High Clay Mineral Content Research Articles

Relationship between Organic Geochemistry and Reservoir Characteristics of the Wufeng-Longmaxi Formation Shale in Southeastern Chongqing, SW China

Shale gas accumulates in reservoirs that have favorable characteristics and associated organic geochemistry. The Wufeng-Longmaxi formation of Well Yucan-6 in Southeast Chongqing, SW China was used as a representative example to analyze the organic geochemical and reservoir characteristics of various shale intervals. Total organic carbon (TOC), vitrinite reflectance (Ro), rock pyrolysis, scanning electron microscopy (SEM), and nitrogen adsorption analyses were conducted, and a vertical coupling variation law was established. Results showed the following: the Wufeng-Longmaxi formation shale contains kerogen types I and II2; the average TOC value at the bottom of the formation is 3.04% (and the average value overall is 0.78%); the average Ro value is 1.94%; the organic matter is in a post mature thermal evolutionary stage; the shale minerals are mainly quartz and clay; and the pores are mainly intergranular, intragranular dissolved pores, organic matter pores and micro fractures. In addition, the average specific surface area (BET) of the shale is 5.171 m2/g; micropores account for 4.46% of the total volume; the specific surface area reaches 14.6%; and mesopores and macropores are the main pore spaces. There is a positive correlation between TOC and the quartz content of Wufeng-Longmaxi shale, and porosity is positively correlated with the clay mineral content. It is known that organic pores and the specific area develop more favorably when the clay mineral content is higher because the adsorption capacity is enhanced. In addition, as shale with a high clay mineral content and high TOC content promotes the formation of a large number of nanopores, it has a strong adsorption capacity. Therefore, the most favorable interval for shale gas exploration and development in this well is the shale that has a high TOC content, high clay mineral content, and a suitable quartz content. The findings of this study can help to better identify shale reservoirs and predict the sweet point in shale gas exploration and development.

Effect of magnetization pretreatment of coal slime water flocculating sedimentation

ABSTRACT Sedimentation of coal slime water (CSW) is an important process in coal processing plants. However, CSW is difficult to settle because of its high content of clay minerals and fine particles. In this paper, the effect of magnetization pretreatment on CSW flocculating sedimentation is investigated. The mechanisms of magnetization pretreatment were studied by measuring CSW’s viscosity and zeta potential and calculating particle’s hydration film thickness using Einstein’s viscosity equation. The results show that the CSW flocculating sedimentation is enhanced by magnetization pretreatment. The optimum dosage of polyacrylamide was 6.0 mg/L and the best magnetic field strength and magnetization time was 0.3 T and 3 min, respectively. As the settling velocity was increased, the turbidity of the supernatant was reduced by magnetization pretreatment. Moreover, after magnetization pretreatment, the absolute value of the zeta potential and the viscosity were reduced, and the hydrated film thickness of coal particles was decreased from 305 nm to 155 nm. Magnetization pretreatment decreased the settling resistance and reduced the electrostatic repulsion and hydration forces between coal particles, thus promoting CSW flocculating sedimentation. It is demonstrated that magnetization pretreatment introduces a new path improve CSW settlement.

Characteristics and Origin of Methane Adsorption Capacity of Marine, Transitional, and Lacustrine Shales in Sichuan Basin, China

Adsorbed gas is an important component of shale gas. The methane adsorption capacity of shale determines the composition of shale gas. In this study, the methane adsorption capacity of marine, transitional, and lacustrine shales in the Sichuan Basin was analyzed through its isothermal adsorption, mineral composition, water content, etc. The results show that the methane adsorption capacity of marine (Qiongzhusi Formation and Longmaxi Formation), transitional (Longtan Formation), and lacustrine (Xujiahe Formation and Ziliujing Formation) shales is significantly different. The Longtan Formation has the strongest methane adsorption capacity. This is primarily related to its high organic matter and organic matter type III content. The methane adsorption capacity of the lacustrine shale was the weakest. This is primarily related to the low thermal evolution degree and the high content of water-bearing clay minerals. Smectite has the highest methane adsorption capacity of the clay minerals, due to its crystal structure. The water content has a significant effect on methane adsorption largely because water molecules occupy the adsorption site. Additionally, the temperature and pressure in a specific range significantly affect methane adsorption capacity.

