Characterization of organic matter-hosted pores by SEM method and their formation mechanisms for shales of Longmaxi Formation, Sichuan basin

In this study, a test was carried out on organic carbon, sulfur content, major and trace elements of 24 shale samples from fresh outcrop sections in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, Shuanghe Town, Changning County, Sichuan Basin. Moreover, this study analyzed the vertical variation characteristics of major and trace element contents and their relationship with sea-basin paleo-sedimentary environment and explored the major controlling factors for organic matter enrichment in Ordovician-Silurian shale of Sichuan Basin. Research results show that the organic carbon content is higher (2.2%-7.76%)in the reservoir intervals about 20 m above Wufeng Formation to the bottom of Longmaxi Formation, then decreased upwards to stable value of 0.81%-1.83%. Major components of shale include SiO2(55.67%), Al2O3(10.13%)and CaO (9.51%). The contents of TiO2and SiO2are lower at the bottom of profile and then increased upwards, indicating the increased input of terrigenous clastics. The SiO2and CaO contents of organic-rich shale are apparently higher than those of organic-poor shale, but the contents of Al2O3, Fe2O3and TiO2are lower. Meanwhile, redox sensitive elements (Mo, U, V, Ni, Co, Cr)and nutritional elements Ba are significantly enriched in the organic-rich interval. Ni/Co, V/Cr, U/Th and the degree of pyritization(DOPT) indicate that Wufeng Formation presents large changes in water mass redox conditions, dominated by suboxic and anoxic conditions. The water mass at the bottom of Longmaxi Formation has a higher reducibility than Wufeng Formation, indicating a euxinic environment with certain content of H2S. However, normal oxygen enrichment environment exists in the middle and upper part. The contents of P and excess Ba (Baxs) in shale demonstrate a higher productivity in the sedimentary stage of Wufeng Formation-Longmaxi Formation, while TOC content has a positive correlation with redox proxies, suggesting that the changes in organic matter content is predominantly controlled by redox environment. The cross plots of Mo/TOC and DOPTshow that redox condition and water retention is the major controlling factor for trace element enrichment in Longmaxi Formation shale and Wufeng Formation, respectively. Organic matter enrichment in Longmaxi Formation is mainly controlled by the degree of seawater anoxic resulting from the rise and fall of sea level. However, anoxic environment in Wufeng Formation caused by water retention leads to favorable preservation of organic matter. © 2015, Editorial Office of ACTA PETROLEI SINICA. All right reserved.

X-ray diffraction is a basic method for determination of shale mineral composition, quantitative accuracy by diffraction by mineral composition, particle size, the diffraction peaks overlap, and other factors. Choose well QQ1#drilling wells hole samples in Sichuan Basin shale mineral components determined, by comparison with two kinds of experimental results, find out the methods of improving the precision for quantitative shale mineral components. Method 1: Whole rock direct determination and quantitative XRD; Method 2: The phase after dissolving stokes gravity separation of different graded mineral XRD measurement and quantitative respectively. Results: showed that through two experimental methods have a greater difference between the percentage of carbonate minerals, directly reflects the XRD quantitative methods in the content of mineral component is not fully trusted. The phase after dissolving and gravity separation XRD test can largely improve the test precision and shale in the mineral content of different granularity of shale diagenesis and shale properties than the single mineral content has more practical significance. ©, 2015, Science Press. All right reserved.

ABSTRACTGeochemical and mineralogical analyses, in addition to isothermal adsorption experiments on field samples, are used to characterise the sedimentary environments, reservoirs and adsorbed gas of the Upper Ordovician Wufeng–lower Silurian Longmaxi formations in the Sichuan Basin and its peripheral areas. The sedimentary environment of the Wufeng and the lower part of Longmaxi formations is a deep-water shelf with five different lithologies identified: siliceous shale, black shale, siltstone, biolithite limestone and bentonite. The black shale in the Wufeng and the lower part of Longmaxi formations is 50 m thick, with an average organic carbon content (TOC) of 3.81 wt% and a maturity (Ro) of 1.62%. Quartz comprises 54.94 vol% of the shale and positively correlates with the TOC. Micropores in the black shale include intergranular pores, intragranular pores, organic matter pores and microfractures. Among these pores, spaces between clay sheets and organic molecules represent a favourable storage space for the accumulation and preservation of oil and gas. The Langmuir volume parameter ranges between 1.52 and 3.01 cm3/g, with an average value of 2.33 cm3/g. The presence of organic matter pores and pores between clay sheets in the black shale is the main and controlling factor for accumulated gas.

