Carboniferous Taiyuan Formation Research Articles

Groundwater chemical characteristics and circulation mode in the Suixiao coal-mining district

Groundwater recharge and runoff conditions are ascertained in the Suixiao coal-mining district using the hydrogen and oxygen isotopes and the trace elements in the unconsolidated pore aquifer of the Cenozoic group, the fissured sandstone aquifer of the Permian system, and the karst fissured limestone aquifer of the Carboniferous Taiyuan Formation and the Ordovician system, which are the main recharge aquifers during coal mining. The main water–rock interactions are pyrite oxidation, cation exchange and adsorption, and carbonate acidification, which are educed by principal component analysis of conventional ions. These results combined with geological conditions prove that hydraulic connection exists generally between the main recharge aquifers, and the groundwater circulation is controlled by faults in the sandstone and limestone aquifers. The water–rock interaction is very weak in the east of the district, which is proved to be a recharge area by Fisher discriminant analysis. This study provides the theoretical basis for the hydrochemistry exploration and the establishment of a water-inrush warning system in a concealed coalfield.

Composition effects on pore structure of transitional shale: A case study of the upper Carboniferous Taiyuan Formation in the eastern uplift of the Liaohe Depression, China

Transitional shale is very different from marine shale with respect to mineral composition and pore structure. Insights into the effects of rock composition on pore structure can aid understanding of the storage and migration mechanisms of shale gas in a reservoir. To investigate the compositional effects on pore structure, a combination of geochemical and petrophysical experiments were conducted on four bulk shale samples and their corresponding extracted organic matter (OM) and clay mineral samples. The experimental results show that the removal of OM using H2O2 during clay mineral extraction oxidizes and removes pyrite and siderite from the samples while other inorganic phases remain unaffected. OM hosted more abundant micropores compared with clay minerals, while mesopores and fine-macropores are dominant in clay minerals. Effects of the OM and clay minerals on pore volume and surface area are quantitatively evaluated on the basis of weight-normalized pore-size distribution (PSD) curves of extracted OM and clay mineral samples. In terms of the CO2 adsorption, contribution from OM and clay minerals to micropore volume (MIV) are 19.43–32.92% and 18.75–39.05% respectively, and to micropore surface area (MIS) are 19.30–32.28% and 16.42–35.72%. Other minerals (other than organic matter and clay minerals) account for 34.29–48.93% of the MIV and 39.75–52.98% of the MIS. N2 adsorption results reveal that the pore volume and surface area are mainly attributable to the clay minerals, providing 54.37–80.75% of the BJH pore volume (BJHV) (average 65.83%) and 70.68–87.04% of the BET surface area (BETS) (average 78.71%). The primary medium for pore development in marine shale is OM, whereas pores in the transitional shale are mostly found in the clay and other inorganic minerals. Therefore, the hydraulic fracturing design and drilling-related operations involved in the exploitation of transitional shale gas should be appropriately adjusted and improved in order to accommodate intrinsic characteristics of the transitional shale gas reservoir.

Hydrogeochemical Characteristics and Water Quality Evaluation of Carboniferous Taiyuan Formation Limestone Water in Sulin Mining Area in Northern Anhui, China.

The Taiyuan formation limestone water in the Huaibei coalfield is not only the water source for coal mining, but also the water source for industry and agriculture in mining areas. Its hydrogeochemical characteristics and water quality are generally concerning. In this paper, conventional ion tests were carried out on the Taiyuan formation limestone water of 16 coal mines in the Sunan and Linhuan mining areas of the Huaibei coalfield. Piper trigram, Gibbs diagram and an ion scale coefficient map were used to analyze the hydrogeochemical characteristics of the Taiyuan formation limestone water. The water quality was evaluated in a fuzzy comprehensive manner. The results show that the main cation and anion contents in the Taiyuan formation limestone water were Na+ > Mg2+ > Ca2+ > K+, SO42− > HCO3− > Cl−. There were differences in the hydrogeochemical types of the Taiyuan formation limestone water in the two mining areas; HCO3-Na type water was dominant in the Sunan mining area and SO4·Cl-Na type water was dominant in the Linhuan mining area. The chemical composition of the Taiyuan formation limestone water is mainly affected by the weathering of the rock and is related to the dissolution of the evaporated salt and the weathering of the silicate. The fuzzy comprehensive evaluation results show that the V-type water accounts for a large proportion of the Taiyuan formation limestone water in the study area and the water quality is poor. This study provides a basis for the development and utilization of the Taiyuan formation limestone water and water environmental protection in the future.