Kaolinite‐based clay ceramics blended with residual fique fibers for potential plastic soil applications

AbstractThe high content of clay minerals within the soil has a significant effect on its engineering behavior; water affects the interaction of their minerals, which influences their plasticity and deformability. Structures made with or on plastic materials can experience problems due to the high deformations presented in soil. Therefore, decreasing the deformability of plastic soils contributes to the stability and serviceability of the structures supported by this kind of soils. This investigation is about the effect of fique fibers contents in the mechanical behavior of a plastic soil based on a kaolinite‐type clay. Fique fiber is natural fiber from Colombia. Soil with additions of 0.0 wt%, 0.2 wt%, 0.7 wt%, and 1.0 wt% of fique fibers was fabricated. The goals by adding the fibers are to increase the compressive strength of the soil and to add a green reinforcement to the soil targeting a circular economy solution. Density, optimal water content, and unconfined compression tests were conducted. The microstructure was characterized with scanning electron microscopy and X‐ray diffraction. Results showed that fique fibers improved not only the compressive strength by almost three times for sample with 1.0 wt% of fibers with respect to the reference soil without fibers but also the ductility increased.

Challenges of terrestrial shale gas exploration and development from Chang 7 shale in the Ordos Basin

There has been great success of marine shale gas exploration and production from Lower Silurian Longmaxi Formation (called Longmaxi shale) in Sichuan Basin, China, while whether commercial gas production could be obtained from lacustrine shales is still a question. The 7th Member of Triassic Yanchang Formation (called Chang 7 shale), which is the exploration target for the only national lacustrine shales gas pilot, was investigated to identify the key issues on the accumulation and development of terrestrial shale gas accumulation. It is considered that there are many differences in terrestrial and marine shales including depositional environment and provenance, distribution characteristics, organic matter types, mineral composition, and pore types. So evaluation standards of the two shales should not be the same. The Longmaxi shale was deposited in deep shelf, widely distributed, stable, mature–overmature (in gas window), presence of plenty micro-pores, brittle, and overpressured, which make it a good condition for shale gas accumulation. The key factors that affect the production of the Longmaxi shale concluded that the shale is rich in organic matter, high thermal evolution, brittle, and overpressured. The Chang 7 shale in Ordos Basin was deposited in deep lake, relative limited distribution, in low stage of thermal evolution (in oil window), high clay mineral content, and conducive to shale gas adsorption. On the basis of comparing the geological features of the two shales, it is found that the shale is richer in the organic matter content than in the Longmaxi shale, while the shale is not mature enough to produce more gas, and the strata pressure is relatively lower which means the shale gas was not preserved very well either. So through this study, it is believed that the shale gas formation condition in the Chang 7 shale may not be as good as in the Longmaxi shale, the absorbed gas may be the future of this shale, and the shale oil should be the focus of petroleum exploration and development in this region.

Adsorption model identification for chromium (VI) transport in unconsolidated sediments

