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But, the earth never rests. Eventually, sea level falls and basins drain causing the surface to be exposed once again. If the climate is right, thick vegetation might reassert itself starting the coal process all over again. This results in multiple coal seams separated by other sedimentary rocks, mainly shale and sandstone.


The massive weight of the overburden gradually heats the buried plant matter and squeezes out much of the water content. The would-be coal also undergoes slow chemical changes. Bacteria feast on the plant matter by reducing its hydrogen and nitrogen content, but much of the carbon remains to become coal’s concentrated, combustible ingredient.


It takes 10 to 15 feet of vegetal matter to develop into one foot of lignite. Thus, a 10-foot seam of North Dakota lignite might represent an original vegetal accumulation exceeding 100 feet. If allowed to cure longer, lignite will


change do not allow the earth to sit still long enough.


Western North Dakota’s lignite resources began forming 50-70 million years ago (by way of reference, dinosaurs became extinct about 65 million years ago) in a swampy, forested environment much like the southeast coast of the contemporary United States. In fact, leaf fossils show that magnolias and other familiar trees and plants flourished here. The vegetation- choked swamps were covered with the mud and sand deposited by meandering streams flowing eastward from highlands to the west.


North Dakota has the world’s largest lignite reserves. The Lignite Energy Council estimates that, at an annual extraction rate of 30 million tons, we have some 835 years of minable lignite. Because it is about 55 percent fixed carbon and 35 percent water. Lignite is not shipped very far and is used primarily as a boiler fuel at power plants close to lignite mines.


Oil and natural gas – tropical seas and lots of mud Although a small minority of scientists questions the traditional explanation for oil and natural gas formation, the common wisdom is that they are the result of the decay of microscopic plants and animals that thrived in ancient tropical seas and lakes. The organic matter also included bacteria that could live in the absence of oxygen. Those conditions prevailed several hundred million years ago in what is now western North Dakota, eastern Montana, southern Canada and a sliver of northwestern South Dakota – the area known as the Williston Basin. The process continues today in tropical and subtropical regions.


eventually transform into sub- bituminous, then bituminous and finally anthracite. With each succeeding step, the coal’s carbon content increases relative to moisture and ash. This proceeds at a snail’s pace and almost never reaches the anthracite stage because plate tectonics and climate


As the microscopic plants and animals died, they sank to the seabed and accumulated in tremendous thicknesses over hundreds of thousands of square miles. They were, in turn, covered by clay, silt and sand sediments being washed into the shallow seas by streams and rivers that were eroding adjacent land masses. Similar to coal formation, the overlying sediments buffered the organic matter from the atmosphere and


Spring 2011 • BAKKEN BREAKOUT


Photo courtesy of Lignite Energy Council


Once lignite is extracted, mined land will be reclaimed and returned to agricultural use. North Dakota was an early leader in mined land reclamation – even before required by federal environmental laws.


did not allow it to rot as on the earth’s surface.


As the sand and mud continued to accumulate, the organic matter was buried deeper, which, along with heat from the earth’s interior, caused its temperature to increase. At this point, the accompanying bacteria began to decompose the hot organic matter and to transform it into crude oil and natural gas. With increasing temperature, heavy crude oil will separate into lighter varieties including natural gas. Therefore, temperature plays a key role in determining crude oil’s density, which people in the oil patch call gravity.


The rock unit in which oil and gas


accumulates is called the source rock. The oil and gas might remain there until tapped by the driller’s bit or they might also migrate to another rock unit, which is called the reservoir rock. It’s important to remember that oil and natural gas do not exist in underground pools. Rather, they inhabit the tiny pore spaces, porosity, in rock units – similar to seawater saturating beach sand.


Sandstone and other grainy rocks make excellent reservoir rocks because they are both porous and permeable. In order for oil and gas to stay put in their reservoir, they must be trapped by geologic structures, non-permeable adjacent rock units and other conditions. There are examples of oil escaping its traps to the earth’s surface


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