Cambrian Shale Research Articles

Study on the Mechanism of Organic Matter Enrichment in Early Cambrian Marine Shales in the Lower Yangtze Area, South China: An Example Using Well JXY1

The abundance of organic matter in shales, which has a direct effect on the hydrocarbon generation potential of shales, is an important organic geochemical parameter for evaluating shale gas reservoirs. The total organic carbon content (TOC content) in shale is controlled by the abundance of original sedimentary organic matter. Therefore, it is very important to study the mechanism of organic matter enrichment in shale. In this paper, the Lower Cambrian marine shales from the Lower Yangtze region are selected as the research subject, most of which originate from a typical area well called Well JXY1. The degree of pyritization (DOP) is used to characterize the redox environment of the water body, while the P/Al ratio is used to analyze the biological productivity of paleoseawater. The paleosalinity of seawater is calculated via carbon and oxygen isotope analysis. In addition, the early Cambrian hydrothermal activities were studied by using core description; Si, Al, Fe, and Mn elemental analysis; and oxygen isotope calculations. The results show that during the early Cambrian Wangyinpu sedimentary period, the seawater was an anaerobic water body with H2S, and the oxygen concentration was approximately 0 mL/L. In the middle stages of the Wangyinpu sedimentary period, the water body had the strongest reducibility and the highest biological productivity. Moreover, the paleoocean in this period between the Yangtze plate and the Cathaysian plate was greatly affected by hydrothermal activities, with temperatures ranging from 90°C to 120°C. Active hydrothermal activities promoted high biological productivity and an anaerobic environment, both of which were conducive to the preservation and enrichment of organic matter, resulting in extremely high TOC content in the Wangyinpu shales (from 6.5% to approximately 16%).

Methane adsorption characteristics of overmature Lower Cambrian shales of deepwater shelf facies in Southwest China

The Lower Cambrian shales of deepwater shelf facies comprise an important part of the marine black shales in southern China. In this study, twenty-four Lower Cambrian shale samples from southwest China were subjected to analysis of geochemistry, pore structure and methane adsorption capacity. The samples have total organic carbon (TOC) contents ranging from 0.25% to 11.3% and equivalent vitrinite reflectance (EqVRo) values in the range of 2.51–4.23%. Specific surface areas determined from N2 adsorption at 77.4 K and specific micropore volumes from CO2 adsorption at 273.15 K are in the range of 3.0–38.6 m2/g rock and 2.5–11.4 cm3/kg rock, respectively, and both of them have positive relationships with TOC values. The Langmuir volumes of these samples at 60 °C range from 0.47 cm3/g rock to 5.96 cm3/g rock and are positively correlated with TOC values, which on the one hand confirms the TOC control on methane adsorption capacity and on the other hand also suggests a weak control of thermal maturity on methane adsorption capacity at overmature stages. Although both specific surface area and specific micropore volume are positively correlated with methane adsorption capacity, the latter seems to have a stronger effect and the specific micropore volume determined from subcritical CO2 adsorption may be considered as a proxy for the volume of adsorbed methane in dry shales. For the extrapolation of measured methane excess adsorption isotherms to geological conditions, the use of a constant or varying Langmuir volume makes no significant difference with respect to the methane storage capacity of shales when the density of the adsorbed methane is allowed to change with temperature. However, the absolute methane adsorption quantity at subsurface is obviously dependent on the use of a constant or varying Langmuir volume, which subsequently affects the estimation of the relative percentages of adsorbed and free methane.

Characteristics of molecular nitrogen generation from overmature black shales in South China: Preliminary implications from pyrolysis experiments

High content of molecular nitrogen (N2) is one of the natural gas exploration risks in petroliferous basins where black shales act as source rocks of either hydrocarbon gas or molecular nitrogen. In this study, two overmature and one low-maturity shale samples and their kerogens were investigated to determine the generation characteristics of molecular nitrogen as well as methane and the differences in the release processes of inorganic nitrogen fixed in ammonium-bearing minerals and organic nitrogen bound in kerogen. The results illustrate that with increasing pyrolysis temperature, the yield of methane first increases and then decreases with an inflection temperature of 650 °C (EqVRo = 3.4%), whereas the yield of molecular nitrogen shows a continuous increase throughout the pyrolysis experiment. The molecular nitrogen during the stage of methane generation (i.e., EqVRo < 3.4%) is more preferentially derived from the inorganic nitrogen in ammonium-bearing minerals, whereas the significant generation of molecular nitrogen from organic nitrogen in kerogen commences only after the methane generation potential is exhausted (EqVRo > 3.4%). The results also reveal that at the experimental maxima of thermal maturity (EqVRo = 4.9%), the generation potential of inorganic molecular nitrogen (mg/g Ninorg) is much lower than that of organic molecular nitrogen (mg/g Norg), indicating that in our experiments the organic nitrogen in kerogen is more easily to be converted into molecular nitrogen than the inorganic nitrogen in ammonium-bearing minerals. All these results indicate that molecular nitrogen content in shale gas may change dramatically during thermal evolution, and source rocks with exceptionally high maturity and high abundance of organic nitrogen likely lead to a high molecular nitrogen risk, particularly in regions of poor preservation conditions of shale gas, which may be the main reasons for the high molecular nitrogen content in the Lower Cambrian shale gas in South China.