Transport of hexavalent chromium, Cr (VI), in groundwater is largely influenced by adsorption and desorption processes. Sediment types can result in substantial difference in adsorption mechanisms. The goal of this work is to explore the adsorption mechanism of Cr (VI) within sediments with different properties and to identify the controlling factors. In this paper, Cr (VI) adsorption onto six different natural unconsolidated sediments (two loamy sands, two sandy loams, one loam, and one silty loam clay) collected from a typical fluvial aquifer in Qiqihar, China, was assessed with batch and column experiments. Equilibrium adsorption capacities and kinetic rates were quantified with batch experiments. Potential controlling effects of pH, solid/solution ratio, grain size, and clay mineral content on adsorption capacities were also analyzed. Adsorption kinetics was evaluated with Elovich, pseudo first- and second-order models, and equilibrium properties were assessed with Henry, Freundlich, and Langmuir isotherms. Model adequacy was discriminated with selection criteria of Akaike information criterion (AIC), modified Akaike information criterion (AICc), Bayesian information criterion (BIC), and Hannan information criterion (HIC). Adsorption and desorption of Cr (VI) onto and from the collected sediment samples under flow conditions were investigated with column experiments. Key results suggest that irreversible chemical adsorption is the dominant mechanism for Cr (VI) adsorption onto the sediment samples. Grain sizes and clay mineral contents are the controlling factors for adsorption capacity. Fine-grained sediments with high clay mineral contents present the highest Cr (VI) adsorption capacity. Chromium (VI) presents high retention on loam and silty loam clay, and high mobility on loamy sand. Results from this study may provide important insight for understanding the transport behaviors of Cr (VI) in shallow aquifers.

Characterization of micropore structure of a middle to deep clastic reservoir: The oligocene reservoir in the Nanpu Sag, Bohai Bay Basin, China

In this paper, the Oligocene reservoirs in the Nanpu Sag of the Bohai Bay Basin are studied by scanning electron microscopy (SEM), thin section observation, XRD, nuclear magnetic resonance (NMR), and mercury injection. The purpose is to judge the microscopic pore structure of the reservoir in the study area through: 1) Combining nuclear magnetic resonance technology with fractal dimension to analyze the pore structure characteristics of sandstone; 2) using mercury injection experiments to judge the connectivity of pore throats; and 3) combining petrophysical properties-NMR-fractal dimension-mercury injection to predict reservoir quality. The results show that the sandstones of the Ed3 and Es1 members in the Nanpu Sag of the Bohai Bay Basin have good physical properties, medium porosity (5.2%–18.7%, average 11.70%) and permeability (0.08mD-111mD, average 22.68mD). Using the T2 spectrum, the fractal dimension of the bound fluid pore and the fractal dimension of the movable fluid pore are determined. The movable fluid pore is more complicated than the bound fluid pore. There is a connection between fractal dimension and mineral composition. High clay mineral content leads to a complex pore network, and the larger the fractal dimension, the more complex the pore shape and the lower the permeability.

Study on the Shale Gas Reservoir-Forming Characteristics of the Taiyuan Formation in the Eastern Qinshui Basin, China.

Shales are widely developed in the strata of the Carboniferous-Permian coal measures in the Qinshui Basin, and these shales have great potential for shale gas exploration. In this paper, the shales of the Taiyuan Formation in the eastern Qinshui Basin are studied. The shales of the Taiyuan Formation in the study area are investigated through field investigation, organic geochemical testing, X-ray diffraction, scanning electron microscopy, high pressure mercury injection, low temperature liquid nitrogen adsorption and PetroMod simulation and through other tests to study the reservoir characteristics, such as organic geochemistry, mineralogy, petrology, pore permeability, and gas burial history. The results show that the shales of the Taiyuan Formation are well developed over the whole area with a thickness of more than 60 m. The average organic matter content is 2.95%, and the kerogen type is type III. The shale maturity (average value is 2.45%) corresponds to the stage of high maturity evolution, indicating that a large amount of shale gas has been generated in this area. A high content of quartz and clay minerals indicates a high fracturability. The nanopores in the shale reservoir are well developed at pore sizes between 2˜10 nm and greater than 1000 nm; however, the pores at the other pore sizes are poorly developed, resulting in weak pore connectivity in the reservoir. According to the results of the PetroMod simulation, the shale of the Taiyuan Formation has undergone two subsidence and two uplift processes. The Yanshanian magmatic intrusion is the key factor for the rapid increase in gas production. In addition, the geological structure of the area is relatively simple, and the burial history and caprock thickness are also the main controlling factors of gas generation and preservation. The shale-sandstone-shale combination and shale-coal-shale combination are the main models of shale gas preservation. This comprehensive study suggests that the shale gas of the Taiyuan Formation in the eastern Qinshui Bain has good potential for exploration and development.