Dissolution pore types of the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin, south China: Implications for shale gas enrichment

The theoretical framework of Unconventional Petroleum Geology has been gradually established along with the rapid progresses in exploration and development of unconventional petroleum resources. It is now imperative for innovative new insights into the unconventional petroleum sedimentology. Herein,the concept and scientific connotations of "Unconventional Petroleum Sedimentology" are proposed and briefly introduced. The research progresses are summarized for the sedimentology of typical unconventional petroleum resources in China,such as Wufeng-Longmaxi Shale gas in Sichuan Basin and tight oil and shale oil of Yanchang Formation in Ordos Basin. Further research themes and challenges of Unconventional Petroleum Sedimentology are discussed. Sedimentary enrichment of unconventional petroleum resources can be closely related with some critical environmental changes,which would be the result of coupling sedimentology from several geological events such as global or regional tectonic activities,sea(lake) level changes,volcanic eruptions,climate changes,anoxic bottom water,biotic mass extinctions and radiations,and gravity currents. For better understanding of Unconventional Petroleum Sedimentology in future,the Earth Systems Science view and "unconventional" insights should be applied to its research by analyzing geological events in details, which can play an important role in the discovery of new unconventional petroleum resources.

Qualitative and quantitative study of micro-pore structures of Longmaxi Formation shale in Fuling area, Sichuan Basin

Types of biogenic quartz and its coupling storage mechanism in organic-rich shales: A case study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China

Formation pressure prediction and high pressure formation mechanisms of shale reservoirs in Fuling area, Sichuan Basin

The Openness Degree Study of the Jiaoshiba Shale Gas, Sichuan Basin, China–Potential Factor Responsible for Reversed Isotope Series

Study on the TOC lower limit of shale oil and gas of freshwater lake facies: a case study on the Jurassic Da'anzhai member in the Sichuan Basin

The statistical discrepancy of physical properties of summer precipitation clouds over the Qinghai-Xizang Plateau,Sichuan Basin and their transition region were analyzed,based on July and August Cloud Sat products from 2008 to 2010. At the same time,four examples from Cloud Sat,FY- 2D TBB data and surface precipitation products were also analyzed to discuss howthe different physical properties lead to the different precipitation of summer precipitation clouds over studying regions. The results showed that:( 1) As to the macrophysical properties of clouds,the main precipitation clouds of the Qinghai-Xizang Plateau were Cu and Ci,lowlevel clouds accounted for the largest proportion,it were Ns and Ci in Sichuan Basin and transition region. Compared with Basin,the clouds base was higher and top was lower in Plateau,it could rain while convection will not deep enough. In three regions,single layer cloud was easier to rain than multilayer one.( 2) As for the microphysical properties of clouds,the main precipitation clouds of the Qinghai-Xizang Plateau was ice cloud,mixed cloud was second,the least was liquid water cloud,mixed cloud accounted for the largest proportion in Sichuan Basin. The clouds were mostly at the beginning of lifetimes in statistics,although the differences were not obvious,the effective radius and distribution width parameter of cloud ice particles over the Qinghai-Xizang Plateau were bigger and wider than those in the other two regions. Their number concentration were similarly,that could quicken the cold cloud processes. It was slower to developed ice cloud processes in Sichuan Basin than that in the QinghaiXizang Plateau.

Mineralogical and petrographic characteristics of the Ordovician-Silurian Wufeng-Longmaxi Shale in the Sichuan Basin and implications for depositional conditions and diagenesis of black shales

Five planetary boundary layer (PBL) parameterization schemes [Yonsei University (YSU), Mellor–Yamada–Janjic (MYJ), Mellor–Yamada–Nakanishi–Niino Level 2.5 (MYNN2), Shin-Hong (SH), and the asymmetric convective model, version 2 (ACM2)] in the Weather Research and Forecast model (WRF v4.0), were used to simulate all well-developed Southwest China vortex (SWCV) processes in the eastern Sichuan basin in 2016. Each level of precipitation prediction was verified, and the L-band radiosonde data with temporal resolution of 1 s were used to reveal the fine structure of the PBL during a midday. The differences between the observation and simulation are assessed, and the reasons are discussed based on the characteristics of the turbulence algorithm used in each scheme. Finally, the parameter of turbulence intensity was adjusted for the ACM2 scheme to improve the structure of the PBL that influences the simulation of the precipitation in the eastern Sichuan basin. The results show that the ACM2 and YSU schemes show a relatively better TS performance. Compared with other schemes, ACM2 has fewer false alarms. The attribute of ACM2 that can modify local or nonlocal algorithms according to the stability of the surrounding environment seems to be more suitable for the Sichuan basin precipitation simulation than the other schemes. However, all PBL schemes show a high false-alarm rate in the prediction of the SWCV precipitation, especially when the precipitation is heavy. The sounding data with 1 s temporal and 3 m spatial resolution further show that all the PBL schemes predict a higher PBL height compared with that of the observations, which means that the simulation has a stronger mixing intensity compared with that of the real atmosphere. By parameter adjustment, using the ACM2 scheme with reduced mixing intensity, the potential temperature and humidity structure in PBL are more aligned with the observations. Further, the potential temperature of the low PBL is low, humidity is high, and false alarm reports of heavy precipitation are reduced, which leads to an improvement regarding the precipitation simulation in the Sichuan basin. The different characters of the PBL schemes that are used in the simulation of the SWCV mainly lead to different positions of the vortex and precipitation intensity. Essentially, these characters are derived from a local or nonlocal attribute and the intensity of vertical mixing. A selection based on regional features of a research object is the key to the accurate simulation of a PBL structure and precipitation process.

