Coal Exploration Geology

 

Coal Exploration Geology

In-situ Analysis - Selective Mining Methods

By

George Stuart Green, B.A. Geol.

© 2015 George S. Green

(pdf) ISBN: 978-0-9919761-9-5

 

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Preface

This paper takes my previous paper, titled; Coal Exploration Dynamics, Geophysical Logging, and In-situ Analysis Selective Mining Methods, © 2013; and adds some new material. Some material has been edited and some material from that earlier paper has been dropped altogether.

My previous paper set out to present and answer the following questions: Is it possible to read a geophysical log taken from a hole drilled through a coal seam, and from that log, determine with some fair degree of accuracy, the ash, moisture and BTU value of the coal within that coal seam; and can this be done for any distinctive layer of coal within any particular coal seam? I will pursue the same questions here.

The direct answer to both questions is yes; and for this work, I used the API scale on a Gamma/Gamma Density geophysical log. I short interval sampled the coal in accordance with, and within a specific set of API numbers. I did this for each, entire section or seam of coal that was encountered through drilling. I did it for a minimum number of holes until I was able to generate a representative, API mediated, sampling profile for every section or seam.

On analysis, I found that each separate sample, when compared to every other sample occurring between a specific set of API numbers, maintained the same range of BTU values; and similar results were found in the case of ash and moisture contents as well. Given the right geological setting, this makes it possible to selectively mine a coal seam; separating coals of varying BTU value and selling those coals at marginally higher prices; thus increasing the value of the deposit.

All of this work can be accomplished by a geophysical tool that runs down the inside the drilling rods; hence the terminology, ‘logging through the rods’.

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In-situ Coal Evaluation Methods

The potential to selective mine any coal seam within any coal deposit, depends upon the internal geometry of that seam and/or deposit. Since deformation processes can alter the geometry and/or chemistry of any coal seam or deposit, I will assume, for purposes of this discussion that both of these criteria are suitable for our intention, as they were for this study.

Un-like a mineral property, where most of the holes that are drilled into a 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 that are 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.

When I was working on B.C. Hydro's Hat Creek coal deposit I came up with the idea that the deposit might be amenable to a method of mining that I referred to as "in-situ Analysis and Selective Mining". It occurred to me that our geophysical logs could be used to accurately indicate the BTU value of any particular layer of coal within any particular coal seam. This study would go on to indicate that accurate values for in-situ ash and moisture could also be determined from our geophysical logs.

To examine this potential, a program of selective sampling had to be initiated for a minimum number of holes. This sampling program had to be guided by the geophysics being run on these holes. The samples that were selected for lab analysis came from specific geophysical "picks". These picks were sections of core that occurred between set geophysical points found on the API scale of the Gamma/Gamma Density geophysical log.

You must look very closely at the coal in your core box to find these points. Lower API numbers will indicate less ash content and the coal will become brighter, harder and cleaner. Higher API numbers will reflect increasing ash content. Finding these cut-off points in the core requires looking very closely at the lithology of your coal; and you can not rely entirely on simple point to point measurements selected along the length of the core; especially if you have to deal with the effect of core loss.

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Figure 1

geo.pic

On the left hand side of a geophysical log there sits a scale from 0 to 50. It is called the American Petroleum Institute scale (A.P.I.). It was set up by the oil and gas industry in the United States for geophysical logging and it measures the background radiation of the layers of rock that have been drilled. This information is displayed as a wavy line (Fig. 1) that runs down the left side of the log. This line represents a graphical representation of the varying degrees of API value associated with the coal.

Down the right side of our geophysical log there is a space for another wavy line which provides for a graphical representation of the density of the rock that has been drilled. For this work, a different geophysical tool shoots a beam of gamma rays into the rock. By measuring the strength of the beam this tool determines the density of the layers of rock that has been drilled. A skilled reader uses both, side by side graphical representations (logs) to determine something of the nature of the rocks that have been penetrated.

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Between these two geophysical logs is an open column where a geologist can pencil-in a sketch of the lithology or type of rock that has been drilled. For my study this space was used to indicate the amount and quality of the coal occurring between a specific set of API numbers. This area was later color coded to reflect API related coal quality.

Figure 2

geo.pic

Recording in Tertiary aged lignite (for example) may begin at about 25 counts per second on the API scale (Fig. 2). As the coal gets cleaner our graphical representation will begin to trace to the left on the API scale; moving towards a lower API number. At Hat Creek, I decided to selectively sample the coal in accordance with and within several specific sets of API values. I could see in the core that the quality of the coal got better as a lower API number was reached. It became cleaner, with a higher percentage of bright bands.