Synergistic Effects of Micronano Structures on Porosity and the Permeability of Shale Under Varying Effective Stresses and Temperatures: A Case Study of Fresh Outcrops from Lower Silurian Longmaxi Formation Shale in the Southern Sichuan Basin, China.

To study the changes in porosity-permeability and the characteristics of the pore structure of shale under stress and high temperature, the Lower Silurian Longmaxi Formation shale in the southern Sichuan Basin, China, was investigated under conditions of continuous pressurization and heating. In addition, the pore compression coefficients and permeability stress sensitivity coefficients were analyzed and quantified. The mineral composition of these black shales was analyzed using scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). Our results reveal that the porosity and permeability of the shales exponentially decreases with increasing stress, reflecting that microfracture development during increasing stress causes the pores in these shale samples to enlarge. However, the pore compressibility and stress sensitivity coefficient vary for each sample, and the quantitative results indicate an overall decrease with increasing stress, suggesting that the shale deformation is nonlinear during stress release. Based on the mineral composition analyses and SEM measurements, we conclude that the nonuniform changes during stress release are related to the complexity of the shale mineral compositions and the different intercrystalline/ interlaminar pores of the different minerals, which are affected in different ways by pressure. The NMR measurements reveal that the mesopores are most developed in the shale samples, the pore volumes of the micro- and macropores are small, and the nanoscale pores are mainly from 1-60 nm in diameter. The different types of pore sizes decrease with increasing stress, indicating that the porosities measured experimentally reflect the synergistic effects of the different pore sizes on porosity. As the temperature increases, the permeability of the shale decreases significantly, which is primarily caused by the thermal expansion effect. The high clay mineral content of the shales also causes their permeabilities to be sensitive to temperature.

Study on Acidification and Injection Technology of Clean Acid for Medium and Low Permeability Heavy Oil Reservoirs

The Shasi 6 sand formation reservoir in Mian 14 area of Bamianhe Oilfield is of high porosity and medium-low permeability, high clay mineral content and strong water sensitivity. In the process of water injection, there are some problems such as high water pressure, increasing under-injection wells, low water well utilization rate and poor conventional acidizing effect. In order to improve the qualification rate of water in this block,a clean retarding acid & anti-swelling and increasing injection system was developed.The comprehensive properties of the system, such as retarding, anti-secondary precipitation and long-term anti-swelling, were evaluated indoors.The experimental results show that the system has the characteristics of long distance and low damage in deep acidification. It has been successfully applied 7 well times in Shasi 6 sand group, and the qualification rate of is 100% with remarkable application effect.

Gas hydrate in-situ formation and dissociation in clayey-silt sediments: An investigation by low-field NMR

The hydrate reservoir in the Shenhu Area of the South China Sea is a typical clayey-silt porous media with high clay mineral content and poor cementation, in which gas hydrate formation and dissociation characteristics are unclear. In this study, the CO2hydrate saturation, growth rate, and permeability were studied in sandstone, artificial samples, and clayey-silt sediments using a custom-built measurement apparatus based on the low-field NMR technique. Results show that the T2spectra amplitudes decrease with the hydrate formation and increase with the dissociation process. For the artificial samples and Shenhu sediments, the CO2hydrate occupies larger pores first and the homogeneity of the sandstone pores becomes poor. Meanwhile, compared with the clayey-silt sediments, CO2hydrate is easier to form and with higher hydrate saturation for the sandstone and artificial samples. In hydrate dissociation process, there exists a protection mechanism, i.e. the dissociation near the center of hydrates grain is suppressed when gas pressure drops suddenly and quickly. For permeability of those samples, it decreased with hydrate forms, and increases with hydrate dissociation. Meanwhile, with the same hydrate saturation, permeability is higher in hydrate formation than in dissociation.

