Brittle Minerals Research Articles

Controlling factors of lamellation fractures in marine shales: A case study of the Fuling Area in Eastern Sichuan Basin, China

The lamellation fractures in marine shales are important to the accumulation and preservation of shale gas, whose characteristics and controlling factors are still unclear. This paper focus on the characteristics descriptions and controlling factor analyses based on core observations, thin sections, and scanning electron microscope (SEM) experiments. The marine shales of Paleozoic Wufeng Formation and Longmaxi Formation in the Fuling Area of Eastern Sichuan Basin are taken as an instance. The lamellation fractures are occurring along the direction of the lamellations, partially of them curved, bifurcated, and converged. The apertures of fractures typically range from 1 to 500 μm. Only a few (<1%) lamellation fractures are filled with quartz, calcite, pyrite, and bitumen. After a series of analyses, developments of lamellation fractures are controlled by total organic carbon (TOC), brittle minerals, laminae, and pyrite. (1) The organic matters control the formation and evolution of lamellation fractures by generating gas and resulting in overpressure. An increase in the TOC content of 1% can increase the lamellation fracture density by 0.26 cm-1. (2) Brittle minerals, especially authigenic quartz, is also an important controlling factor to the development of lamellation fractures. When BI increases by 10%, the lamellation fracture density increases by 19%. (3) Besides, siliceous and carbonate laminae in the shale are more suitable for lamellation fractures to develop. When the laminae density is smaller than 4 cm−1, it is easier to develop lamellation fractures with higher laminae density. However, too densely-distributed laminae will suppress the formation of lamellation fractures. (4) Higher pyrite content is good for the development of lamellation fractures. Lamellation fracture density will increase by 9%, with pyrite content increased by 1%. For the shale at the bottom of the Wufeng Formation - Longmaxi Formation, lamellation fractures develop extensively due to the high TOC and pyrite content, great brittleness, and moderate laminae density. The density of lamellation fractures decreases from the bottom to the top.

How argillaceous reservoirs exhibit better quality than silty mudstones? Anomalous behavior of shale gas-bearing properties of continental fine-grained sediments in Southwest China and its possible forcing mechanisms

Achieving a thorough understanding of how primary sedimentary granularity drives considerable heterogeneity in internal reservoir attributes of terrigenous fine-grained deposits is of great significance. We investigated the quantitative differentiation and its corresponding driving forces of physical reservoir properties and pore-structure characteristics of silty-mud sediments in the Upper Triassic Xujiahe Formation (SW China) using a multi-method approach. The results show that the micro-mesopore volume and surface area of mudstones/shales are apparently higher than those of silty mudstones and a remarkable threefold rise in average permeability also presents. Extensively distributed bitumen pores occurring mostly along brittle mineral grains or forming clay-organic complexes make considerable contributions to shrinking microcracks. Furthermore, an evidently higher concentration of clay minerals in mudstone/shale reservoirs is primarily responsible for development of the two types of clay inter-crystalline pores distributed along grain aggregates and between well-oriented platelets. These two major causes facilitate the formation of micro-bedding fractures/non-bedding microfractures and connected fracture and pore-fracture networks, and also high-quality argillaceous reservoirs by strongly enhancing storage spaces and seepage capacities. Finally, a conceptual model is established for interpreting a differential reservoir-forming mechanism and corresponding two-sided effects on petrophysical and reservoir quality properties for continental silty-mud sediments.

New cognition on pore structure characteristics of Permian marine shale in the Lower Yangtze region and its implications for shale gas exploration

