Dazhen Tang

Advanced characterization of physical properties of coals with different coal structures by nuclear magnetic resonance and X-ray computed tomography

In order to understand the correlation between coal structure and physical property of coal, samples with different coal structures were collected from the Late Permian period coal seams in the Laochang area, Yunnan Province, China. A set of experiments were carried out to quantitatively characterize the physical properties of coals with different coal structures using advanced and nondestructive low-field nuclear magnetic resonance (NMR) and X-ray computed tomography (X-CT). The experimental results gave us confidence to conclude that the evolution of the coal structures can be divided into five stages with stress increasing: the fractures closing stage, the microfractures development stage, the cracks development stage, the shear deformation stage, and the plastic deformation stage. Each stage corresponds to a different coal structure with unique physical characteristics. The undeformed coal is dominated with pores and a small amount of poorly connected fractures. In the proto-cataclastic stage, the volume of the mesopores, macropores and fractures sharply decreases with stress increasing. The coal rock becomes more compacted. Additionally, the connectivity between fractures and pores becomes worse. The cataclastic coal has well-developed mesopores, macropores and fractures but few micropores and transition pores. The connectivity between fractures and pores is most conducive to the exploitation of coalbed methane. In the mylonitic coal stage, the plastic deformation occurs, resulting in the reduction and discontinuity of mesopores, macropores, and fractures. Moreover, the undeformed coal has the best homogeneity, and the mylonitic coal has the highest heterogeneity, resulted from the uneven distribution of the maceral, pores, fractures, and minerals caused by later stress effect. Furthermore, the CT porosities have a good positive correlation with the permeability; the average CT number, the standard deviation of CT number have a negative correlation with the permeability.

Geochemistry of sulfur and elements in coals from the Antaibao surface mine, Pingshuo, Shanxi Province, China

Abstract Coal-bearing strata in the Antaibao surface mine, Pingshuo mining district Shanxi, China occur in the Taiyuan Formation (Upper Carboniferous) and Shanxi Formation (Lower Permian). The most productive coal seams in the mine are the Nos. 11, 9 and 4. Twenty-five samples of coals, partings, and roof shales from the Nos. 11, 10, 9, 5 (Taiyuan Formation), and 4 (Shanxi Formation) seams in the Antaibao mine were analyzed for ash, and total sulfur contents and forms of sulfur. The No. 11 seam is a medium- to high-sulfur coal (1.3–3.1% S). A coal sample (14.1% ash) from the No. 10 seam contains 3.4%, and high ash coal (37.4% ash) from the same seam contains only 0.4% total sulfur. The No. 9 seam is a low- to medium-sulfur coal (0.6–1.3% S). The highest sulfur content among the samples is 5.8% total sulfur from a coal sample of the No. 5 seam. The No. 4 seam is low-sulfur coal (0.29–0.54% S). The remarkable variation in sulfur content among the coal seams in the Taiyuan and Shanxi Formations could reflect different depositional environments; from coastal to lower deltaic plain for the Taiyuan, and from upper deltaic plain to fluvial basin for the Shanxi Formation. The sulfate sulfur comes mainly from the oxidation of pyrite. Sixteen samples were analyzed for 26 major, minor and trace elements by X-ray fluorescence spectrometry. The Sr content is significantly higher in coals from the No. 4 seam in the Shanxi Formation than those from Nos. 11 to 5 seams. Chlorine, Sr, Ca and P are elements bound organically; while Si, Al, Ti, Fe, K, Na, Mg, Cr, V, Ni, Rb, Ba and Nb are mainly associated with minerals.

Geological and hydrological controls on water coproduced with coalbed methane in Liulin, eastern Ordos basin, China

Significant amounts () of water are currently being extracted from coalbed methane (CBM) wells in Permian–Carboniferous coal in the Liulin area of the eastern Ordos basin, China. Waters coproduced with CBM have common chemical characteristics that can be an important exploration tool because they relate to the coal depositional environment and hydrodynamic maturation of groundwater and can be used to guide CBM development strategies. The CBM production targets of the No. 3 and 4 coal seams from sandstone in the Shanxi Formation and No. 8, 9, and 10 coal seams in the karst of the Taiyuan Formation were deposited in fluvial-deltaic and epicontinental-sea environments, respectively. This paper combines CBM geology, hydrogeology, CBM recovery, and laboratory data to define mechanisms of CBM preservation including the important influence of groundwater. Relevant indices include fluid inclusions as an indicator of the hydraulic connection between the coal seam reservoir and the overlaying strata and the ensemble characteristics of total dissolved solids (TDS) contents of water, water production rates, and reservoir temperatures as an indication of the current hydraulic connection. The TDS contents of waters from the No. 3 and 4 and No. 9 and 10 coal seams are double those from the subjacent karst No. 8 coal seam, indicating the important control of fast flow in karst. Low-salinity fluid inclusions from the roof of the subjacent-karst No. 8 coal seam also indicate an enduring hydraulic connection with overlaying strata during its burial history. Relatively low current temperatures in the No. 8 (subjacent-karst) coal seam also infer a strong hydraulic connection and active flow regime. Deuterium concentrations are elevated in the mudstone-bounded No. 9 and 10 coal seams, further confirming low rates of fluid transmission. The gas contents of coal seams from the Taiyuan Formation are higher than those from the sandstone-bounded coal seams in Shanxi Formation, also correlating with low rates of water transmission and low permeability. Conceptual models for these fluvial-deltaic and epicontinental-sea environments that are consistent with geology, gas content, and gas and water production rate histories are of gas-pressure sealing for the Shanxi Formation and hydrostatic-pressure sealing for the Taiyuan Formation. These results confirm the important controls of hydrogeological conditions on the preservation of CBM and the utility of hydrogeological indicators in prospecting for CBM.

