Occurrence and Potential for Coalbed Methane Extraction in the Depocenter Area of the Upper Silesian Coal Basin (Poland) in the Context of Selected Geological Factors

Abstract

Coalbed methane (CBM) is the only unconventional gas in Poland with estimated recoverable resources. The prospects for developing deep CBM have been explored in recent years by drilling deep exploration wells within the depocenter of the Upper Silesian Coal Basin. The purpose of this study is to analyze the occurrence and potential for CBM extraction in this area of the basin, which can be considered prospective due to the confirmed presence of significant amounts of gas and thick coal seams at depths > 1500 m. The study examined the vertical and horizontal variability of the gas content in the studied area, the coal rank in the seams, thermal conditions, and coal reservoir parameters. The gas content in the seams, reaching more than 18 m³/t coal^daf at a depth of 2840 m, and indicative estimated gas resources of 9 billion m³ were found. The high gas content is accompanied by positive thermal and coal rank anomalies. The permeability and methane saturation of the coal seams are low, and therefore, potential methane production may prove problematic. However, the development of CBM extraction technologies involving directional drilling with artificial fracturing may encourage gas production testing in the study area.

1. Introduction

Coalbed methane (CBM), belonging to unconventional gas resources, is still an attractive source of energy due to the possibility of borehole extraction without the necessity to extract the coal itself. CBM drilling technology was initiated and expanded in the USA in the second half of the 20th century; however, it is currently most developed in Australia and China [1]. Global CBM resources are estimated at 113–184 trillion m³ [2]. Countries with the largest CBM potential resources include Russia, the USA, Canada, Australia, Indonesia and Poland [2]. In the latter of the unconventional reservoirs, only coalbed methane (CBM) has defined and calculated recoverable resources and developed reserves of 106,362.35 and 10,564.32 million m³, respectively, for the Upper Silesian Coal Basin (USCB) only [3]. The volume of recoverable resources of CBM is comparable to the volume of conventional gas resources in Poland; however, due to different methods of resource calculation and uneven levels of deposit assessment, the vast majority of these resources (>90%) belong to the lowest category of appraisal corresponding to possible resources [3].

The exploration and appraisal of CBM deposits in Poland has a long history [4,5,6]; however, to date, industrial production of this gas has not been initiated except for minor production of coal-mine methane and abandoned-mine methane. Among the numerous projects aimed at initiating industrial production of CBM, it is worth mentioning the Geometan project implemented in 2017–2019 to demethanation of coal seams with surface boreholes a few years before the start of coal mining and thus reduce the methane hazard. The following undertaking is the deep exploratory well Orzesze-1 with a depth of 3710 m carried out in 2019–2020 to check the possibility of unconventional gas deposits developing in the place of the deepest subsidence and the largest thickness of coal-bearing sediments (depocenter) in the USCB [7].

The extraction of so-called deep coalbed methane, defined as occurring at depths > 1500 m, is of interest due to, among other things, the expected significant gas reserves thicker as well as regularly occurring coal seams and has been practiced in the USA and China [8,9]. Deep CBM production tests performed in the Laramide Basin in the western United States have confirmed the feasibility of cost-effective gas production to depths of approximately 3000 m [9].

The results of gas content tests in coal seams carried out in the central part of the USCB with a maximum thickness of carboniferous strata proved positive for the amount of gas present in coal seams and are encouraging for the initiation of more advanced tests for its extraction [7].

This paper aims to analyze the variability of gas content in selected wells with depths > 1500 m in the depocenter area of the USCB in the context of the amount of gas present (estimated resources), the spatial distribution of the resources, and its production potential. The study will also address factors associated with gas-bearing reservoirs, such as formation temperature, coal rank, and selected geological features of the area (carboniferous lithology and fault tectonics).

2. Study Area

2.1. Location

The study area is situated in the central part of the USCB within the Main Syncline and includes the area of the depocenter of the basin, i.e., the maximum depression of the carboniferous coal-bearing series. The thickness of the carboniferous sequence exceeds 4000 m at this location. The boundaries of the study area were chosen to include deep wells (>1500 m deep) located in the hanging wall of the Bełk Fault, which ensures that the depocenter area is recognized for its gas-bearing capacity at the locus of accumulation of a significant amount of gas, generally located in the hanging wall of regional faults [10]. The defined sector has an area of 53.4 km² and includes, among others, the eastern part of the former Dębieńsko mine, the southern part of the Budryk mine, and the western part of the Bolesław Śmiały mine (Figure 1).

2.2. Outline of Geological Structure

In the study area to the geological prospection/exploration depth, there are carboniferous formations belonging to the Mississippian and Pennsylvanian, covered by Triassic and Miocene strata in the form of isolated patches and quaternary sediments (Figure 2). The upper part of the Mississippian (Serpukhovian) and the entire Pennsylvanian (Bashkirian and Moskovian) are coal-bearing and developed in the form of molasses deposits consisting of alternating packages of sandstones, siltstones, and mudstones with numerous coal seams. The carboniferous coal-bearing formation has been subdivided into three series, differing in the proportion of sandstone layers and the number and thickness of coal seams [4,11]. The Paralic Series (Serpukhovian) contains distinctive horizons with marine faunas, indicative of periodic inundation by the sea, the Upper Silesian Sandstone Series (Bashkirian) contains thick sandstone packages with thick coal seams, while the Mudstone Series (Moscovian) contains fine clastic packages (siltstone and mudstone) and numerous but rather thin coal seams, sandstones are in the minority. The carboniferous roof is erosional. Overlying the eroded carboniferous surface lie, for most of the area, quaternary sediments up to 50 m in thick and isolated slabs of Triassic and Miocene deposits were found, with a total thickness of up to 100 m.

