Annales Societatis Geologorum Poloniae (2003), vol. 73: 139-143.
ANOMALOUS DRILL-CORE PARTING IN THE SEDIMENTS OF THE UPPER SILESIAN BASIN
Alena KOZUSNIKOVA1, Vera FIALOVA1 & Ivan HOCH2
1 Institute o f Geonics A S CR, Studentska 1768, 70800 Ostrava-Poruba, Czech Republic, e-mail: kozusnik@ ugn.cas.cz 2 M ining Exploration and Safety a.s., Ostrava-Karvina Mines,Rude armady 637, 73921 Paskov, Czech Republic
e-mail: Ivan.Hoch@ dpb.cz
KoZusrrikova, A., Fialova, V. & Hoch, I., 2003. Anomalous drill-core parting in the sediments o f the Upper Silesian basin. Annales Societatis Geologorum Poloniae, 73: 139-143.
A bstract: Anomalous jointing o f drillcores is sometimes observed in prospect boreholes in the Upper Silesian basin. The drillcore is jointed into discs, slices or porringers that are not resemblant to natural jointing. This occurs in various types o f sediments, but especially in brittle rocks. Orientation o f jointing planes is perpendicular to the borehole axis and does not depend on the distance from the front o f the core run. Anomalous jointing reflects locally increased stress in the layer.
Key w ords: core, anomalous parting, state o f stress.
Manuscript received 11 June 2003, accepted 7 November 2003
INTRODUCTION
The F reiberg m ethod (T hom a, 1964) is one o f the m eth ods used in the prognostics o f rock-burst and gas-burst o c
currences. It em ploys anom alous parting o f drillcores into bow ls, discs, p lates or ro lle rs (further referred to as discs).
A nom alous parting has b ee n observed in m ines in R ussia, G erm any, and Poland in connection w ith bursts o f sand
stones, salts, an d gases. G re at attention has been g iven to the study o f this phenom enon in th e m ines o f D onbas (Zabi- gailo et al., 1974).
A ccording to the findings o f Z abigailo et al. (1974), separation o f rock into d iscs is an indicator o f rock p otential for failure, b u t the author h im s e lf adm its a lim ited applica
bility o f this m ethod. F o rm atio n o f discs during drilling d e
pends on the state o f stress o f the m assif, and the strength and strain characteristics o f rocks. A ccording to the above author, partin g o f a drillco re from the m a ssif is m ore pro b able in brittle rocks. S eparation into discs m ay o ccur in the event o f a certain com bination o f these properties. The d e
pendence betw een the in cidence o f d isc form ation and the petrographic com position o f sandstones found in the D o n bas area, how ever, can n o t b e applied to the U p p er Silesian B asin (U SB ). This fact is pro b ab ly caused b y h igher diage- netic lithification o f the sandstones n ear O strava, w ith p a r
tial elim ination o f the effec t o f individual rock constituents.
MATERIAL AND METHOD
A nom alous drillcore parting has been studied in b o re holes drilled from the surface, as w ell as in m ine boreholes in the U SB for the last 15 -2 0 years. Intervals o f anom alous parting have been observed in beds o f sandstone, conglom erate, and coal (Fig. 1). In b oreholes drilled from the sur
face, these effects w ere clearly identified in one case during the above-m entioned period. A m ore com m on phenom enon w as tapering o f th e d rillcore into a steep cone, m o stly on 20 cm segm ents o f sandstone, som etim es even at b o th their ends. C onsultation w ith drilling technologists did n o t ex
clude a p o ssibility o f co re-breaker involvem ent in this phe
nom enon. A t a depth o f 1,322 m o f borehole N P 820, a sandstone segm ent w ith such a type o f ending w as d isinte
g rated into a series o f cones in serted into one an o th er (Fig.
2F); m oreover, occurrence o f bow l-shaped partin g w as reg istered in the p roxim ity o f this find. T hese effects are m ore frequently observed in drillcores d erived from subsurface drilling, probably due to a sm aller core diam eter; hence, also a sm aller strength n ecessary fo r the disk separation.
