Dolomitic Shale Research Articles

Genetically linked litho- and organo-facies in meter-scale cycles of lacustrine deposits within the Permian Lucaogou Formation in the Bogda Mountains, Northwest China: Implications for paleoenvironment and organic matter origin

A close correlation between lithofacies and organofacies in meter-scale high-order cycles composed of lacustrine sediments enables comparison and refinement of lithofacies-defined cyclostratigraphy. Four lithofacies and four organofacies have been identified in fluctuating profundal high-order cycles in the lower-Permian Lucaogou Formation, southern Bogda Mountains, NW China. The four lithofacies include interbedded and interlaminated coarse siltstone and very fine sandstone, black shale, wackestone and dolostone, and calcareous and dolomitic shales. Four distinctive organofacies have been identified, on the basis of geochemical composition of organic matter and specific biomarker proxies related to organic matter types, rather than to depositional conditions and thermal maturity. The four organofacies are associated with the four lithofacies in the meter-scale high-order cycles, suggesting litho- and organo-facies may be genetically linked and may have been controlled by lake contraction and extension. The study shows that the lithofacies-derived and environment-defined high-order cycles can be delineated and substantiated by geochemical proxies-defined organofacies. This study also demonstrates that a holistic approach combining litho- and organic geochemical data is useful in reconstruction of meter-scale lacustrine cycles in a half-graben.

Geochemical implications of uranium-bearing thucholite aggregates in the Upper Permian Kupferschiefer shale, Lubin district, Poland

New inorganic and organic geochemical data from thucholite in the Upper Permian (Wuchiapingian) Kupferschiefer (T1) shale collected at the Polkowice-Sieroszowice Cu-Ag mine in Poland are presented. Thucholite, which forms spherical or granular clusters, appears scattered in the T1 dolomitic shale at the oxic-anoxic boundary occurring within the same shale member. The composition of thucholite concretions and the T1 shale differs by a higher content of U- and REE-enriched mineral phases within the thucholite concretions compared to the T1 shale, suggesting a different mineralising history. The differences also comprise higher Ntot, Ctot, Htot, Stot contents and higher C/N, C/S ratios in thucholite than in the T1 shale. The hydrocarbon composition of the thucholite and the surrounding T1 shale also varies. Both are dominated by polycyclic aromatic compounds and their phenyl derivatives. However, higher abundances of unsubstituted polycyclic aromatic hydrocarbons in the thucholite are indicative of its pyrogenic origin. Pyrolytic compounds such as benz[a]anthracene or benzo[a]pyrene are more typical of the thucholite than the T1 shale. Microscopic observations of the thucholite and its molecular composition suggest that it represents well-rounded small charcoal fragments. These charcoals were formed during low-temperature combustion, as confirmed by semifusinite reflectance values, indicating surface fire temperatures of about 400 °C, and the absence of the high-temperature pyrogenic polycyclic aromatic hydrocarbons. Charred detrital particles, likely the main source of insoluble organic matter in the thucholite, migrated to the sedimentary basin in the form of spherical carbonaceous particulates, which adsorbed uranium and REE in particular, which would further explain their different contents and sorption properties in the depositional environment. Finally, the difference in mineral content between thucholite and the T1 shale could also have been caused by microbes, which might have formed biofilms on mineral particles, and caused a change in the original mineral composition.