Shale of the Middle–Upper Permian Dalong Formation and Gufeng Formation in the Lower Yangtze region is characterized by large thickness, high total organic carbon (TOC), wide distribution and moderate organic thermal evolution degree, so it may be the next important field of shale gas exploration. In order to point out the target and direction of shale gas exploration and development in this region, this paper selects the Dalong Formation and Gufeng Formation shale in the Xuanjing area of Lower Yangtze as the research object to quantitatively describe the development characteristics of shale pores of different scales by means of scanning electron microscopy (SEM), high pressure mercury injection, CO2 and N2adsorption. Then, the fractal dimension of sample pores is calculated by using the FHH (Frenkel–Halsey–Hill) model. Finally, combined with TOC and mineral compositions, the relationship between pore structure and fractal dimension is discussed. And the following research results are obtained. First, dominant lithofacies of Dalong Formation are argillaceous-rich siliceous shale and argillaceous-rich/siliceous mixed shale, which has smaller specific surface area and pore volume, larger average pore diameter. The pore development is mainly controlled by clay mineral content. Dominant lithofacies of Gufeng Formation is siliceous shale, which has a larger specific surface area and pore volume, and smaller average pore diameter. The pore development is controlled by organic matter and brittle mineral content. Second, the influencing factors of fractal dimension can be attributed to the controlling factors of micropore development. The fractal dimension D1of Dalong Formation ranges from 2.4515 to 2.5513 (average 2.5227), and D2ranges from 2.5817 to 2.6578 (averagely 2.6246). The fractal dimension D1of Gufeng Formation ranges from 2.5817 to 2.6578 (averagely 2.6246), and D2ranges from 2.7227 to 2.871 (averagely 2.813). Gufeng Formation shale presents the characteristics of more complex pore structure. Third, the fractal dimension D1is more sensitive to specific surface area, pore development and mineral composition, while D2is more effective in characterizing the average pore diameter. In conclusion, Dalong Formation shale with high D1and D2and Gufeng Formation shale with low D1and high D2can be taken as the favorable exploration targets of Permian marine shale gas in the Lower Yangtze region, and the regional overpressure zones (belts) with weaker structural deformation in hydrocarbon rich sags will be the favorable shale gas exploration targets in this area.

The Characteristics and Main Controlling Factors for the Formation of Micropores in Shale from the Niutitang Formation, Wenshuicun Section, Southwest China

The characteristics of shale micro-pore development and its main influencing factors have important theoretical guiding significance for shale gas exploration and resource evaluation. In order to clarify the micro-pore development characteristics of lower Cambrian shale and the main controlling factors of micro-pore development, we used the lower Cambrian Niutitang formation shale, in the Wenshuicun section of the Guizhou Province in southwest China. The micro-pore development characteristics of the shale in the region were studied by argon ion profile field emission scanning electron microscopy and a low-temperature liquid nitrogen adsorption and desorption experimental system. The relationship between micro-pore and kerogen maceral composition, total organic carbon (TOC) content and different mineral content was analyzed in combination with mineral and geochemical characteristics. Inorganic pores (clay mineral pores, dissolution pores and pyrite intergranular pores) and micro-fractures (clay mineral shrinkage crack, tectonic fractures and overpressure fractures) were the main type of pore developed in the shale of the Niutitang formation in the Wenshuicun section, and no organic pores had developed. The pore size of shale is usually 2–50 nm, accounting for 58.33% of shale pores, e.g. mesopores. Clay mineral content has an obvious positive correlation with macropore volume and average pore diameter, and an obvious negative correlation with micropore volume. In addition, the content of feldspar in brittle minerals has a strong negative correlation with macropore volume and average pore diameter, and a strong positive correlation with micropore volume and BET-specific surface area. TOC content and the content of different kerogen macerals have no obvious correlation with the development of shale micropores in this region. It is concluded that inorganic mineral composition is the main controlling factor of micro-pore development within lower Cambrian shale, and organic matter abundance and maceral content have little influence on the micro-pore development. This study provides a case study for the characteristics of micropores in lower Cambrian shale in China.

Characteristics of Pore Structure and Gas Content of the Lower Paleozoic Shale from the Upper Yangtze Plate, South China

This study is predominantly about the differences in shale pore structure and the controlling factors of shale gas content between Lower Silurian and Lower Cambrian from the upper Yangtze plate, which are of great significance to the occurrence mechanism of shale gas. The field emission scanning electron microscopy combined with Particles (Pores) and Cracks Analysis System software, CO2/N2 adsorption and the high-pressure mercury injection porosimetry, and methane adsorption were used to investigate characteristics of overall shale pore structure and organic matter pore, heterogeneity and gas content of the Lower Paleozoic in southern Sichuan Basin and northern Guizhou province from the upper Yangtze plate. Results show that porosity and the development of organic matter pores of the Lower Silurian are better than that of the Lower Cambrian, and there are four main types of pore, including interparticle pore, intraparticle pore, organic matter pore and micro-fracture. The micropores of the Lower Cambrian shale provide major pore volume and specific surface areas. In the Lower Silurian shale, there are mesopores besides micropores. Fractal dimensions representing pore structure complexity and heterogeneity gradually increase with the increase in pore volume and specific surface areas. There is a significant positive linear relationship between total organic carbon content and micropores volume and specific surface areas of the Lower Paleozoic shale, and the correlation of the Lower Silurian is more obvious than that of the Lower Cambrian. The plane porosity of organic matter increases with the increase in total organic carbon when it is less than 5%. The plane porosity of organic matter pores is positively correlated with clay minerals content and negatively correlated with brittle minerals content. The adsorption gas content of Lower Silurian and Lower Cambrian shale are 1.51–3.86 m3/t (average, 2.31 m3/t) and 0.35–2.38 m3/t (average, 1.36 m3/t). Total organic carbon, clay minerals and porosity are the main controlling factors for the differences in shale gas content between Lower Cambrian and Lower Silurian from the upper Yangtze plate. Probability entropy and organic matter plane porosity of the Lower Silurian are higher than those of Lower Cambrian shale, but form factor and roundness is smaller.