Division of coalbed methane desorption stages and its significance

Abstract To quantitatively analyze the effect of desorption characteristics on productivity, a division method of coalbed methane (CBM) desorption stages was established based on the Langmuir adsorption isothermal theory, and the indicating significance of the method was analyzed with cases of CBM production. The desorbed efficiency was used to quantitatively characterize the CBM desorbed rate under different reservoir pressures. On the basis of key nodes on mathematical curves, the starting pressure, turning pressure and sensitive pressure were defined. For different coal samples, the corresponding desorbed efficiencies of the three nodes were always constants, therefore the CBM desorption process was divided into four stages, i.e., inefficient, slow, fast and sensitive desorption stages. Studies reveal that, fast and sensitive desorption stages contribute a lot to CBM productivity, while inefficient and slow desorption stages have very little contribution; absorption capacity, gas-bearing property and reservoir pressure are the key factors for the desorption characteristics. The production status of southern Qinshui Basin and Liulin area in Ordos Basin in Northwest China demonstrates that the method can guide the CBM development effectively.

Distribution of stable carbon isotope in coalbed methane from the east margin of Ordos Basin

The commercial recovery of methane from coal is well established in the coalbed methane (CBM) blocks at the east margin of Ordos Basin, China. CBM forms with various carbon isotopic ratios (δ13CPDB) due to the carbon isotopic fractionation in biogenical or thermogenical processes. Based on the geologic evolution of coalbed reservoir and studies on the characteristics of δ13CPDB values distributed spatially (e.g., horizontal CBM well location area, vertical coal burial zone, coal rank, etc.) and temporally (e.g., geologic evolution history), we explored the formation mechanism of carbon isotopic of methane. The relatively low δ13CPDB values are widely distributed along the research area, indicating a trend of “lighter-heavier-lighter” from north to south. From a combination analysis of the relationship between δ13CPDB and the relative effects, the essential aspects in determining CBM carbon isotope being light in the study area are: the genesis of secondary biogas in the north; water soluble effects in the active hydrodynamic areas in the middle; desorption fractionation effect promoted by tectonic evolution in the south; and the sudden warming hydrocarbon fractionation accelerated by magmatic event in particular areas (e.g., Linxian).

Controlling factors of underpressure reservoirs in the Sulige gas field, Ordos Basin

Abstract Based on the study of low-pressure distribution characteristics, the main factors that control the formation of low-pressure reservoirs in the Sulige gas field are analyzed from several aspects, such as structural evolution, sedimentary characteristics and formation fluid features. It is suggested that the gas pool pressure is low and affected greatly by buried depth. Its roof and floor developed several layers of uncompacted mudstone, indicative of good property and pressure sealing conditions. The formation water also indicates favorable preservation conditions. The reservoir pressure of the Sulige gas field experienced 4 major evolutionary stages: the normal pressure stage of the Late Triassic-early Jurassic, the pressure rising stage of the Middle Jurassic-Late Jurassic, the sustainable pressure increasing stage of the Early Cretaceous, and the pressure decreasing stage of the late Early Cretaceous. The pore rebound and temperature decrease caused by intense tectonic uplift after the late period of Early Cretaceous, caused the reservoir pressure of the Sulige gas field to reduce by 0.673 MPa and 23.08% of the original strata pressure, respectively. The low pressure of the Sulige gas field is related to sedimentary assemblage, tectonic evolution and hydrocarbon accumulation.

Genetic relationships between swamp microenvironment and sulfur distribution of the Late Paleozoic coals in North China

The genetic relationships between microenvironment of the Late Paleozoic peat-forming swamp and the sulfur contents of coal in North China have been studied by using coal-facies parameters involving gelification degree, tissue preservation index, vegetation index, transportation index, groundwater influence index, water medium indicator and swamp type index, etc. Among the various controlling factors of swamp microenvironment, swamp water medium elaborates a dominant action to sulfur accumulation in the marine-influenced coals; while coal-forming plant type, hydrodynamic state and water covering depth are more important to sulfur accumulation in the fresh water-influenced coals. Geological fractionation of sulfur isotopes reflects that sulfur accumulation experienced multi-stages evolution. Pyrite sulfurs formed earlier than organic sulfur and the sulfur isotopic δ34Sp shows lower values than organic sulfur isotopic δ34So. In the brine-influenced coals, sulfur accumulation processed relatively a long time span, the distribution of sulfur isotopes dispersed, and the coals are provided with high sulfur contents. In the fresh-water-influenced coals, sulfur accumulation occurred mainly at the syngenetic-penesyngenetic stage and the early diagenetic stage, and the total sulfur is lower and mainly composed of organic sulfur.