STUDY OF MICROSCALE PORE STRUCTURE AND FRACTURING ON THE EXAMPLE OF CHINA SHALE FIELD

Accurate characterization of pores and fractures in shale reservoirs is the theoretical basis for effective exploration and development of shale oil and gas. Currently, the scientific community has developed methods for determining the characteristics of the pore and fracture structure of shales with different resolutions, and also discussed the mechanisms of the influence of the characteristics of pores and cracks on the filtration of shale oil and gas. In this work, firstly, a three-dimensional model of a variable-scale pore-fracture structure was created. Secondly, on the basis of data obtained using computed tomography, the characteristics of the development of pores and cracks at different geometric sizes (25 and 2 mm) were statistically analyzed. Third, pore and fracture structures of mixed, felsitic and laminated felsitic shale have been relatively studied. The results show that: (a) the fractured porosity of group I felsite shale is 0.36–0.89 %; mixed shale of group I is 0.34–0.47 %; mixed shale of group III is 0.12–0.86 %; mixed shale of group III is 0.13–0.15 %. (b) Compared to mixed shale of group I, fracturing in felsite shale of the same group is more developed. In addition, the average fractured porosity of felsite shale is 0.28 % greater than that of mixed shale. After analyzing group III, it was noticed that the cracks of the mixed dolomite shale are less developed compared to the mixed shale of the same group, while the average porosity of the cracked shale is 0.33 % less than that of the cracks of the mixed shale (c). Various components of shale samples are distinguished on a gray scale. High density components are bright white (e.g. pyrite); medium density components — light gray (for example, quartz, carbonate and clay minerals); low-density components — dark gray (for example, organic matter); cracks — black (d). The volume of matrix components is relatively high and constitutes the bulk of the total volume of shale samples. The distribution of high-density minerals has a certain randomness. High-density ellipsoidal or irregular minerals (mainly pyrite) range in diameter from a few microns to hundreds of microns, and their volume is relatively small (e). Three types of microscale fractures are mainly developed in shale: microscale fractures within mineral particles (intragranular fractures), microscale fractures between mineral particles (intergranular fractures), and shrinkage fractures. Intragranular cracks are mainly microscale cracks formed by rigid particles that collapse in a certain direction under stress.

Quantitative Characterization of Inorganic Pores in Sinian Doushantou Dolomitic Shale Based on FIB-SEM in Western Hubei Province, China.

Unlike traditional shale gas reservoirs, where organic matter pores dominate, inorganic pores are the primary reservoir space in the Sinian (Ediacaran) high-maturity Doushantou dolomitic shale in western Hubei Province, China. The inorganic pore characteristics of Doushantuo shale and its influence on shale gas aggregation were investigated by examining the TOC content, thermal maturity, mineralogical composition, and field-emission scanning electron microscopy (SEM) and focused ion beam scanning electron microscopy (FIB-SEM) of drill cores. The results show that the shale mineral composition in the study area is primarily dolomite and plate-shaped interparticle-intercrystalline pores associated with dolomite are widespread inorganic pores in dolomitic shale. Interparticle-intercrystalline pores account for 75% of the total pores, with a pore size distribution mainly between 50 and 300 nm, as extracted from the 3D pore network model (PNM). Compared with organic pores, interparticle-intercrystalline pores provide greater space for gas storage and have a strong coupling relationship with the hydrocarbon generation and evolution of organic matter. Therefore, the inorganic pores in the Doushantuo Formation play a vital role in the enrichment and accumulation of shale gas. This study aims to establish a scientific basis for understanding the enrichment mechanism of shale gas in Doushantuo dolomitic shale and other inorganic pore-dominated shales in southern China.

Influences of different alkaline and acidic diagenetic environments on diagenetic evolution and reservoir quality of alkaline lake shales

Thin section and argon-ion polishing scanning electron microscope observations were used to analyze the sedimentary and diagenetic environments and main diagenesis of the Permian Fengcheng Formation shales in different depositional zones of Mahu Sag in the Junggar Basin, and to reconstruct their differential diagenetic evolutional processes. The diagenetic environment of shales in the lake-central zone kept alkaline, which mainly underwent the early stage (Ro<0.5%) dominated by the authigenesis of Na-carbonates and K-feldspar and the late stage (Ro0.5%) dominated by the replacement of Na-carbonates by reedmergnerite. The shales from the marginal zone underwent a transition from weak alkaline to acidic diagenetic environments, with the early stage dominated by the authigenesis of Mg-bearing clay and silica and the late stage dominated by the dissolution of feldspar and carbonate minerals. The shales from the transitional zone also underwent a transition from an early alkaline diagenetic environment, evidenced by the formation of dolomite and zeolite, to a late acidic diagenetic environment, represented by the reedmergnerite replacement and silicification of feldspar and carbonate minerals. The differences in formation of authigenic minerals during early diagenetic stage determine the fracability of shales. The differences in dissolution of minerals during late diagenetic stage control the content of free shale oil. Dolomitic shale in the transitional zone and siltstone in the marginal zone have relatively high content of free shale oil and strong fracability, and are favorable “sweet spots” for shale oil exploitation and development.

Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

AbstractExtensive phosphorite deposition is observed in the Neoproterozoic after a prolonged hiatus during most of the Mesoproterozoic era. This event is thought to represent an important record of major palaeoenvironmental, palaeoceanographic and biotic changes that shaped Neoproterozoic ecosystems, suggesting close relationships between phosphogenesis and the preservation of key Ediacaran biotas. However, high‐grade Ediacaran phosphorite deposits are relatively uncommon, diminishing the opportunity to test current phosphate mineralization–deposition models and their implications for Neoproterozoic research. In this scenario, widespread Ediacaran phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group – Central Brazil) are poorly explored in international literature. Nevertheless, recent advances in phosphate exploration gave access to continuous drill core sections and freshly opened mine pits, revealing an unprecedented record of complex phosphatic successions featuring the occurrence of Ediacaran microfossils assigned to the Doushantuo–Pertatataka assemblages. This work seeks to constrain main lithofacies, sequence stratigraphy and depositional settings from these phosphatic successions in order to analyse the sedimentary evolution of the unit under the current Neoproterozoic phosphorite research framework. These results indicate that the Bocaina Formation records secular sustained phosphate deposition. These deposits are related to unprecedented, microbialite reef rim phosphorites deposited during a lower accretionary rimmed platform stage, followed by the deposition of Doushantuo‐like, whole platform phosphorites associated with a later, drowned platform stage, therefore, reinforcing the evidence for the operation of strong allogeneic controls on phosphate mineralization–concentration. In addition, this study concludes that fossiliferous Ediacaran phosphatic deposits such as the Bocaina Formation are important to understanding Neoproterozoic phosphogenic events, because they may record the transition from a Precambrian to Phanerozoic‐like phosphogenesis associated with the instauration of the Ediacaran–Cambrian phosphatic taphonomic window. This evidence hints that the growing dataset from the Bocaina Formation may bring new, exciting perspectives for Neoproterozoic research as a whole.

Petrography and mineralogy control the nm-μm-scale pore structure of saline lacustrine carbonate-rich shales from the Jianghan Basin, China

Carbonate-rich shale in salt lakes has become a potential target for petroleum exploration. A full understanding of storage space development characteristics is the basis for the efficient development of this kind of shale. In this study, the complex mineral composition and multiple pore structure of a typical carbonate-rich shale from the Paleogene Xin'gouzui Formation in the Jianghan Basin were systematically analyzed. The mixed shale and dolomitic shale were the main lithofacies types in the study area. Both of them were dominated by interparticle (interP) pores. However, their pore sizes are noticeably different. Pores with a size of <100 nm are the main compositions of the mixed shale, with an average volume proportion of 78.24%. Conversely, the dolomitic shale develops mainly micro-sized pores and micro-fractures, with a ratio of 70.46%. The difference in pore structure for different types of shales is closely associated with mineral composition. When particles of different sizes are mixed, most of the interP pores are prone to be filled by fine minerals (e.g., clay), resulting in relatively small pore sizes for the mixed shale. Homogeneous carbonate minerals are favorable for the residual of larger size interP pores. However, recrystallization and secondary enlargement of carbonate minerals can also lead to a significant decrease in pore diameter of interP pores. With these results, a storage space development model for different lithofacies shales was established. This work is crucial to further reveal the reservoir mechanisms and hydrocarbon scales of an argillaceous dolomite reservoir.

Study on dynamic fracture growth mechanism of continental shale under compression failure

Microscale and nanoscale fractures induced by stress transfer can effectively improve pore connectivity, which can provide significant benefits by improving well productivity in shale reservoirs. In this research, representative samples of four shale lithofacies are investigated. The rock deformation and fracture propagation of shale samples under stress are numerically simulated by integrating various technical approaches, such as characterization of the mineral composition and distribution, digital core modeling, micromechanical tests, and finite element method-based simulations. The results show that the numerical model developed in this research can accurately replicate the microscale characteristics of the heterogeneity of shale and thus effectively clarify the effects of control of the composition, morphology, and distribution of minerals and structural defects on induced fractures in rock masses. Under uniaxial compression, the failure mechanism of rock is dominated by tensile fracturing, which generates tensile fractures parallel to the loading direction. However, compression‒shear fractures perpendicular to the loading direction also form due to the presence of lamination seams. The content of brittle minerals in different lithofacies is relatively stable, but the characteristics of the morphology and distribution of such minerals are highly differentiated, which leads to significant differences in mechanical properties and failure mechanisms. Both laminated felsic and diamictic shale (LFS and LDS) can present high fracture complexity in cases of low fracture energy. In argillaceous shale (AS), fractures initiate at low strengths, yet fracture propagation requires high energy. In contrast, the fracture initiation pressure and fracture propagation energy consumption of dolomitic shale (DS) are both high and unfavorable for the propagation of induced fractures. This research provides a technical approach for studying the formation mechanism and dynamic evolution of induced fractures, and the corresponding findings are of great theoretical significance for the identification of sweet spots and design of shale reservoirs for fracturing applications.