Pore Structure and Fractal Characteristics of Organic-Rich Lacustrine Shales of the Kongdian Formation, Cangdong Sag, Bohai Bay Basin

In order to examine the pore structure and reveal the fractal geometric nature of shales, a series of laboratory experiments were conducted on lacustrine shale samples cored from the Kongdian Formation. Based on the low temperature nitrogen adsorption, fluorescent thin section and field emission scanning electronic microscope, a comprehensive pore structure classification and evaluation were conducted on shale samples. Fractal dimensions D1 and D2 (with relative pressure of 0–0.45 and 0.45–1.00, respectively) were obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) method. With additional means of X-ray diffraction analysis, total organic carbon content analysis and thermal maturity analysis, the relationships between pore structure parameters, fractal dimensions, TOC content and mineral composition are presented and discussed in this paper. The results show that interparticle pores and microfractures are predominant, whereas organic matter pores are rarely found. The pore morphology is primarily featured with wide-open ends and slit-shaped structures. In terms of pore scale, mesopores and macropores are predominant. The value of fractal dimension D1 representing small pores ranges from 2.0173 to 2.4642 with an average of 2.1735. The value of D2 which represents large pores ranges from 2.3616 to 2.5981 with an average of 2.4960. These low numbers are an indication of few pore types and relatively low heterogeneity. In addition, smaller D1 values reveal that large pores have more complicated spatial structures than smaller ones. The results of correlation analysis show that: 1) D2 is correlated positively with specific surface area but negatively with average pore diameter; 2) D1 and D2 literally show no obvious relationship with mineral composition, TOC content or vitrinite reflectance (Ro); 3) both total Barrett-Joyner-Halenda (BJH) volume and specific surface area show a positive relationship with dolomite content and a negative relationship with felsic minerals content. These results demonstrate that the pore types are relatively few and dominated by mesopores, and the content of brittle minerals such as dolomite and felsic minerals control the pore structure development whilst organic matter and clay minerals have less influence due to low thermal maturity and abundance of clay minerals.

Chemostratigraphic Analysis of Wufeng and Longmaxi Formation in Changning, Sichuan, China: Achieved by Principal Component and Constrained Clustering Analysis

The increasing proportion of unconventional worldwide energy demands have consistently promoted the necessity for exploring a precise, high-resolution, objective, and quantitative stratigraphic division method for macroscopically homogeneous mudstone successions. The chemostratigraphy can resolve this problem well, although it has been applied successfully in North America, but not systematically studied in China for shale gas exploration and development. This work has conducted a chemostratigraphic analysis of Wufeng and Longmaxi Formation on the Changning section of Sichuan Province, southwestern China, to testify its applicability for shale gas exploration in China. Principal component analysis (PCA) was first employed to reduce the dimensionality of datasets. Three chemofacies, including detrital (K, Ti, Fe, Al, Na, Mg, Cr, Zr, Rb), authigenic (Ca, Sr, Mn, Si, S, Ba), and redox-organic (P, V, Ni, Zn, Cu, TOC), were found. Subsequently, constrained clustering analysis was utilized for the zonation of each chemofacies into chemozones. Consequently, the whole Changning section was divided into twelve chemozones (CZ I–CZ Ⅻ). The geochemical interpretation for these chemozones can be resolved from the regional changes in paleogeography and paleoceanography during the Late Ordovician to Early Silurian period. Thus, a three-stage geochemical evolution along the Changning section can be classified: (1) the siliceous and anoxic deposits of Wufeng Formation (CZ I–CZ III) with high TOC contents; (2) the siliceous and anoxic sedimentary rocks of bottom Longmaxi Formation with still higher TOC (CZ Ⅳ); (3) the calcarous-detrital and oxic sediments for the rest of Longmaxi Formation (CZ Ⅴ–CZ Ⅻ). In considering their high content of TOC and abundant brittle siliceous minerals, the CZ (I–Ⅳ, 0 m–33.6 m) are thought to be the most preferable sweet spot for shale gas exploration.