Formation Environment and Development Models for the Lower Cambrian Source Rocks of the Southern North China Plate, China.

In this article, the lower Cambrian marine shale of the southern part of the Ordos basin is taken as an example to investigate the formation environment and development models of source rocks with high organic abundance in the ancient strata of the southern North China plate. The lower Cambrian shale source rocks had a thickness of 10–80 m, with high total organic carbon contents (an average of 2.68% and maximum of 9.39%) and favorable organic matter types—mainly sapropelinite macerals—and yet low S1, S2, and hydrogen index, which indicate that the source rocks have relatively high thermal maturity. Therefore, it is demonstrated that marine shale of the lower Cambrian proves to be excellent gas source rocks at the southern part of the Ordos basin and are presently distributed mainly in the south part of the North China plate. During the early Cambrian period, paleoproductivity was relatively high, as indicated by the rich C27 sterane contents, high sterane/hopane ratios, and high tricyclic terpane/hopane ratios in biomarkers and high P and Ba values in trace elements. The tectonic setting promoted the formation of highly abundant source rocks with favorable preservation conditions, as suggested by the relatively high V/Cr ratios and low Zr/Rb and Pr/Ph ratios. Thus, the paleoproductivity and preservation conditions were the main factors controlling the occurrence of highly abundant source rocks. The formation of highly abundant source rocks was under the joint effects of the tectonic setting and the undercompensation environments of the passive continental marginal slope of the Paleo-Qinling Ocean and deepwater bays. The paleo-geomorphologic troughs and/or fault depressions (because of tectonic movement) and deepwater bay environments together influenced the development scale of highly abundant source rocks, whereas the deepwater shelf environment of the continental marginal slope ensured the high quality of the source rocks.

Spatiotemporal evolution and causes of marine euxinia in the early Cambrian Nanhua Basin (South China)

Marine redox conditions and their dynamic variations in the early Cambrian are thought to have strongly influenced the evolution of early animals. However, the patterns and mechanisms of contemporaneous marine redox variation remain poorly understood. The Nanhua Basin (South China) was a failed rift basin located between the Yangtze Block (NW margin) and Cathaysia Block (SE margin) in the early Cambrian. In this study, we conducted a comprehensive study of marine redox conditions, sulfur cycling, and chemical weathering intensity of source areas for the lower Cambrian shales of the Qingxi Formation in the basinal Shimensuidao section, which was geographically close to the Cathaysia Block. We integrated these results with previously published data from the nearby Silikou section as well as the coeval inner-shelf Xiaotan, upper-slope Daotuo, lower-slope Longbizui, and basinal Zhalagou sections, where were closer to the Yangtze Block. At Shimensuidao, iron species and redox-sensitive trace element data suggest dominantly euxinic conditions for the organic-rich black shale interval (lower member; LM) and variable redox conditions (oxic-ferruginous-euxinic) for the overlying organic-lean shales (upper member; UM) of the Qingxi Formation. Integrated redox results show a similar lessening of euxinic conditions from LM to UM across the entire basin and that euxinia developed along both the Yangtze and Cathaysia basin margins, but it was mainly in upper-slope areas (e.g., Daotuo) on the Yangtze margin and reached basinal areas (e.g., Silikou and Shimensuidao) on the Cathaysia margin. A decline of δ34Spy from LM to UM was observed at Shimensuidao and other sections, which can be explained by decreasing microbial sulfate reduction rates due to decreasing organic availability and increasing riverine sulfate flux at study sections. Lower δ34Spy values in shallow-marine relative to deep-marine areas suggest increased terrestrial sulfate availability to the shallower sites. Chemical index of alteration (CIA) values at Shimensuidao are uniformly high for both the LM (CIAcorr1: 82.5 ± 0.4, CIAcorr2: 88.6 ± 0.5) and UM (CIAcorr1: 80.2 ± 2.3, CIAcorr2: 86.2 ± 2.5) compared with the low-to moderate values at Daotuo (CIAcorr2: 61.9 ± 3.9 for LM, 71.3 ± 4.8 for UM). This pattern is consistent with intense chemical weathering of source areas on the Cathaysia Block and weaker chemical weathering of source areas on the Yangtze Block. Based on these observations, we propose that strong chemical weathering of the Cathaysia Block led to regionally high terrigenous sulfate fluxes, resulting in expansion of mid-depth euxinia into deep basinal areas on the Cathaysia margin of the Nanhua Basin. In contrast, weak chemical weathering of the Yangtze Block yielded lower terrigenous sulfate fluxes, limiting euxinia to upper slope areas on the Yangtze margin of the basin. Weaker euxinia during UM deposition may have been due to its decreasing organic availability and/or small seawater sulfate reservoir caused by persistent euxinic consumption during LM deposition. Our results highlight the complex controls and the key role of continental weathering and resulting terrigenous sulfate fluxes in marine euxinic variations in early Cambrian marginal seas, providing new insights into the co-evolution of continents, oceans, and early animals.