Mineralogical and geochemical characterization of lacustrine calcareous shale in Dongying Depression, Bohai Bay Basin: Implications for paleosalinity, paleoclimate, and paleoredox conditions

Shale is an important self-sourced reservoir rock in many sedimentary basins worldwide. In this research work, a geochemical history of variations in the sedimentary environment of the lacustrine basin is analyzed based on extensive shale samples collected from the lower 3rd member (Es3L) and upper 4th member (Es4U) of the Paleogene Shahejie Formation in the Boxing Sag (Dongying Depression, East Coast of China). The representative shale samples were analyzed for total organic carbon (TOC) contents, mineralogical composition, the concentration of different elements (major, minor, and trace), kerogen types and vitrinite reflectance (Ro), and different sedimentary structures to understand the evolution of the depositional settings and their effect on the distribution of shale lithofacies in this basin. The TOC content in this shale ranges from 0.08 % to 13.6 %. Ro values range from 0.55 to 1.19 % with type-I to type-II kerogens. The studied shale is composed of calcite (average 37.83 wt%), dolomite (avg. 10.47 wt%), quartz (avg. 23.8 wt%), plagioclase (avg. 3.91 wt%), clay (avg. 20.84 wt%), and pyrite (avg. 2.85 wt%). Seven lithofacies have been established including mixed shale, calcareous shale, laminated limestone, argillaceous shale, dolomitic shale, siliceous shale, and siliceous dolomite based on mineral composition, TOC contents, and sedimentary structures. Different geochemical proxies including Sr/Ba, Ca/Ca + Fe, C-values, CIA values, Ti/Al, Al/Al + Fe, Al/Ti, V/V + Ni, Ni/Co, and Fe/Ca + Mg were used to interpret salinity, paleoclimate, detrital influx, provenance, paleoredox and water depth of the depositional environment in the studied shale. The detailed interpretation of the sedimentary environment suggested that during the deposition of Es3L shale, the terrigenous influx was high in shallow to a deep open lacustrine system with higher primary biogenic productivity. Semi-humid climatic conditions with higher precipitation rates prevailed during the deposition of this shale. On the other hand, relatively limited terrigenous input with low bioproductivity prevailed during the deposition of Es4U shale in the study area. Semiarid to humid climatic conditions with strong evaporation prevailed during the deposition of this shale. In this research work, a detailed depositional model of the Es3L–Es4U shale in Boxing Sag is established. The interpretations deduced from the current research work are likely to expand the knowledge of shale lithofacies and the genesis of lacustrine fine-grained sedimentary rocks, as well as can offer a theoretical foundation for lacustrine shale oil exploration and development.

Geneses of multi-stage carbonate minerals and their control on reservoir physical properties of dolomitic shales

The formation and distribution of carbonate minerals have important influences on reservoir development. The dissolution-precipitation effect plays an important role in controlling the reformation of reservoir physical properties. High-quality source rocks with abundant reserves exist in the Lower Permian Fengcheng Formation, Junggar Basin, China, and the basin reservoir contains a large amount of carbonate minerals. Multi-stage cementation and dissolution of these carbonate minerals have important influences on diagenesis-porosity evolution of the reservoirs. Because of the coupled influences of deep fluid transformation and volcanic activity in the diagenetic evolution of the Fengcheng Formation, it has experienced a complex evolution of carbonate minerals. In this study, through core observation, thin section, X-ray diffraction mineral composition, carbon and oxygen isotope, fluid inclusion homogenization temperature, and laser Raman and cathodoluminescence analysis, the material sources, development period, dissolution-precipitation reactions, and transformation mechanisms of carbonate minerals in the diagenetic evolution of the Fengcheng Formation reservoir were evaluated in detail. Finally, the controlling effect of multi-stage cementation of carbonate minerals on the reservoir physical properties was determined. The results showed that there are three stages of carbonate minerals in the Fengcheng Formation. The material source of the early carbonate was mainly terrigenous debris, and the formation temperature range was 30–70 °C. The material source of middle-late carbonate minerals was mainly decarboxylation of organic acid, and the formation temperature was 60–110 °C. In addition, some were generated through hydrothermal metasomatism at high ambient temperatures, up to 120 °C. In areas where early carbonate minerals are well developed, a large number of microcracks was more likely to form under the influence of tectonic movements, resulting in high-quality reservoirs. The formation of late carbonate minerals, which is the main reason for reservoir densification in the later stage of diagenesis, resulted in poor reservoir physical properties. This study provides information useful for predicting the distribution areas of favorable reservoirs in the Fengcheng Formation, and a theoretical basis and guidance for improving the accuracy of shale oil exploration and development.