Geochemical characteristics of trace elements and carbon isotopes in Taiyuan Formation: a case study of Huainan Coalfield

This paper reports systematically the redox sensitive elements (Mo, V, Cd, Cr, U and Th) and carbon and oxygen isotope data of the Late Carboniferous Taiyuan Formation limestone in the Huainan Coalfield in the southern margin of North China Platform, combined with the observation of limestone slice, and tries to explain the sedimentary environment of each layer of limestone. Through the analysis of the diagenesis of each layer of limestone, except for L7 and L12 limestone, the lithogenesis change of the remaining limestone becomes weak, which generally reflects the geochemical characteristics of the paleosol during sedimentation. There are three negative shifts of δ13C in Taiyuan Formation limestone, which may be related to the enrichment of redox sensitive elements (Cd, Cr, Mo and V) in limestone. In addition, the value of the above redox sensitive elements shows a negative drift. Combining the low V/Cr ratio and high Th/U ratio, it can be concluded that limestone of the Taiyuan Formation is likely to be deposited under an oxidizing condition.

Distribution characteristics and geochemistry mechanisms of carbon isotope of coalbed methane in central-southern Qinshui basin, China

Coalbed methane (CBM) as an environmentally clean energy resource, its origin and geochemistry characteristics play critical roles for the exploration and development of CBM resource. In this paper, the carbon isotopic distribution laws and the geochemistry mechanisms of CBM were analyzed in the No. 3 coal seam of Permian Shanxi Formation and the No. 15 coal seam of Late Carboniferous Taiyuan Formation, central-southern part of Qinshui basin, China. The research results revealed that the δ13C1 values of the No. 3 coal seam distribute between −47.3‰ and −26.5‰ (average −36.4‰), which are generally larger than those of the No. 15 coal seam that range from −48.6‰ to −27.5‰ (average −38.3‰). As a whole, the δ13C1 values of CBM in the two coal seams show a gradually decreasing tendency from southeast to northwest. Then, due to the greater generation and the higher adsorption capacity of 13CH4 in the high rank coal, the δ13C1 values show a strongly positive correlation with the thermal evolution degree. The gas content and burial depth also have positive correlations with the δ13C1 values, but the hydrodynamic activity presents a negative relationship along with the δ13C1 values. Actually the burial depth, geological structure and hydrodynamic activity influence the distribution of CBM carbon isotopes by affecting the gas content. Moreover, there also exists an obvious fractionation phenomenon for CBM carbon isotopes under the post-reformation effects. On the one hand, owing to the preference for 13CH4 adsorption and postponement for 13CH4 desorption in the coal reservoir, the desorption-diffusion-migration effect that occurs during tectonic evolution contributes a weak “vertical zone” characteristic. On the other hand, the lateral groundwater circulation and the vertical thermal fluid circulation under an abnormally paleo geotemperature field have a good effect on 13CH4 dissolution. Finally it concluded that the geological period from the Late Jurassic to the Late Cretaceous is a crucial stage in causing this fractionation phenomenon and determining the thermal evolution feature and gas-bearing property, thus forming the current distribution of CBM reservoir and geochemistry characteristics in the central-southern Qinshui basin.

Three-dimensional seismic analysis of Late Paleozoic coal-bearing series reflections in the Hangjinqi, North Ordos Basin, China

