For many years in the 1980s, the most prolifically producing oil wells in the onshore 48 states were in Nevada. Nevada? Yep – not the first place you think of for oil, but there’s oil there, in some pretty unusual traps.
Oil was first discovered in Railroad Valley, in desolate central Nevada, back in 1954. It was kind of a fluke – the seismic data they had were pretty poor, and they drilled one thing but found another. The oil reservoir at Eagle Springs Field is mostly fractured volcanic rocks called welded tuffs – essentially, the result of hot ash erupted from a volcano perhaps 10 million years ago – just yesterday, geologically speaking, during the Cenozoic era. The ash fell and landed while still pretty hot, hot enough to weld itself together into a hard, almost glassy rock. Such rock is pretty easy to fracture naturally, and the fractures trap the oil.
|Oil Fields of Railroad Valley (data from Nevada BuMines;
interpretation by Gibson)
OK, so Mississippian source rocks and Cenozoic volcanics as reservoirs. Aren’t we in the Devonian this month? Yes. Hang on, we’ll get there.
Fast forward to 1976. Another oil field was discovered in Railroad Valley. Trap Spring Field was also in fractured volcanic rocks, but it was across the valley from Eagle Springs. Eagle Springs was a small but steady producer, with today something like 5 million barrels total produced in 60 years. Trap Spring was better, and it has yielded around 15 million barrels in less than 40 years. For perspective, the United States today consumes close to 20 million barrels of oil every day.
The discovery of Trap Spring stimulated a renewed interest in Nevada. At the time, even major oil companies, like Gulf Oil where I worked, were interested. My first work on trying to understand the geology and to use geophysical data to predict where analogs to the existing production might be found began in 1978. And my most recent work on Nevada was this year.
In 1983 another oil field was discovered, in another corner of Railroad Valley. This one was entirely different from the others in terms of the reservoir. Instead of fractured volcanics, the reservoir was extremely porous dolomite – Devonian dolomite, buried within the Cenozoic sands and gravels that fill the basins of Nevada. Nevada’s basins and ranges are formed by long normal faults – the kind formed by pulling apart, extension of the earth. Think of the basins as the parts that dropped down, and the mountain ranges as the high-standing parts that were left back, that did not subside. As the faulting continues, and one side goes down and the other side goes up, relatively, you get these alternating high ranges and low basins. And of course you get erosion of the mountains, dumping sediment into the adjacent basins. Some of the basins in Nevada have more than 10,000 feet of sediment that was eroded off the mountain ranges, and most of that has happened in the past 10 to 15 million years. All of the known oil in Nevada is trapped in various kinds of rock that’s been dumped into the basins.
So back to the new oil field discovered in 1983, named Grant Canyon. If all the oil is in the Cenozoic fill in the basin, how can I say it’s in a Devonian dolomite? Think of a fairly rapidly downdropping basin. Fairly rapidly means just a few million years. That can make a pretty steep scarp, the face of the mountain range. Steep scarps lend themselves to massive landslides on occasion – and that appears to be what happened here. A huge slice of the mountain range – composed of those Devonian dolomites and other rocks – slumped off the mountain and into the basin, maybe 6 or 8 million years ago. And then it was buried by more and more sediment coming off the mountain front, until that huge landslide was buried under around 3500 to 5500 feet of later sediment. You can think of it as a landslide, as I described it above, but it’s probably a little more accurate to think of it as another fault that dropped part of the mountain front down into the basin. Either a large landslide or a small fault block. The entire area of material is less than a square mile.
What’s the big deal? Well, in those highly porous Devonian dolomites, oil migrating up from the Chainman shale accumulated. Most of the time you should think of oil in rocks as simply filling the tiny pore spaces between grains of rock, but in this case it’s actually fair to visualize a real pool of liquid oil down there. Some of the porosity in these rocks is called cavernous porosity – essentially, little caves eroded out of the carbonate. With a really good seal, an impermeable layer of rock sitting above it, the Devonian dolomite became a small, but excellent oil reservoir.
How excellent? For about 9 years, from 1983 through 1992, the two wells in Grant Canyon Field yielded close to 6,000 barrels per day – the most of any wells in the onshore 48 states. I’ve said it before, but as a reminder and for perspective, the average US oil well produces 10 barrels per day. 6000 is Saudi Arabian levels. The total volume was nothing like a Saudi Arabian field, but Grant Canyon and the associated Bacon Flat Field produced about 25 million barrels over about 30 years.
There have been several other important oil discoveries in Railroad Valley and some in Pine Valley, further north. The last large discovery came in 1986.
In 2009 and 2010 I did some work for an Irish oil company in Hot Creek Valley, across one mountain range to the west of Railroad Valley. I used a predictive model based on analysis of gravity, magnetic, and geologic data to point to possible analogs to the existing production in Railroad Valley. The company used my recommendations to do a lot of additional work, including geochemical surveys and other approaches, and in 2012 they drilled the second exploratory well ever located in Hot Creek Valley. The 400 barrels per day that they tested was deemed non-commercial, but I can tell you that as far as I am concerned, I felt like I had found oil. It was for me a proof of the concept used to identify analogs to existing production, and I was really happy! The last I’ve heard, the company is using the information it gained in the first well to plan a second well. Stay tuned.
—Richard I. Gibson
My Nevada oil exploration page