth transition away from oil and gas, toward other options with much lower greenhouse gas emissions, such as wind and solar, and also make stronger efforts for energy efficiency. On the other hand, if our estimates and forecasts for oil and gas are too optimistic, we could wind up in a bind, dependent on fossil fuels that are significantly more expensive than we'd expected.

In a recent feature for Nature, "The Fracking Fallacy," I reported on differing forecasts for the future of shale gas. If the US continues to try to extract natural gas as fast as possible, and also to export as much as possible, this could lead to much higher energy prices that would likely have a large impact on the economy. As one researcher I spoke with put, we could be "setting ourselves up for a major fiasco." I'm also following fracking trends in my ongoing data journalism project, The Frack Lab.

How long can North Dakota's boom continue? And how will it fare if oil prices remain around their current relatively low price, of about $50 a barrel? It could be that companies have stopped drilling in some areas, or started drilling in new areas. It could be that they're drilling at much higher density in areas that are particularly attractive—especially since oil prices have dropped drastically over the past six months. Tracking how many wells have been drilled in recent months, and where they've been drilled, could show how companies are responding to lower oil prices. I wanted to do this to supplement my own reporting, but also to let anybody who’s interested explore the data more easily.

There are some maps out there already, including an online interactive map from North Dakota’s Department of Mineral Resources (ND DMR), and other groups have created their own maps of fracked wells across the nation, like the non-profit FracTracker. To get an impression of the extent of the boom, there are the mind-bending graphics in this New York Times piece, showing a tangle of lines obscuring the sky. But I haven’t seen any interactive maps out there that let ordinary citizens easily explore the extent of this boom.

To make my map (see a larger version here), I took the same data the New York Times used and plotted it out using the open-source program TileMill, creating an interactive map that allows you to, um, drill into the data. It appears the New York Times used a large shapefile available from the ND DMR, for nearly all horizontal wells, showing their curvy paths in detail. To get this file, I went to their ArcIMS Server, and clicked on the button in the upper-right, which opened up a list of shape files. The one I used was “Horizontals_Lines.zip”. I unzipped it, created a new project in Tilemill and added the shapefile as a layer.

On top of this layer, I’ve added the locations of each wellhead, with the most recently drilled ones highlighted in red. This wellhead data is available only through ND DMR’s basic subscription at $50 a year. If you’re into this kind of data, it’s definitely worth the price, since it gives you access to all sorts of details—monthly production rates for each well and well files that give extensive details, including which rock layer the well is tapping. (Yes, I’m obsessed.)

With the wells drilled during 2014 highlighted in red, it's clear that drilling has contracted to focus mainly on a core area in the center, rather than pushing out into new areas.

Also, North Dakota allows drillers to put new wells on “confidential” status, typically for about six months. I’ve marked those in orange. The details on these wells—where exactly each horizontal well threaded its way through the rock, when the wells started producing oil, and how much they yielded—is kept private until confidential status expires. Since the horizontal well paths are kept private, they don’t show up on this map, so you’ll see lone orange dots with no horizontal wells—not because they’re not there, but because the data isn’t available yet. Since most wells get put on confidential status initially, the locations of these wells gives you an idea of where companies have been drilling most recently.

From afar, it’s just a forest of lines. But if you zoom in, you’ll see that some areas have wells tightly packed in, whereas other areas have a solo wells. From what I’ve read—such as in assessments like those from geologist David Hughes in his recent report “Drilling Deeper”—the densely drilled areas are those where wells yield the most oil. These are the “sweet spots,” as the industry calls them.

To help distinguish the drilling hotspots from the less attractive areas, I’ve made all the wellhead location markers partially transparent. This is because companies often drill several wells on a single “pad,” an area of they clear where they set up the drilling rig and park all the trucks that bring in the millions of gallons of water that they pump into the wells at enormous pressures to fracture the rock, and the thousands of tons of sand that the water forces into those fractures. (The sand stays behind, holding the cracks open so that the oil and gas will continue to flow out.)

Even when you zoom in as close as the web version of this map will allow, what looks like a single well could actually be a few wells nearly on top of each other, so to show the real density of the wells, I had to make the markers partially transparent.

The companies are able to drill out from the pad at an angle, so as they tunnel down, they’re also veering off to the side. Then when they approach the horizontal rock formation they’re targeting, they turn the drill so it travels horizontally. They’re able to drill with amazing accuracy, steering through rock layers that are often just 50 feet or so thick, with the well typically staying within 10 feet or so of its target path. If you look around, you can see where these pads are, with a bunch of wells radiating from them, then all turning and running parallel to each other.

There are also large areas that have been drilled in the past, but have few red dots. These seem to be areas where companies have ceased drilling in the past year or so. My guess is that these did not yield enough oil to be profitable—even before the price drop that started this summer. The orange dots are fairly tightly clustered in the center, and in a few outlying areas. You don’t see orange dots around the edges of the area that has been drilled so far. This also suggests that in more recent drilling, companies are concentrating on the central core, and a few smaller outlying areas that are also attractive. The data here are up-to-date as of December 17.

This map alone won't answer those big questions I had, about how long the boom in North Dakota might last. But tracking how the boom is unfolding is one crucial part of understanding the potential for the long-term.