The study of sand bodies and coalbeds that formed during strong events is conducive to understand the relationship between source rocks and reservoirs. Two sets of Late Paleozoic coal-bearing sequences including the Late Carboniferous Taiyuan Formation and Early Permian Shanxi Formation, with an accumulated thickness of more than a half wavelength (90 m), were deposited in the Hangjinqi region on the northern margin of the Ordos Basin. These strata appear as two peaks and two troughs, three peaks and three troughs in the 3D seismic data. We analyzed this coal-bearing seismic response through a combination of well-data statistical analysis, the establishment of forward modeling, and the creation of crossplots. The coal-seam thicknesses of the Taiyuan and Shanxi Formations and the spacing between the two sets of coal indicate the major roles in the merging and bifurcation of events. The formation of three peaks and three troughs requires that the thicknesses of the two sets of coal are greater than 2 m. The spacing between the two sets of coal must also exceed 10 m with a detectable lateral extension, which would otherwise generate two peaks and two troughs. The two peaks and two troughs indicate a combined relationship between the thin sandstones at the bottom of the Shanxi Formation and the thick coalbeds in the Taiyuan and Shanxi Formations. The three peaks and three troughs suggest a combined relationship between the thick sandstones at the bottom of the Shanxi Formation and the thin coalbeds in the Taiyuan and Shanxi Formations. The drilling results clarified that the well situated at the two peaks and two troughs regions is accompanied by higher gas productivity, revealing that the coal-seam thickness has a significant influence on gas accumulation. The presence of two peaks and two troughs indicates favorability for natural gas exploration.

Groundwater Source Discrimination and Proportion Determination of Mine Inflow Using Ion Analyses: A Case Study from the Longmen Coal Mine, Henan Province, China

Complex hydrogeological conditions in China’s coal mines have contributed to frequent mine water disasters. A simple and effective method to determine water inflow sources and paths is therefore essential. The Longmen Mine, located in Henan Province, in central China was used as a case study. A Piper diagram and cluster analysis were used to screen the characteristic values of 18 water samples from potential aquifers. A comprehensive fuzzy evaluation of the groundwater ions was carried out to determine the main source of the total mine inflow. Then, based on conservation of ionic masses, a matrix function was established to calculate the groundwater recharge composition. Finally, using measured water inflows for the Cambrian limestone aquifer, the calculated and observed results were compared. The results showed that the Carboniferous Taiyuan Formation limestone aquifer (the L7 limestone aquifer) accounts for 60.8% of the total mine inflow, while the Cambrian limestone and roof sandstone aquifers account for 34.8 and 4.4% of the inflow, respectively. The normal mine inflow totals about 19,200 m3/day, of which 6,840 m3/day is from the Cambrian limestone aquifer. This agrees well with the calculated value of 6,720 m3/day. Thus, the method is feasible and reliable.

Geological controls on variable gas concentrations: A case study of the northern Gujiao Block, northwestern Qinshui Basin, China

A systematic investigation was performed on the variation of gas content and its geological controls based on the diverse geological data and the experimental results of 72 coal samples from 24 coalbed methane (CBM) wells in the northern Gujiao Block, NW Qinshui Basin, China. Coal rank varies from low-volatile bituminous coal to medium-volatile bituminous coal, with maximum vitrinite reflectance (Ro,max) ranging from 1.21 to 1.92%. CBM in the northern Gujiao Block is mainly produced from the relatively thick coal seams (∼1.0 m-7.0 m), which are distributed in the Upper Carboniferous Taiyuan formation and Lower Permian Shanxi formation. The total gas content of coal seams ranges from 3.83 to 16.82 m3/t, with increasing gas content from marginal to central areas. Highly variable gas content reflects the comprehensive effect of various geological factors and coal reservoir properties. The results of the study show that the structural types, roof lithology, burial depth and hydrodynamic conditions are key factors controlling the gas content, whereas coal thickness and coal properties have no or little correlation with the gas content. In the northern Gujiao Block, three geological features controlling gas content are identified: 1) hydrodynamic trapping of gas in the graben structure with an adequate burial depth and impermeable roof, 2) gas loss by hydrodynamic flushing and normal faults in the syncline structure, and 3) gas migration by intensive movement of groundwater in the shallow burial area. The first mechanism is considered to cause the high gas content of central study area, whereas the other two mechanisms can explain the low gas content in the north-western and south-eastern regions.

