The influence of tectonic deformation on some geochemical properties of coals—a possible indicator of outburst potential

Abstract

Nearly all outburst-prone coal is known to have been tectonically deformed in fault and fold zones which underwent one or more episodes of strong structural deformation. The physical properties of deformed coal, such as strength coefficient and rate of gas desorption, have been broadly studied and used to predict outbursts in the past. However, it can be difficult to reliably determine these physical coefficients from core sampling operations because the coal can be inadvertently pulverized during drilling. The development of a new index which predicts the outburst potential of coal seams and is not influenced by sampling procedures could be beneficial to mining safety technology.

Tectonic deformation may not only affect the original physical structure of a coal but also may bring about changes in chemical properties. Any chemical property with the potential to identify outburst-prone coal must also be stable and independent of the physical changes imposed by drilling. Most of the chemical characteristics of coal display no obvious difference between normal undeformed coal (which is not subject to outbursts) and deformed coal. However, recent studies have shown that the chloroform extract yield is a very good geochemical index for differentiating these coals.

Nineteen coal samples from five mines experiencing outbursts in the Pingdingshan coalfield, Henan, China, were analyzed. Their mean maximum vitrinite reflectance in oil ranges from 0.97% to 1.28%. All of the samples were prepared to −180 μm (−80 mesh US) and subsamples were extensively extracted with chloroform in a Soxhlet apparatus at 75 °C. The mean extract yield of the 10 deformed coals is 1.45% whereas that of the nine normal coals is 0.44%. We conclude that deformed coal has a higher chloroform extract yield than equivalent coal which has not been tectonically disturbed.

Chloroform extraction, therefore, can be used to recognize deformed coal and the potential to undergo further deformation. Chloroform extract yield is an accepted measure of the hydrocarbon-generating potential of coals and other rocks. The results presented here, therefore, suggest that tectonic deformation may modify the hydrocarbon generating potential of coals. The adsorption of soluble organic matter expelled during and following tectonic deformation onto enhanced surface areas may have contributed to the higher extract yields of the deformed coal.

Introduction

Coal and gas outbursts have occurred in nearly all the major coal producing countries. In the last 150 years, possibly over 30,000 outbursts have occurred throughout the world (Beamish and Crosdale, 1998). This mining hazard still represents a severe threat to miners in some countries. In China, the first recorded instantaneous outburst of coal and gas was at Liaoyuan colliery in Northeast China in 1950 (Peng, 1990). More than 14,000 outbursts have occurred throughout northeastern, eastern, southern and central China, and over 200 collieries have experienced these events since then. The biggest outburst, recorded in Tianfu coal mine, Sichuan Province, ejected 12,780 t of coal and rock and 1,400,000 m³ of gas. The second biggest outburst, in Yutianbo coal mine, Sichuan Province, ejected 8327 t of coal (Zhang, 1992). During violent instantaneous outbursts of coal and gas, miners have been injured by projected coal or moving equipment, or suffocated by the gas. In the worst accidents, coal miners were killed by gas explosions which followed the outbursts.

The establishment of corrective methodology and research into outbursts has progressed continuously in China since 1953 (Peng, 1990). However, although methods for both prediction and abatement have been investigated, increase in both mining activity and depth of mining have resulted in a rapid growth in the frequency of outbursts. Coal beds with lower gas contents and representing no danger of outbursts at shallow levels can become outburst prone beds at greater depth (Zhang, 1992).

Indices for the prediction of outburst tendency for a particular mine or region are selected according to the prevailing outburst mechanism and history. Current outburst theory recognizes stress, gas content, and the physical–mechanical properties of coal as the three principal factors influencing instantaneous outbursts Hargraves, 1983, Hargraves, 1993, Peng, 1990, Lama and Bodziony, 1996, Lama and Bodziony, 1998, Beamish and Crosdale, 1998. Because the measurement of stress in situ is difficult, most assessment indexes employed in outburst prediction are based on the gas content and coal strength. The thresholds are based on data obtained from past outbursts. One important index that combines gas emission rate and coal strength for wide application in China for regional outburst assessment is the K value (Wang and Yang, 1980):

$$\text{K=}\text{Δ}\text{p}{}_{\mathrm{10–60}}\text{/f}$$

where Δp_10–60=emission rate of gas released from the coal sample and f=Protodyakonov strength coefficient of the coal sample.

When gas pressure reaches or exceeds 0.6 MPa/cm², outburst tendency increases with increasing K value (Wang and Yang, 1980): K<15, a coal seam is not liable to outburst; 15≤K<40, a coal seam is liable to outburst; K≥40, a coal seam has serious outburst tendency.

As shown in Eq. (1), outburst tendency, or K value, is directly proportional to gas emission rate Δp_10–60 and inversely proportional to coal strength (f). When Δp_10–60 is constant, the outburst tendency is determined only by coal strength.

The Protodyakonov strength coefficient (f) is measured by a simple test designed to determine the degree of pulverization of rock upon impact Protodyakonov, 1963, Lama and Bodziony, 1996. In China, a modification of the test is accepted for coal and microstructurally altered coal Peng, 1990, Jiang and Li, 1992. Each coal sample is pulverized into a size fraction of 1–3 mm for deformed coal and 10–15 mm for normal coal. Forty grams of the size fraction are placed at the bottom of a steel cylinder of 76 mm internal diameter and struck n times with a 2.4-kg piston falling through a distance of 600 mm. Generally, n=3 for deformed coal samples and n=5 for normal coal samples.¹ Five such tests are performed for a total of 200 g of broken coal which is combined and sieved though a 0.5-mm screen. The material that passes through the screen is placed into graduated cylinder of 23 mm diameter and compressed by a weighted piston. The height of the resulting cylindrical mass of compacted fines, l, is recorded in milliliters. The dimensionless strength coefficient, or f value, is given by:

$$\text{f=20n/l}$$

However, measurement of strength coefficient is not possible when coal cores are inadvertently pulverized during drilling because it is difficult to collect the required fragment size. This situation is aggravated during drilling in the Pingdingshan coalfield's medium volatile bituminous coal which can be heavily cleated; thus, the K value is not an effective index in this situation.

The coal strength coefficient f is used alone as an index of outburst potential under certain conditions. Coal strength coefficient decreases with increasing degree of deformation through the sequence of normal, cataclastic, granular and mylonitic coals and outburst tendency. This has been confirmed by the research results from the Pingdingshan mine nos. 8 and 12 in Henan (Peng and Cao, 1993). Cao (1995) has pointed out that coal strength or microstructural types can be used alone to predict outburst tendency in mining areas where the depth is greater than that of the shallowest depth of outburst-prone coal.

Because of the influence of drilling process, neither coal strength nor microstructural type can be exactly measured or differentiated using core samples and, therefore, cannot be used for determining outburst tendency. Consequently, the development of a new index to predict the outburst tendency of medium and low volatile bituminous coal could be very important for the prediction during exploration of the likelihood of outbursts. Research involving coals from the Pingdingshan bituminous coalfield, Henan Province, suggests that the organic solvent extract yield provides the means of differentiating normal and deformed coals of medium volatile bituminous coals. As a geochemical parameter, it might be independent of changes imposed by drilling.