Geological conditions and reservoir characteristics of various shales in major shalehosted regions of China

China is home to shales of three facies:Marine shale,continental shale,and marine-continental transitional shale.Different types of shale gas are associated with significantly different formation conditions and major controlling factors.This study compared the geological characteristics of various shales and analyzed the influences of different parameters on the formation and accumulation of shale gas.In general,shales in China’s several regions exhibit high total organic carbon(TOC)contents,which lays a sound material basis for shale gas generation.Marine strata generally show high degrees of thermal evolution.In contrast,continental shales manifest low degrees of thermal evolution,necessitating focusing on areas with relatively high degrees of thermal evolution in the process of shale gas surveys for these shales.The shales of the Wufeng and Silurian formations constitute the most favorable shale gas reservoirs since they exhibit the highest porosity among the three types of shales.These shales are followed by those in the Niutitang and Longtan formations.In contrast,the shales of the Doushantuo,Yanchang,and Qingshankou formations manifest low porosities.Furthermore,the shales of the Wufeng and Longmaxi formations exhibit high brittle mineral contents.Despite a low siliceous mineral content,the shales of the Doushantuo Formation feature a high carbonate mineral content,which can increase the shales’brittleness to some extent.For marine-continental transitional shales,where thin interbeds of tight sandstone with unequal thicknesses are generally found,it is recommended that fracturing combined with drainage of multiple sets of lithologic strata should be employed to enhance their shale gas production.

Reservoir Characteristics and Preservation Conditions of Longmaxi Shale in the Upper Yangtze Block, South China

The Upper Ordovician-Lower Silurian Longmaxi Shale in the Upper Yangtze block represents one of the most important shale gas plays in China. The shale composition, porosity, organic thermal maturity, and methane sorption were investigated at the Qilongcun section in the Dingshan area, southeastern Sichuan Basin. The results show that the Upper Ordovician-Lower Silurian Longmaxi Shale contains： （1） sapropelic I organic matter; （2） a 40-m thick bedded sequence where total organic carbon （TOC） content is 〉 2%; （3） a 30-m thick layer at the base of the Longmaxi Shale with a brittle mineral content higher than 50%; and （4） a mean methane adsorption capacity of 1.80 cm3/g （7 MPa pressure）. A positive correlation between TOC and sorbed gas indicates that organic matter content exerts an important control on methane storage capacity. Based on the analysis of the shale reservoir characteristics, the lower member of the Longmaxi Shale can thus be considered a favorable stratum for shale gas exploration and exploitation. It has similar reservoir characteristics with the Longmaxi Shale in the Jiaoshiba area tested with a high-yield industrial gas flow. However, based on tectonic analysis, differences in the level of industrial gas flow between the low-yield study area and the high-yield Jiaoshiba area may result from different tectonic preservation conditions. Evidence from these studies indicates the shale gas potential of the Longmaxi Shale is constrained by the reservoir and preservation conditions.

Development of a CH4 dehydroaromatization–catalyst regeneration fluidized bed system

A pilot-scale methane dehydroaromatization–H_2regeneration fluidized bed system(MDARS)was developed.In the MDARS,the catalyst circulation between a fluidized bed reactor and a fluidized bed regenerator with the help of a catalyst feeder allowed methane dehydroaromatization(MDA)and H_2regeneration to be carried out simultaneously,which is good for maintaining a stable MDA catalytic activity.A fixed bed reactor(FB)and a single fluidized bed reactor(SFB)were also used for a comparative study.The experimental results showed that the catalytic activity in the MDARS was more stable than that in the FB and SFB reactors.The effects of some parameters of MDARS on the CH_4conversion and product selectivity were investigated.To verify the feasibility and reliability of the MDARS,an eight-hour long-term test was carried out,which demonstrated that the operation of the MDARS was stable and that the catalytic activity remained stable throughout the entire experimental period.