Source rock characterization and oil-to-source rock correlation of a Cambrian -Ordovician fold-and-thrust belt petroleum system, western Newfoundland

Abstract In this study, we use biomarker and isotope data to address the complexities of petroleum system analysis in fold-thrust systems, specifically related to hydrocarbon generation and migration, such as variable thermal maturities of source rocks, complex migration pathways, and mixing. Source rocks, extracts, and oil samples are taken from Paleozoic rocks along the Appalachian structural front in western Newfoundland. Oil seeps along this fold-and-thrust belt have motivated episodic exploration efforts over the last 150 years. However, economic development has been unsuccessful to date, in part because of the complex nature of oil-to-source relationships. Pyrolysis analyses identify promising source intervals in Lower Ordovician (Floian) formations, with an excellent source potential containing type I/II organic matter (TOC up to 9.35%, HI up to 840). A second good source interval, identified within the late Cambrian (Furongian) continental slope and rise sediments, contains type II/III organic matter (TOC up to 2.34%, HI = 380). Source rock samples from outcrops are marginally mature to mature with Tmax values between 436 and 447. Geochemical analyses of source rock extract imply a clastic shale-dominated source rock with a minor contribution from carbonate source rocks. Chemometric analyses yield 3 extract groups. Groups 1 and 2 contain lower Ordovician samples with organic matter derived from oxidized micro-plankton with varying amounts of Gleocapsomorpha prisca. Extract group 3 originates from Cambrian samples and shows evidence of a bacterial-derived organic matter. Similar genetic relationships among 10 oil samples from natural oil seeps and abandoned well sites are indicated by high-resolution biomarker analyses. Seven oil samples originated from Ordovician source rocks that produced high API oils with low bisnorhopane/hopane, tricyclic/hopane ratios, and high sterane concentrations, supporting interpretation of a source containing algal-derived organic matter. Three oils originated from Cambrian shales. These high API oils show high bisnorhopane/hopane and tricyclic/hopane ratios, low sterane concentrations, and low carbon isotope values, all indicative of organic matter derived from bacteria, generated over a wide range of maturities (0.98–1.26 %Ro). Thermal maturity-sensitive biomarkers and naphthalene ratios show that source rocks generated oils with a wide range of maturities related to their dipping character in the imbricated thrust stack. The Cambrian source generated oils that locally experienced secondary cracking during Acadian inversion, whereas the Ordovician source generated oil with low maturities after the Acadian inversion. Oil operaters exploring fold-and-thrust belt petroleum systems may expect extreme changes of oil properties (for example, API gravity) in spacially restricted areas.

Nitrogen-rich gas shale logging evaluation and differential gas-bearing characterization of lower Cambrian formation in northern Guizhou, south China

Abstract There are significant differences in the gas content between the two sets of lower Paleozoic shale in south China. The nitrogen content of the lower Silurian shale gas that has been commercially developed is very low, whereas that of the lower Cambrian shale (LCS) is relatively high. Shale has complex gas-bearing mechanistic characteristics. The genesis of nitrogen is complex and sources are diverse. A gas containing high nitrogen and low hydrocarbons increases the risk and difficulty of developing a shale gas resource. Based on the key geological parameters of the LCS in the northern Guizhou area (NGA), this study investigates the characteristics of nitrogen in the highly mature shale and discusses the logging evaluation method for nitrogen-bearing shale gas reservoirs in this complex tectonic area. Changes in the gas-bearing characteristics of the LCS in the study area are complex. The shale gas composition was found to vary widely due to differential enrichment; the methane content was between 1.2% and 82.4%, and the nitrogen content was between 0.03% and 97.4%. The nitrogen-rich shale formations exhibited a high compensation density (2.46–2.58 g/cm3), high compensation neutron (10.2–17.9%), and low acoustic time difference (192–230 μs/m). There were two different types of resistivity responses in nitrogen-bearing shale gas reservoirs, which had distribution characteristics of 21–56 Ω m and 100–1000 Ω m. With these response characteristics, the logging evaluation of nitrogen-bearing shale gas reservoirs was interpreted by selecting differential gas-sensitive parameters, and different gas-bearing types were divided into dry, nitrogen, mixed, and methane gas layers. Furthermore, the identification and computational evaluation of the gas composition (methane/nitrogen) and content were carried out quantitatively. Differential gas-bearing property was characterized and that is beneficial to the exploration practice in the study area.

