Silurian Black Shales Research Articles

The radioisotopically constrained Viséan onset of turbidites in the Moravian-Silesian part of the Rhenohercynian foreland basin (Central European Variscides)

The Březina Formation represents the initiation of siliciclastic flysch turbidite sedimentation at the eastern margin of Bohemian Massif or within the Rhenohercynian foreland basin. Its deposition started after drowning of the Devonian carbonate platform during Visean (Mississippian) times, resulting in a significant interval of black siliceous shale and variegated fossiliferous shale deposition in a starved basin. Near the top of the Březina Formation an acidic volcanoclastic layer (tuff) of rhyolitic composition has been dated with high precision U–Pb zircon chemical abrasion isotope dilution method at 337.73 ± 0.16 Ma. This new radiometric age correlates with the previously inferred stratigraphic age of the locality and the current calibration of the Early Carboniferous geologic time scale. Shales of the Březina Formation pass gradually upwards into the siliciclastics of the Rozstani Formation of the Drahany culm facies. Thus our new age offers one of the few available radioisotopic constraints on the time of onset of siliciclastic flysch turbidites in the Rhenohercynian foreland basin of the European Variscides.

A Combination of N2 and CO2 Adsorption to Characterize Nanopore Structure of Organic‐Rich Lower Silurian Shale in the Upper Yangtze Platform, South China: Implications for Shale Gas Sorption Capacity

AbstractThe pores in shales are mainly of nanometer‐scale, and their pore size distribution is very important for the preservation and exploitation of shale gas. This study focused on the organic‐rich Lower Silurian black shale from four wells in the Upper Yangtze Platform, and their TOC, mineralogical composition and pore characterization were investigated. Low pressure N2 and CO2 adsorption were conducted at 77.35 K and 273.15 K, respectively, and the pore structures were characterized by modified Brunauer‐Emmett‐Teller (BET), Dubinin‐Radushkevich (DR), t‐plot, Barrett‐Joyner‐Halenda (BJH) and density functional theory (DFT) methods and then the relationship between pore structure and shale gas sorption capacity was discussed. The results indicate that (1) The Lower Silurian shale has high TOC content of 0.92%–4.96%, high quartz content of 30.6%–69.5%, and high clays content of 24.1%–51.2%. The total specific surface area varies from 7.56 m2/g to 25.86 m2/g. Both the total specific surface area and quartz content are positively associated with the TOC content. (2) Shale samples with higher TOC content have more micropores, which results in more complex nanopore structure. Micropore volumes/surface areas and non‐micropore surface areas all increase with the increasing TOC content. (3) A combination of N2 and CO2 adsorption provides the most suitable detection range (∼0.3–60 nm) and has high reliability and accuracy for nanopore structure characterization. (4) The TOC content is the key factor to control the gas sorption capacity of the Lower Silurian shale in the Upper Yangtze Platform.

Regional depositional changes and their controls on carbon and sulfur cycling across the Ordovician-Silurian boundary, northwestern Guizhou, South China

The controls of regional depositional environment on stable isotope records spanning the Ordovician-Silurian boundary on the Yangtze Platform have seldom been investigated. The objectives of this study include reconstruction of regional depositional settings and assessment of how sedimentary processes may have influenced carbon- and sulfur-isotopic fractionation. Seven shale facies have been recognized in the Wufeng-Longmaxi interval from two locations. Completely bioturbated claystone of the basal Wufeng Formation accumulated on a shallow oxygenated muddy shelf. Overlying faintly banded black siliceous shale suggests deposition under deep anoxic conditions interrupted by episodes of dysoxia. Muddy fossiliferous facies of the Guanyinqiao Formation reflects a glacial sea-level lowstand setting, and the observed proximal to distal heterogeneity of facies matrix supports shallowing in the proximal area. Overlying faintly banded black shale of the Longmaxi Formation tells of a post-glacial transgression. The carbonaceous deposits are overlain by banded gray and dark gray muddy siltstones and suggest a shoaling upward trend. The shallow-water facies display relatively heavier δ13Corg values, whereas deep-water facies are characterized by lower δ13Corg values. Discrepancies between Hirnantian positive δ13Corg excursions in proximal and distal areas probably reflect spatial gradient in seawater δ13CDIC induced by glacioeustasy. Deposits accumulated under oxic and physically dynamic conditions display strong positive δ34Ssulfide excursions, perhaps a consequence of diagenetic sulfate reduction occurred at some depth below the sediment-water interface and removed from the sulfate pool of overlying water column. Negative relationship between δ34Ssulfide and TOC indicates that redox conditions and physical reworking of sediment at the sediment-water interface controlled the rate of microbial sulfate reduction and sulfur isotope fractionation. Overall, co-variation of lithofacies and isotopic records suggests that local heterogeneity of seawater chemistry and sedimentary dynamics may exert partial controls on biogeochemical processes and amplify the magnitude of the Hirnantian isotopic excursions in the proximal area.