The study area, like the entire USCB, is tectonically involved. The strata generally run in an NE-SW direction and dip to the SE at an angle of several to 50°. To the west, N-S-oriented fold structures appear, which include the Knurów Anticline adjacent to the Orlova Thrust, which is regarded as the western boundary of the disjunctive part of the USCB. The fault network of this part of the basin is built of several systems with distinct geometry and origin [4,12]. Among them, the system with sub-latitudinal principal faults is of great importance [13]. One such dislocation is the WNW-ESE Bełk Fault forming the southwestern boundary of the study area, which is regional with throw up to several hundred meters to the south. In addition, in the study area, there are numerous faults with small throws dividing the coal-bearing series into blocks. Due to the borehole reconnaissance of the study area, these faults have not been definitively confirmed, but they have been found in neighboring mines.

The sub-latitudinal fault systems have been active in various tectonic regimes, beginning with the syn- and post-sedimentary periods in the Carboniferous and Permian, through the Triassic and Cretaceous to the Neogene [13]. Neotectonic activity is also recorded in these systems, which results in contemporary movements on the faults [14]. During periods of reactivation, the recurring left-lateral oblique-slip character of principal faults was noted [13]. Thanks to the horizontal component of such displacement, alternating segments with compressional (watertight) and tensile (water-bearing, conducive to fluid migration) conditions were episodically formed in the zones of these dislocations [13].

2.3. Methane Occurrence

Methane in the coal-bearing series is genetically related to coal since it is formed as a result of chemical alterations of organic matter caused by the coalification process. The main factor driving this process is the temperature of the rock mass and the pressure of the overburdened rock. The amount of generated methane depends on the thermal maturity of organic matter, i.e., the more mature the substance, theoretically, the more methane can be produced [15,16]. The coal-bearing series in the USCB reached its greatest depth at the turn of the Carboniferous and Permian, and at that time, the highest temperature affected the coal seams, causing the currently observed coal rank and generating significant amounts of methane estimated at trillions of m³ [17]. However, the sorption capacity of coal is limited and dependent on pressure and temperature, so not all the gas produced remains in the coal, and the excess is expelled outside the deposit. Therefore, only an insignificant part of the generated gas is accumulated in coal seams today [17,18]. Another genetic type is methane produced by bacteria (microbial methane), but its presence was not detected in the study area.

There are two basic forms of methane occurrence in coal-bearing formations: sorbed methane, which is physically and chemically bound to the coal substance and located in coal micropores, and free methane, which fills macropores and fractures in the coal seam and is present in gangue rock (sandstone) and fault zones. Free methane and sorbed methane exist in mutual equilibrium expressed by gas pressure, an increase of which promotes gas adsorption by coal, while a decrease induced, for example, by geological processes or mining activities, causes gas desorption and migration toward other parts of the deposit or into the atmosphere [19].

The spatial distribution of the gas content in the USCB is related to its geological development. The long period of uplift of the basin lasting from the end of the Permian to the Paleogene caused erosion of the upper parts of the coal-bearing complex, changing hydrogeological conditions and, consequently, resulting in the escape of methane into the atmosphere. The free migration of gases was facilitated by the lack of a tight overburden of the coal-bearing series. Thus, the carboniferous rock mass was naturally degassed to depths varying from 500 to 1000 m, depending on local lithological and tectonic conditions [4,19].

The study area lies in the northern region of the USCB [4], where up to a depth of 500–600 m from the ground surface the carboniferous coal-bearing series has been naturally degassed in the geological past and the gas content in the coal seams is very low <2.5 m³ CH₄/Mg coal^daf, deeper the amount of methane increases very rapidly until it reaches a methane-bearing maximum with a gas content of 10–18 m³ CH₄/Mg coal^daf (Figure 2).

3. Methods

The study is based on archival data from 11 boreholes drilled to identify coal deposits and methane conditions over an area of more than 53 km². The depths of the boreholes range from 1612 to 3710 m, but gas-bearing tests were carried out to a depth of 2840 m in the Orzesze-1 borehole, the deepest in the USCB. The type and number of data obtained are shown in

Table 1. Type and number of data from boreholes used in the study Th—coal seam thickness, G—gas content, M—moisture content, A—ash content, T—temperature, Ro—vitrinite reflectance, “-“ no data.