Analysis of Gas Composition and Nitrogen Sources of Shale Gas Reservoir under Strong Tectonic Events: Evidence from the Complex Tectonic Area in the Yangtze Plate

Strong tectonic movement brings great risk to exploration of shale gas in southern China, especially in Lower Cambrian shale with complex tectonic backgrounds, which has good hydrocarbon-generation matter but low or no gas content. In this paper, the Lower Cambrian shale from the southeast Chongqing region, located in the Upper Yangtze Platform, and the Xiuwu Basin, located in the Lower Yangtze Platform, were selected as the research objects. First, the gas components in shale gas samples were measured, then analysis of nitrogen isotopic was used to reveal the nitrogen sources. Using regional geological backgrounds, core description, and seismic interpretation, combined with the perpendicular and parallel permeability test and focused ion beam–helium ion microscopy (FIB–HIM) observation, the reasons for high content of nitrogen in the Lower Cambrian shale from the Xiuwu Basin and the Southeast Chongqing region were clarified. The results indicate that the main sources of nitrogen in the Lower Cambrian shale gas at the Southeast Chongqing region is the thermal evolution of organic matter and atmosphere. Nitrogen in the atmosphere is filled into the shale reservoir through migration channels formed by detachment layers at the bottom of the Lower Cambrian, shale stratification planes, and widespread thrust faults. Nitrogen was also produced during the thermal evolution of organic matter. Both are responsible for the low content of hydrocarbon and high content of nitrogen of shale gas in the Southeast Chongqing region. Further, the main sources of nitrogen in the Lower Cambrian shale gas at the Xiuwu Basin is the upper mantle, superdeep crust, and atmosphere. Nitrogen in the atmosphere is also filled into the shale reservoir through migration channels formed by detachment layers at the bottom of the Lower Cambrian, shale stratification planes, and widespread thrust faults. Nitrogen was also produced by volcanism during the Jurassic. Both are the causes of the low content of hydrocarbon and high content of nitrogen in shale gas in the Xiuwu Basin. Finally, destruction models for shale gas reservoirs with complex tectonic backgrounds were summarized. This research aimed to provide a theoretical guidance for shale gas exploration and development in areas with complex tectonic backgrounds.

Numerical Test Study of the Microscale Failure Modes and Fractal Analysis of Lower Cambrian Shale Based on Digital Images

To reveal the effect of confining pressure on the mechanical properties and rupture modes of quartz‐bearing shale, the shale core of the no. 3 block of Fenggang, Guizhou Province, China, was analyzed by nuclear magnetic resonance, polarized light microscope thin section observation and identification, and core X‐ray whole‐rock minerals diffraction analysis to determine the distribution of shale minerals in the Niutitang Formation. On the microscale, based on digital image processing technology, this paper characterizes the nonuniformity of minerals in shale, a numerical model that can reflect the true microstructure of shale. Then, the failure process of shale under different confining pressures was simulated. The results show that when the shale is loaded with vertical displacement under different confining pressures, the compressive strength and elastic modulus of the sample change significantly. The failure mode can be roughly divided into three types: the inverted V‐shaped (0 MPa, 2 MPa, and 4 MPa), V‐shaped (6 MPa and 8 MPa), and inverted Z‐shaped (10 MPa). Since the development of fractal theory provides a new space for studying the damage and fracture of rocks, the damage evolution and failure process of shale can also be regarded as the fractal process of cracks, in which the fractal dimension is the core parameter. The calculation is different under different stress levels. The fractal dimension under the condition of confining pressure shows that the value of the fractal dimension is greatly affected by the effect of confining pressure. When the fractal dimension is higher, the fracture mode is more complicated, and the internal damage degree is more serious. The research results provide important theoretical guidance for shale gas fracturing production.