Petrography and geochemical characteristics of Chang 7 sandstone in Yanhe profile, Ordos Basin: Implication for paleoenvironment and tectonic background

The Chang 7 oil layer from the upper Triassic Yanchang Formation is an important layer for hydrocarbon exploration. Most studies on the Chang 7 oil layer have focused on the source rocks, while research on the sandstone is still inadequate, especially on the petrography and geochemical characteristics. Using seven sandstone samples of the Chang 7 oil layer in the Yanhe profile, the grain-size analysis, major elements, trace elements, and rare earth elements were tested. The results find that the sandstone of fine-grained sediments of the Chang 7 oil layer is dominated by arkose with a minor number of lithic arkose. The range of grain size (Mz) is from 2.72 to 3.92 Φ, and the C value and M value of the sandstone samples suggest characteristics of turbidity deposition. The Al/Si ratios of all of the samples imply high clay mineral content. The results of trace and rare earth elements demonstrate the reducing condition, freshwater, and cold and dry weather. The provenance of the sandstone samples is mainly from island arc acidic volcanic rock, and the type of provenance is mixed with sedimentary rock, granite, and alkaline basalt. The tectonic background is continental island arc. This study provides a systematic geologic foundation for the formation of sandstone of Chang 7 oil layer in Ordos Basin.

Experimental Study on the Self-Healing Behavior of Fractured Rocks Induced by Water-CO2-Rock Interactions in the Shendong Coalfield

This study experimentally investigated the self-healing behavior, referring to the naturally occurring water permeability decrease, of fractured rocks exposed to water-CO2-rock interaction (WCRI). The experiment was conducted on prefractured specimens of three rock types typical of the Shendong coalfield: coarse-grained sandrock, fine-grained sandrock, and sandy mudrock. During the experiment, which ran for nearly 15 months, all three specimens exhibited decreasing permeabilities. The coarse- and fine-grained sandrock specimens exhibited smooth decreases in permeability, with approximately parallel permeability time curves, whereas that of the sandy mudrock specimen decreased rapidly during the initial stage and slowly during later stages. The sandrock specimens were rich in feldspars, which were dissolved and/or corroded and involved in ionic exchange reactions with CO2 and groundwater, thereby generating secondary minerals (such as kaolinite, quartz, and sericite) or CaSO4 sediments. These derivative matters adhered to the fracture surface, thereby gradually repairing fractures and decreasing the water permeability of the fractured rocks. In comparison, the sandy mudrock had a high content of clay minerals, and the water-rock interaction caused rapid expansions of illite, mixed illite-smectite, and other clay minerals, thereby narrowing the fractures and causing the rapid permeability decrease during the initial stage. In later stages, the derivative matters generated by the dissolution and/or corrosion of feldspars and other aluminum silicate minerals in the mudrock filled and sealed the fractures, causing the slow permeability decreases during the later stages, as in the sandrock specimens. Neutral and basic groundwater conditions facilitated better self-healing of fractured mudrocks rich in clay minerals, whereas acidic groundwater conditions and the presence of CO2 facilitated better self-healing of fractured sandrocks. Thus, this study’s results are of significant value to aquifer restoration efforts in the Shendong coalfield and other ecologically vulnerable mining areas.