Pore structure characterization for organic-rich Lower Silurian shale in the Upper Yangtze Platform, South China: A possible mechanism for pore development

The Lower Silurian shale in the Upper Yangtze Platform has become one of the main targets of the exploration and development of marine shale gas in South China. This study is focused on the organic-rich Lower Silurian black shale from four wells in the Upper Yangtze Platform, and their organic geochemistry, petrology, porosity and pore characterization were investigated. A combination of field emission scanning electron microscopy (FE-SEM), low pressure N2/CO2 adsorption and mercury intrusion porosimetry (MIP) measurements was conducted to characterize the pore structure and the effects of organic matter and inorganic minerals on porosity were discussed. Then the contribution of individual mineral components to total porosity was analyzed quantitatively. The results indicate that (1) FE-SEM images show that there are mainly three types of pores in the Lower Silurian shale: OM pores, pores of framework minerals, and pores in clay minerals. (2) CO2 adsorption results suggest that the micropores of the shale samples are mainly contributed by the pores with width between 0.4 and 0.7 nm, while N2 adsorption results show that the mesopores of the shale samples are mainly contributed by the pores with width between 2 and 10 nm, which is consistent with MIP results. (3) The TOC content shows a weakly positive correlation with porosity, suggesting that organic matter has an important influence on shale porosity, but it is not the most essential factor. When the maturity is lower than 2.6%, the porosity increases with increasing maturity, and when the maturity value is greater than 2.6%, the porosity decreases gradually. (4) The presence of rigid grains led to the formation of rigid frameworks, which can prevent pores from collapsing and benefit the preservation of OM pores and pores in clay minerals. Pores of framework minerals are mainly generated by the corrosion effect of organic acids produced in the process of hydrocarbon generation. (5) Organic matter has the largest pore volume per unit mass, followed by clay minerals, and then framework minerals. This means that organic matter and clay minerals contribute significantly to shale pore space given the same mass percentage condition; the matrix pores are mainly composed of pores in clay minerals and OM pores, which account for approximately 90% of the matrix pores.

Crude oil and condensate in the Ordovician–Silurian formations of the Eastern European: The Western Black Sea Basin

ABSTRACTThe Lower Paleozoic sediments of Eastern Europe extend from Poland to the Black Sea Coasts. The Lower Ordovician Bakacak Formation and Middle Ordovician–Silurian Karadere, Ketencikdere, and Fındıklı formations of the Western Black Sea Basin contain mudstones and black shales with sandstone and limestone intervals. The mudstones of the Lower Ordovician do not display any potential. But some intervals of the Middle Ordovician–Silurian black shales have potential for unconventional exploration because total organic carbon (TOC) of organic matter contents ranges from 0.11% to 1.94%. Hydrogen index (HI) values of the Karadere, Ketencikdere, and Fındıklı formations are ranging 0–139 mg oil/g TOC. Vitrinite reflection equivalent (Roe) estimation from HI shows that the shales locate within the oil, wet gas-condensate, and dry gas zone. In-situ oil and condensate volumes of the black shales have been calculated between 0 and 2812 mg HC/g TOC. These volumes indicate presence of potentially producible hydrocarbon in the some intervals of the Middle Ordovician–Silurian shales. Mineralogical properties of the Karadere, Ketencikdere, and Fındıklı formations are suitable for hydraulic fracturing because the shales are dominated by quartz/carbonate. Quartz ratio ranges from 12% to 91% with an average 53%. Total calcite/dolomite content is an average of 13/0.5%. Total clay mineral chance between 0% and 39% with an average of 21%.