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Figure 3

geo.pic

In the example shown here (Fig. 3) near the top of this particular hole, we see a (.7 meter layer) of coal that exists between 15 and 25 API. Looking at the next interval, we can see a 3 meter section of coal that exists between 5 and 15 API. The coal between 5 and 15 API is visually distinct. It is hard, bright, with a conchoidal fracture and lots of bright bands.

Immediately beneath this layer is another 3 meter section of coal that runs from 15 API to 25 API. It is black, layered, coal, with minor shale.

Immediately below this, there is a 4.0 meter section of coal that runs from 25 API to 35 API. It is dull, black, coal with significant shale.

Below this, there is one more 3.5 meter sample section that runs from 35 to 45 API. It is essentially, carbonaceous shale.

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For this study, I sampled each instance of coal within a 5 to 15 API range and sent it to our lab for analysis. Next, I took each instance of coal between 15 and 25 API and sent it in for analysis. Then I did the same thing for any coal occurring between 25 and 35 API; and once again for coal occurring between 35 and 45 API. I did this for the entire hole thus "selectively sampling" the entire hole in accordance with these specific API designations. The same thing was done for a second hole.

I began with a range of 5 to 15 API because this quality of coal when examined in the core box, could be visually identified as predominately clean coal; above 15 API the lithology of the coal began to change. Hat Creeks' best coal occurred between 5 and 15 API. At Hat Creek, these various ranges in API and coal lithology were designated as follows:

5 to 15 API: clean coal; (color coded: black)
15 to 25 API: shalely coal; (color coded: orange)
25 to 35 API: coaly shale; (color coded: pale blue)
35 to 45 API: carbonaceous shale; (color coded: green)

On lab analysis we found the following results;

5 to 15 API = 14,000 to 12,000 Btu's
15 to 25 API = 12,000 to 10,000 Btu's
25 to 35 API = 10,000 to 8,000 Btu's
35 to 45 API = 8,000 to 6,000 Btu's

Because I color coded these layers of coal (Fig. 3) we could easily see their thickness and where these various qualities of coal were in our cross sections; and depending on the geometry of the deposit itself, this allowed us to look at the possibility of selectively mining these coals so as to remove coals of variable Btu value and sell that coal at marginally higher prices. Thus increasing the value of the deposit and giving those concerned a better handle on the amount of waste rock being generated.

There was a consistency between API related assays and relative ash and moisture contents as well; although ash and moisture accuracies widened with rising API numbers.

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The American school of thought recognizes two types of organic material in coal; Anthraxylon and Attritus. The British recognize four; Vitrain, Fusain, Clarain, and Durain. The Australian's prefer counting "bright bands". The British designations may have more relevance for my studies and if you look very closely at your coal, you may see that the API “picks” you select may indeed correspond to these particular variations in coal lithology.

I began this approach with Roke Oil Enterprises, Ltd. of Calgary. Roke was the company providing the geophysical logging services for the 77-78 Hat Creek exploration program and they could see the merit in perusing this line of reasoning. As a preliminary approach to what came later, I gave them the lab results for one hole and they predicted the results (intervals specified) for two other holes; using only the geophysical data from those two other holes. They did quite well.

Roke would go on to continue this line of thinking and develop what they identified as their "Coal/Water/Ash Program". A program which allows for the in-situ analysis and potential selective mining of relevant coal properties by using limited lab results and detailed geophysical logs. It has proven to be a useful program and has been applied to other coal exploration programs in Canada serviced by Roke Oil Enterprises, Ltd.

To be clear; the "selective mining" part of this approach was first disgust in 1975 (as per this author's experience) while he was employed by an Edmonton based coal company. At that time, this potential only centered on "selective mining" studies. The potential as a geophysically mediated 'in-situ analysis' capability was the author's subsequent contribution to the topic and was confirmed with studies carried out by the author in 1977-78 on B.C. Hydro's Hat Creek coal property.

It was in March of 1978, that I began to see that our sampling might be better off if it were tied to the API scale on our geophysical logs. Up to this point, we had been looking at different sampling intervals that were related to the 'ash' content of the coal, and we were moving those sampling intervals vertically, up and down the hole looking for the optimal outcome for mining purposes and later use in a thermal power plant.