Study of structural properties of acid-treated natural sediment and its application as a sustainable catalyst

This paper aims to characterize a natural sediment from Bahía Blanca estuary (Argentina) as a new clay source and determine the catalytic capacity after its structural modification by an acid activation process. The acid treatment was performed by employing H2SO4 at different concentrations at 353 K and 60 min. The raw material and acid-activated samples were characterized by XRD, XRF, FTIR, N2 physisorption, TGA, SEM, EDX, potentiometric titration and pyridine adsorption monitored by FTIR. Moreover, the catalytic activity on lauric acid esterification with methanol was evaluated as a test reaction. From the structural characterization, it was determined that the material studied contains mainly Ca-montmorillonite, illite, albite and quartz. Likewise, the results obtained showed that the acid-treatment impacts the material characteristics, predominantly surface acidity and textural properties. Particularly, it was evidenced that the samples treated under medium acid activation conditions lead to an increase in acid site number and strength and specific surface area and consequently improve the catalytic activity. The results obtained evidenced that this low cost and widely available raw material presents an interesting mineralogical composition with a high clay mineral content. Additionally, the catalytic performance showed a good conversion in the test reaction regarding other heterogeneous catalysts. Hence, considering these results, this natural sediment merits further research on its purification and application.

Elimination of total coliforms and Escherichia coli from water by means of filtration with natural clays and silica sand in developing countries.

This research presents the results of a pilot system used for water treatment to be supplied to single-family or small housing nucleus at rural sector. The pilot system is constituted by an up-flow aerated filter with ceramic carrier, followed by a second down-flow filter of silica sand. The pilot plant was installed in a rural sector of the province of Loja, Ecuador, with the main purpose of monitoring the reduction in bacteria in the water for human consumption. Two natural clays from the area were tested. They were transformed into sphere-type ceramics, and then, they were placed in the pilot filters. These both natural clays are inorganic materials with high content of aluminosilicate clay minerals with favourable characteristics to achieve greater efficiency for the reduction in bacteria. The acclimatization time of 60days allowed to achieve optimum treatment efficiency performance until 90days of operation. The physicochemical parameters, namely temperature, pH, dissolved oxygen and total dissolved solids, were monitored throughout the experiments. Regarding microbiological parameters, up to 99% of total coliforms removal was reached for the clay called CF-CC and 79% for the other, called CF-CV. In addition, 100% of total coliforms and Escherichia Coli removal was reached for both clay systems. Social and economic viability of the proposed treatment is also analysed in the manuscript.

Pore development of the Lower Longmaxi shale in the southeastern Sichuan Basin and its adjacent areas: Insights from lithofacies identification and organic matter

X-ray diffraction (XRD), field emission/focusing ion beam scanning electron microscopy (FE-/FIB-SEM), nitrogen gas adsorption (N2GA), and geochemical analyses were performed on the Lower Longmaxi shale in the southeastern Sichuan Basin and its adjacent areas to reveal the controls of shale lithofacies and organic matter (OM) on pore development. The results showed that the Lower Longmaxi shale primarily comprises three types of lithofacies based on shale compositions and sedimentary environment: OM-rich siliceous shale (ORSS), OM-moderate mixed shale (OMMS) and OM-lean argillaceous shale (OLAS). ORSS comprises high contents of OM and siliceous minerals (quartz and feldspar), deposited in a deep-water shelf and early semi-deep-water shelf environment, while OLAS consists of high clay mineral and low OM contents, primarily formed in a shallow-water shelf environment. OMMS witness a transition from ORSS to OLAS in sedimentary environment and shale components. OM hosts a higher volume of pores per unit weight compared to inorganic minerals, corresponding to 0.1878 ml/g and 0.00396 ml/g, respectively. OM-hosted pores are dominant in ORSS, constituting 72%, whereas OLAS mainly consists of mineral-hosted pores, comprising 63.2%. OM-hosted and mineral-hosted pores in OMMS exist in similar proportions, namely, 56.4% and 43.6%, respectively. Differentiated diagenetic paths resulting from sedimentary environment are responsible for the differences in pore development under various shale lithofacies. The thermal maturity, macerals, and abundance of OM all play a significant role in the morphology and development of OM-hosted pores. OM-hosted pores are primarily founded in migrated OM (mainly sapropelic type), instead of structured OM (mainly humic type), in the form of secondary pores that are derived from dry gas-generation stage. Furthermore, OM-hosted pore development in shale gas reservoirs is slowed or even reduced because of the weakening of shale's resistance to compaction resulting from excessively high OM content.