Effect of the properties of Silurian shales from the Barrandian Basin on their methane sorption potential

High-pressure methane sorption isotherms were measured on seven representative samples of Silurian shales from the Barrandian Basin, Czech Republic. Excess sorption measurements were performed at a temperature of 45°C and at pressures up to 15MPa on dry samples, using a manometric method. Experimental methane high-pressure isotherms were fitted to a modified Langmuir equation. The maximum measured excess sorption parameter (nmax) and the Langmuir sorption capacity parameter (nL) were used to study the effect of TOC content, organic maturity, inorganic components and pore size distribution on the methane sorption capacity. The values of nmax ranged from 0.050 to 0.088mmol.g−1, and the values of nL ranged from 0.068 to 0.133mmol.g−1. The studied shale samples with random reflectance Rr of graptolite 0.56–1.76% had a very low TOC content from 0.34 to 2.37wt% and dominant mineral fractions. Illite was the prevailing clay mineral (0–51%). Organic matter of the Silurian black shales was consisted of residues of graptolites, chitinozoans, two types of bitumen including dispersed and massive bitumens, recycled organic matter, and organic detritus. In the shales, the occurrence of fractures parallel with the original sedimentary bending was highly significant. A greater proportion of fragments of carbonaceous particles of graptolites and bitumens in the Barrandian shales had a smooth surface without pores. The sample porosity Porcalc ranged from 4.6 to 18.8%. In most samples, the micropore volumes were markedly lower than the meso- and macropore volumes. No relation has been proven between TOC-normalized excess sorption capacities or the TOC-normalized Langmuir sorption capacities and thermal maturation of the shales. The methane sorption capacities of shale samples show a positive correlation with TOC and a positive correlation with the clay content. The assumption that the sorption capacity is a function mainly of the microporous system of shales was not confirmed. The highest sorption capacity was observed in shale samples with the lowest volume of micropores and the highest TOC content, indicating that the organic matter content and the microporosity of clay minerals are the principal factors affecting the sorption capacity of shale samples.

The climatic significance of Late Ordovician‐early Silurian black shales

AbstractThe Ordovician‐Silurian transition (∼455–430 Ma) is characterized by repeated climatic perturbations, concomitant with major changes in the global oceanic redox state best exemplified by the periodic deposition of black shales. The relationship between the climatic evolution and the oceanic redox cycles, however, remains largely debated. Here using an ocean‐atmosphere general circulation model accounting for ocean biogeochemistry (MITgcm), we investigate the mechanisms responsible for the burial of organic carbon immediately before, during, and right after the latest Ordovician Hirnantian (445–444 Ma) glacial peak. Our results are compared with recent sedimentological and geochemical data. We show that the late Katian time slice (∼445 Ma), typified by the deposition of black shales at tropical latitudes, represents an unperturbed oceanic state, with regional organic carbon burial driven by the surface primary productivity. During the Hirnantian, our experiments predict a global oxygenation event, in agreement with the disappearance of the black shales in the sedimentary record. This suggests that deep‐water burial of organic matter may not be a tenable triggering factor for the positive carbon excursion reported at that time. Our simulations indicate that the perturbation of the ocean circulation induced by the release of freshwater, in the context of the post‐Hirnantian deglaciation, does not sustain over sufficiently long geological periods to cause the Rhuddanian (∼444 Ma) oceanic anoxic event. Input of nutrients to the ocean, through increased continental weathering and the leaching of newly exposed glaciogenic sediments, may instead constitute the dominant control on the spread of anoxia in the early Silurian.

Brittleness evaluation of the Upper Ordovician Wufeng–Lower Silurian Longmaxi shale in Southern Sichuan Basin, China