Nuclear magnetic resonance response characteristics and quantitative evaluation method of fluid saturation of lacustrine shale oil

The quantitative evaluation of fluid saturation is important for formation evaluation of shale oil. However, there is currently no effective method to identify the fluid occurrence state and quantitative evaluate the fluid saturation of lacustrine shale oil because of the complexity of diagenetic minerals and pore types. In this paper, a method is proposed for the quantitative evaluation of fluid saturation based on nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements for shale samples of Fengcheng Formation, Mahu sag in Junggar Basin, China. These studies revealed that the shale oil rocks mainly contain quartz, feldspar, dolomite, calcite and clay minerals, both develop organic and inorganic pores. The fluids mainly occur in the form of bitumen, clay bound water, bound water, bound oil and movable oil in this study area. Based on the findings from these experiments, a mixed rock index (MI) and mud index (SI) were proposed to divide the shale oil formations into three types, including sand shale, dolomitic shale and mudstone. A T1−T2 2D NMR fluid occurrence state characterization map was established to identify the different fluids by the MI, SI and NMR characteristics. Furthermore, a method was proposed to quantitatively calculate the coefficient distribution of bound and movable fluids for shale oil in different formations. Finally, the proposed method was successfully applied into the lacustrine shale oil in the Fengcheng Formation to identify fluid occurrence state and evaluate fluid saturation quantitatively.

Control of complex lithofacies on the shale oil potential in ancient alkaline lacustrine basins: The Fengcheng Formation, Mahu Sag, Junggar basin

The study of ultra-deep (>4500 m) oil and gas reservoirs in hydrocarbon-bearing basins is a hot spot for oil and gas exploration in the world today, the heterogeneity of lithofacies is a key factor influencing the potential of deep-buried shale oil. It is highly important to better understand the control of complex rock on the shale oil potential for shale oil exploration and development. The Fengcheng Formation (P1f) in the Mahu Sag is a type of ancient alkaline lacustrine shale oil, which was chosen as a research example in this paper. Four lithofacies were identified based on mineralogy: felsic, lime, dolomitic and mixed shale lithofacies. P1f exhibits a very high shale oil potential, and the oil saturation index (OSI) can reach 176 mg HC/g TOC, far exceeding the lower limit of effective benefits. The shale lithofacies of P1f is complex, and the complex lithofacies has a controlling effect on shale oil potential, specifically in terms of hydrocarbon generation, storage and hydrocarbon migration. The different lithofacies differ in various aspects and are controlled by different factors. Felsic shale is an enriched oil shale with a high hydrocarbon generation potential and high storage capacity that can receive foreign hydrocarbons. Mixed shale is a self-generating and self-accumulating oil shale with a low hydrocarbon production potential and low storage capacity. Lime shale is a partial reservoir-type oil shale with a low hydrocarbon production capacity and satisfactory storage capacity that can receive foreign hydrocarbons. Dolomite shale is a partial source rock-type oil shale exhibiting a high hydrocarbon generation capacity and low storage capacity, which can expel hydrocarbons. Studies have indicated that felsic shale exhibits a notable hydrocarbon generation potential, high storage capacity, easy foreign hydrocarbon migration, high brittle mineral content, and easy fracture production. This shale type is the most favorable shale oil exploration target in P1f within the Mahu Sag. In addition, this study reveals the relationship between rock lithofacies and shale oil potential, which provides a new idea for the potential evaluation of similar lacustrine shale oil.