Influence of detrital composition on the diagenetic history of tight sandstones with implications for reservoir quality: Examples from the Permian Xiashihezi Formation and Carboniferous Taiyuan Formation, Daniudi gas field, Ordos Basin, China

Tight-gas reservoirs, characterized by low porosity and low permeability, are widely considered to be the product of post-depositional, diagenetic processes associated with progressive burial. This study utilizes a combination of thin section petrography, scanning electron microscopy, microprobe and back scatter electron analysis, stable isotope geochemistry and fluid inclusion analysis to compare the diagenetic history, including porosity formation, within sandstones of the second member of Carboniferous Taiyuan Formation (C3t2) and the first member of Permian Xiashihezi Formation (P1x1) in the Ordos Basin in central China.In the P1x1 member, relatively high abundances of metamorphic rock fragments coupled with a braided river and lacustrine delta environment of deposition, produced more smectite for transformation to illite (50–120 °C). This reaction was driven by dissolution of unstable minerals (K-feldspar and rock fragments) during the early to middle stages of mesodiagenesis and consumed all K-feldspar. Abundant intragranular porosity (average values of 2.8%) and microporosity in kaolinite (average values of 1.5%) formed at these burial depths with chlorite and calcite developed as by-products.In the C3t2 member, relatively low abundances of metamorphic rock fragments coupled with an incised valley-coastal plain environment of deposition resulted in less smectite for transformation to illite. High K+/H+ ratios in the early pore waters related to a marine sedimentary environment of deposition promoted this reaction. Under these conditions, K-feldspar was partially preserved. During the middle to late stages of mesodiagenesis, K-feldspar breakdown produced secondary intragranular (average values of 1.4%) and intergranular pores (average values of 1.2%). Release of K+ ions promoted illitization of kaolinite with quartz overgrowths and ferrous carbonates developed as by-products.This study has demonstrated that whereas both members are typical tight-gas sandstones, they are characterized by quite different diagenetic histories controlled by the primary detrital composition, especially during mesodiagenesis. Types of secondary porosity vary between the two members and developed at different stages of progressive burial. The content of unstable detrital components, notably feldspar, was the key factor that determined the abundance of secondary porosity.

Analytical modeling of mercury injection in high-rank coalbed methane reservoirs based on pores and microfractures: a case study of the upper carboniferous Taiyuan Formation in the Heshun block of the Qinshui Basin, central China

The coalbed gas reservoirs in the Qinshui Basin in central China are highly heterogeneous; thus, the reservoir characteristics are difficult to assess. Research on the pore structure of a reservoir can provide a basis for understanding the occurrence and seepage mechanisms of coal reservoirs, rock physics modeling and the formulation of rational development plans. Therefore, the pore structure characteristics of the coalbed gas reservoirs in the high rank bituminous coal in the No. 15 coal seam of the Carboniferous Taiyuan Group in the Heshun coalbed methane (CBM) blocks in the northeastern Qinshui Basin were analyzed based on pressure mercury and scanning electron microscopy data. The results showed that the effective porosity system of the coal reservoir was mainly composed of pores and microfractures and that the pore throat configuration of the coal reservoir was composed of pores and microthroats. A model was developed based on the porosity and microfractures of the high rank coal rock and the mercury injection and drainage curves. The mercury injection curve model and the coal permeability are well correlated and were more reliable for the analysis of coal and rock pore system connectivity than the mercury drainage curve model. Coal rocks with developed microfractures are highly permeable; the production levels are often high during the initial drainage stages, but they decrease rapidly. A significant portion of the natural gas remains in the strata and cannot be exploited; therefore, the ultimate recovery is rather low. Coal samples with underdeveloped microfractures have lower permeabilities. While the initial production levels are lower, the production cycle is longer, and the ultimate recovery is higher. Therefore, the initial production levels of coal reservoirs with poorly developed microfractures in some regions of China may be low. However, over the long term, due to their higher ultimate recoveries and longer production cycles, the total gas production levels will increase. This understanding can provide an important reference for developing appropriate CBM development plans.

Genetic types and sources of Lower Paleozoic natural gas in the Daniudi gas field, Ordos Basin, China