Rock brittleness is now one of the significant parameters for geological evaluation of shale gas reservoirs due to the wide application of hydraulic fracturing in shale gas development. Currently, the evaluation of shale brittleness is generally based on mineral composition or elastic parameters, which both derive from the exploration experiences in North America. Due to the easier access of mineral composition data through x-ray diffraction method than rock mechanical data through array sonic logging method, the mineral-based brittleness indices have stronger practicability. However, there is not a unified definition of brittle minerals yet, which now usually refers to quartz or quartz and carbonate minerals. Considering the specific conditions of Wufeng–Longmaxi Formation in Southern Sichuan Basin; depositional environment of calcareous shelf; and higher content of calcite, dolomite, and pyrite, we redefined brittle minerals according to the elastic parameters of each mineral, taking quartz, dolomite, and pyrite which best satisfy the standard of “Young’s modulus>30 GPa and Poisson’s ratio<0.25” as brittle minerals to establish a new brittleness index, which was proved to be more accurate and applicable than the current mineral-based brittleness indices. Then we applied the new evaluation model to Well W201, and selected “sweet spot” intervals combining with lithology, total organic carbon (TOC), thickness, and other geological parameters. The results showed that the “sweet spot” interval of this well is the black siliceous shale developed in Rhuddanian–middle Aeronian stage with an average brittleness index of 50.66%, TOC of 2.3–6.2%, and total thickness of about 19 m.

Silurian black shales of Western Ukraine: petrography and mineralogy

Silurian black shales of Western Ukraine: petrography and mineralogy

Effect of thermal events on maturation and methane adsorption of Silurian black shales (Checa, Spain)

The present work is devoted to the study of the relationship among composition, properties and methane adsorption of selected Silurian black shales from the Bádenas Formation, located near Checa town (Spain). These outcropping samples were collected in a formation, which is not considered as target for shale gas exploration. The clay minerals are predominant in the whole-rock samples and mostly composed of illite (both potassium and ammonium phases), rectorite and pyrophyllite with subordinated kaolinite and chlorite. This peculiar mineralogical assemblage is related with burial and thermal events having affected the shales since their deposition. Differences in TOC (1.92–7.62wt%), BET specific surface (27–34m2/g) and thermal maturity (Tmax of 459–484°C) were observed prior to experimental adsorption. Excess adsorption isotherms for methane were measured on dry bulk samples at 30°C, 50°C and 80°C, up to 3.5 (MPa) by the use of a combined manometric-calorimetric device. The maximum methane excess adsorption capacities within this pressure range varied from 0.021 to 0.168mmol/g at 30°C. TOC seems to be the primary factor affecting methane adsorption capacity but nonlinear regression of adsorption capacity with TOC content was observed. A modified Langmuir-type adsorption function, with a term taking into account the volume of the adsorbed phase, provide a good representation of the measured excess adsorption isotherms. Additionally, the heats of adsorption were found nearly constant, with an approximate value of 30kJ·mol−1, indicating that physical adsorption process controls the adsorption of methane molecule to the shale samples.

Application of low pressure gas adsorption to the characterization of pore size distribution of shales: An example from Southeastern Chongqing area, China

Pore size distribution is vital in the preservation and exploitation of shale gas; the pores in shales are mainly of nanometer scale. This study focuses on the pore structures of the Upper Ordovician-Lower Silurian black shales that were sampled from the Well YC7 in southeastern Chongqing area. Nitrogen and carbon dioxide gas adsorption were conducted at 77.4 K and 273.15K, respectively. Pore structures were characterized by the modified BET, BJH, DFT, and Stoeckli methods. The results show that: (1) micropores, mesopores, and macropores are all well developed in both the Lower Silurian and Upper Ordovician shales; (2) the mean diameter of micropores for our samples is approximately 1.26 nm; and (3) the micropore size distribution curves derived from the Stoeckli method together with the CO2 adsorption data at 273.15K can be well connected with the calculated DFT model values through nitrogen adsorption data at 77.4 K, indicating that a full pore size distribution curve could be achieved for micropores, mesopores, and a portion of macropores in the shales by combing the N2 and CO2 adsorption data.

The geochemical characteristics and factors controlling the organic matter accumulation of the Late Ordovician-Early Silurian black shale in the Upper Yangtze Basin, South China