Borehole	Symbol	Depth (m)	Th (%)	G (m3/t)	M (%)	A (%)	T (°C)	Ro (%)
Bolesław Śmiały 1M	BS 1M	1836	34	34	34	34	6	3
Bolesław Śmiały 2M	BS 2M	1834	27	27	27	27	6	5
Dębieńsko-Głębokie 2	DG-2	1726	27	27	27	27	6	-
Dębieńsko-Głębokie 3	DG-3	1950	42	42	40	40	7	-
Dębieńsko-Głębokie 4	DG-4	2000	34	34	31	31	7	4
Dębieńsko-Głębokie 6	DG-6	2000	36	36	35	35	7	-
Dębieńsko-Głębokie 7	DG-7	2000	37	37	37	37	7	-
Orzesze-1	Or-1	3708	41	41	41	41	4	3
Ornontowice 42	On-42	1612	33	33	28	28	6	3
Szczygłowice IG-1	Sz IG-1	2000	45	45	44	44	6	5
Paniowy IG-1	Pn IG-1	1948	32	32	31	31	6	4
In total			388	388	375	375	68	27

.

Due to the depocenter area of the USCB and, therefore, the focus on deep gas, the wells were selected to be deeper than 1600 m and the recorded gas content high (>8 m³/t coal^daf) and as little disturbed by mining activities as possible. These conditions are fulfilled in the area located between the Bełk Fault in the south, the decommissioned Dębieńsko mine in the west, and the working Budryk mine in the northwest. The eastern boundary is represented by the approximate border of the depocenter marked by the location of the Paniowy IG-1 borehole.

Two groups of parameters were analyzed. The first group consists of values related to the amount of gas in the deposit, i.e., gas content, thickness of coal seams, coal rank expressed by the vitrinite reflectance, and the temperature of the rock mass. They are important in the case of spatial changes in the quantity of gas in the rock mass and affect the amount of gas-in-place resources. The parameters from the second group determine the gas volume that can be extracted from the deposit and include gas saturation, permeability, and hydrogeological conditions [20]. The results from both groups of parameters indicate trends in gas content variability in the study area and indicate the possibilities of extracting CBM as a raw material.

The main parameter used in this study is the gas content in the coal seams, defined as the volume of gas contained in a mass unit of dry ash-free (pure) coal substance, expressed as m³ of gas per 1 ton of pure coal substance. Seam gas content was determined using two methods. The KPG hermetic canisters method involves placing a 0.1 m piece of coal core in a hermetically sealed container and subjecting it to two-stage vacuum degassing at a pressure of approximately 7 mmHg [4]. The amount of gas obtained is measured and converted per unit mass of pure coal substance. Gas content was measured in all 11 boreholes using this method. In boreholes Bolesław Śmiały 1M, 2M, and Orzesze-1, the free degasification USBM method was additionally applied at atmospheric pressure [21]. This is based on cyclic (daily) measurements of gas release through a coal core placed in 0.3 or 0.6 m containers until, for 5 consecutive readings, the amount of gas falls to zero or the amount of gas released is less than 5–10 cm³/day. Both methods usually give similar results.

In the study area, the spatial distribution of gas was analyzed both vertically in 11 wells and horizontally between wells. Vertical variability was analyzed by drawing up a summary graph of the variability of gas content in the 11 boreholes. Horizontal variability was analyzed by separating 4 levels: −970, −1220, −1470 and −1720 m a.s.l. For each level, the average gas content in individual wells was calculated by selecting data from an interval of 250 m above the level, i.e., for example, for the −970 m a.s.l., data were collected from an interval of −720 to −970 m a.s.l., and for the −1220 m a.s.l. from an interval of −970 to −1220 m a.s.l., etc. For the mean gas content values calculated in this way, horizontal variability maps, as well as a gas content cross-section were made. The maps and cross-sections were produced in the Surfer 12 computer program using the natural neighbor interpolation method.

The estimated gas resources were then calculated using the formula:

Q = Vc × (G − Gr) × b × T

(1)

where Q—estimated gas resources, Vc—mass of coal in seams with thickness < 0.6 m within the surveyed area, G—average gas content for the horizon, Gr—average residual gas content, b—coal mass correction factor according to dry ash-free basis, T—temperature factor.

Resources were calculated for each of the levels mentioned above and then summed. The desorbing gas content was used, i.e., the total gas content minus the residual gas content, which is the amount of gas present at atmospheric pressure and, therefore, not desorbed. It was determined using the USBM method. Coal seams with a thickness of >0.6 m were considered for the calculation of resources.

The resources calculated in this way provide information on the amount of gas currently accumulated in seams (gas in place) but not on the amount of gas that can be recovered from the reservoir during drilling, i.e., the recoverable resources. To calculate the recoverable resources, it is necessary to establish the so-called recovery factor, i.e., the ratio of recoverable gas to the total amount of gas in the reservoir. This factor varies and depends on parameters related to the productivity of the field, i.e., the permeability of the reservoir, its gas saturation, waterlogging, etc. It is most often determined after the test phase of gas production. As the trial exploitation was not carried out in the study area, it is not possible to establish this coefficient precisely. Therefore, an attempt was made to roughly estimate gas production possibilities based on reservoir permeability.