High negative surface charge increases the acidification risk of purple soil in China

Purple soils, developing from rapid physical weathering of nutrient rich sedimentary rock and classified as Inceptisols or Entisols in United States Department of Agriculture (USDA) Taxonomy, are the most important agricultural soils in the Sichuan basin of southwestern China. A portion of purple soils has been acidified and the acidification risk of these soils need further understanding. Here, electrodialysis (ED) was used to simulate the process of natural acidification of soils. We investigated the acidification risk of purple soils by ED treatments and compared its acidification characteristics with that of variable charge soils (Oxisol and Alfisol). Our results showed that the degree of acidification of purple soils after ED treatments was much higher than that of variable charge soils, although the purple soils contained more exchangeable base cations. There was no significant correlation between the purple soil acidity before and after ED treatments, however, the pH of purple soils after the ED treatments had a significant negative correlation (r = −0.680, P < 0.01) with the soil cation exchange capacity (CEC). We concluded that CEC (i.e., effective negative surface charges) played a decisive role in determining the acidification potential of purple soils. High content of clay minerals and low content of Fe/Al oxides in the purple soils lead to the higher amount of effective negative surface charge than that of variable charge soils, which induced the rise in the amount of H+ and Al3+ absorbed on the surface of purple soils and increased the acidification degree of purple soils remarkably. Therefore, compared to the variable charge soils, the purple soils had a higher acidification risk. These findings will provide a useful reference for the understanding of the acidification characteristics of purple soils.

Enrichment and exploration of deep lacustrine shale oil in the first member of Cretaceous Qingshankou Formation, southern Songliao Basin, NE China

Pure shales in the first member of Qingshankou Formation (simplified as Qing 1 Member) in the southern Songliao Basin, i.e., the semi-deep and deep lacustrine shales, are characterized by a high content of clay minerals and poor hydrocarbon mobility, making the development of shale oil difficult. According to the drilling and testing results, the shale of Qing 1 Member can be classified into 3 lithofacies, i.e., bedded argillaceous shale, laminated diamictite shale, and interbedded felsic shale. The TOC and brittle minerals control the enrichment of shale oil, of them, TOC controls the total oil content, in other words, the total oil content increases with the increase of TOC; while the laminae made up of brittle minerals contain a large number of bigger intergranular pores which are favorable enrichment space for movable shale oil. In consideration of the origins of the 3 lithofacies, two shale oil enrichment models are classified, i.e., the deep lacustrine high-TOC bedded argillaceous shale (Model-I) and the semi-deep lacustrine moderate-high-TOC laminated diamictite shale (Model-II). In the Model-I, the shale is characterized by high hydrocarbon generation ability, high total oil content, abundant horizontal bedding fractures, and vertical and high angle fractures locally; the complex fracture network formed by horizontal bedding fractures and vertical fractures improve the storage capacity and permeability of the shale reservoir, increase the enrichment space for movable oil. In the Model-II, the shale is characterized by good hydrocarbon generation ability and fairly high total oil content, and as the brittle laminae contain large intergranular pores, the shale has a higher movable oil content. Based on the two models, shale oil sweet-spot areas of 2880 km2 in the southern Songliao Basin are favorable for further exploration. Aimed at the difficulties in reservoir fracturing of the lacustrine shale with a high content of clay minerals, the composite fracturing technology with supercritical carbon dioxide was used in the shale oil reservoir for the first time, realizing large-scale volume fracturing in shale with a high content of clay minerals and strong heterogeneity, marking a breakthrough of oil exploration in continental shale with a high content of clay minerals in China.

