10 Geological Causes of Coalification MARLIES TEICHMÜLLER and ROLF TEICHMÜLLER
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Geologisches
Landesamt
Nordrhein-Westfalen,
Krefeld,
Germany
The effect of pressure, temperature, and time on the
diagenesis and
metamorphism of
coals
is
tested by geological observations and by experiments.
Geological arguments are taken mainly
from rank sections and
rank maps, based on
vitrite analyses. Temperature has most influence on the chemical reactions during the coalification process.
Not
only
the
maximum
temperature
reached but also the duration of exposure is important. The same rank may be reached either by a rapidly acting high temperature or by a long acting lower temperature.
Pressure retards the
chemical
coalification,
reactions during
but
it
changes the physical structure of coal. Tectonic pressure can cause occasional and
restricted in-
crease of rank.
|n a l l coal basins the geologist observes a more or less regular alteration of the coals w i t h increasing depth. L o w rank coals most noticeably lose moisture; i n higher rank coals the volatile matter decreases markedly w i t h increasing depth; i n the case of anthracites a loss of hydrogen is most characteristic. Other coal properties also change—e.g., the carbon content, the calorific value, and the optical reflectivity of the vitrinites. A l l these phenomena have been described b y geologists under the general term "coalification." T o study coalification quantitatively and determine its degree ( " r a n k " ) , it is necessary to work always w i t h one and the same penological constituent since the changes occur differently i n the different macérais. Vitrinite is best suited to comparative rank investigations since it is the most abundant component, is relatively easy to isolate, and changes i n the most consistent manner Regarding the type and extent of rank change w i t h depth, investigations of vitrites out of deep borings are most informative. Over the past 10 years approximately 12 deep borings have been put d o w n i n the N o r t h of the R u h r district i n order to establish the extent of 133 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
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134
COAL SCIENCE
the reserves of coal. Since the pattern of coal utilization largely depends o n rank, the range of coalification featured b y these boreholes, w h i c h have a depth of u p to 1200-1700 meters, was studied i n detail. F o r example, i n the Nordlicht-Ost 1 borehole (Figure 1) coal rank has been determined b y meas uring various properties (volatile matter, moisture, carbon content, calorific v a l u e ) . T h e unfaulted, fully cored profile, featuring a productive Carboniferous of 1250-meter thickness overall, covers, of course, only a limited range of coal rank. T o form a picture of the way i n w h i c h rank increases w i t h depth from the h i g h volatile bituminous coals to the anthracites, it is necessary to link the rank profiles of individual boreholes i n a meaningful order. Patteisky and M . Teichmuller have done this ( 1 7 ) , a n d Figure 2 shows the results i n d a f. )
moisture l°/o)
calorific »alue (kcal/kq a.f|
NordUcht
carbon | % d . a f i
hydrogen i^od-af)
Figure 2. The increase of rank of coal (vitrite) with depth, based on data from deep borings with flat lying beds (scheme according to Patteisky and A i . Teichmuller (17))
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
10.
C O U M S of Coalification
TEICHMULLER AND TEICHMULLER
depth ft. m
135
The increase of rank of coal The disappearance of heavy The decrease of pore volume (decrease of volatile matter) hydrocarbons in coal and rockl in sandstones
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2000
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Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 2, 2017 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch010
7000220QH
2400H 8 ooo H
26004
90002800H
3000H 10000-
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composition of the rest gas IN}- and CO, f rte)
pore volume
Figure 3. Changes of coals (vitrites) and rocks with increasing depth in the Munsterhnd 1 well (according to M . Teichmuller (26) Gedenk ( 5 ) and Tunn (31)) simplified form. A t first volatile matter decreases very little w i t h depth, a n d the values for the l o w rank bituminous coals show marked scatter expressed b y the broadness of the coalification band. O n proceeding d o w n w a r d , how ever, a considerable decrease is noted i n the range of the medium volatile bituminous coals to the semianthracites. W i t h increasing depth the contents of moisture, carbon, a n d hydrogen, as well as the calorific value likewise show changes of varying degree, w h i c h are characteristic for each property as w e l l as for the different ranges of rank. This shows that the evidential value of the different measures of rank vary w i t h the individual rank range. Therefore we used different measures of rank i n our studies depending o n the rank range being considered. These rank investigations included not only individual boreholes a n d shafts but also larger geological cross-sections and whole coal districts, as for instance the R u h r Basin ( 1 8 ) , the Saar Basin ( 2 ) , a n d the Basin of W e a l d e n coals i n L o w e r Saxony (28). F r o m such areas rank maps were drawn representing the regional rank changes of one a n d the same seam.
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
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