Coal deposits occur in sedimentary strata that have some lateral extent. They can also be affected by deformation processes that can alter their internal geometry. Depending on the degree of deformation, the internal geometry of any coal deposit can range from very little deformation to highly deformed. For purposes of this discussion we will look at a simple, horizontal coal deposit.
Un-like a mineral property, where most of the holes that are drilled into that property are 'cored' for recovery of the sub surface rock, coal seams do not need this concentration of core recovery. With coal, only a certain percentage of the holes drilled into a deposit need to be cored and assayed. The rest need only to be drilled and geophysically logged. Accurate coal quality determinations for mining purposes can be determined from the geophysical logs once a minimum assay profile has been generated.
Initially a coal seam can be core-drilled and the core recovered for study. After the hole is drilled and the core is recovered, the hole is geophysically logged; using among other logs, a gamma/gamma density log. This geophysical tool is small enough to be lowered down the center of the drill rods. Logging through the rods has been done since 1976 when advances in tool design reduced the size of these tools. Logging open hole before that date carried with it a risk of loosing an expensive tool and a radio active source down the hole. There are no problems with logging coal through the rods.
A geophysical log provides a graphic profile of a coal seam. Coal seam profiles have unique characteristics that can be cross-correlated to coal quality. It is possible to read a geophysical log and determine with some fair degree of accuracy, the ash, moisture and BTU values for any seam or any distinctive layer of coal within any seam. This capability makes it possible, given the right geological setting, to separate the various grades of coal within a seam and set the deposit (or seam) up to be selectively mined. This process has been described as an "in-situ analysis, selective mining method.
This potential was first looked into in 1975 (as per this author's experience) and studies into this capability were carried out (by this author) in 77-78 in British Columbia on the Hat Creek coal deposit. The Hat Creek deposit is about eighteen hundred feet thick (600 meters). Its internal geometry is that of an asymmetrical syncline orientated north to south, with a reverse faulted east limb. Even so, it was possible to set this property up for insitu analysis and selective mining methods. Horizontal coal deposits are easier targets for in-situ analysis and selective mining methods.
A certain percentage of holes are cored. These holes are geophysically logged and these logs, along with a litho-log are used to generate a graphic profile of every seam. Each separate layer of coal is split out for analysis according to its unique graphic profile and in relation to its vertical location within a seam. The coal that is split or separated in this way is assayed for its respective ash, moisture and BTU (British Thermal Unit or Kilo-Joule) values.
If these assays reveal differing layers of coal with different ash, moisture and Btu values, it sets up the possibility for mining these layers of coal as separate products with separate market values. This kind of study when done for the Hat Creek property actually increased the value of the property.
With a certain percentage of holes drilled, cored and logged in this fashion it becomes possible to drill off the rest of the deposit without the need to core and assay every hole, thus achieving a significant saving in exploration expenses.
Prior to this capability, coal seams were examined as one unit of coal. The analysis on this coal was performed as a bulk sample analysis. The end product was a composition of hot and not so hot coals, with some degree of ash thrown in as an admixture from the mining process. Selective mining capabilities make it possible to split out and identify higher quality coal from lower quality coal at the mine face.
With minerals exploration, there are reporting standards as set out under what is called; National Instrument 43-101. It is un-clear to this writer at this time whether there are such reporting standards in effect pertaining to anything other than "bulk sample type" reserve calculations for coal. It is not known whether or not those responsible for setting these standards even know what is involved in this kind of an approach to coal exploration.
Nevertheless, this capability exists and it has been field tested and proven to be effective. This author is one of the people responsible for pioneering this capability. I do not know however, how effectively it has been picked up by others in the coal exploration industry.
The author was involved in these kinds of studies in 1975 as an employee with an Edmonton based coal mining company. These methods of analysis were expanded upon in 1977 -78 while serving as a principle coal geologist working on B.C. Hydro's Hat Creek coal deposit. They resulted in an increase in the value of the deposit, coupled with a decrease in the waste product associated with the mining of the deposit.
Not all coal deposits are amenable to these sorts of studies. Some that are highly deformed are not good candidates. Some deposits that have been subjected to hydrothermal alteration may not be good candidates either. Deposits that are primarily low grade lignite may not be good candidates. Hat Creek is classified as a "lignite deposit" but was amenable to this kind of study because of the thickness and geometry of the deposit.
Property that has demonstrated this kind of value added potential is going to be viewed as being more valuable as an asset class, than property without this kind of work being done.
The other kind of insitu analysis program that can be run during an exploration program is one for insitu coalbed methane (CBM). This is done using a "Quad Neutron" tool.
All coal deposits contain coalbed methane. This tool can detect the potential for insitu gas in the coal and/or sedimentary strata enclosing the coal. It can also measure the degree of porosity of those sediments and give some indication as to whether or not any 'gas' may or may not flow. A core sample (canister test) may be necessary to determine if the gas is indeed coalbed methane or carbon dioxide, or some other gas.
It's a good idea when evaluating any coal seam, that might be a candidate for underground mining, to evaluate that seam for its contained coalbed methane, and do it at the time of an initial exploration program. The determination of coalbed methane, its location and potential concentration, might help future miners mitigate and possibly avoid, accidents caused by coalbed methane.
These kinds of studies can help determine if tapping a coal deposit for its coalbed methane is a viable and profitable option prior to mining. A Quad Neutron analysis could help in this determination.
The other thing to be aware of is the potential for intersecting shallow gas during the exploration process. Natural gas as shallow as 300 feet, is known to exist in the substrata of the Alberta foothills. Companies new to coal exploration need to be made aware of this potential and the associated dangers when making their exploration plans.
From my experience, logging through the rods began in earnest in 1977. Yet, still today, there are engineers and geologists that do not know this or believe that it can not be done. It works fine, and here is some insight into what happened to bring it about.
Maybe I had something to do with it, I do not know; however, as I was running exploration programs for coal in Alberta in 1975 there was a geophysical logging company working with me. They were logging open hole and every once and awhile, they would get a tool (with a radio active source) stuck in a hole. On one occasion it took 12 hours of work to get the tool out of the hole. I was not happy about this or any prospect of loosing a radio active source down a hole. So, I went to the company supplying the geophysical services and I asked them, "Why do not you log through the rods?"
The company president looked at me and thought about it for a minute. He did not dismiss the idea. Two years later, when I encountered them again on another coal property, they were logging through the rods! They had re-designed the tool to log through the rods, tested it and found that doing it this way worked just fine. They would later go on to invent and design what they called a "Quad Neutron" tool to log for coalbed methane. And they would be able to do it again, through the rods! The Quad Neutron has two radio active sources in it, and it too works just fine, through the rods.
Now, geologists no longer have to run the risk of loosing a radio active source down a hole. And you still provide your client with valuable, accurate exploration information. Other kinds of logs, that do not involve the use of radio active material, can be run open hole.
There are occasions where a hole may collapse when the rods are pulled. Logging through the rods enables a geologist to recover valuable geological information before the rods are removed.
The author has been a coal geologist for many years. Participating as an exploration geologist on some 10 coal exploration projects; directing exploration activities on 7 of those 10 projects. Having logged over 250,000 feet of hole, some 80% of it being coal from the very thick Hat Creek coal deposit.
He has done work for a two Calgary based consulting firms; two Vancouver based consulting firms, two large mining companies and worked as an independent contract geologist; all in coal in either B.C. or Alberta.
He has written several papers on coal exploration, exploration practice and the geology of the Hat Creek coal deposit. He also produced a film about coal exploration.
Thank you for your interest.
He has a web site which he is developing: www.geologyrocks.ca
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