The molecular composition, stable carbon and hydrogen isotopes, and light hydrocarbons of the Lower Paleozoic natural gas in the Daniudi gas field in the Ordos Basin were investigated to study the geochemical characteristics. The Lower Paleozoic gas in the Daniudi gas field displays methane contents of 87.41–93.34%, dryness coefficients (C1/C1–5) ranging from 0.886 to 0.978, δ13C1 and δ13C2 values ranging from −40.3 to −36.4‰, with an average of −38.3‰, and from −33.6 to −24.2‰, with an average of −28.4‰, respectively, and δD1 values ranging from −197 to −160‰. The alkane gas generally displays positive carbon and hydrogen isotopic series, and the C7 and C5–7 light hydrocarbons of the Lower Paleozoic gas are dominated by methylcyclohexane and iso-alkanes, respectively. The Lower Paleozoic gas in the Daniudi gas field is mixed from coal-derived and oil-associated gases, similar to that observed in the Jingbian gas field. The oil-associated gas in the Lower Paleozoic gas is secondary oil cracking gas and displays a lower cracking extent than that in the Jingbian gas field. The coal-derived gas in the Lower Paleozoic gas in the Daniudi gas field migrated from the Upper Paleozoic gas through the window area where the iron–aluminum mudstone caprocks in the Upper Carboniferous Benxi Formation were missing. The oil-associated gas in the Lower Paleozoic gas in the Daniudi gas field was probably derived from presalt source rocks in the Lower Ordovician Majiagou Formation rather than the limestone in the Upper Carboniferous Taiyuan Formation. It seems unlikely that the marlstone in the Upper Ordovician Beiguoshan Formation and shale in the Middle Ordovician Pingliang Formation on the western and southwestern margins of the Ordos Basin contributed to the oil-associated gas in the Lower Paleozoic gas in the Daniudi gas field.

Genetic Types and Source of the Upper Paleozoic Tight Gas in the Hangjinqi Area, Northern Ordos Basin, China

The molecular and stable isotopic compositions of the Upper Paleozoic tight gas in the Hangjinqi area in northern Ordos Basin were investigated to study the geochemical characteristics. The tight gas is mainly wet with the dryness coefficient (C1/C1–5) of 0.853–0.951, andδ13C1andδ2H-C1values are ranging from-36.2‰to-32.0‰and from-199‰to-174‰, respectively, with generally positive carbon and hydrogen isotopic series. Identification of gas origin indicates that tight gas is mainly coal-type gas, and it has been affected by mixing of oil-type gas in the wells from the Shilijiahan and Gongkahan zones adjacent to the Wulanjilinmiao and Borjianghaizi faults. Gas-source correlation indicates that coal-type gas in the Shiguhao zone displays distal-source accumulation. It was mainly derived from the coal-measure source rocks in the Upper Carboniferous Taiyuan Formation (C3t) and Lower Permian Shanxi Formation (P1s), probably with a minor contribution from P1s coal measures from in situ Shiguhao zone. Natural gas in the Shilijiahan and Gongkahan zones mainly displays near-source accumulation. The coal-type gas component was derived from in situ C3t-P1s source rocks, whereas the oil-type gas component might be derived from the carbonate rocks in the Lower Ordovician Majiagou Formation (O1m).

Sedimentary characterization of the Upper Paleozoic coal-bearing tight sand strata, Daniudi Gas Field, Ordos Basin, China

The coal-bearing strata of the Upper Paleozoic (from the Taiyuan Formation to the lower member of the Shanxi Formation) are the most important units that have high gas production in the Daniudi gas field, which is a typical tight-sandstone reservoir with high heterogeneity in the Ordos Basin, China. Based on an integrated investigation of well logs, cores, SEM and 3-D seismic data, we delineated the sedimentary facies of the coal-bearing strata and divided the succession into sequenced stratigraphic units of different depositional systems. A sedimentary hiatus was documented for the first time in the study area and forms the sequence boundary between the Lower Pennsylvanian Carboniferous Taiyuan Formation (Ct1) and the Upper Pennsylvanian Carboniferous Taiyuan Formation (Ct2). The coal-bearing strata in Ct1 are indicative of a barrier coastal deposition system. Tidal channels are identified by their fine-grained, cross-stratified character. The sands in the tidal channels are well sorted, and the quartz content is above 95%. The coalbed located beside the sandstone is thought to be a lagoon. Gas-bearing, coarse-grained sandstone in the coal-bearing strata spanning from the Ct2 to the lower members of the Shanxi Formation (P1s) is interpreted as a fluvial-dominated braided delta that is divided into four third-order sequences. The coal-bearing strata are composed of sandstone, mudstone and coalbed from base to top in each sequence. Braided-river deposits form the lowstand system tract (LST) within each sequence. A shelf and lake depositional environment containing dark gray mudstone forms the transgressive systems tract (TST). The highstand systems tract (HST) deposits form the swamp coalbed in each sequence.