Identification of fluid types and their implications for petroleum exploration in the shale oil reservoir: A case study of the Fengcheng Formation in the Mahu Sag, Junggar Basin, Northwest China

The Fengcheng Formation in the Junggar Basin is a typical shale oil reservoir. Fluid type identification and controlling factors on fluid combination types are significant challenges in shale oil exploration and development. By using two-dimensional (2D) nuclear magnetic resonance (NMR) technology, the accurate boundary values of each fluid type can be determined. Combined with thin sections, scanning electron microscopy (SEM), cathode-luminescence (CL), X-ray diffraction (XRD) and NMR, the effect of lithologies on fluid combination types has been evaluated. The results show that a total of six fluid types can be identified in the Fengcheng Formation, including free oil, adsorbed oil, kerogen, hydroxyls, adsorbed water and free water. Their boundary values on the NMR T1-T2 map can be determined by comparing fluid signals under the original state and water-saturated state after oil washing. In this study, different fluid types eliminated kerogen and hydroxyls can be combined into four fluid combination types defined as Type 1 (free oil, adsorbed oil and adsorbed water), 2 (free oil, adsorbed oil, free water and adsorbed water), 3 (adsorbed oil, free oil and free water) and 4 (movable fluids), existing in the dolomitic shale, mixed shale, felsic shale and sandstone, respectively. Meanwhile, the geometric means of T2 (T2gm) and T2 curves indicate that the pore size can effectively distinguish different lithologies and can be considered as an essential factor influencing the occurrence state of fluids. As the average pore size increases, free water emerges in the mixed shale, and adsorbed water disappears in the felsic shale. Furthermore, after the lithology transforms into sandstone, the fluid type is simple, and only movable fluids exist. Finally, the oil test results imply that the reservoirs with fluid combination Type 4 and Type 3 have relatively high oil production. Therefore, the subsequent orientation for petroleum exploration in shale oil reservoirs targets the sandstone and felsic shale interlayers in the Fengcheng Formation.

Morphological classification and three-dimensional pore structure reconstruction of shale oil reservoirs: A case from the second member of Paleogene Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, East China

This study combines large volume three-dimensional reconstruction via focused ion beam scanning electron microscopy (FIB-SEM) with conventional scanning electron microscope (SEM) observation, automatic mineral identification and characterization system (AMICS) and large-area splicing of SEM images to characterize and classify the microscopic storage space distribution patterns and 3D pore structures of shales in the second member of the Paleogene Kongdian Formation (Kong 2) in the Cangdong Sag of the Bohai Bay Basin. It is shown that: (1) The Kong 2 Member can be divided into seven types according to the distribution patterns of reservoir spaces: felsic shale with intergranular micron pores, felsic shale with intergranular fissures, felsic shale with intergranular pores, hybrid shale with intergranular pores and fissures, hybrid shale with intergranular pores, clay-bearing dolomitic shale with intergranular pores, and clay-free dolomitic shale with intergranular pores. (2) The reservoir of the intergranular fracture type has better storage capacity than that of intergranular pore type. For reservoirs with storage space of intergranular pore type, the dolomitic shale reservoir has the best storage capacity, the hybrid shale comes second, followed by the felsic shale. (3) The felsic shale with intergranular fissures has the best storage capacity and percolation structure, making it the first target in shale oil exploration. (4) The large volume FIB-SEM 3D reconstruction method is able to characterize a large shale volume while maintaining relatively high spatial resolution, and has been demonstrated an effective method in characterizing the 3D storage space in strongly heterogeneous continental shales.

Petrological and organic geochemical characteristics of the Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin, NW China: Implications on the relationship between hydrocarbon accumulation and volcanic-hydrothermal activities