To explore the population-genetics characteristics of Mycobacterium tuberculosis (MTB) prevailing at the Sichuan basin of China. A total of 413 MTB strains collected from Sichuan basin were genotyped by large sequence polymorphism (LSP) and 15 loci variable number tandem repeat (VNTR). Difference between the distribution of lineage population was analyzed by χ(2)-test and the discriminatory ability of each VNTR locus was evaluated under the Hunter-Gaston Discriminatory Index (HGI). Both phylogeny on population level and genetic structure were demonstrated through N-J tree and the Minimal Spanning Tree (MST). Genetic differentiation of different lineage strains was analyzed by Analysis of Molecular Variance (AMOVA). Time of the Most Recent Common Ancestor (TMRCA) was calculated based on the Bayesian model. Four hundred and thirteen MTBs were divided into two major lineages, in which the Beijing lineage accounted for 56.2% (232/413) and the Euro-American lineage for 43.8% (181/413). There was no significant difference of population distribution between the two lineages (P > 0.05). The N-J tree of Beijing lineage MTB presented distinctly "star-like" and 72.4% strains were grouped to one clonal complex in MST. The Euro-American lineages MTB presented "branch-like" in N-J tree and were grouped into multiple clonal complexes in MST. There was significant genetic differentiation in Beijing lineage MTBs between Chongqing and Sichuan (FST = 0.018 91, P < 0.05), but not in the Euro-American lineage MTB (FST = 0.005 19, P > 0.05). TMRCA of the largest clonal complex in Euro-American lineage MTB appeared to be 723 (95% CI: 517-946) years. Both Beijing lineage and Euro-American lineage MTBs were competitively prevalent in the Sichuan basin. There was some difference noticed between the two lineages referring to genetic differentiation. The invasion of Euro-American lineages MTB to the basin area might be associated with a war occurred in this area about 720 years ago.

As an unconventional oil and gas reservoir, the diagenesis and evolution of continental shale controls the formation and occurrence of inorganic and organic pores. In order to quantitatively characterize the pore characteristics of a continental shale reservoir and their influence on the evolution of the diagenesis stage, the characteristics of organic and inorganic pore types of continental shale in the Da’anzhai section of the lower Jurassic Ziliujing Formation were identified by means of X-ray diffraction mineral composition analysis and argon ion polishing scanning electron microscope measurements and observations, and the influence control of the diagenesis stage on the pore development of the continental shale reservoir and its control were clarified. The results show the following: ① The organic matter pores in continental shale are developed in large quantities, including organic matter pores in the mineral asphalt matrix and organic matter pores in the kerogen; the pore types of inorganic minerals are very rich, the main pore types are linear pores between clay minerals, intergranular (intergranular) pores, and intragranular corrosion pores, and microcracks are also developed. ② When affected by compaction diagenesis, the inorganic pores of continental shale decrease with an increase in the burial depth and diagenesis degree. ③ The burial depth of continental shale is 2000–3000 m in the middle of diagenetic stage A, and a large number of organic matter pores and dissolved inorganic pores develop at this depth, meaning that the total porosity of shale increases significantly. The burial depth of continental shale is 3000–4000 m at diagenetic stage B, where kaolinite and other clay minerals are dehydrated and converted into illite, the brittleness of shale is increased, and the interior of the shale is subject to external stress, causing microcracks to form. In the late diagenetic stage, when the buried depth of the continental shale is more than 4000 m, the organic matter is subject to secondary cracking and hydrocarbon generation, the organic pores of shale increase in number again, and the inorganic pores decrease in number due to compaction. In conclusion, we found that the burial depth is the main control factor for the development of pores and microfractures in continental shale reservoirs; diagenesis caused by burial depth is the main factor affecting the development of pores and microfractures in continental shale reservoirs; and the shale burial depth in this case is more than 3500 m, which is in the middle of diagenetic stage B. Inorganic porosity in shale is reduced, and the number of microfractures is increased. When the shale is buried more than 4000 m deep in the late diagenetic stage, the thermal evolution of organic matter in shale is high, and methane gas is generated in large quantities, which is conducive to the formation and development of organic matter pores in continental shale.