The discovery and significance of a sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China: Discussion

The paper by Jiang et al. (2012) documented a hiatus and overlying incised-valley fill between the Lower Pennsylvanian Taiyuan Formation (Ct1) and the Upper Pennsylvanian Taiyuan Formation (Ct2). Herein, we provide credible evidence suggesting that the incised-valley fill mentioned by Jiang et al. (2012) is a barrier island. Our work also indicates that the contact between Ct1 and Ct2 is continuous and without hiatus. The Paleozoic Ordos Basin has two stages of sedimentary environments: the early Paleozoic marine carbonate deposition and the Late Paleozoic (Carboniferous to Permian) transformation from paralic to terrestrial. Most scholars believe that the Carboniferous Taiyuan Formation is a barrier–lagoon–tidal flat system (Zhao and Liu, 1990; Hao et al., 2007; Luo et al., 2011) that formed during the relatively large-scale transgression at the North China block. The predominant sedimentary environment of the Taiyuan Formation is the barrier shoreline and has been documented as a barrier island, lagoon, and swamp (Chen et al., 2001; Yang et al., 2005; Hao et al., 2007; Luo et al., 2011). In particular, Jiang et al. (2012) confirmed the facies of Ct1 as a barrier–tidal system and interpreted the interval of Ct2 as a northeast–southwest-trending incised-valley fill and further claimed a hiatus between the Ct1 and Ct2 units. Well D23 locates in the incised-valley fill, as mentioned in figure 14 of Jiang et al. (2012). Its core of the lower Ct2 (above the bottom coal of D23; Figure 1) suggests that it is tidal in origin based on the following sedimentary structures: lens (starved ripples), ripples, and flaser beddings; herringbone bedding; and tidal bundles (couplet) (Figure 1). The fining-upward succession with erosive-based structures of tidal origin is a tidal inlet (Figure 1C, H). The coarsening-upward succession with parallel …

The discovery and significance of a sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China: Reply

We thank Zheng et al. (2016) for their comments on our previous work of sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China (Jiang et al., 2012). Zheng et al. (2016) have different opinions on sedimentary facies and sequence stratigraphy interpretation in the study area. They question our interpretation based on their understanding of sedimentary facies, seismic geometry, orientation of sand body, and sand body thickness. Here, we provide additional evidence to reinforce our interpretation. The structure of this reply follows the order of the questions raised by Zheng et al. (2016). The interpretation of depositional environment for sand bodies in the Upper Pennsylvanian Taiyuan Formation (Ct2) has been controversial for decades. Some previous studies suggested that Ct2 represents a barrier–lagoon–tidal flat system (e.g., Hao et al., 2007; Wang et al., 2007). Zheng et al. (2016) also made such an interpretation. However, northeast–southwest-trending sand bodies in Ct2 were interpreted as fluvial–deltaic channels by many other studies (e.g., Li et al., 1995; Liu, 1998; Fu et al., 2003; Guo et al., 2009; Zhang et al., 2009; Luo et al., 2011). The braided stream fills of the incised valley were documented by extensive erosional surfaces, conglomerate and/or mud clast fluvial lag deposits, and trough cross bedding. Erosional surfaces are one of the most common features observed in cores of Ct2 interval (Figure 1; see also figure 4 in Jiang et al., 2012). The poorly sorted gravels are aligned in various directions, indicating a rapid deposit without significant tidal reworking (Figure 1E). In contrast, the sedimentary structures that Zheng et al. (2016) interpreted as tidal–barrier island signatures, such as mud drapes and cross bedding, can be also found in fluvial deposits. Figure 1. Sedimentary features of fluvial deposit in the …

Isotopic composition and content of coalbed methane production gases and waters in karstic collapse column area, Qinshui Coalfield, China