Analysis of Lower Cambrian shale gas composition, source and accumulation pattern in different tectonic backgrounds: A case study of Weiyuan Block in the Upper Yangtze region and Xiuwu Basin in the Lower Yangtze region

Marine shale gas exploration in southern China has successes and failures. Under the condition of great hydrocarbon generation material basis, shale gas wells drilled from some shale gas blocks are rich in methane, while the wells in other shale gas blocks with high nitrogen and low hydrocarbon gas, which indicates that they have different accumulation mechanisms. Therefore, the study of gas composition in shale will help us to figure out the mechanism of shale gas accumulation and loss. In this paper, the Lower Cambrian shale from Wei-201 well in Upper Yangtze Weiyuan Block and Jiangye-1 well from Lower Yangtze Xiuwu Basin are selected as research object, and shale samples are used for tests and experiments including analysis of gas composition and nitrogen isotope, test of porosity and TOC content, overburden permeability test, permeability test before and after methane adsorption under different osmotic pressure, permeability test parallel and vertical to the bedding surface, FIB-SEM (Focus Ion Beam Scanning Electron Microscope) and FIB-HIM (Focused Ion Beam Helium Ion Microscope). Finally, the reasons for the difference in the gas components of the Lower Cambrian shale gas in Weiyuan Block and Xiuwu Basin are studied by means of seismic interpretation, core description and outcrop observation besides the tests and experiments. The results show that the gas components of the Lower Cambrian shale in Weiyuan Block, the Upper Yangtze, mainly consist of methane, derived from liquid hydrocarbon cracking. The sealing capacity of roof and floor, the great self-sealing of shale and the flat anticline structure contribute to the high methane content in shale gas. The Lower Cambrian shale gas in Xiuwu Basin, the Lower Yangtze, is mostly nitrogen, which is derived both from atmosphere and deep crust-upper mantle. The detachment layer at the bottom of the Lower Cambrian, the widely developed deep faults and the Jurassic volcanic activity are the reasons for the high nitrogen and low hydrocarbon of shale gas. Based on the above analysis, the patterns are summarized for shale gas accumulation in the simple anticline background and reservoir destruction in the complex syncline background.

Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China

Shale gas reservoir performance and canister desorption experiments of the Lower Cambrian organic-rich shales in the eastern Upper Yangtze Platform reveal a significant difference in shale gas content between the Dabashan arc-like fold-thrust belt in northeastern Chongqing (Deformed Zone) and the slightly folded area in southeastern Chong (Non-deformed Zone). Integrated pore characterization methods including scanning electron microscopy (SEM), low-temperature N2 and CO2 adsorption, and mercury injection capillary pressure (MICP) analyses were comparatively conducted in both areas in order to examine shale gas reservoir pore size variations and thus the possible microscopic pore structure controls on shale gas enrichment.The Lower Cambrian shales in both Deformed Zone (DZ) and Non-deformed Zone (NDZ) were deposited in the deep-water shelf and show similar organic matter richness and thermal maturity. The majority of organic matter (OM)-hosted pores in DZ samples are in nanoscale size range with the dominance of micro-fractures within the OM or at the interface of OM and minerals. In contrast, OM-hosted meso-(2–50 nm) to macropores (>50 nm) are the dominant pore types in the NDZ samples. OM-hosted micropores (<2 nm) are abundant in both zones. Helium ion microscopy observations further confirm the presence of OM-hosted micropores in the studied samples. Mineral-hosted pores in carbonate minerals are abundant in both zones, while dissolution rims around carbonate minerals are more abundant in NDZ samples.The Dabashan arc-like fold-thrust belt took place by the end of the Late Triassic, while the Lower Cambrian shales have reached thermal maturity peak. OM-hosted meso-(2–50 nm) to macropores (>50 nm) in DZ samples are most probably collapsed during structural deformation related to tectonic compression, while micropores due to their smaller size survived the tectonic stress. The OM-hosted micropores are the main storage space for adsorbed gas in the DZ area. The dominance of micro-pores in DZ and lack of connection between those pores and matrix pores led to higher gas content in DZ samples. On the contrary, the well-connected OM-hosted pore network in NDZ allows easier gas flow in the rock matrix that eventually led to significant gas leakage during uplift and exhumation and lower gas content in this zone. The results of this study suggest that structural deformation can potentially change the pore structure of shales and thus shale gas content which has major significance for shale gas exploration and development in south China where had experienced complex tectonic movements.

