In the late 1960's a coal exploration program was undertaken in the Denver Basin in a search funded by Public Service Company of Colorado. The objective was to determine if coal beds existed which could economically be mined and used to fire existing power plants. This paper will describe the program, the results of the program and efforts to develop the deposits found. Included will be a description of how 100% recovery was obtained from coring operations.
The coal beds in the Denver formation are unusual in that, for the most part, they contain local partings of crystalline kaolin, which apparently developed from chemical reactions when volcanic ash fell in the developing coal swamps. Such minor interbeds decrease the overall Btu value of the coal for conventional combustion purposes, but are not expected to interfere with the generation of coalbed methane from these deposits. Therefore analyses of the coal without the partings is the best indication of coal quality in this situation.
Back in about 1965 Page Jenkins, an independent geologist in Casper, got the idea that there might be strippable coal deposits in the Denver Basin suitable for use as fuel in steam power plants operated by Public Service of Colorado. With this concept in mind he approached Public Service to determine if they had any interest; they did and they agreed to fund an exploration program. I was working in Utah at the time and Page called me to see if I would run the program. I moved to Aurora with the task of searching for substantial strippable coal deposits, which then meant within 150 feet of the surface.
A search was commenced immediately for data concerning coal deposits which might exist in this basin. Some of the old records related to previous coal mining operations were available from the Bureau of Mines, USGS and Colorado Geological Survey, but they mostly contained information about shallow known coal occurrences from small surface mines. The type of information we needed was not available. Colorado state law required that records of water wells be filed at the state engineer's office, and these records appeared to offer the best hope for clues to what coal beds might lie in the subsurface throughout the basin.
At the time there was underground coal mining going on in the Laramie formation near the town of Erie north of Denver. I personally went underground in one of those mines and found the coal bed they were mining to be about 7 to 8 feet thick with gentle vertical flexures, and the bed was about 200 feet below the surface. A vertical shaft had been sunk to reach the coal and miners were drifting in the bed. Operations ceased just a couple of years after my visit. This coal was too thin to be of interest for a substantial open pit mining operation and, at that location at least, too deep to be worth serious consideration for our program. In addition, there was a substantial amount of real estate development going on nearby.
At the state engineer's office I found that some of the drillers' logs on file, mostly hand-written, appeared to contain very good information, while others must have been made after several beers at the local bar and were, therefore, of highly questionable value. Our area of search extended from southeast of Fort Collins south within the basin to east of Colorado Springs. From the outset, we recognized that coals within the Laramie formation were likely to be of higher rank than those of the Denver formation, so a determination was made to focus on those areas first where open pit mining seemed possible.
In addition to figuring out how best to carry out a fast and cost-effective evaluation program, we also faced the problem of how to gain access to properties where we might wish to conduct drilling. Having been involved with a substantial amount of uranium drilling in Wyoming in the early 1960's, I knew a great many holes could be drilled inexpensively with a simple rotary rig, using portable mud pits, and that we could obtain electric logs at a good price through Century Geophysical, whose uranium logging business had fallen off substantially due to industry cutbacks. A driller who owned his own Mayhew 750 drilling rig and a water truck was found in Ft. Collins and Century sent a probe truck down from Casper.
The access problem to properties was solved mostly through obtaining permission, from the County Commissioners of the several Colorado counties involved, to drill within the rights of way of county roads. This approach allowed us to move quickly and efficiently as we evaluated the areas where we had some information about coal occurrences and some areas where there was no information at all about coal occurrences. We also obtained permission to drill, at no cost, from local landowners. We could drill two to three 200 foot holes per day even with moving several miles between locations. We drilled about 600 holes in the program.
Our exploration in the Laramie formation was limited, but we found about what we expected: coal beds too thin and too erratic to be of interest for a substantial open pit operation. However, a couple of water well logs for wells drilled near the town of Watkins east of Aurora indicated substantial coal thicknesses at reasonable depth in the Denver formation. In addition, the logs had been made by one of the drillers who made excellent logs. Within a matter of weeks of starting the program, we were drilling in the Watkins area and began confirming what the water well drillers had recorded: substantial beds of lignitic coal. At one location one of the beds was found to be 55 feet thick and it was apparent we could be on to deposits with tens of millions of tons of resources. There appeared to be three separate coal beds, although our search for shallow coal, and our budget, precluded us from drilling deep holes to follow all of the beds. We drilled dozens of holes on one/half to two mile spacing to provide evaluation data. Much of the prospective land lay within the Union Pacific land grant checkerboard. UP did not have a reputation for being cooperative.