Meso-Cenozoic Tectonothermal History of Permian Strata, Southwestern Weibei Uplift: Insights from Thermochronology and Geothermometry

This study provides an integrated interpretation for the Mesozoic-Cenozoic tectonothermal evolutionary history of the Permian strata in the Qishan area of the southwestern Weibei Uplift,Ordos Basin.Apatite fission-track and apatite/zircon(U-Th)/He thermochronometry,bitumen reflectance,thermal conductivity of rocks,paleotemperature recovery,and basin modeling were used to restore the Meso-Cenozoic tectonothermal history of the Permian Strata.The Triassic AFT data have a pooled age of^180±7 Ma with one age peak and P(χ2)=86%.The average value of corrected apatite(U-Th)/He age of two Permian sandstones is^168±4 Ma and a zircon(U-Th)/He age from the Cambrian strata is^231±14 Ma.Bitumen reflectance and maximum paleotemperature of two Ordovician mudstones are 1.81%,1.57%and^210℃,~196℃respectively.After undergoing a rapid subsidence and increasing temperature in Triassic influenced by intrusive rocks in some areas,the Permian strata experienced four cooling-uplift stages after the time when the maximum paleotemperature reached in late Jurassic:(1)A cooling stage(~163 Ma to^140 Ma)with temperatures ranging from^132℃to^53℃and a cooling rate of^3℃/Ma,an erosion thickness of^1900 m and an uplift rate of^82 m/Ma;(2)A cooling stage(~140 Ma to^52 Ma)with temperatures ranging from^53℃to^47℃and a cooling rate less than^0.1℃/Ma,an erosion thickness of^300 m and an uplift rate of^3 m/Ma;(3)(~52 Ma to^8 Ma)with^47℃to^43℃and^0.1℃/Ma,an erosion thickness of^500 m and an uplift rate of^11 m/Ma;(3)(~8 Ma to present)with^43℃to^20℃and^3℃/Ma,an erosion thickness of^650 m and an uplift rate of^81 m/Ma.The tectonothermal evolutionary history of the Qishan area in Triassic was influenced by the interaction of the Qinling Orogeny and the Weibei Uplift,and the south Qishan area had the earliest uplift-cooling time compared to other parts within the Weibei Uplift.The early Eocene at^52 Ma and the late Miocene at^8 Ma,as two significant turning points after which both the rate of uplift and the rate of temperature changed rapidly,were two key time for the uplift-cooling history of the Permian strata in the Qishan area of the southwestern Weibei Uplift,Ordos Basin.

Sedimentary environment controls on the accumulation of organic matter in the Upper Ordovician Wufeng–Lower Silurian Longmaxi mudstones in the Southeastern Sichuan Basin of China

The Upper Ordovician–Lower Silurian mudstones(including the Wufeng, Guanyinqiao and Longmaxi Formations) in the Sichuan Basin are some of the most important shale gas plays in China. In order to enhance our understanding of the process of formation of organic carbon up to 10%, optical, microscopy and geochemical methods have been used to investigate the petrographic and geochemical characteristics of the formation. Firstly, three mudstone lithofacies were identified based on a wide variety of mudstone laminations. These are:(a) indistinctly laminated mudstone;(b) parallellaminated mudstone; and(c) nonparallel-laminated mudstone. Then, combining with the evidence from depocenter migration, Th/U ratios and total organic carbon, the abundant organo-minerallic fabrics suggest that organic carbon was preferentially deposited and preserved in anoxic, low energy and stagnant water conditions during deposition of the Wufeng and Longmaxi Formations. On the contrary, the Guanyinqiao Formation with poor organic carbon was deposited in oxic and high-energy water conditions.