Quest carbon capture and storage offset project: Findings and learnings from 1st reporting period

Quest is a fully integrated Carbon Capture and Storage (CCS) project that started CO2 injection in August of 2015. The Quest CCS Project is located near Fort Saskatchewan, Alberta, Canada. It includes a capture facility which uses a Shell amine technology, a pipeline of about 65 km length, and three injection well pads. Each injection well pad has an injection well, a deep monitoring well, and shallow groundwater wells. The storage complex is geologically defined by the injection reservoir, a deep saline aquifer called the Basal Cambrian Sand (BCS) (about 45 m thick) and several seals, including the Middle Cambrian Shale (about 50 m thick) and Lotsberg Salts (about 120 m thick). As of August 2018, over three million tonnes of CO2 have been safely injected and permanently stored in the BCS. The Alberta Carbon Competitiveness Incentive Regulation (CCIR) requires the use of standard methods of quantification for reporting greenhouse gas (GHG) emissions for facilities with over 100,000 tonnes of carbon dioxide equivalent (CO2e) per year. An emission offset project is required to comply with CCIR, associated standards and protocols, to demonstrate a reduction in the specified gas emissions and, in the case of Quest, geological sequestration. Quest is the first CCS project to implement an offset project in the context of commercial scale on-shore CO2 geological sequestration within a saline aquifer. Quest uses the Quantification Protocol for CO2 Capture and Permanent Storage in Deep Saline Aquifers, from Alberta Environment and Parks. An offset project must develop an offset project plan (OPP) which demonstrates how the project meets the requirement of the protocol, describes how GHG emissions reductions are achieved, identifies risks associated with the quantification of emission reduction benefits, and describes methodologies used to quantify sources and sinks. Subsequent to completing the OPP, an offset project will put together offset project reports (OPR) to report on the net reductions of GHG emissions for a specific period. The intent of this paper is a) to provide an overview of the OPP and OPR for the Quest CCS project, and b) to discuss learnings from the initial compilation and submission of offset project reports. The key learning at this time is associated to the equipment improvements to the injection gas online analyzer.

Various controlling factors of matrix-related pores from differing depositional shales of the Yangtze Block in south China: Insight from organic matter isolation and fractal analysis

Abstract In this study, matrix-related pores from differing depositional shales were explored comparatively. Among of them, Lower Cambrian shale (3.83%Ro) and Lower Silurian shale (2.61%Ro) were marine sediments with abundant oil-prone kerogen and rich siliceous minerals, while Upper Permian shale (2.44%Ro) were transitional sediments with redundant gas-prone kerogen and rich clay. The morphology and geometry of pores were investigated via fractal analyses based on N2 adsorption and direct imaging. The effects of organic matter (OM) within different shales were also highlighted through N2 adsorption before and after OM isolation. Lower Cambrian shale possessed the lowest pore volumes (PV) (averaging 0.0109 ml/g) and the lowest pore surface areas (PSA) (averaging 9.09 m2/g) as well as the smallest average pore diameters (APD) (averaging 5.47 nm). Dissolved pore with dead-end openings was the main type. The PV and PSA of isolated OM were only approximately 1 and 2 times higher than that of corresponding samples, respectively. In contrast, Lower Silurian shale possessed the highest PV (averaging 0.0109 ml/g) and the highest PSA (averaging 9.09 m2/g) as well as relatively large APD (averaging 13.43 nm). Organic-hosted pores (OMP) with cellular structure is the main type. The PV and PSA of isolated OM were approximately 8.5 and 3 times higher than that of corresponding samples, respectively. Upper Permian shale with the largest averaging pore diameters (averaging 18.82 nm) presented high PV (averaging 0.0209 ml/g) similar to that of Lower Silurian shale, and a low PSA (averaging 10.85 m2/g) like that of Lower Cambrian shale. Pore associated with clay flakes was the main type. The PV and PSA of isolated OM were only approximately 0.6 times and 1 times higher than that of corresponding samples, respectively. For marine shale, matrix-related pore features are synergy effects of the matrix basis where pre-existing space controls the occurrence of porous OM and functions as the shelter for OMP with an appropriate thermal maturity. However, extensive diagenesis can overprint the effects of matrixes on pore properties, because oil-prone kerogen is sensitive to thermal maturity. The specific material composition of transitional shale limit pore properties, because dominant structured OM is thermally stable with limited migration ability and pore contribution. Hence, diagenetic differences and material diversities may be attributed to the discrepancies of pore properties between marine shale and transitional shale.

Geologic and geochemical characteristics of the Cambrian marine shale in the Tarim Basin of China and their implication for the prediction of shale-gas resources