Our driller was instructed to lay out a cutting sample for each 5 foot interval drilled and I was making lithologic logs of each drill hole. One of the surprising observations made near Watkins, when I was with the drilling rig one day in 1965, was crystal faces which sparkled in the sun as they emerged in the water and mud from the drill hole. At the time, it was hard to imagine what this could be. Hand lens examination revealed tan, soft, hexagonal crystal faces which later turned out to be kaolinite of such an astonishing large size that the crystals were visible with the naked eye.
The only explanation for the presence of this mineral in a coal deposit seemed to be that there was probably volcanic activity in Colorado, or upwind, at the time the Paleocene coal swamps existed, although I have not investigated such sources. Chemical reactions may have caused the ash to dissolve and the aluminum silicate then reformed as kaolinite, perhaps with some living organisms involved in the process.
But back to the coal: up to this time all of our drilling had been simple rotary drilling with Adrag@ bits. ---- In order to evaluate the deposits further, it was apparent we must begin core drilling. We obtained some of the best core barrels then available and attempted coring, but the results were not good. 40% to 50% recovery was obtained for the first couple of holes and this is simply unacceptable when you need all of the data from the total beds to make an appropriate analysis. Our driller pointed out to me that he had on hand an old core barrel, which was only an empty pipe with some tungsten carbide inserts on the end. No core catcher, no inner barrel, nothing else. I told him to go ahead and try it. He would commence coring at the top of the coal bed using little rotation, slight water pressure, and little drilling pressure and proceed to drill about 8 feet for a 10 foot core barrel. To catch the core he would shut off the water and Ajam@ the last few inches into the core barrel. When he brought the barrel out of the hole we had 100% recovery, and this technique provided 100% recovery on the large majority of the core holes we drilled in this stage of the program. We note that subsequent investigators have had great difficulty with core recovery, no doubt using a different approach.
Analysis of the cores, when the kaolinitic partings were included, provided results which were often below 5,000 BTU, with high ash content. In order to determine the actual heat content of the coal itself, it appeared that we would have to split out the kaolin partings. I estimate that the partings constitute less than 10% of the total coal bed interval.
Under this approach, we found that the coal analysis generally went up in the range of 6,000 to 7,500 BTU with much less ash. In other words, more similar to coals from the Powder River Basin.
However, it also became clear that this coal, without expensive beneficiation, would not compete with inexpensive coal from Wyoming. Public Service Company abandoned the program and funding ended, but not before we had defined several hundred million tons of lignitic coal. With the approval of the previous participants, I then took over the program personally and found another interested party in Cameron Engineers of Denver, a consulting firm. Making synthetic gas from coal was quite the rage at the time and John Hand of Cameron decided this project would be a good candidate and, further, that opening a large mine east of Denver would provide a site for waste disposal from metro Denver in addition to taking Denver sewage water to use in the gasification plant. Plans included use of the kaolin as a refractory or possibly even for paper coatings. A great number of environmental and other studies were carried out, copies of most of which I have maintained in my library.
AMAX Coal became temporarily interested and carried out drilling and studies. Later the Colorado Geological Survey studied the area, including drilling, with much of their data derived from the 1960's drilling and the work of Cameron and AMAX. Total Denver formation lignitic coal resources are approximately 13 billion tons. There are several hundred million tons in beds more than 20 feet thick.
No one up to about the mid 1990's was thinking coalbed methane as a recoverable resource, but we at Aspen Exploration have recently begun looking at the area as a possible CBM play. We have not attempted any CBM production but we have observed degassing water from some water wells in the Watkins area and have ignited some of the gas emanating from one well. At this time there appears to be no reason why this area should not be a CBM producer. Aspen Exploration has prepared coal resource maps and cross sections for the area of the play with the thickest and most continuous coal intervals.