Experimental Study on the Development Characteristics and Controlling Factors of Microscopic Organic Matter Pore and Fracture System in Shale

Research on microscopic pore and fracture system of shale is a hot spot in the field of unconventional petroleum geology. Micro- and nano-scale organic matter pores in shale play a vital role in the accumulation of hydrocarbons. Research on the types, evolution rules and controlling factors of organic matter pore-fracture system in shale reservoirs can provide scientific guidance for the prediction of shale “sweet spots”. In this paper, taking the shales from the Napo Formation, Oriente Basin, the Shahejie Formation, Zhanhua Sag, Bohai Bay Basin, and the Longmaxi and Wufeng Formations, Sichuan Basin as an example, the developmental characteristics of organic matter pore-fracture system were systematically studied using thin section, argon ion profiling scanning electron microscopy, X-ray diffraction, N2 adsorption and desorption, geochemistry experiments, and image processing technology. The types of shale organic matter pores were divided into kerogen-hosted pores, organic matter microfractures (intra-organic matter and organic matter edge microfractures), and asphalt pores (or intra-asphalt pores). The circumferences of organic pore were generally within 100 nm, and the areas of most pores were smaller than 1,000 nm2. Face rates of the organic pores were generally less than 1.5%, and the proportion of shale samples with a shape factor of 1 reached more than 50%. In addition, the deviation angles of organic matter pores at (0°, 45°) reached 90%, which showed that most of the organic matter pores tended to be oriented pores. The increase in the degree of thermal evolution provided driving force for the formation of circular pores in the organic matter. Internal factors (abundance of organic matter, kerogen types, and maturity) and external factors (diagenesis and mineral composition) controled the development of shale organic matter pores. Maturity, TOC content and inorganic minerals such as clay and pyrite content were positively correlated with the development of organic matter pores. However, brittle minerals caused a decrease in the face ratio of organic matter pores. Diagenetic compaction caused the organic matter pores being deformed or eventually disappeared. This research can provide scientific guidance for the high-efficiency exploration of hydrocarbons in shale.

Difference in pore structure characteristics between condensate and dry shale gas reservoirs: Insights from the pore contribution of different matrix components

A breakthrough in shale gas exploration was made in the lacustrine shale of Qianfoya Formation located in the Sichuan Basin; a gas analysis has shown that the reservoir therein is a condensate gas reservoir. To enhance the production of shale gas in the Sichuan Basin, reduce the exploration risk, and facilitate development, the marine dry gas shale of the Longmaxi Formation was selected as a comparison to analyze the pore structure characteristics of condensate and dry gas reservoirs. The results showed that the dry gas shale reservoir is mainly developed as organic matter (OM) pores, followed by clay mineral (CM) pores. In condensate gas shale reservoirs, the OM pores, CM pores, intragranular dissolution (ID) pores, and microfractures are common. In the dry gas shale reservoir, the surface porosities of the organic matter, clay minerals, brittle minerals, and microfractures were 10%–65%, 0%–23%, 0%–22%, and 0%–13%, respectively. Micropores were mainly provided by OM pores; mesopores were mainly provided by OM and CM pores; macropores were mainly provided by OM, CM, and ID pores. In the condensate gas shale reservoir, the surface porosities of the organic matter, clay minerals, brittle minerals, and microfractures were 0%–52%, 6%–34%, 0%–18%, and 0%–21%, respectively. Micro-mesopores were mainly provided by OM and CM pores, while macropores were mainly provided by CM pores and microfractures. Because of the influences of material composition, diagenetic evolution, and hydrocarbon generation at the burial depth, there were significant differences in the surface porosities of the matrix components and the distribution and proportions of the different pores between the dry and condensate gas shale reservoirs. This essentially led to a difference in the reservoir space.