Finds o f anom alous p arting w ere recorded in a database w ith ca. 230 cases registered to-date. The database also in
cludes finds from boreholes in P oland (e.g., Jaw o rzn o 6757 d), and sam ples from a skip p it at Slany. M any exam ples w ere, how ever, observed at jo in t core inspections attended by the s ta ff o f the O strav a -K a rv in a M ines, and n o t included in the database.
140
A. k o z u s n i k o v A e t a l.Borehole: CSM 1152/99
Borehole: Staric-750/99
Fig. 1. Intervals of anomalous drillcore parting in boreholes CSM 1152 and Staric II - 750
DISCUSSION
T he follow ing facts w ere ascertained b ased on the analysis o f individual finds:
1. T he effects o f anom alous drillcore p arting include (Fig. 2): a) plates, b) discs, c) bow ls, d) rollers, e) p red isp o sition, f) a series o f cones inserted into one another.
2. Intervals o f anom alous drillcore parting are several centim etres to several m etres thick. D isc thickness m ay vary even in a single interval, and discs m ay be com bined w ith rollers.
3. T he described anom alous p arting is different from n atural disintegration o f sedim ents, w hich is controlled p ri
m arily by th e ir bedding. T hin sections orientated p erp e n d icular to bedd in g show accum ulations o f m ica, coal m atter and other inhom ogeneities on the bedding planes. T his d is
integration is typical especially for claystones, siltstones o r lam inated sandstones w ith tab u lar bedding, w hich w ere not included in this study.
A nom alous parting originates in the course o f drilling, regardless o f the orientation o f th e borehole relative to the bed. A xes o f bow ls o r o th e r types o f anom alous partin g are alw ays p arallel to the b o reh o le axis. T he cores som etim es show only indications o f this phenom enon, characterized by circular fractures on the circum ference o f the core, also o ri
entated p erpendicular to th e b orehole axis. T hese d isco n ti
nuities (this is the term w e u se for these hair-thin fractures for sim plicity) w ere studied in thin sections o f p erpendicular orientation (F ialova & K oZusnikova, 1986). T hey have the form o f p arallel fractures dilated to 0.04 m m , accom panied b y a zone 1.29 m m thick o f a n etw o rk o f m inute d iscontinu
ous fractures (Fig. 3A). T he rest o f area o f the thin section
otherw ise is featured quartz grains w ith fractures o f various orientations.
D iscontinuities have both intra- and intergranular course, som etim es w ith tooth-like parting o f q uartz grains (Fig. 3B). T hese fractures do not resu lt from accum ulation o f m inerals o f low er strength at th e site o f the discontinuity.
It can be presu m ed th at in such cases the stress in the bed w as too low , or the strength o f the ro ck w as so high th at the plates did not separate com pletely. T hese cases are com m only observed in conglom erates o r in som e types o f sand
stone.
4. It can be inferred from the available finds th at the for
m ation o f discs is n o t restricted exclusively to th ick sand
stone com plexes or to a particu lar position w ithin these com plexes (the base, the top etc.). A nom alous partin g can equally be observed in all C arboniferous strata in the U SB , and it is independent o f the distance from the b orehole co l
lar. H igher frequency w as, how ever, encountered at stra- tigraphic levels th at bear sandstone beds confined to a p ar
ticular stratigraphic interval (e.g., the Saddle M em ber, Low er P etrkovice M em ber).
5. S everal intervals w ith anom alous parting w ere som e
tim es encountered in a single sandstone com plex (m ax. 6 in
tervals in borehole Staric 770). In som e cases, the first drilled interval has the hig h est thickness and th e last drilled interval has th e low est thickness (gradual stress release in the bed?).