Organic-rich black shale of the Upper Yangtze Basin from the Late Ordovician and Early Silurian is considered an excellent source rock in South China. The formation and preservation conditions of this resource are revealed by its geochemical characteristics in this study. Geochemical indices, including redox indices (V/(V + Ni), V/Cr, V/Sc, and Ni/Co) and primary productivity indices (P/Ti and Ba/Al), and paleoclimate, clastic flux and sedimentary rate analyses are presented to investigate the accumulation mechanism of organic matter. Redox indices suggest that a stagnant, anoxic environment predominated in the Upper Yangtze Basin during accumulation of Wufeng and Longmaxi formations. In contrast, ventilated and oxygenated marine conditions pervaded the Upper Yangtze Basin during deposition of Linxiang and Guanyinqiao formations. The concentrations of V and U demonstrate that accumulation of organic matter was mainly controlled by redox conditions. Besides, such factors as clastic fluxes, fresh water inflows or a mixed deposition with a rapid sedimentary rate cannot be ignored due to their influences on organic matter enrichment and preservation. However, weak co-variance relationship of TOC content and productivity proxies, including P/Ti and Ba/Al, demonstrates that the accumulation of organic matter was not controlled by primary productivity. Results of the present study suggest a depositional model that stresses the importance of tectonic movements and glacial events on the accumulation and preservation of organic matter. The model shows that the Upper Yangtze Basin was a semi-restricted basin system influenced by the isolation of Xuefeng, but also it implies that oxygen-depleted bottom water of the basin favored the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich black shale.

The lithofacies and reservoir characteristics of the Upper Ordovician and Lower Silurian black shale in the Southern Sichuan Basin and its periphery, China

The black shale of the Upper Ordovician and Lower Silurian is a significant target for shale gas exploration in the Southern Sichuan Basin. In this study, we introduced a lithofacies classification for shale based on rock mineral composition. Because the pore structure of gas shale reservoirs are complex and greatly affect the gas storage and transport in shale, four different methods of low-pressure nitrogen gas adsorption, high-pressure mercury intrusion, scanning electron microscopy (SEM) and gas expansion methods were used to investigate the reservoir pore structure and storage space. Combined with X-ray diffraction, total organic matter content (TOC), gas content, methane adsorption, porosity and permeability and wireline data, the key factors that affect shale gas content and storage of shale gas were analyzed.According to our shale classification, three main lithofacies, i.e., calcareous mudstone (CM), high calcareous mixed mudstone (HCMM) and low calcareous mixed mudstone (LCMM), were identified in our study area. The experiment indicates that the shale has a high TOC, thermal maturity and gas content. The methane adsorption isotherms show that the sorbed gas content has a positive correlation with TOC. Difference of TOC between the three is small. Nitrogen adsorption indicates that mesopores dominate the shale pore composition and organic matter is the main source of mesopores. Mercury injection shows that the macropore volume accounts for approximately 14–22% of the total pore volume. No obvious pore structure differences were found for different lithofacies. However, taking the porosity into consideration, LCMM has the largest macropore volume. The SEM observations revealed that organic matter pores, interparticle pores and intraparticle pores are the main pore types. Interparticle pores mainly develop in LCMM which has a relatively high quartz and low carbonate content, it may be the major reason for high porosity, high macropore volume, and high free gas content of LCMM. Free gas and absorbed gas are the major storage form of shale gas. Given similar temperature and pressure conditions, the total organic matter content is the major factor that affects the adsorbed gas content. Therefore, with high free gas and adsorbed gas content, LCMMs with high total organic matter contents would be the most favorable type of lithofacies in this region.

Remagnetization of lower Silurian black shale and insights into shale gas in the Sichuan Basin, south China

AbstractThe organic‐rich lower Silurian shale of the Longmaxi Formation in the Sichuan Basin is the most important target for shale‐gas exploration in China. Most Paleozoic rocks of the Sichuan Basin have experienced extraordinarily pervasive remagnetizations. To test a hypothesized connection between hydrocarbon generation and remagnetization and contribute to shale‐gas exploration in the region, we undertook an integrated magnetic, geochemical, and petrographic study of 160 specimens from the shale. The results suggest that the shale contains a reliable remanent magnetization (Dec = 41.4°, Inc = 40.8°, and α95 = 6.8°). The magnetization predates tilting, and the paleopole plots close to the Late Triassic segment of the south China apparent polar wander path. The rock magnetic data and scanning electron microscope (SEM) observations confirm that framboidal magnetites carry the bulk of the magnetization, which suggest a Late Triassic chemical remanent magnetization in the shale. 87Sr/86Sr and magnetic analyses indicate that the amount of magnetite was unaffected by fluid alterations around the veins but is strongly covariant with the amount of total organic matter. Moreover, SEM observations reveal possible evidence of the replacement of pyrite framboids by magnetite, probably in the presence of organic acids. These analyses, therefore, suggest that the remagnetization was caused by organic maturation rather than orogenic fluids and that the maturation occurred in the Late Triassic. This timing of organic maturation has been validated by independent modeling studies and provides important constraints on the complex thermal history of the Longmaxi Shale as well as contributing to shale‐gas exploration efforts.