Although the Cambrian marine shale in the Tarim Basin is usually considered to be the source rocks for overlying conventional reservoirs, its unconventional shale-gas potential has not yet been evaluated. To indicate the unconventional resource play potential and highlight more favorable zones, we conducted a comprehensive lithologic, petrophysical, and geochemical analysis of 228 drill core samples from 14 wells and 167 outcrop samples from eight geologic sections in the basin. The results indicate that the Cambrian marine shale was deposited in basinal and slope environments dominated by shale and siliceous shale. The average total organic carbon (TOC) content of the Є1xd (Xidashan) and Є2m (Moheershan) Formations ranges from 0.5% to 2.5%, and that of the Є1y (Yuertusi) Formation ranges from 1.0% to 7.0%. Phytoplankton is dominant in the Є1xd and Є2m Formations, whereas benthic algae are more prevalent in the Є1y Formation. The organic matter ranges in maturity from high to postmature. The shale consists of abundant meso- and macropores. The mineral matter is mostly composed of quartz ([Formula: see text] in most samples) followed by carbonate minerals, with minor contributions made by clay minerals ([Formula: see text]). The analysis of methane adsorption isotherm indicates a positive correlation between the gas and TOC content and a negative correlation between the gas content and burial depth implying the good adsorption capacity of the shale-gas play. Resource estimation of the Middle-Lower Cambrian indicates that the Yuli and Ruoqiang areas contain a potential [Formula: see text] of shale gas, and they are the most favorable zones for further exploitation.

Applications of chemostratigraphy in a characterization of shale gas Sedimentary Microfacies and predictions of sweet spots —taking the Cambrian black shales in Western Hubei as an example

Exploratory research was conducted in this paper on the correlation of chemical elements with sedimentary microfacies and shale gas sweet spots through cluster analysis, factor analysis, and so on, using a large amount of geochemical data of elements obtained from shale gas exploration of the Cambrian Niutitang Formation in Western Hubei. This paper screened the chemical element associations and parameter systems that can reflect sedimentary microfacies and sweet spot evaluations, proposed a classification method for the chemostratigraphic facies of shales, and finely depicted the sedimentary chemical microfacies of the shales in the Cambrian Niutitang Formation. A chemostratigraphic lithofacies paleogeography map of SQ1 (the first sequence strata) in the Cambrian Niutitang Formation in Western Hubei was compiled. Sweet spots were predicted and evaluated, and favourable areas were predicted, thereby providing important support for making a breakthrough in the exploration of the Cambrian shale gas in the Yichang area. Chemostratigraphy has a higher precision and a low characterization cost of reservoir sedimentary microfacies; especially in areas lacking drilling data and seismic data and having a low exploration degree, chemostratigraphy is an important means for identifying of shale sedimentary sequences and predicting shale sweet spot sections.

A Cause Analysis of the High-Content Nitrogen and Low-Content Hydrocarbon in Shale Gas: A Case Study of the Early Cambrian in Xiuwu Basin, Yangtze Region

There are successes and failures in the exploration of marine shale gas in South China. In some shale gas plays with great basis for hydrocarbon generation, a phenomenon exists that gas loggings reflect low gas bearing in some of the wells and the gas is dominated by nitrogen rather than hydrocarbon gas. The study of nitrogen concentration in shale gas contributes to solve the question that how shale gas diffuses in complex tectonic areas, which helps to figure out the preservation requirements and accumulation mechanisms of shale gas and avoid exploration crisis. This study focused on the lower Cambrian shale in Xiuwu Basin, Lower Yangtze Region, with emphasis on the well Jiangye-1, using gas component analysis, stable nitrogen isotope analysis, overburden permeability tests in parallel and perpendicular directions, and FIB-HIM experiments, also combining with core description, outcrop observation, and seismic interpretation to explore the causes of the high-content nitrogen and low-content hydrocarbon in the lower Cambrian shale gas. The results show that the nitrogen of the lower Cambrian shale in Xiuwu Basin is derived from the atmosphere and the deep crust-upper mantle. The bedding planes and the detachment layer at the bottom of the lower Cambrian compose the lateral pathways, and the widespread deep faults are the vertical pathways for shale gas migration and diffusion. Combining these two, an effective pathway network was built, favorable to gas exchange between the shale gas interval and the atmosphere, partly leading to the concentration of nitrogen and the diffusion of hydrocarbon gas. In the Jurassic, the magmatic activities occurred frequently in the surrounding areas, which not only brought nitrogen from the deep crust-upper mantle but also increased the value of paleo-heat flow even though the basin began to uplift, which promoted the graphitization of organic matter and the collapse of organic pores and accelerated the loss of shale gas. Based on the study above, an explanation model was summarized to expound the causes of high-content nitrogen and low-content hydrocarbon in shale gas plays near the plate-active region in Xiuwu Basin, Lower Yangtze Region.