Lithofacies types and assemblage features of continental shale strata and their implications for shale gas exploration: A case study of the Middle and Lower Jurassic strata in the Sichuan Basin

Fine identification and division of lithofacies types of continental shale strata is an important basis for the evaluation of shale gas exploration and development potential. At present, however, there is no consensus on the identification standard and division scheme of shale lithofacies. Taking the continental shale strata of the Middle–Lower Jurassic in the Sichuan Basin as an example, this paper established a lithofacies division method by means of core observation, whole-rock mineral X-ray diffraction analysis, thin section analysis, total organic carbon (TOC) measurement and helium porosity measurement after analyzing whole-rock mineral composition and shale characteristics. Then lithofacies types of shale strata were identified and divided, and characteristics of lithofacies assemblages in different scales were investigated. Finally, their significance for shale gas exploration was discussed. The following research results were obtained. First, 20 shale lithofacies types of 6 categories are totally identified in this continental shale strata using the newly established three-step lithofacies division method (whole-rock mineral composition partition–TOC classification–correction and improvement of mineral texture and sedimentary structure). Among them, medium–high TOC clay shale lithofacies, lamina–thin layer clay shale lithofacies and low–medium TOC silty shale lithofacies are dominant, followed by low–medium TOC shell limy clay shale lithofacies, and TOC bearing and low TOC silty clay shale lithofacies. Second, the average TOC and the average porosity of clay shale lithofacies and shell limestone clay shale lithofacies are higher than those of silty and silty clay shale lithofacies. It is indicated that mineral composition and lithofacies types of shale have a certain impact on gas source and reservoir performance. Third, three types of assemblages are identified in the continental shale strata, including mudstone–limestone assemblage, mudstone–sandstone assemblage and mudstone–limestone–sandstone mixed assemblage, which reflect the sedimentary characteristics of distal region, proximal region and transitional region in the lacustrine environment, respectively; and that the characterization of different lithofacies assemblages is conducive to recognizing the differences between different shale sedimentary environments. Fourth, fine identification and statistic of the number and frequency of limy shell laminae and thin layers in the terrestrial organic-rich shale with high clay mineral content can provide a basis for the fracturability evaluation of gas-rich zones and the optimization of optimum exploration and development intervals.

Porosity model and pore evolution of transitional shales: an example from the Southern North China Basin

The evolution of shale reservoirs is mainly related to two functions: mechanical compaction controlled by ground stress and chemical compaction controlled by thermal effect. Thermal simulation experiments were conducted to simulate the chemical compaction of marine-continental transitional shale, and X-ray diffraction (XRD), CO2 adsorption, N2 adsorption and high-pressure mercury injection (MIP) were then used to characterize shale diagenesis and porosity. Moreover, simulations of mechanical compaction adhering to mathematical models were performed, and a shale compaction model was proposed considering clay content and kaolinite proportions. The advantage of this model is that the change in shale compressibility, which is caused by the transformation of clay minerals during thermal evolution, may be considered. The combination of the thermal simulation and compaction model may depict the interactions between chemical and mechanical compaction. Such interactions may then express the pore evolution of shale in actual conditions of formation. Accordingly, the obtained results demonstrated that shales having low kaolinite possess higher porosity at the same burial depth and clay mineral content, proving that other clay minerals such as illite–smectite mixed layers (I/S) and illite are conducive to the development of pores. Shales possessing a high clay mineral content have a higher porosity in shallow layers (< 3500 m) and a lower porosity in deep layers (> 3500 m). Both the amount and location of the increase in porosity differ at different geothermal gradients. High geothermal gradients favor the preservation of high porosity in shale at an appropriate Ro. The pore evolution of the marine-continental transitional shale is divided into five stages. Stage 2 possesses an Ro of 1.0%–1.6% and has high porosity along with a high specific surface area. Stage 3 has an Ro of 1.6%–2.0% and contains a higher porosity with a low specific surface area. Finally, Stage 4 has an Ro of 2.0%–2.9% with a low porosity and high specific surface area.