The coal permeability test was performed in the laboratory of the Oil and Gas Institute in Krakow on cylinder-shaped samples with a diameter of 1 inch and a length of 3–4 cm. The coal samples came from the Orzesze-1 well [7]. Nitrogen was passed through these samples at a given pressure, and the amount of gas flowing per unit of time was measured. The measurement was repeated several times for different pressure values, and then the permeability was calculated using the Darcy equation. The permeability results are presented for two pressure variants at 500 psi load corresponding to the near-wellbore zone disturbed by drilling works and at 2900 psi corresponding to the undisturbed rock mass. A total of 40 measurements from each variant were used. The variation of permeability of each variant with depth is presented.

The gas saturation of the coal seams was calculated using 11 measurements of the coal sorption capacity from the Orzesze-1 borehole and data on the measured gas content of the seam according to the following formula

S = G/Gc × 100 (%)

(2)

where S—saturation of the seam with gas, G—measured gas content in the seam, and Gc—sorption capacity of the seam.

In addition, the results of gas production tests conducted in other areas of the USCB were used.

The high gas content in the studied area is accompanied by a thermal positive anomaly [22,23] and an increased coal rank of the seams expressed by a slightly higher vitrinite reflectance in comparison with neighboring areas [24]. Therefore, rock mass temperature data were measured in 11 boreholes, and vitrinite reflectance data were collected. The sources of data on rock temperature and vitrinite reflectance are literature data [22,24]. Measurements of vitrinite reflectance were performed in the laboratories of geological companies (e.g., Katowice Geological Enterprise and Polish Geological Institute, Warsaw, Poland). Temperature measurements are taken in boreholes after drilling is completed, and about 120 h have elapsed to eliminate disturbance to the natural temperature of the rock mass caused by drilling [22]. The variability of these parameters was presented as a map of temperature variability and the vitrinite reflectance at −1000 m a.s.l. The aim is to investigate how temperature and coalification degree have contributed to shaping the recent gas-bearing nature of the study area.

4. Results and Discussion

4.1. Gas Content

Table 2. Minimum, maximum, average gas content and total thickness of coal seams in boreholes at individual levels.

Borehole	Level (m a.s.l.)		Methane Content (m3/t coaldaf)					Total Seams Thickness (m)
	From	To	Minimum	Maximum	Average	Standard Deviation	Data Number
BS 1M	−720	−970	3.40	7.35	5.48	1.62	3	2.25
	−970	−1220	5.24	5.91	5.72	0.38	5	4.4
	−1220	−1470	3.70	10.56	7.45	1.84	15	15.15
BS 2M	−720	−970	0.02	7.34	2.85	2.13	7	5.75
	−970	−1220	2.29	4.26	3.28	0.64	6	4.6
	−1220	−1470	2.00	6.19	3.50	1.38	7	5.8
DG-2	−720	−970	5.41	9.22	7.08	1.43	4	4.3
	−970	−1220	3.73	9.18	6.43	1.73	12	16
	−1220	−1470	5.26	8.35	7.18	1.16	4	5.4
DG-3	−720	−970	7.94	12.96	10.45	2.51	2	1.8
	−970	−1220	3.30	9.22	5.47	1.82	6	8.7
	−1220	−1470	4.53	12.08	9.47	2.37	10	12.15
	−1470	−1720	9.05	16.05	11.72	2.66	6	8.2
DG-4	−720	−970	8.88	9.02	8.95	0.07	2	1.4
	−970	−1220	2.12	5.82	4.34	1.24	5	4.8
	−1220	−1470	5.46	10.87	7.76	1.94	10	14.4
DG-6	−720	−970	3.01	5.03	3.81	0.87	3	2.9
	−970	−1220	2.75	3.31	3.03	0.28	2	1.2
	−1220	−1470	2.97	7.19	4.37	1.65	4	5.2
	−1470	−1720	4.57	11.89	6.71	3.02	4	7.4
DG-7	−720	−970	1.23	2.53	1.72	0.46	5	5.4
	−970	−1220	0.96	3.39	2.29	1.01	3	2.6
	−1220	−1470	3.72	6.18	5.24	0.96	5	4.2
	−1470	−1720	4.87	9.13	7.22	1.38	6	8.9
Or-1	−720	−970	9.91	12.41	10.98	1.05	3	2.35
	−970	−1220	4.61	7.36	5.66	0.84	9	12.27
	−1220	−1470	7.60	14.71	10.63	2.11	9	12.56
	−1470	−1720	9.94	14.32	12.19	1.79	3	5.32
	−1720	−1970	12.20	13.15	12.72	0.34	6	12.99
On-42	−720	−970	6.74	17.11	10.93	4.46	3	3.1
	−970	−1220	3.13	8.92	6.09	1.69	9	11.5
Sz IG-1	−720	−970	2.65	10.22	4.70	2.38	9	11.25
	−970	−1220	2.10	5.05	3.39	0.93	6	9.35
	−1220	−1470	0.96	4.01	2.90	1.16	4	14.7
	−1470	−1720	2.74	5.27	4.00	1.27	2	1.5
Pn IG-1	−720	−970	3.72	9.32	5.83	1.90	6	7.25
	−970	−1220	2.17	6.70	4.38	1.52	6	7.65
	−1220	−1470	2.12	6.13	4.18	1.52	6	14.65
	−1470	−1720	5.74	7.57	6.96	0.86	3	6.3

and Figure 2 show the vertical distribution of gas content in 11 wells drilled in the study area. This distribution coincides with the USCB’s northern pattern of the gas content proposed by Kotas [4]. The data presented show that up to a depth of about −250 m above sea level (500–600 m below ground level), the gas content is low or very low (<1 m³/t coal^daf). This is due to the natural process of degassing of the rock mass in the geological past between the Permian and Miocene periods (several hundred million years) due to the steady uplift of the carboniferous rock mass and erosion of its upper parts (see Section 2). The lack of a tight carboniferous overburden allowed the gas to escape into the atmosphere.