The coupling relationship between volcanic-hydrothermal activities and hydrocarbon generation has been a common understanding in recent years. However, there is a lack of theoretical and practical guidance in the application hydrocarbon production. The fine-grained sedimentary rocks of the middle Permian Lucaogou Formation in the Jimsar Sag in the Junggar Basin, NW China, are critical source rocks and have been simplify regarded as a mudrocks for target for tight oil exploration. Recently studies show that rocks were deposited in an intracontinental lacustrine environment with multiple volcanic-hydrothermal activities. Their complex micro-scale lithological characteristics restrict the understanding and further exploration and development of tight oil. Hence, this study focuses on the detailed lithological and their organic geochemical characteristics of the Lucaogou Formation using microscopic thin-section observation, scanning electron microscopy, X-ray diffraction, Rock-Eval, saturated hydrocarbon chromatography and mass spectrometry. The following key conclusions were obtained. First, the massive mudrocks are sub-divided into lithofacies for hydrocarbon generation, including tuffaceous shale, calcareous shale and dolomitic shale, and, microinterval reservoirs, including dolostone and sedimentary tuff. Second, the origins of organic matter in Lucaogou Formation were mainly plankton, bacteria with minor of land plants. The depositional environment is high salinity and anoxic conditions. Materials derived from volcanic-hydrothermal activities, lacustrine precipitation, and terrestrial transportation mixed with various proportions to form this formation. Third, a mixture of carbonate minerals, felsic minerals and clay minerals is the best source rock, and the higher the mixture, the better. This research not only provides insight into the influence of volcanic-hydrothermal activities on lithology and organic matter enrichment, but also providing a potential scheme for the tight oil exploration and development in similar rifted geological settings. • A mixture lithology of carbonate minerals, felsic minerals and clay minerals is the best source rock. • The better the mixture of carbonate minerals, feldspar minerals and clay minerals, the better the quality of hydrocarbon source rock. • Volcanic-hydrothermal activities are favorable to the enrichment and preservation of organic matter. • The Lucaogou formation was formed in a high salinity and anoxic condition, and the biological source was mainly plankton.

Comparative analysis of non-recursive three-dimensional (3D) modifications of Hoek-Brown failure criterion

A comparative analysis of three-dimensional (3D) modifications of Hoek &amp; Brown (1988) rock failure criterion was carried out. The correlations between failure stress and the other principal stresses were first determined using polyaxial test data for five geomaterials including KTB amphibolites, Westerly granite, Dunham dolomite, Shirahama sandstone and Yuubari shale. The prediction accuracies of five non-recursive, three-dimensional modifications to Hoek-Brown failure criteria and the original two-dimensional Hoek-Brown criterion were later investigated using root mean square error and coefficient of determination as measures of misfit. The results reveal that the intermediate principal stress significantly affects strength in geomaterials like the Dunham dolomite. It also moderately affects the strength of geomaterials like the KTB amphibolites, the Westerly granite and the Yuubari shale. Moreover, the intermediate principal stress has mixed effects on strength in the Shirahama sandstone. In addition, the original Hoek-Brown failure criterion could still be used with reasonable accuracy for geomaterials whose strength shows low dependence on the intermediate principal stress. While a three-dimensional Hoek-Brown criterion must be used for geomaterials like the Dunham dolomite, whose strength shows a high dependence on the intermediate principal stress. The original Hoek-Brown failure criteria should be used with caution for geomaterials like the Shirahama sandstone, the KTB amphibolites, the Westerly granite, and the Yuubari shale, whose strength shows either mixed or intermediate dependence on the intermediate principal stress. Average prediction accuracies followed the order: simplified Priest (2012), Ma et al. (2020), and Jiang &amp; Zhao (2015). Both original Hoek &amp; Brown (1988) and Li et al. (2021) criteria were tied, while Liu et al. (2019) was the least.

Sedimentary Environment and Organic Matter Accumulation of Wufeng-Longmaxi Shales, Southwest Yangtze Platform: Insights from Geochemical and Petrological Evidence

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Foration is the most developed strata of shale gas in southern China. As several shale gas wells were drilled in the mountain-basin transition zone in the Southwest Sichuan basin, the attraction was drawn by this area. While, due to the complex sedimentary environment adjacent to the Kangdian Uplift, the area favorable for organic-rich shale development is still in fog. Further study is necessary to obtain a more complete understanding of organic matter enrichment in the marginal region around Sichuan Basin. We therefore focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales. In this study, a combination of methods was applied, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. According to the mineralogical and petrological analyses, five lithofacies have been defined, including mudstone, bioclastic limestone, silty shale, dolomitic shale and carbonaceous siliceous shale. And for the geochemical characterization, the detrital influx, redox conditions and paleo-productivity have been studied. The results showed that Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment.