The Qinshui Coalfield is one of the most important coalbed methane (CBM) coalfields in China with vast CBM resources, in which the Carboniferous Taiyuan Formation is one of the main coal-bearing sequences. Eleven CBM production gas samples and associated production waters were collected from the Taiyuan Fm. in the karstic collapse column (KCC) area of Sijiazhuang mining area, Qinshui Coalfield. The gas molecular and isotopic compositions and water quality, water carbon and hydrogen isotope compositions were analyzed. Results in this study reveal that CBM from Taiyuan Fm. of Sijiazhuang is dominated by CH4 (95.9–99.4mol%, air-free basis) with minor amounts of N2 (average: 1.07%), CO2 (average: 0.25%), and ethane (average: 0.02%). The carbon isotope ratios of the production CH4 range from −40.8‰ to −33.2‰, with an average of −37.1‰, and the corresponding hydrogen isotope ratios of CH4 −196‰ to −178‰, with an average of −186‰ (n=11). Thermogenic methane is the primary source of CBM from the Taiyuan Fm. of Sijiazhuang, and its estimated proportion is calculated to range from 72% to 95%.The type of CBM production water is Na–HCO3, and the concentrations of total dissolved solids range from 1282mg/mL to 1718mg/mL, with an average of 1417mg/mL. The δD values of the water samples range from −74.8‰ to −61.1‰, the δ18O values from −9.6‰ to −8.0‰. The isotope compositions of water samples fall to the right of the GMWL, suggesting a combination of fluid–rock interaction under high temperature conditions and evaporation.There is a good correlation between the distribution of KCCs and the isotopic compositions of CBM production gases and waters in the study area. KCCs in this area can be the free pathways going through the surface, the limestone beds and the 15# coal seam of Taiyuan Fm. KCC presence makes the CH4 carbon isotopic composition become more enriched in 13C in shallower areas, because of the stronger desorption–diffusion–migration isotope fraction effect of CBM in the north of Sijiazhuang mining area. In contrast, the δ13C1 and δ13CCO2 values are lighter in deeper areas in the southwest of the study area. This could be due to the stronger groundwater stripping process in that area. The results can be used for CBM exploration and exploitation in KCC areas.

Genetic types of natural gas and filling patterns in Daniudi gas field, Ordos Basin, China

The genetic types, source precursors and filling pattern of natural gas in the Upper Carboniferous Taiyuan Formation, Lower Permian Shanxi Formation and Lower Shihezi Formation gas reservoirs of Daniudi gas field were investigated using chemical composition as well as carbon and hydrogen isotopic compositions. Geochemical analysis of natural gases in 25 representative wells shows that natural gas in the Daniudi gas field is composed predominantly of hydrocarbons with a dryness coefficient of 0.884–0.978. The carbon isotopic values of ethane and propane are higher than −28‰ and −25‰, respectively, and the hydrogen isotopic values of methane are lower than −180‰, indicating that natural gas in the Daniudi field is a typical coal-type gas, derived mainly from humic organic matter in the transitional facies of the Carboniferous–Permian age. Hydrogen isotopic values of CH4 and H2 display a good positive correlation, suggesting that both were controlled by thermal maturity. When the mixing of ethane generated from mudstone and coal with the same kerogen type and similar thermal maturity occurred, the carbon isotopic values of ethane barely reflect the thermal maturity. Although the fractionation of hydrogen isotopes of ethane is significantly higher than that of carbon, hydrogen isotopic values of ethane in natural gas reservoirs evidently are not related to thermal maturity. The Daniudi natural gas reservoirs represent both self-sourced and near-source accumulations. The natural gas accumulations in the Late Triassic–Early Jurassic periods are mainly of the self-sourced type, while accumulations in the Late Jurassic–Early Cretaceous period comprise both self-sourced and near-source patterns, and the natural gas reservoirs formed after the Late Cretaceous period are mainly of the near-source type.

Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China

Significant progress has been made in coalbed methane (CBM) exploration and development in the eastern margin of the Ordos Basin where nearly 2000 CBM wells have been drilled, achieving a maximum gas production rate of 16000 m3/d by 2013. The geological evolution of the eastern Ordos Basin plays an important role in the CBM formation. This study is focused on the interrelationship between structural geology, gas accumulation and production characteristics of the No. 4 + 5 coal in the Permian Shanxi Formation as well as the No. 8 + 9 coal in the Carboniferous Taiyuan Formation. This research is based on the data collected from CBM production wells and coal samples from coalmines and exploration wells. The results show that thermogenic gas is the dominant CBM source in the study area and there are two significant generation periods, the coalification in the Triassic and the magmatic thermometamorphism during the Yanshan movement. Combining the structure background and hydrogeological conditions, the monoclinic-hydraulic sealing model was proposed as the representative CBM enrichment model. Different types of structures are also classified, and their influence on the CBM accumulation is discussed. Compressional structures formed during the Yanshan movement are conducive to CBM enrichment and retention; however, the tensional structures formed during the Himalaya movement may have led to CBM dissipation. Combining the structural effect on the CBM production with CBM exploration and development practices in the study area, the following three types of high gas production models are summarized: updip of the monocline, the axial part of the anticline or nose structure, and the structural high far from the normal fault.