Formation of fractures in carbonate rocks by pad acid fracturing with different states of carbon dioxide

Carbonate outcrops were taken from Ma 51 sub-member in the Lower Paleozoic in the Yan’an gas field to conduct true tri-axial hydraulic fracturing experiments with water, liquid CO_(2) and supercritical CO_(2). CT scan was applied to analyze initiation and propagation laws of hydraulic fractures in carbonate rocks. The experiments show that supercritical CO_(2) has low viscosity, strong diffusivity and large filtration during fracturing, which is more liable to increase pore pressure of rocks around wellbore and decrease breakdown pressure of carbonate rocks. However, it would cost much more volume of supercritical CO_(2) than water to fracture rocks since the former increases the wellbore pressure more slowly during fracturing. For carbonate rocks with few natural fractures, tensional fractures are generated by fracturing with water and liquid CO_(2), and these fractures propagate along the maximum horizontal principal stress direction;while fracturing with supercritical CO_(2) can form shear fractures, whose morphology is rarely influenced by horizontal stress difference. Besides, the angle between propagation direction of these shear fractures near the wellbore and the maximum horizontal principal stress is 45°, and the fractures would gradually turn to propagate along the maximum horizontal principal stress when they extend to a certain distance from the wellbore, leading to an increase of fracture tortuosity compared with the former. For carbonate rocks with well-developed natural fractures, fracturing with fresh water is conducive to connect natural fractures with low approaching angle and form stepped fractures with simple morphology. The key to forming complex fractures after fracturing carbonate rocks is to connect the natural fractures with high approaching angle. It is easier for liquid CO_(2) with low viscosity to realize such connection. Multi-directional fractures with relatively complex morphology would be formed after fracturing with liquid CO_(2).

The influences of composition and pore structure on the adsorption behavior of CH_(4) and CO_(2) on shale

CO_(2) enhanced shale gas recovery(CO_(2)-ESGR)has attracted extensive attention as it can improve the shale gas recovery efficiency and sequestrate CO_(2) simultaneously.In this study,the relationship between mineral composition,pore structure,CH_(4) and CO_(2) adsorption behavior as well as selective adsorption coefficient of CO_(2) over CH_(4)(αCO_(2)/CH_(4))in marine and continental shales at different temperatures was investigated.The results illustrated that shale with higher total organic carbon(TOC),higher clay minerals and lower brittle mineral contents has a larger micropores and mesopores volume and specific surface area.TOC content was positively correlated with fractal dimension Df.Both CH_(4) and CO_(2) adsorption capacity in shale have positive correlations with TOC and clay mineral content.CO_(2) adsorption capacity of the all the tested shale samples were greater than CH_(4),and theαCO_(2)/CH_(4) of shale were larger than 1.00,which indicated that using CO_(2)-ESGR technology to improve the gas recovery is feasible in these shale gas reservoirs.A higher TOC content and in shale corresponding to a lowerαCO_(2)/CH_(4) due to the organic matters show stronger affinity on CH_(4) than that on CO_(2).Shale with a higher brittle mineral content corresponding to a higherαCO_(2)/CH_(4),and no obvious correlation betweenαCO_(2)/CH_(4) and clay mineral content in shale was observed due to the complexity of the clay minerals.TheαCO_(2)/CH_(4) of shale were decreased with increasing temperature for most cases,which indicated that a lower temperature is more favorable for the application of CO_(2)-ESGR technique.

Photo-Induced Water Oxidation Based on a Mononuclear Cobalt(II)Complex

Photo-induced water oxidation based on first row transition metal complexes has drawn much attention recently as a part of the efforts to design systems for solar fuel production.Here,the classic tetradentate ligand TPA(tris(2-pyridylmethyl)amine)is used together with cobalt(II)in CH_(3)CN to form a mononuclear cobalt complex[Co(TPA)Cl]Cl.Single crystal X-ray diffraction shows that[Co(TPA)Cl]Cl is composed of discrete cationic units with a penta-coordinate cobalt center,along with chloride counter ions.In borate buffer,the Co complex acts as a water oxidation catalyst,as shown by the presence of a catalytic wave in electrochemistry.Under visible light irra-diation,in the presence of photosensitizer and electron acceptor,the Co complex catalyzes O2 evolution with a turnover frequency(TOF)of 1.0 mol(O_(2))·mol(Co)^(-1)·s^(-1)and a turnover number(TON)of 55 mol(O_(2))·mol(Co)^(-1)in pH 8 borate buffer.