Genetic Mechanism of Multi-Stage Carbonate Minerals and its Control on Reservoir Physical Properties in Dolomitic Shales

Genetic Mechanism of Multi-Stage Carbonate Minerals and its Control on Reservoir Physical Properties in Dolomitic Shales

Characteristics and environmental significance of concretions in the Lower Cambrian Qiongzhusi Formation in the Middle–Upper Yangtze region, China

Based on the outcrop and drilling data of the Sichuan Basin and its periphery, this paper reveals the concretion development characteristics of the Lower Cambrian Qiongzhusi Formation and its significance to the environment. Three preliminary conclusions are drawn as follows. (1) The exposure layers and lithofacies characteristics of the concretions in the Qiongzhusi Formation are very different from area to area. In the eastern Sichuan-western Hubei-Hunan & Guizhou and southern Sichuan depression areas, the concretionsare developed in the section of SQ1, and are mainly composed of calcareous siliceous mixed shale facies, siliceous shale facies, dolomitic shale facies and dolomite facies. The clay content is generally less than 25% and is significantly less than the surrounding rocks, and the TOC is generally higher in the lower concretion layers and less in the upper concretion layers. In the central Sichuan and northern Sichuan depression areas, the concretions appear in the sections of SQ1 and SQ2, and are dominated by calcareous shale facies and marl facies. The calcic content is obviously higher than that of surrounding rocks, and the clay content is less than that of surrounding rocks, and the TOC is less than 1.50%. Because mineral composition difference is high, most of the GR curves of the concretions show trough response characteristics, reflecting the central region enriched with calcium or siliceous, and the edge enriched with clay and organic matter. (2) The concretion layers generally occur in carbonaceous shale, silty shale and siliceous shale with a TOC over 0.7%, and are especially developed in carbonaceous shale and siliceous shale of SQ1, and occasionally appear in grey silty shale with a TOC over 0.2%, showing that the concretion layers are basically symbiotic with grey shale (especially with high-quality shale). (3) The concretions are a kind of important sedimentary response of rifting activities in Yangtze marine basin. According to the distribution characteristics of concretions and black shale in the Qiongzhusi Formation, the rifting activities of Yangtze marine basin can be divided into three periods including development period (SQ1), adjustment period (SQ2) and abortion period (SQ3). In the period of SQ1, controlled by massive rifting and upwelling ocean currents in Yangtze marine basin, the concretions and organic-rich shale were widely developed in the Lower Qiongzhusi Formation. In the periods of SQ2 and SQ3, while rifting activity of Yangtze marine basin decreased and aborted, and while sea level fell and upwelling ocean currents receded to the southeast, the concretions and organic-rich shale just appeared in the local depression areas in small scale.

Sweet spot evaluation and exploration practice of lacustrine shale oil of the second member of Kongdian Formation in Cangdong sag, Bohai Bay Basin

Based on core, thin section, X-ray diffraction, rock pyrolysis, CT scanning, nuclear magnetic resonance and oil testing data, the macro and micro components, sedimentary structure characteristics, of Paleogene Kong 2 Member in Cangdong sag of Huanghua depression and evaluation standard and method of shale oil reservoir were studied to sort out the best shale sections for shale oil horizontal wells. According to the dominant rock type, rhythmic structure and logging curve characteristics, four types of shale lithofacies were identified, namely, thin-layered dolomitic shale, lamellar mixed shale, lamellar felsic shale, and bedded dolomitic shale, and the Kong 21 sub-member was divided into four quasi-sequences, PS1 to PS4. The PS1 shale has a porosity higher than 6%, clay content of less than 20%, and S1 of less than 4 mg/g; the PS2 shale has well-developed laminar structure, larger pore and throat size, better connectivity of pores and throats, high contents of TOC and movable hydrocarbon, S1 of over 4mg/g, clay content of less than 20%, and porosity of more than 4%; PS3 shale has S1 value higher than 6 mg/g and clay content of 20% - 30%, and porosity of less than 4%; and PS4 shale has lower TOC content and low oil content. Shale oil reservoir classification criterion based on five parameters, free hydrocarbon content S1, shale rhythmic structure, clay content, TOC and porosity, was established. The evaluation method of shale oil sweet spot by using the weighted five parameters, and the evaluation index EI were proposed. Through comprehensive analysis, it is concluded that PS2 is best in quality and thus the dual geological and engineering sweet spot of shale oil, PS3 and PS1 come next, the former is more geologic sweet spot, the latter more engineering sweet spot, and PS4 is the poorest. Several vertical and horizontal wells drilled in the PS2 and PS3 sweet spots obtained high oil production. Among them, Well 1701H has produced stably for 623 days, with cumulative production of over 10000 tons, showing bright exploration prospects of Kong 2 Member shale oil.