The depth of the roof of the occurrence of methane coal seams is determined by the gas-bearing surface of 4.5 m³/t coal^daf (Figure 3), which in the study area is located at a depth of −300 to −1250 m above sea level (a.s.l.) and shows a morphologically varied character constituting a kind of dome with a culmination in the area of the Orzesze 1 and Ornontowice 42 wells in the central part of the area. The location of the roof reaches a level of −300 to −450 m a.s.l. here, and from this point, the roof dips in all directions and reaches its minimum value in the area of the Dębieńsko-Głębokie 6 and 7 boreholes located near the Bełk Fault in the south of the study area (Figure 3). Below the depth of the surface of the roof of the methane seams, the gas content increases rapidly, reaching high values often >8 m³/t coal^daf. This is a high methane-bearing zone, the depth range of which has not been recognized. Data from the Orzesze-1 well show that this zone does not disappear at a depth of −2500 m a.s.l. (2800 m below ground level) (Figure 2), making it very extensive vertically and reaching >2250 m in thickness. Within it, two depth sub-zones of increased gas content are noticeable: the first at a depth of −750 m a.s.l. (about 1000 m below ground level) with a maximum value of about 17 m³/t coal^daf, and the second broader one in the depth interval from −1250 m a.s.l. (1500 m below ground level) to the reconnaissance limit at a depth of −2500 m a.s.l. (about 2800 m below ground level). Gas content here ranges from 12 to 18 m³/t coal^daf. The two zones are separated by an interval of reduced gas content up to 10 m³/t coal^daf.

The horizontal distribution of gas content is shown in Figure 4 and Figure 5. It can be seen that the average gas content of the seams increases towards the center of the area and in the vicinity of the wells Dębieńsko-Głębokie 2 and 3, Ornontowice 42, Orzesze 1 and Bolesław Śmiały 1M it takes maximum values from 7 to 11 m³/t coal^daf at the level of −1470 m a.s.l. From the visualization of the data presented in the geological-gas cross-section (Figure 6) and maps (Figure 3, Figure 4 and Figure 5), it can be seen that the distribution of gas content in the study area has a dome shape with a peak in the central part of the area, from which the values decrease in all directions.

The composition of the gas is characterized by the predominance of CH₄ (>90%). Of note is the occurrence of CO₂ at a level of about 15%, not previously recorded in the USCB at depths > 2300 m below ground level (Figure 7). The presence of CO₂ is accompanied by a decrease in methane content to about 80%, which may be due to the replacement of CH₄ by CO₂. Other gas components, namely higher hydrocarbons and N₂, are in the minority (up to 2–3%, Figure 7).

4.2. Coal Rank

Figure 8 shows the variation of coal rank of the seams determined using the average vitrinite reflectance (Ro) for a depth interval of −750 to −1000 m a.s.l. A clear upward trend toward the west is evident, which is consistent with the results of earlier studies [11,24]. The minimum value of Ro was recorded in the Paniowy IG-1 borehole in the eastern part of the area (<1%), while the maximum value was recorded in the Dębieńsko-Głębokie 8 borehole (1.34%) in the southwestern part. According to Kotas [11], the coal rank of the seams in the western and central parts of the USCB is zoned. There are alternating areas of high and low degrees of coalification at the same depth with a simultaneous depth-dependent decrease in the coalification field towards the east, as can be seen in Figure 8. The study area partly coincides with the latitudinal positive coal rank anomaly described by Kotas et al. [25], occurring in the Leszczyny, Orzesze, and Ornontowice areas.

Vertically, the coal rank shows an increase in depth according to Hilt’s rule. Ro measured in boreholes BS-1M, BS-2M, and Orzesze 1 ranges from 0.75% at a depth of 450–880 m through 1.60% at a depth of 1700–1800 m to 2.90% at a depth of about 3000 m. This is manifested by the occurrence of coal rank ranging from high volatile bituminous coal in the carboniferous roof part through medium and low volatile bituminous coal to anthracite at depths > 2500 m [7].