6. F our boreholes drilled from the bo tto m o f a shaft w ere inspected at the F renstat M ine; one o f them is vertical w hereas the others are inclined b y up to 40° from th e v erti
cal. D iscs in all boreholes are o rientated p erp e n d icu la r to the core axis. In the siltstone succession w ith tw o quartzite
ANOMALOUS DRILL-CORE PARTING IN THE SEDIMENTS
Fig. 2. Types o f anomalous drillcore parting: A. plates; B. discs; C. bowls; D. rollers; E. predisposition; F. a series o f cones
Fig. 3. The zone o f fracturing, transmitted light, crossed nicols: A. parallel fractures; B. tooth-like fracture o f quartz grains
142
A. KOZUSNIKOVA ET AL .\
\ N
\°* 4
\ %
\ % N \
\ %
\ S
N Q>
I
H 13-85
▼---
&
N
70°-80°
N \ \ H 12A-85
■H 12-85
Fig. 4. Location of boreholes with the occurrence o f discs (CSA Mine, block 8)
beds below the P rokop S eam , interestingly, the occurrence o f discs is restricted o n ly to brittle quartzites, even though the sam e d rilling technique w as em ployed.
7. M o st boreholes w ith the occurrence o f anom alous p arting are situated in th e p ro x im ity o f faults (m ax. 100-150 m ), m ost typically n orm al faults (Fig. 4).
8. Intervals w ith anom alous parting in boreholes lo
cated close to each o th e r g enerally occu r at the sam e stra- tigraphic level (H 13, H 12 an d H 12 A).
9. A m athem atical m odel w as form ulated (M alik et al., 1990) to explain the o rigin o f d isc-shaped parting, p resu m a
bly resu ltin g from stress in the bed or in the m assif. The po w er thrust effect o f the co re b it on the ro ck an d the d y nam ic effects o f drilling w ere n eglected for sim plicity. I f these factors w ere o f decisive im portance, p arting w o u ld ap p ear in the sam e rocks at an y depths, w hich is n o t the case.
P arting w as p roved to b e con tro lled b y m aterial properties and the stress tensor, w h ich characterizes the original rock m a ssif and changes up o n drilling.
10. A nom alous p artin g is presen t in brittle rocks, in sandstone an d coal beds, less frequently in conglom erates.
Its occurrence is, how ever, n o t co ntrolled b y the p roportions o f m atrix an d other rock constituents, pro v id ed that the lat
te r do n o t alter rock b rittleness. R ocks w ith the m o st fre
quent occurrence o f discs m ostly show brittleness co effi
cient (uniaxial com pression strength vs. tensile strength ra
tio) h ig h e r than 10.
11. N o d irect co rrelation w as found betw een disc occu r
rence and positio n in th e bo reh o le b y com paring records in the drill rec o rd (depths o f co re runs) and disc occurrence (Fig. 5).
CONCLUSION
A s indicated by the d ata obtained, anom alous drillcore parting is a significant g eo m echanical criterion o f prim ary stress state o f th e rock m assif. A s such, it should be consis
tently follow ed and recorded. T he d ata suggest th a t the disc occurrence alone cannot be u se d for ro ck -b u rst prognostics directly, b u t indicates locally increased stress in the m assif.
T hus, it indicates a situation in w h ich ro ck-burst m a y b e ex
pected w ith a certain probability, due to the com b in atio n o f natural and technological conditions.
Depths of occurrence of disc-like parting (m)
218.70.
218.80
219.90- 220.15 220.35 220.50 220.65.
220.70
222.80 222.85
223.60 223.65:
223.80 223.90
224.70
225.50
217.10 Depths of runs (m)
219.10
219.90
'220.10
221.10
disc-like parting run
run 222.60
.223.95
224.70
225.50
M 1 : 50
Fig. 5. Intervals o f anomalous parting occurrence vs. core runs
ANOMALOUS DRILL-CORE PARTING IN THE SEDIMENTS
143
Acknowledgements
The research has been financially supported by the Czech Academy o f Sciences (project No. K 1012601) and the Grant Agency o f the Czech Republic (grant No. 105/02/0895).
REFERENCES
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Malik, J„ Fialova, V., KoZusnikova, A. & Polacek, J., 1990. Model o f origin o f anomalous drill-core parting. (In Czech). Uhli, 38, 2: 70-75.
Thoma, K., 1964. Ergebnisse der Untersuchung von Kembohrun- gen in Kalikombinat “Werra.“ Freiberger Forschungshefte, A, 304:91-113.
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