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What I did not realize, at that time however, was that this particular sampling regimen was a holdover from Hydro's 1975 work; and that that work had been presupposed to an approach to mining that had to do with looking at the deposit as a kind of, bulk-sample type coal deposit, where everything was to be lumped into several large blocks of coal (plus ash) for easy mining. In 1975, it may not have seemed plausible to approach the mining of the Hat Creek deposit from any other point of view. From my perspective however, coal is mined for its BTU value and not necessarily for its ash content.

It was moreover, not possible in 1975 to get good geophysical logs on the Hat creek coal deposit. Geophysical logging at that time, was done 'open hole' and it was (and still is) very difficult to get any hole drilled into the Hat Creek deposit to stay open. The coal seams there contain numerous beds of bentonitic clay and these beds of clay will swell up on becoming wet; and especially so when their confinement pressure is released. There are as well, many fault zones which contain crushed coal and these are prone to collapse when the drilling rods are removed. In at least one instance the rods got stuck in the hole and could not be removed.

It was not until 1976, that Roke Oil Enterprises developed the technology to log through the drilling rods. This provided us with the higher quality geophysical logs that we needed to go after and consider, the in-situ analysis part of the selective mining program. I was on the trail of this beast even as we were still operating under the old 1975 ‘ash based’ mining model. We had adopted that old approach without realizing where it had come from and it took some time for us to recognize the counterproductive effect that it was having on our efforts.

Under the previous sampling arrangement, our constant adjustments in looking for the optimal coal-to-ash ratio, were causing grievous headaches for our engineering staff. They were charged with studying various mining scenarios and the confusion that the old approach was causing was not appreciated. It was not intentional however, and was not recognized for what it was. When we switched to the API based sampling schedule, and developed its corresponding color scheme, things began to make better sense for everyone concerned.

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Now we had a sampling approach that was set to a specific set of API values which also had a corresponding horizontal orientation that could be used for correlation and mining purposes. This allowed us to outline several layers of coal that could be selectively mined for the value of their BTU content. This increased the value of that coal and hence the overall value of the deposit. It reduced the amount of ash in the final product and provided our engineering staff with more efficient control over the mining process.

The geometry of the deposit was a contributing factor as well, which helped advance the selective mining potential of the property. Our re-worked geological interpretation of the property helped redefine and clarify the structural geology and internal geometry of the deposit. We also had excellent lab work being done on site which saved us time in processing our results. Our geological team provided us with consistent, accurate lithological logs that were very helpful with our correlation work.

Our geophysical company’s ability to log through the rods had the effect of improving drill program performance and optimizing rig utilization; even as they provided us with high quality geophysical logs.

Unfortunately, recessions and budget restrictions in 1980 and 81 caused a shut down of Hydro's Hat Creek work and the closing of their Thermal Division. I never got an opportunity to complete or refine my work. We did this work without the aid of modern computers and I often wonder what the work would have looked like if it had been run through an appropriate computer program?

I am of the opinion that more study needs to be done with the API and perhaps the use of a new kind of geophysical tool that can detect and determine something of the nature of the carbon content of the coal. This may help to refine our understanding of the measurements of BTU or Kilo Joule output relative to an API signature.

End.

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About the author

The author has been a coal exploration geologist for many years. Participating as an exploration geologist on some ten coal exploration projects; directing exploration activities on seven of those ten projects. 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 British Columbia or Alberta.

He began at Hat Creek by mapping Trench 'A' and Trench 'B'. He moved on from there to log core, and concurrent with that work, he began to draw up the 1977-78 cross sections. As he did this, he set out to do and complete, a through review and re-interpretation of the structural geology of the Hat Creek coal deposit. He was eventually joined in this effort by Mr. Hun Kim (Hydro's on-site geologist) and together they would proceed to carry out a through correlation effort, involving all of the litho logs and all of the geophysical logs that were generated by the 1975 and the 1977-78 exploration programs.

The results of that work, along with the API based in-situ analysis work led to the development of a set of colored cross sections and a plan view geology map of Deposit Number One. That work then set the standards for the studies that went into the selective mining part of the Hat Creek exploration program.

The author has written several papers on coal exploration, advances in exploration practice, and the geology of the Hat Creek coal deposit. He is the originator of Riemannian Geology; wherein he applies to geology, geological exploration and exploration practice Professor Bernhard Riemann’s concept of geometry (circa 1854).

You can read these and other papers on his web site at the address below.

Thank you for your interest in my work.

www.geologyrocks.ca

Return to:My research papers list.