The organic geochemistry, pore structure and methane adsorption/storage capacity of lacustrine shales from the Cretaceous Madongshan Formation, Liupanshan Basin, China

Lacustrine shale from Madongshan Formation in Liupanshan basin has been the important target strata for shale gas exploration in northwest China. However, the system cognition in generation, reservoir conditions and geological gas-in-place (GIP) estimation of Madongshan shale is quite limited. In this research, the shale strata are evaluated in terms of shale quality, pore structure and methane adsorption/storage capacity. The results show that Madongshan shales mainly belong to mixed mudstone and siliceous mudstone and have the relatively high brittle minerals content and low clay minerals contents, indicating a favorable condition for hydraulic fracturing. The Rock-Eval pyrolysis reveals that the shales can be classified as good to very good to source rock with relatively high TOC over 2% and contain type Ⅰ and Ⅱ kerogen in the early maturation stage with a fixed amount of gas can be produced. In terms of pore structure, the Type IV isotherms and Type H3 hysteresis loops of shale samples show that mesopores and macropores with slit-like pore types are the dominated space in Madongshan shales. The maxima of methane adsorption capacity (n0) ranged from 2.32 ml/g to 5.96 ml/g are significantly by organic matters and clays. Despite the relative importance of organic carbon to methane adsorption capacity, influence of TOC on pore surface area and volume is negative interaction, perhaps reflecting the dissolution of methane in the residual bitumen. Thus, an assessment on lacustrine shale gas with low maturity needs to consider the influences of solute methane in residual bitumen, apart from adsorption methane, so as to obtain a reliable shale gas resource. The clays show a complex relationship of influence on n0 and it may be related to the pore structure and the difference of methane capacity between clays and organic matters. At last, a modified and simpler equation which can eliminate the errors is adopted to calculate the methane storage capacity of Madongshan shales. And the methane storage capacities of Madongshan shales range from 2.06 ml/g to 2.16 ml/g.

Discovery of shale oil in alkaline lacustrine basins: The Late Paleozoic Fengcheng Formation, Mahu Sag, Junggar Basin, China

A new type of shale oil in alkaline lacustrine sediments has been discovered in the Late Paleozoic Fengcheng Formation, Mahu Sag, Junggar Basin, China. The fine-grained sedimentary rocks deposited in this alkaline lacustrine environment can be divided into four types and eight sub-types: mudstone (with no alkali minerals), including massive dolomitic mudstone, and massive and laminated calcareous mudstone; dolomite, including massive argillaceous dolomite (with alkali minerals), and massive and laminated argillaceous dolomite (with no alkali minerals); evaporites; and pyroclastic rocks. The massive argillaceous dolomite (with alkali minerals) and pyroclastic rocks have the highest shale oil potential, with average oil saturation index (OSI) values of 344.67 and 124.65 mg HC/g TOC, respectively. Shale oil exploration in the representative well MY1 indicates that the Fengcheng Formation is thick and contains abundant natural fractures, brittle minerals, and mobile oil. The entire Fengcheng Formation is oil-bearing and contains three concentrated stratigraphic intervals of shale oil (i.e., sweet spots). Well MY1 indicates that, compared with source rocks developed in marine and sulfate-type saline basins, the fine-grained sedimentary rocks deposited in alkaline lacustrine environments can also have high shale oil potential. The co-existence and regular distribution of conventional and unconventional reservoirs in the Fengcheng Formation indicate that it is an ideal exploration target for multiple resource types.

Mechanical characteristics and influencing factors of shallow igneous rocks in Junggar Basin

The mechanical properties of rocks play a pivotal role in hydraulic fracture propagation. In this study, mineral composition experiments, uniaxial and triaxial tests, shear tests, and fracture toughness tests were performed to analyze the rock mechanics characteristics of igneous rocks and their relationship with confining pressure, mineral composition, lithology, and lithofacies. The results show that the confining pressure has a significant influence on the elastic modulus and compressive strength of the igneous rock, but an insignificant effect on Poisson’s ratio. The contents of clay minerals and brittle minerals significantly affect the mechanical properties. However, the lithology and lithofacies do not have an evident influence on the rock mechanical properties. In this study, a dimensionless elastic modulus, namely, the ratio of the elastic modulus to the confining pressure is proposed. The average dimensionless elastic modulus under different confining pressures exhibited a good linear correlation with the confining pressure. This parameter can be used to predict the elastic modulus. The results of this study can provide a basis for the hydraulic fracturing design of shallow igneous rock formations in the Junggar Basin in China.