Characterization of microscopic pore structures in Lower Silurian black shale(S1l), southeastern Chongqing, China

Due to the large thickness and richness of organic matter in Longmaxi shale, southeastern Chongqing is considered as the most promising area for shale gas exploration and development and was a focus of numerous research interests in China. Characterization on the pore system of organic rich shale is significant for the reserve estimation and better understanding the production mechanism of shale gas plays. Shale composition, detected by X-ray diffraction, indicates that brittle minerals, especially quartz was the most prevalent component，with average contents of 41.72 wt.%. Pore types was classified by using a combination of argon-iron milling and field emission scanning electron microscopy (FESEM), and six types of pores were observed in rock images. High pressure mercury intrusion and low pressure N2 adsorption were performed to extract the pore size distribution of Longmaxi shale. The results show that the nanopore was the major pore type in shale samples, which accounts for 95.6% of the pore volume. In the nanopore system, pore with diameters between 2 nm and 10 nm is the major component that contributes 76% of the total pore volume and 78% of the whole inner surface area. Furthermore, the dominant pore shape was interpreted from the adsorption-desorption hysteresis loop shape, and the typical slit-shaped pores were identified in the examined shale samples. Based on single factor analysis method, organic matter richness is the main controlling factor for the volume of nanopore and the specific surface area. No direct correlation between brittle minerals and nanopores, but high quartz and calcite content makes it much easier for the formation to generate natural fractures and to be hydraulically fractured. The study can inspire and guide shale gas exploration and exploitation southeastern Chongqing to some extent.

Pore structure of Cambrian shales from the Sichuan Basin in China and implications to gas storage

The microstructure of black siliceous shale from the lower Cambrian Niutitang Formation, Sichuan Basin in China was investigated by the combination of field emission scanning electron microscope (FE-SEM) and argon ion beam milling. The nanometer-to micrometer-scale pore systems of shales are an important control on gas storage and fluid migration. In this paper, the organic porosity in shale samples within oil and gas window has been investigated, and the formation mechanism and diagenetic evolution of nanopores have been researched.FE-SEM reveals five pore types that are classified as follows: organic nanopores, pores in clay minerals, nanopores of framework minerals, intragranular pores in microfossils, and microfractures. Numerous organic nanopores are observed in shales in the gas window, whereas microfractures can be seen within the organic matter of shales in the oil window. Microfractures in oil window shales could be attributed to pressure buildup in the organic matter when incompressible liquid hydrocarbon are generated, and the orientation of microfractures is probably parallel to the bedding and strength anisotropy of the formation. Pores in clay minerals are always associated with the framework of clay flakes, and develop around rigid mineral grains because the pressure shadows of mineral grains protect pores from collapse, and the increasing of silt content would lead to an increase in pressure shadows and improve porosity. Nanopores of rock framework are probably related to dissolution by acidic fluids from hydrocarbon generation, and the dissolution-related pores promote permeability of shales. Porosity in the low-TOC, low-thermal-maturity shales contrast greatly with those of high-TOC, high-thermal-maturity shales. While the high-TOC shales contain abundant organic microporosity, the inorganic pores can contribute a lot to the porosity of the low-TOC shales.

Geologic characteristics of hydrocarbon-bearing marine, transitional and lacustrine shales in China