Division of shale sequences and prediction of the favorable shale gas intervals: an example of the Lower Cambrian of Yangtze Region in Xiuwu Basin

Abstract It is a common method to use sequence stratigraphic theory to identify favourable intervals in hydrocarbon exploration. The Lower Cambrian shale of Well Jiangye-1 in Yangtze Region in Xiuwu Basin was chosen as the research object. The content of excess silicon of siliceous minerals in shale was calculated quantitatively, and the concentration distribution of Al, Fe, Mn showed that the excess silicon is of hydrothermally origin and the shale deposited in an environment with hydrothermal activity. Using U/Th values in the study, combined with lithology and logging data, in order to divide sequences of the Lower Cambrian shale in Yangtze Region in Xiuwu Basin. The result shows that the shale of the Lower Cambrian shale is recognized as 1 2nd sequence (TST-RST, TST = Transgressive systems tract; RST = Regressive systems tract) and then further subdivided into 5 3rd sequences (SQ1-SQ5). During the deposition of SQ2 and SQ3, hydrothermal activity was active, and their excess silicon content was generally above 20%-30%. Rising sea level and active hydrothermal activity were beneficial for the enrichment of siliceous minerals and organic matter. Based on the comparison of the reservoir parameters, it tells that SQ2 and SQ3 have relatively higher content of TOC, higher content of brittle minerals (such as siliceous minerals, carbonate minerals and so on), larger effective porosity and higher content of gas, which make it as the most favourable intervals of the Lower Cambrian in Xiuwu Basin.

Geological controls on the accumulation of shale gas: A case study of the early Cambrian shale in the Upper Yangtze area

Abstract The results of sedimentological, mineralogical, geochemical and petrophysical studies on the early Cambrian shale in the Upper Yangtze area are summarized to provide insights on the paleoenvironmental conditions, diagenetic evolution and preservation conditions that give rise to shale gas accumulation. The lower Cambrian organic-rich Qiongzhusi shale is mainly developed in the outer shelf facies of an intracratonic sag [average total organic carbon (TOC): 1.3–2.5%] and in the southeast margin (average TOC: 3.5–5.5%) of the Upper Yangtze area. The thermal maturity values (Reqv) of the Qiongzhusi shale range from 2.19 to 4.23%, while first increasing and then decreasing from the center of the Sichuan Basin outward. The average porosities of the Qiongzhusi shale from the outer shelf facies association (FA5) in the intracratonic rift range from 1.5 to 3.41%, and those from the outer shelf facies association (FA4) in the southeast Sichuan and Guizhou areas range from 1.32 to 2.56%. The organic matter (OM) content and the development degree of OM pores are the main factors that control the porosity and pore structure of the Qiongzhusi shale. For the outer shelf deposits in the intracratonic rift, a quartz (detrital origin) content ranging between 25 and 35% is most favorable for providing an effective support framework for OM pore development due to the mixture of brittle and clay minerals. For the outer shelf deposits in the southeastern margin of the Upper Yangtze area, the high OM content and poor compaction resistance cause a poor development of OM pores, with a smaller average pore diameter than that in the outer shelf shale of the intracratonic rift. In addition, the carbonization of OM in the overmature Qiongzhusi shale change the microstructure and decrease the OM pore volume. Due to poor preservation conditions, the gas content is generally low in the organic-rich shale in the eastern and southeastern parts of the Upper Yangtze region. The Qiongzhusi shale in the interior of the Sichuan Basin, with good preservation conditions, high pressure coefficients, and high gas contents, is a favorable area for shale gas accumulation and potential exploitation.

Source Analysis of Silicon and Uranium in uranium-rich shale in the Xiuwu Basin, Southern China

Abstract Uranium deposits are crucial resources for the development of the nuclear energy. Among known sources of uranium, the uranium-rich shales have recently obtained significance. In this paper, the Lower Cambrian Wangyinpu Formation shale in the Xiuwu Basin, southern China, has been studied using a combination of techniques including element analysis (Al, Fe, and Mn), δ30Si silicon isotopic analysis, δ18O oxygen isotopic analysis, study of core samples. It has been observed that significant hydrothermal activity occurred in the Xiuwu Basin during the Early Cambrian period. The results show that 20%–40% of the silicon in most of the sections of the Lower Cambrian Wangyinpu Formation were inherited from the hydrothermal fluids, with temperatures ranging between 75∘C and 102∘C. It is concluded that more than 90% of the uranium in most of the sections of the Lower Cambrian Wangyinpu shale was derived from submarine hydrothermal fluids, while less than 10% from the terrigenous detritus. The enrichment of uranium in the basin was observed in the Middle-Upper part of the Wangyinpu Formation and the geological resources estimated to a tune of ~4.9×103 t. In this paper, we proposed a model for silicon and uranium enrichment in the Lower Cambrian shale controlled by hydrothermal activity in the Xiuwu Basin. This model also provides a scientific rationale for uranium further exploration and exploitation of the uranium resource.

Pore structure characteristics of the Lower Cambrian black shale in the Cengong block, southeastern Guizhou area

Pore structure characteristics of the Lower Cambrian black shale in the Cengong block, southeastern Guizhou area