Differences of Main Enrichment Factors of S1l11-1 Sublayer Shale Gas in Southern Sichuan Basin

In this study, shale cores from 20 wells in the S1l11-1 sublayer of Longmaxi Formation buried in shallow shale (&lt;3500 m) and deep shale (&gt;3500 m) in the southern Sichuan Basin, China were collected to compare their pore structures and gas-bearing properties using multiple experiments. Results showed that the deep layer has relatively lower brittle mineral content, which is disadvantageous in terms of the higher requirements it imposes on hydraulic fracturing. Results also showed that the most important factor controlling the differential enrichment of S1l11-1 shale gas in southern Sichuan Basin is porosity. Moreover, the porosity composition of shallow shale and deep shale has significant differences: the porosity of shallow shale is dominated by organic pores, while for deep shale, both organic and inorganic pores are important. The inorganic pores provide significant storage space for free gas in deep shale; their contribution warrants more attention. We also found that the difference in organic porosity of the shallow and deep shale samples resulted from large differences in pore development ability, while the highest inorganic porosity was concentrated near the optimal mineral composition when the content of quartz plus feldspar plus pyrite was about 70%. This study revealed the primary factor controlling the difference in gas content between shallow and deep shale and detailed the characteristics of microscopic pore structure, providing a basis for the exploration and development of deep shale gas in the Wufeng-Longmaxi Formation in the southern Sichuan Basin.

Experimental investigation of water sensitivity effects on microscale mechanical behavior of shale

Drilling and multi-stage hydraulic fracturing bring a large amount of water into clay-bearing shale reservoirs, which may lead to attenuation of fracturing effects and wellbore instability. However, a thorough understanding of changes in shale micromechanics and corresponding mechanisms when exposed to water remains unclear. In this work, samples were selected based on clay enrichment. Subsequently, the contact resonance (CR) technique based on atomic force microscopy (AFM) was performed to characterize the micromechanics of shales after exposure to water. Visual phenomena provided by environmental scanning electron microscopy (ESEM) assisted to explain the underlying mechanisms. It was found that the hydration effect lowered both the storage modulus and stiffness of samples, but with different contributions from brittle minerals and clay, as well as variations depending on bedding plane orientation. Due to the difference in composition, the investigated terrestrial shale material exhibited stronger water sensitivity and anisotropy, with a general decrease of 15%–25% in storage modulus, compared with variations of -5%–15% in modulus of marine shale samples. Moreover, microscopic observation experiments revealed that the reduction of capillary force and the interlaminar swelling of clay particles might have been active after water adsorption, which led to the decreases of modulus and stiffness values. However, the swelling-caused confining effect or void space closure during the water imbibition process might have offset this weakening effect and even increased storage modulus. At mesoscale , excessive shrinkage caused the growth of micro-cracks, which in turn significantly attenuated overall mechanical properties. • Comparison of clay composition between terrestrial and marine shale based on clay enrichment. • AFM contact resonance method to characterize the micromechanical properties of shale before and after water adsorption. • Investigation of mechanisms of microscale mechanical changes in hydration and dehydration processes.

Sawtooth patterns in flexural force curves of structural biological materials are not signatures of toughness enhancement: Part II

Stiff biological materials (SBMs), such as nacre and bone, are composites that display remarkable toughness enhancements over their primary constituents, which are brittle minerals. These enhancements are thought to be a consequence of different mechanisms made possible by the SBMs’ internal lamellar architecture. One such mechanism is the Cook–Gordon (crack-arrest-and-reinitiation) mechanism, whose operation manifests in flexural tests as a sawtooth pattern in the force–displacement curves. The curves from flexural tests carried out on marine sponge spicules, which also possess a lamellar architecture, also display a sawtooth-pattern, suggesting the presence of the Cook–Gordon mechanism. Intriguingly, the spicules were recently found not to display any significant toughness enhancement. To resolve this apparent contradiction, in the preceding paper (Kochiyama et al., 2021), we put forward the hypothesis that the sawtooth pattern was due to the spicules slipping at the tests’ supports. In this paper, we present a model for the spicule’s flexural tests in which we allow for the possibility for the specimen to slip at the test’s supports. We model contact between the specimen and the test’s supports using the Coulomb’s friction law. By choosing experimentally reasonable values for the friction coefficient, we were able to get the model’s predictions to match experimental measurements remarkably well. Additionally, on incorporating the spicules’ surface roughness into the model, which we did by varying the friction coefficient along the spicule’s length, its predictions can also be made to match the measured sawtooth patterns. We find that the sawtooth patterns in the model are due to slip type instabilities, which further reinforces the hypothesis put forward in our preceding paper.