Organic-rich shales spanning in age from Pre-Cambrian to Quaternary were widely deposited in China. This paper elaborates the geology and unique characteristics of emerging and potential hydrocarbon-bearing shales in China. The Pre-Cambrian Sinian Doushantuo to Silurian black marine shales in the intra-shelf low to slope environments were accumulated in South China and Tarim Platform in Northwest China. These marine shales with maturity (Ro) of 1.3–5% are in dry gas window. During Carboniferous to Permian, the shales associated with coal and sandstones were mainly deposited in coastal swamp transitional setting in north China, NE China, NW China and Yangtze platform in South China. These transitional shales are generally clay rich and are potential gas-bearing reservoirs. Since Middle Permian, the lacustrine shales with total carbon content (TOC) up to 30% and Ro mainly in oil window are widely distributed in all the producing basins in China. The lacustrine shales usually have more clay mineral content than marine shales and are characterized by rapid facies change and are interbedded with carbonates and sandstone. The high quality shale reservoir with high TOC, hydrocarbon content and brittle minerals content is usually located at transgressive systems tract (TST) to early highstand systems tract (EHST) interval deposited in anoxic depositional setting. Recent commercial shale gas production from the Silurian Longmaxi marine shale in the southeastern Sichuan Basin, preliminary tight oil production associated with lacustrine hydrocarbon-bearing shale intervals and hydrocarbon shows from many other shales have proven the hydrocarbon-bearing shales in China are emerging and potential shale gas and tight (shale) oil plays. Tectonic movements could have breached the early hydrocarbon accumulation in shales and tectonically stable areas are suggested to be favorable prospects for China shale plays exploration and production.

Organic Carbon and Stable C-O Isotopic Study of the Lower Silurian Longmaxi Formation Black Shales in Sichuan Basin, SW China: Paleoenvironmental and Shale Gas Implications

In order to better decipher the paleoenvironment of the Early Silurian black shale in the southern Sichuan Basin and constrain their shale gas potential, organic carbon and stable C-O isotopic studies have been performed on the Longmaxi Formation from Well N203. The organic carbon isotopic studies show a relative low δ13Corgvalue at the lower part of the Longmaxi Formation, corresponding to the negative stable carbon isotope (δ13C) excursion and positive oxygen isotope (δ18O) excursion. These isotopic features indicate an anoxic environment at the lower part of the Longmaxi Formation, which might be related to the high atmosphere temperature and sea-level rise. The anoxic environment is also proved by framboidal pyrites studies, which present diameter of most framboids in the range of 3–5 μm. The anoxic environment promoted the deposition of organic carbon, leading to the high TOC content at the lower part. This anoxic environment only lasted for a short period, with a gradually evolution to oxic environment at the middle and upper part of the Longmaxi Formation, evidenced by very low TOC content, increasing δ13Corgvalues, positive δ13C values and negative δ18O values. Our studies have further constrained the best source of rocks for shale gas at the lower part of the Longmaxi Formation.

Vitrinite equivalent reflectance of Silurian black shales from the Holy Cross Mountains, Poland

Abstract A number of independent methods have been used to measure the thermal maturity of Silurian rocks from the Holy Cross Mountains in Poland. Black shales are characterized by diverse TOC values varying from 0.24-7.85%. Having calculated vitrinite equivalent reflectance using three different formulas, we propose that the most applicable values for the Silurian rocks are those based on Schmidt et al. (2015) equation. Based on this formula, the values range from % 0.71 VReqvVLR (the vitrinite equivalent reflectance of the vitrinite-like macerals) to % 1.96 VReqvVLR. Alternative, complementary methods including Rock Eval pyrolysis and parameters based on organic compounds (CPI, Pr/n-C17, Ph/n-C18, MPI1, and MDR) from extracts did not prove adequate as universal thermal maturity indicators. We have confirmed previous suggestions that Llandovery shales are the most likely Silurian source rocks for the generation of hydrocarbons in the HCM.

Dehydration Effect on the Pore Size, Porosity, and Fractal Parameters of Shale Rocks: Ultrasmall-Angle X-ray Scattering Study

The characterization of pore networks is extremely important in understanding transport and storage phenomena in unconventional gas and oil reservoir rocks. An ultrasmall-angle X-ray scattering (USAXS) measurement has been performed on Silurian black shales from the Baltic Basin, Poland, from a wide range of depths along a burial diagenetic sequence. This study provides insight into the nature of the pore structure, including the pore size distribution, total porosity, and fractal dimensions of the rocks. Samples were measured in both their air-dried state, equilibrated at ∼50% relative humidity, and prior to dehydration by drying at 200 °C to make a comprehensive comparison of the pore structure changes induced by dehydration. Two trends were observed: porosity values decreased with depth as expected from the models of porosity evolution with burial and increased upon sample dehydration. The USAXS-measured porosity values show very good correspondence with the measurements by immersion porosity methods. ...