4.3. Formation Temperature

In the study area, temperature was measured in all 11 boreholes. Its changes are observed both vertically and horizontally. Temperature values were shown to increase with depth from about a dozen degrees near the carboniferous roof to >100 °C at a depth of about 3000 m in the Orzesze-1 well. The geothermal gradient varies between the carboniferous stratigraphic series and in the study area is about 3.25 °C/100 m for the Mudstone Series, 4.00–4.50 °C/100 m for the Upper Silesian Sandstone Series, and about 3.50 °C/100 m for the Paralic Series [22]. The horizontal distribution of temperature in the study area shown in Figure 9 indicates a general increase in values to the northeast toward the Szczygłowice IG-1 borehole with a clear positive anomaly in the region of the Orzesze-1 borehole. This picture shows general agreement with the results presented by Karwasiecka [22]. A positive thermal anomaly in the Ornontowice and Orzesze area is evident at every documentation level shown in the Geothermal Atlas of the USCB [22]. Both this anomaly and others found in the USCB generally coincide with the position and direction of regional sub-latitudinal dislocations, of which the Bełk Fault is one in the study area. This localization of thermal anomalies indicates heat transport by these dislocations from great depths toward the surface. The source of heat could be the decay of radioactive elements (radiogenic heat) [26] and/or additional heating after the tectonic inversion of the basin in the Asturian phase [27]. Circulating hydrothermal solutions in the Mesozoic may have played a special role in this case, leading, for example, to the formation of giant zinc-lead ore deposits bound to the Triassic formations in the northern margin of the USCB [28]. This heating is unlikely to have led to a renewal of the coalification process [29] but may have remobilized methane accumulated in the coal seams. The two processes may overlap and are presumably linked to heat sources that are not very deep. Previous interpretations of the heat transport possibility in the area of large dislocations in the south of the USCB, in the Jastrzębie and Czechowice regions, have shown that the depth of these sources may be about 10 km from the ground surface, where crystalline rocks (granites and gneisses) occur that underlie the USCB sedimentary series and maybe the provider of radiogenic heat [22,30]. Analysis of the maps in Figure 4, Figure 5, Figure 8, and Figure 9 indicates some convergence of methane content in seams, high coal rank, and positive thermal anomaly. However, the trends of changes in vitrinite reflectance in Figure 8 and temperature in Figure 9 do not completely coincide. The high formation temperature at the level of −1000 m, culminating in the area of the Orzesze-1 well (53.5 °C), is accompanied by lower vitrinite reflectance (1.01%) compared to the western part of the area. This is because the vitrinite reflectance shows a regional USCB trend of increasing from east to west, which results from the rate of subsidence of the area of deposition of carboniferous sediments and, consequently, the course of the coalification process at the turn of the Carboniferous and Permian [29]. In turn, the thermal anomaly occurring in the research area (Figure 9) is the result of the subsequent heating of the rock mass, probably initiated in the Asturian phase and then continued in the Mesozoic [27,28]. Thus, it overlaps the previously existing coal field of the basin.

The relationship between gas-bearing capacity and temperature was postulated by Tarnowski [10], who indicated the presence of magmatic intrusions around which the pressure of gas in the seams and gas-bearing capacity were low or zero, while at some distance from them, a sort of halo was formed with rapidly increasing gas pressure and high gas content.

The observed increase in gas content towards the central part of the study area (Figure 4 and Figure 5) and, thus, the delineated dome (see Section 4.1) may indicate that gas migration occurred from the bottom up above the local hypothetical heat source. This may be pointed out by variations in the molecular gas composition manifested by the fact that higher hydrocarbons (C₂H₆ and C₃H₈) and CO₂ are found deeper than methane (Figure 7), the opposite of the gas composition resulting from the origin of its components [16]. On this basis, it can be assumed that the gas found in the so-called first subzone of increased gas content may be of migratory origin. Migration pathways are faults and accompanying fractures and permeable sandstones. The lack of information as to the fading of the deeper methane subzone at a depth of 2800 m makes it difficult to assess the mode of gas migration at this depth. Studies on the isotopic composition of carbon in methane indicate that up to a depth of about 1000 m, there is migration gas, and below that, indigenous gas occurs [31].

The coincidence of temperature anomalies with increased gas content is also observed in other areas of the USCB, especially in the south (Jastrzębie, Pszczyna, and Czechowice–Dziedzice regions) [22].

4.4. Gas Operating Conditions

The estimated gas-in-place resources are summarized in

Table 3. Summary of estimated gas resources at individual levels and in total in the study area. B—recalculation coefficient for the dry and ash-free basis of coal, G—gas content, Gr—residual gas content, M—moisture content, A—ash content, Q—methane resource.

Level (m a.s.l.)		Coal Mass (t)	B	G (m3/t)	Gr (m3/t)	M (%)	A (%)	Q (m3)
From	To
−720	−970	329,415,243.91	0.86	5.70	0.69	1.41	12.80	1,275,154,180.25
−790	−1220	573,079,043.18	0.86	5.10	0.95	1.25	13.03	1,837,238,965.39
−1220	−1470	767,209,827.76	0.87	6.96	1.40	1.21	12.16	3,329,197,046.04
−1470	−1720	458,504,627.04	0.85	8.46	1.53	1.36	14.11	2,421,325,988.99
In total								8,862,916,180.67

. They amount to 8.9 billion Nm³ for the entire field, with the largest resources (3.3 billion Nm³ of gas) recorded for the −1470 m a.s.l. This is due to the highest average thickness of coal seams at this level. The resource estimate presented does not mean the actual amount of gas to be extracted (see Section 3), as this can be calculated based on a recovery factor that depends on the reservoir parameters of the coal seams. These parameters include permeability of coal seams, hydrogeological conditions, and saturation of coal seams with gas.