Shale Gas Exploration Potential in Southwestern Shandong Province of Eastern China

The shale layers of the Permian Shanxi Formation and Taiyuan Formation in the southwestern part of Shandong Province are marine-continental transitional sedimentary facies. When compared with the Ordos Basin, which was a typical breakthrough in marine-continental transitional shale gas exploration, the geological characteristics, exploration, and development prospects of marine-continental transitional shale gas in the southwestern section of Shandong Province are more defined. The shale gas deposits of the Shanxi and Taiyuan Formations in southwestern Shandong Province have the following geological characteristics. The sedimentary environments of Shanxi and Taiyuan Formations are considered to be stable and characterized with widely distributed organic-rich shale, which can be used as composite evaluation layers. The total thickness of the shale range is between 140 and 350 m, with an average of 230 m. The main types of organic matter in the Shanxi Formation are Type Ⅲ and Type Ⅱ2 kerogen, while the main types of organic matter in the Taiyuan Formation are rock types. The abundance and maturity of the organic matter are high, and the gas generating capacity is strong. The reservoir rock mineral composition is complex in the region. The clay mineral content is relatively high and the brittle mineral content is rich. Also, the shale fracturing ability is good. The region has the characteristics of a low porosity and low permeability reservoir, with relatively good reservoir capacity. Three primary points of interest have stably been developed within the region, which are located in the upper high water level system, near the initial flood surface, and the majority of flood surface, including a transgressive system tract and high level system tract. The total gas content is estimated to range between 0.03 and 4.47 m3/t, with an average of 0.30 m3/t. The shale gas resources are rich, including one favorable Class I location with an area of 819.06 km2 and a resource amount of 985.84 × 108 m3; four favorable Class Ⅱ locations with an area of 1,979.68 km2 and a resource amount of 2,278.14 × 108 m3; and five prospective locations with an area of 8,385.52 km2 and a resource amount of 7,299.48 × 108 m3. Therefore, the region was considered to have major exploration potential.

Salinized lacustrine organic-rich shale influenced by marine incursions: Algal-microbial community, paleoenvironment and shale oil potential in the Paleogene Biyang Depression, East China

Salinized lacustrine organic-rich shale (SLORS) in the Paleogene has been considered as one of the most important shale oil reservoirs in China. Although deposition of the Paleogene SLORS in East China is generally concomitant with marine incursions, the influence of marine incursions on the algal-microbial community, paleoenvironment and shale oil potential of SLORS is still controversial. Here we examine this issue using a case of the Paleogene Biyang Depression (Nanxiang Basin) in East China, based on the integration of bulk geochemical, mineralogical, organic petrographic, biomarkers, elemental geochemical, and shale reservoirs analyses. Marine incursions were occurred during deposition of the SLORS in the Paleogene Biyang Depression as evidenced by the presence of marine organisms and marine-derived biomarkers (24-n-propylcholestanes). Lake transgressions, global sea-level rise, and extensional tectonic setting were responsible for the occurrence of marine incursions. The algal-microbial community change is associated with marine incursions, from dominance of green algae with a weak influence of marine incursions to dominance of red (brown) algae and coccolithophores with a strong influence of marine incursions, accompanied by enhanced bacterial activity. With the stronger influence of marine incursions, higher salinity, more reducing conditions, higher productivity, and lower detrital influx were present in the paleo-lake, resulting in the deposition of SLORS with higher shale oil potential, which was characterized by higher free shale oil contents, more abundant pores and fractures in nano- to micro-meter scale, and higher brittle mineral contents. In summary, our data suggest that marine incursions have a positive influence on the development of biodiversity in paleo-lakes, and the deposition of SLORS with high shale oil potential. Therefore, the influence of marine incursions should be fully considered during the palaeoecological reconstruction and the shale oil exploration of SLORS, especially during high sea-level stages. In addition, the results provide guidance for future profitable shale oil exploration in East China and other similar areas.

Jurassic Terrestrial Shale Gas Potential of the Northern Kashi Sag in the Tarim Basin, Northwestern China