Geologic and hydrological controls on coal reservoir water production in marine coal-bearing strata: A case study of the Carboniferous Taiyuan Formation in the Liulin area, eastern Ordos Basin, China

Coalbed methane (CBM) is commercially explored and produced in the continental Permian Shanxi Formation of the southern Qinshui and eastern Ordos Basins of China. However, CBM production in marine Carboniferous Taiyuan Formation is hindered by a lack of knowledge about the controlling factors on water production. The Liulin area of the eastern Ordos Basin is one of the trail areas for CBM development in China. In this study, water yield characteristics and their influencing factors during CBM production in the Liulin area were investigated by fluid inclusion studies, core slice observations, and stable isotope analyses of groundwater. The results demonstrated that the recharge area for this limestone aquifer of the Taiyuan Formation is the exposed carbonate in the northeastern Liulin area. The thickness along with the dissolved pores and fractures of the limestone aquifer were closely related to the water production of the CBM well. From an engineering point of view, hydraulic fracturing allows the direct movement of water from coal seams to the roof limestone aquifers, which was a key factor in the high water yield of CBM wells in the Liulin area. Finally, based on the structure and distribution of roof limestone as well as water production data, the geologic scenarios for water production in the Taiyuan Formation was divided into four types. These divisions indicated that the best prospective target areas for CBM production should have low numbers of faults, small limestone thicknesses, relatively large burial depths and normal reservoir pressures and temperatures. The results of this study are helpful for CBM exploration and production in the marine coal-bearing strata of China.

Geochemical characteristics of the coal gangues from the Du’erping coal mine, Xishan coalfield, North China

Inductively coupled plasma mass spectrometry, inductively coupled plasma optical emission spectroscopy, hydride generation-atomic fluorescence spectrometry, emission spectrometry, X fluorescence spectrometry, and X-ray diffraction were employed to study the geochemistry and mineralogy of coal gangues from Nos. 2, 3, and 8 coal seams of the Du’erping coal mine, Xishan coalfield, Taiyuan, Shanxi Province. The study revealed that compared with the sedimentary cover, upper continent crust, Carboniferous-Permian coal from North China, as well as most coal in China, coal gangues from Nos. 2 and 3 coal seams are rich in Li, Be, Sc, Cr, Cu, Ga, Ba, Th, Nb, Cd, Pb, Ta and rare-earth elements, and coal gangues from No. 8 coal seam are rich in Li, Sc, V, Cr, Ga, U, and rare-earth elements. Compared with the Carboniferous-Permian coal from North China and most coal in China, coal gangues from Nos. 2, 3 and 8 seams are rich in Rb, V, Cs and Sr. Therefore, The Du’erping coal gangues in the Xishan coalfield are rich in most hazardous trace elements and rare-earth elements, wherein the contents of Ga and Li reach the industrial grade and have significance for industrial utilization. On the whole, coal gangues of the Shanxi Formation from the Permian are rich in more trace elements than those of the Carboniferous Taiyuan Formation. The distributions of REE show obviously dipping rightwards with negative Eu anomalies. The contents of rare-earth elements in the three seams are quite different. All of the above indicate that the source of the rare-earth elements is terrigenous debris. Minerals in No. 2 seam identified by X-ray diffraction mainly include quartz, kaolinite, in addition to calcite, pyrite, apatite, epidote, and epsomite. No. 3 seam mainly contains quartz, kaolinite, in addition to a small amount of sodium feldspar, calcium nitrate, iron ore, gypsum, and vivianite. No. 8 seam mainly contains kaolinite, dickite, quartz, illite, and a small amount of hematite and U. The correlations between major elements and trace elements in coal gangues of the Du’erping coal mine analyzed by using SPSS (Statistical Product and Service Solutions) indicate that the trace alkali elements and rare-earth elements occur mainly in such clay minerals as kaolinite.