4.4.1. Permeability

This parameter is responsible for the migration of fluids (gas) in the rock mass. In the case of coal seams, it is determined by the presence of a complex system of fractures (cleat system), among which one can distinguish fractures of the primary system (face cleats) and perpendicular subordinate system (butt cleats). In addition, master cleats occur, i.e., fractures that cut not only the coal seam but also the surrounding rocks [32,33]. In the study area, permeability was measured only in the Orzesze-1 borehole on 40 coal samples, and its variation in depth is shown in Figure 10. Figure 10a illustrates permeability results at a seal of 500 psi, which corresponds to the conditions of the rock mass relaxed by drilling the borehole, while the results illustrated in Figure 10b correspond to a 2900 psi seal adequate to the conditions of an intact rock mass. In both cases, the permeability of the coal seams is low, ranging from 0.004 to 47.6 mD (average 3.97 mD) for a 500 psi load and from 0.0001 to 0.34 mD (average 0.05 mD) for a 2900 psi seal. Values >2 mD appear only in the set of results for 500 psi loading in 19 samples and a value exceeding 10 mD in only 2 cases for the same data set. The highest permeability occurs in the depth interval from about −1430 to −1600 m a.s.l. (1740–1900 m below ground level) (Figure 10), but in general, the permeability of coal seams is too low (<0.1 mD in the bulk of samples for the dataset at 2900 psi loading), considering the gas flows in a profitable amount. This is most likely due to the great depth of the survey (>800 m below the ground surface), at which significant rock mass stress (>800 m column of rocks) causes the tightening of fractures and thus reduces the permeability of coal seams. The petrographic structure of the coal substance and the tectonic involvement of the study area are also not negligible [5]. Low coal permeability (up to 0.1 mD) is also a feature of other regions of the USCB, and higher values appear rarely [5].

4.4.2. Hydrogeological Conditions

The study area lies within the hydrogeologically open region of the USCB, where there is free media communication between the ground surface and the carboniferous complex. Quaternary and partly Triassic and Miocene formations occurring in the overburden are not a barrier to migrating meteoric waters. Aquifers occur in the quaternary and carboniferous. In the latter, sandstones, primarily of the Upper Silesian Sandstone Series, are water-bearing; however, in the Mudstone Series, the Orzesze strata also contain inserts of water-bearing sandstones several meters thick. The sandstones of the Upper Silesian Sandstone Series have the best filtration parameters, but at great depths (>1000 m), the filtration conditions worsen significantly, and the rock mass becomes practically impermeable. The filtration coefficient at this depth drops below 10⁻⁸ m/s. The water is also becoming increasingly saline (6 to more than 60 g/L), containing sulfates and chlorides.

From the perspective of methane exploitation, water pumping is necessary because of the need to lower the hydrostatic pressure to at least the critical desorption pressure, which enables the initiation of methane desorption from the coal seam and, thus, its exploitation. The amount of water pumped out varies and depends on the duration of operation and the watering of gas exploitation intervals. In the southern part of the USCB, an average of 1.1 to 2.7 thousand cubic meters of water was pumped out during the methane production tests, with 23–40 thousand cubic meters of gas captured during the 116–130 days of the test [5]. Due to the salinity of the pumped water, plans should be made for its effective disposal or injection back into the rock mass.

4.4.3. Sorption Capacity and Methane Saturation of the Seams

Figure 11 shows the sorption isotherm versus measured gas content for a sample from seam 420/1, at a depth of 2234 m in the Orzesze-1 well. The sorption capacity of coal at this depth is about 25 m³ CH₄/t coal^daf with a measured gas content of 15 m³/t coal^daf. It follows that the saturation of the sample with methane is only 60%. The situation is similar for other coal samples from this borehole, whose sorption capacity is in the range of 16–40 m³ CH₄/t coal^daf and gas content is 8–18 m³ CH₄/t coal^daf, which means saturation from 42 to 92%, on average 42–60% [7]. Similar results were obtained from the Bolesław Śmiały-1 well. This shows significant undersaturation of coal seams with gas under reservoir conditions. In contrast to other USCB regions, where methane saturation increases with depth and at >1000 m from the ground is >90% [34], here we are dealing with saturation much lower regardless of depth.

According to previous global studies [35,36], the saturation of coal seams with methane is linked to the geological evolution of coal basins, among other factors. When a coal seam reaches its final degree of coalification, the seam is mostly saturated with methane at 100%, and the excess gas is expelled outside. As a result of the subsequent uplift of the coal-bearing series and the lowering of the temperature of the rock mass, the seam increases its sorption capacity, and if there is no resumption of gas generation, the seam becomes undersaturated with methane. This phenomenon also occurred in the USCB [5], which was uplifted from the Permian to the Neogene, causing the temperature of the rock mass to decrease to the current values and, as a result, undersaturation of the seams with methane over a significant area.

Thermal events in the geological past described in Section 4.3, which can cause methane remobilization, probably contributed to the secondary degassing of coal seams and further increase of their undersaturation. The positive thermal anomaly in the study area may thus have played a role in the observed incomplete saturation of the seams with methane.

The phenomenon of undersaturation of seams with gas under high-pressure conditions at great depth (>1500 m) has an unfavorable effect on the extraction of methane from the coal seam through the production well due to the observed delay in gas flow after the critical desorption pressure is reached.