The Differences of Physical Properties of Coal Reservoirs and Their Origin Mechanism between Zhijin and Panxian Areas, Western Guizhou, China

Western Guizhou has abundant coal resources, and it is considered as the most prospective area for coalbed methane (CBM) production in Southern China. To explore the potential of CBM production in this area, this study systematically addressed the differences of physical properties of coal reservoirs in the two typical areas (Zhijin and Panxian) of Western Guizhou, and on the base of that, deeply analyzed the forming mechanism of their differences. The results show that: the coal reservoirs in the Zhijin area are mainly high and medium rank coals with low porosity and poor permeability, and the pore structure is mainly dominated by micropores and transition pores; while the coal reservoirs in the Panxian area are mainly low and medium rank coals with high porosity and favorable permeability, and the pore structure is dominated by macropores and mesopores. The Langmuir volume and pressure of coal reservoirs in the Zhijin area are higher than those in the Panxian area, which indicates that coal reservoirs in the Zhijin area have much better adsorption capacity. The differences of physical properties of coal reservoirs in Western Guizhou are primarily controlled by the evolution degree of coal reservoirs. The evolution degree is dominated by the depositional and burial history as well as the tectonic thermal events afterwards: the depositional and burial history determines the difference of coal ranks between Zhijin and Panxian areas; while the tectonic thermal events are the main controlling factors of the internal differences of the coal ranks. Resulting from the various depositional and burial history or tectonic thermal events, different evolution process and physical-chemical structure generate, which in return determine the physical properties of coal reservoirs in Western Guizhou.

Hydrocarbon-Generation in Cambrian-Silurian High- to Over- Mature Source Rocks, Middle and Upper Yangtze Region, China

Two main Cambrian-Silurian high- to over-mature hydrocarbon source rocks occur in the Middle and Upper Yangtze region: the Lower Cambrian Niutitang Formation (mostly shale), and the Lower Silurian Longmaxi and Upper Ordovician Wufeng formations (mostly graptolite shale). Due to several complicated tectonic events occurred in the study area, the source rocks experienced inhomogeneous uplift, deformation, and burial, resulting in discontinuous and episodic hydrocarbon generation evolution. Based on a detailed study of the tectonic burial history and an analysis of the current degree of thermal evolution of the source rocks, this paper studied the burial history, the thermal history, and the hydrocarbon-generation history of the Cambrian-Silurian source rocks. It used basin modeling technology and thermal simulation tests on source rocks to reveal the process and the regional regularity of the hydrocarbon-generation evolution, and to expound the tectonic phases and the time-spatial distribution of hydrocarbon generation. It shows that the region from western Hubei to eastern Chongqing is the major source area of Lower Cambrian rocks, and the region from southeastern Sichuan to central Guizhou is the major source area of Lower Silurian rocks. Reconstruction of hydrocarbon-generation history allows simulated estimation of the remaining hydrocarbon-generation potential of the high-over mature marine source rocks.

Composite Petroleum System and Advantageous Exploration Targets in the Kongquehe Area of Tarim Basin

Abstract Guided by the description methods and evaluation routine for composite petroleum system in superimposed basins, the composite characteristics of the petroleum systems in the Kongquehe area of the Tarim basin was studied and the geological evolution processes of hydrocarbon from source to trap was revealed. The petroleum systems in the Kongquehe area, mainly through fault composite linkage combined with unconformity surface composite linkage, constitute the Cambrian € (!)–Lower Ordovician O 1 (!)–Carboniferous C (*)–Triassic T (*)–Jurassic J (*) + Cambrian € (!)–Lower Ordovician O 1 (*)–Silurian S (*)–Devonian D (*)–Jurassic J (*) modified composite petroleum systems (*). There are three critical periods for composite petroleum system, such as the end of Devonian for forming tectonic framework and paleoreservoir, the end of Jurassic and Cretaceous for hydrocarbon transformation, redistribution, and adjustment. The research area may be classified into three types — destroyed dissipation zone, transformed adjustment zone, and deeply-buried preservation zone, amongst which the latter two zones are favorable for hydrocarbon accumulation. Especially, the Upper Paleozoic and Mesozoic faulted anticline traps of the Longkou anticline and the Weimake–Kaiping anticline and the residual Lower Paleozoic fault paleo-anticline traps in the transformed adjustment zone are the best prospecting targets.