Evaluation of terrestrial shale gas resource potential is a hot issue in unconventional oil and gas exploration. Organic-rich shales are widely developed in the Jurassic strata of Tarim Basin, but their shale gas potential has not been described well. In the study, the Lower-Middle Jurassic fine-grained sedimentary rocks (Kangsu and Yangye Formations) in northern Kashi Sag, northwestern Tarim Basin, were taken as the study object. The comprehensive studies include petrology, mineralogy, organic geochemistry, and physical properties, which were used to characterize the organic matter and reservoir characteristics. Results show that the Jurassic terrestrial shale in the northern Kashi Sag was mainly deposited in lakes, rivers, and deltas. The thickness of black lacustrine shale developed in the Early-Middle Jurassic in the study area is generally over 100 m. The total organic carbon (TOC) content is rich, averaging 2.77%. The vitrinite reflectance ( R o ) values indicate that the Lower Jurassic shale organic matter is in the early mature–mature stage, while the Middle Jurassic is in the mature stage. Besides, organic matter is primarily II and III in kerogen types. The whole shale contains a large number of clay minerals, especially illite. The average brittle minerals such as quartz and feldspar are 28.67%, and the average brittleness index is 38.63%. Nanoscale pores containing intergranular pores, dissolution pores, and organic pores, coupled with microcracks, are well developed in Jurassic shale. The sample’s average pore volume is 0.017 cm3/g, and the specific surface area is 9.36 m2/g. Mesoporous contribute the most to pore volume, while the number of microporous is the largest. Both of them provide most of the surface area for the shale. Combined with regional geologic settings, we propose that the Jurassic terrestrial shale has good-excellent shale gas exploration potential and development prospects.

Shale Gas Reservoir Evaluation by Geophysical Measurements: A Case Study of the Upper Ordovician–Lower Silurian in the Fenggang Block, Northern Guizhou Province

AbstractWith the aid of geophysical measurements, including seventeen two‐dimensional (2D) seismic lines and the well logging curves of well FGY1, the structure and reservoir characteristics of the Upper Ordovician–Lower Silurian strata in the Fenggang block, northern Guizhou Province, were analyzed thoroughly to identify desert areas and favorable intervals. The results show that Longmaxi‐Wufeng is the most prospect‐rich formation, consisting of a thick succession of overmature black shale, this formation remaining partially in the Suiyang, Fenggang and Jianchaxi synclines. The Longmaxi‐Wufeng shale, especially the lower member, was deposited in a reducing low‐energy environment with relatively high U content and a low Th/U value. In this shale, the organic matter type (sapropelic and humic‐sapropelic), total organic carbon (TOC) content, gas content, gas adsorption capacity, vitrinite reflectance and brittle mineral content are profitable for shale gas preservation and development. The fractures of this shale were closed because of its high overburden pressure. The gas adsorption capacity of this shale increases with increasing TOC content and Ro. In the Longmaxi‐Wufeng Formation at well FGY1, the most favorable intervals are in the depth ranges of 2312.4–2325.1 m and 2325.8–2331.1 m.

Diagenetic evolution sequence and pore evolution model of Mesoproterozoic Xiamaling organic-rich shale in Zhangjiakou, Hebei, based on pyrolysis simulation experiments

Organic-rich shale is an unconventional and complex petroliferous system that integrates a “source-reservoir-cap”, and the coupled evolution of hydrocarbon generation, diagenesis and nanoscale pores is the key problem that affects shale gas accumulation. Current research methods for this issue mainly include direct observation and physical simulation. Direct observation ignores the heterogeneity and regional differences of the natural samples, and physical simulation mostly lacks an intuitive characterization and cannot clearly show the relationship between minerals and pore evolution characteristics in the same region. To avoid the effect of heterogeneity on the researched samples and clearly and intuitively reveal the coupled evolutionary relationship among the hydrocarbon generation of thermal maturation, diagenesis and nanopore structures in shale, this research included hydrous pyrolysis experiments on low mature marine shale samples to achieve various maturities and diagenesis stages. The pyrolysis products at each stage were recovered and subjected to an ongoing multidisciplinary analytical program. The results show that an increased temperature intensifies the evolution of dissolution pores in unstable brittle minerals, promotes clay mineral conversion, and accelerates the development of clay mineral pores and organic pores. The pore volume (PV) of the nanoscale pores reached its minimum at 300 °C, increased to its maximum at 500 °C and decreased thereafter. The surface area (SA) of the nanoscale pores reached their minimum at 300 °C and then continuously increased, while those of macropores reached their maximum at 500 °C and then decreased. The diagenesis of the pyrolysis products were mainly dissolution, clay mineral transformation, the thermal maturation of organic matter (OM), compaction and cement filling. The diagenetic evolution was divided into four stages, and the diagenetic evolution sequence and pore evolution model of the shale were established. Hydrocarbon generation, diagenesis and nanoscale pore evolution have synergistic effects, which have great significance for shale reservoir evaluation and shale gas accumulation, exploration and exploitation. The conceptual evolution model provides a quantitative prediction method for hydrocarbon generation, diagenesis and nanoscale pore variations.