4.4.4. Gas Production Prospects

Factors such as hydrostatic pressure, permeability, and saturation of seams with methane determine the success of methane well production. Hydrogeological conditions are also important. Methane extraction from deep coal seams in the depocenter area of the USCB can be particularly difficult due to the low permeability of coal seams. The results of studies and observations carried out during borehole methane production in the USA have shown that at high hydrostatic pressures at depth (>1000 m), a significant reduction is necessary to achieve the critical desorption pressure, so vast amounts of water must be pumped out [37]. The low permeability of coal seams can make this very difficult and, therefore, slow down the process of achieving the critical desorption pressure and thus delay the flow of gas into the well. The low saturation of the seams with gas can be an additional negative factor here.

However, the advantage of the deep interval for potential gas production is the high gas content in the seams, in most cases exceeding 4.5 m³/t coal^daf at depths below −720 m a.s.l. (about 1000 m below ground level), and thick coal seams (>0.6 m). Among the thickest are seams of the Upper Silesian Sandstone Series (2.5 to 11.8 m thick), but they occur at depths > 2000 m in the study area. The considerable thickness of the coal seams and the high gas content have determined the estimated gas resources at 8.9 billion m³. As already mentioned, this figure is only an indicative estimate and applies only to gas in place. The low permeability of the reservoir, which significantly determines the amount of gas production, leads us to conclude that the extraction rate will not be high. According to international experience, at a permeability of 1 mD, the utilization of gas-in-place resources is up to 25%, at 5 mD up to 47%, and at 25 mD up to 75%, respectively [20,38,39]. Taking the above into account and considering the low permeability of the reservoirs, only periodically increasing above 1 mD at a seal of 500 psi, the resource utilization rate in the study area will be low, below 50%.

Nevertheless, it should be remembered that the technology of exploitation, which has greatly modernized over the past two decades, is of great importance in the amount of gas extracted. A major innovation was the introduction of horizontal boreholes in the USA at the turn of the 21st century, which, covering a much larger area of coal seam drainage, contributed to a significant increase in gas extraction and the coverage of production in areas previously considered unpromising [40]. Artificial fracturing of coal seams also proved to be very helpful, leading to an increase in permeability and, therefore, improving the gas yield from the wells. Horizontal borehole technology combined with artificial fracturing was introduced in the USCB after 2010, following the failed tests of gas production with vertical boreholes providing access to multiple seams simultaneously, conducted in the basin in the 1990s [41]. The USCB’s breakthrough was the use of a doublet of combined vertical and horizontal boreholes, with the vertical one acting as a production well. This ensured efficient dewatering of the deposit [41]. Tests conducted with this method in the southeastern part of the USCB (Międzyrzecze region) gave very good results, as daily gas production of 10,000 m³ was achieved, and after pressure stabilization, about 5000 m³. The trials were carried out at a depth of 1000 m in the Upper Silesian Sandstone Series by the Polish Oil and Gas Company and the Polish Geological Institute under the Geometan project. This shows that the use of new technologies can significantly increase the amount of gas extracted and significantly improve the recovery rate from a reservoir, even in problematic areas for gas production. Global experience shows that increased gas production from a well can also be achieved by combining hydraulic splitting technology with the use of directional drilling [42] or by applying coiled tubing fracturing technology implemented in several horizontal well clusters [43].

The combination of methods applied in the USCB and in other countries prompts us to consider the possibility of conducting CBM production tests in the depocenter area of the basin, which, if the results are positive, may contribute to expanding the gas resource base in Poland, given that the composition of the gas extracted from the virgin areas of the basin allows for a wide range of uses.

5. Conclusions

1. In the study area, the gas content of the coal seams is arranged in a zonal manner. Up to a depth of about 500–600 m from the ground, there is a naturally degassed zone, below which a vertically extensive high-methane zone is present with two sub-zones visible, an upper zone with gas content up to 17 m³/t coal^daf and a lower zone with gas content up to 18.6 m³/t coal^daf at a depth of 2840 m. The two zones are separated by an interval of reduced gas content.

2. Horizontally, the distribution of gas content is observed in the form of a dome with a maximum in the central part of the study area, from which gas volume decreases in all directions.

3. The dome of gas content coincides with the positive temperature anomaly and the coal rank of the seams. The transport of heat by faults from a deep source causing secondary migration of methane and its accumulation at some distance from the hypothetical heat source is not excluded.

4. Significant estimated methane resources of 8.9 billion m³ are accompanied by not very favorable parameters related to reservoir productivity, i.e., low permeability of seams (in a significant part of the profile not exceeding 2 mD) and low saturation of seams with methane (on average 40–60%). The poor permeability is due to the considerable depth of the seams in the depocenter area of the basin. These parameters mean that the gas extraction rate from the reservoir may be low.

5. Leveraging new CBM production technologies used globally and in the USCB, for example, the use of hydraulic fracturing technology combined with horizontal boreholes in the form of well doublets can help improve reservoir productivity.

6. Given the significant estimated gas-in-place resources and the track record of deep gas production worldwide, it is worthwhile to conduct gas production tests in the study area, as the gas resource base in Poland could be expanded if successful.