Niutitang Formation Research Articles

Influence of different thermal evolution stages on shale pore development

ABSTRACT Different thermal evolution degrees on pore development were studied by cryogenic nitrogen adsorption experiment and microscopic photometric analysis. The organic matter Ro, the pore volume, and the proportion of different pore types are different in the Qingshankou Formation, Longmaxi Formation, and Niutitang Formation. However, the relationship between pore development and thermal evolution is consistent. With increasing the degree of the thermal evolution of organic matter holes appear three different change rules. When the organic matter Ro is less than 1.2, shale porosity is mainly influenced by physical compaction and cementation, shale organic hole filling. With the increase of Ro, the porosity tends to decrease, and when Ro is between 1.2 and 3.0, the organic matter of pyrolysis hydrocarbon generation and organic matter hydrocarbon generation hole and corrosion hole a lot of development, the stage total shale porosity is lower before the rising trend, organic matter porosity increased with the increase of Ro. When Ro is greater than 3.0 (overmaturity stage), graphitization occurs. Under the influence of deep burial and slow compaction, the porosity is decreasing continuously.

The occurrence of vanadium in nature: its biogeochemical cycling and relationship with organic matter—a case study of the Early Cambrian black rocks of the Niutitang Formation, western Hunan, China

The occurrence of vanadium in nature: its biogeochemical cycling and relationship with organic matter—a case study of the Early Cambrian black rocks of the Niutitang Formation, western Hunan, China

Geological controls of shale gas accumulation and enrichment mechanism in Lower Cambrian Niutitang Formation of western Hubei, Middle Yangtze, China

Geological controls of shale gas accumulation and enrichment mechanism in Lower Cambrian Niutitang Formation of western Hubei, Middle Yangtze, China

An experimental investigation into the oxidative dissolution of typical organic-rich shale from China

Oxidizing stimulation of organic-rich shale was proposed to enhance the rate of methane extraction from the matrix. Previous studies lacked attention on the dissolution characteristics of organic matter (OM) during the dissolution process performed using oxidative fluids. In this study, the organic-rich shale samples obtained from the Longmaxi Formation, Niutitang Formation, and Yanchang Formation were used for dissolution experiments. Crushed shale samples were treated with deionized water, hydrogen peroxide (H2O2), or sodium hypochlorite (NaClO) for 240 h, respectively. By using a combination of X-ray Diffraction, rock-eval pyrolysis, total organic carbon analysis, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, the composition and organic geochemical characteristics of the treated shale samples were determined. The results showed that H2O2 induces the dissolution of both the pyrite and carbonate minerals, and NaClO more readily dissolves pyrite and not carbonate minerals. Higher removal efficiency of OM for all four shale samples occurs when treated with NaClO than with H2O2. The oxidative dissolution of OM in these shale samples occurs as two dominant modes: one is the oxidation cracking of C–C, C–H such as in the Niutitang or Yanchange formations; the other is oxidation cracking of C–O, such as in the Longmaxi Formation samples. Both the Longmaxi Formation samples, which showed higher C–O contents compared to samples from the other two formations, exhibited better oxidizability than others when treated with H2O2 or NaClO. Oxidation products such as Fe2+/Fe3+ and H+ enhance the oxidizing ability of H2O2 and NaClO toward cracking of OM. However, the consumption of H+ due to the carbonate minerals may impair the oxidizing ability of H2O2, but not that of NaClO. Furthermore, the presence of an organo-clay complex has an adverse influence on the removal of OM. The results indicated that the type of OM, the content of the pyrite and carbonated minerals, and the physical protection of the organic carbon by clays should be taken into account, when selecting a shale reservoir suitable for oxidizing stimulation and associated optimization of the oxidant during the fracturing operation design.

Quantitative Characterization and Determination of the Main Factors That Control Fracture Development in the Lower Paleozoic Shale in Southeastern Chongqing, China

The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.

Fractal characteristics of shale pore structure and its influence on seepage flow.

The migration law of shale gas has a significant influence on the seepage characteristics of shale, and the flow of the gas is closely related to the pore structure. To explore the influence of shale pore parameters on permeability in different diffusion zones, the pore structure of the shale in the Niutitang Formation in Guizhou, China, was analysed based on liquid nitrogen adsorption experiments and nuclear magnetic resonance experiments. The relationship among fractal dimension, organic carbon content (TOC) and BET-specific surface area was analysed based on the fractal dimension of shale pores calculated using the Frenkel–Halsey–Hill model. Shale permeability was calculated using the Knudsen number (Kn) and permeability equation, and the influence of the fractal dimension and porosity in different diffusion zones on shale permeability was analysed. Previous studies have shown that: (i) the pores of shale in the Niutitang Formation, Guizhou are mainly distributed within 1–100 nm, with a small total pore volume per unit mass, average pore diameter, large BET specific surface area and porosity; (ii) fractal dimension has a negative correlation with average pore diameter and TOC content and a quadratic relationship with BET specific surface area; and (iii) permeability has a positive correlation with Kn, porosity and fractal dimension. In the transitional diffusion zone, fractal dimension and porosity have a significant impact on permeability. In the Knudsen diffusion zone, porosity has no obvious effect on permeability. The methodologies and results presented will enable more accurate characterization of the complexity of pore structures of porous media and allow further understanding of the seepage law of shale gas.

The influence of syngenetic hydrothermal silica fluid on organic matter preservation in lower Cambrian Niutitang Formation, South China

Early Cambrian shale-chert couplets are widely distributed in South China, within which abundant organisms have been detected. Cherts in early Cambrian are known to be influenced by hydrothermal activity. However, the influence of syngenetic hydrothermal activity on organism preservation has not yet been well evaluated. Litho-, chemo- and bio-stratigraphic data have documented that the syngenetic hydrothermal silica fluid activity from the Majiang-Yangtiao section of the early Cambrian Niutitang Formation in Guizhou province, SW China.Geochemical compositions [Fe/Ti, Al/(Al + Fe), Al–Fe–Mn ternary diagram and Fe/Ti vs. Al/(Al + Fe + Mn) diagram] indicated that the chert, siliceous shales and mudstones were strongly, moderately, and weakly influenced by hydrothermal silica fluids, respectively. Mudstones contained much more terrigenous elements (e.g. Al and Zr). The total organic carbon (TOC) content and concentrations of nutrient elements (Cu, Zn, P, Ba) were highest in siliceous shales, followed by mudstone and lowest in chert. This could be due to a high silica dilution from hydrothermal fluid in chert. Compared with the contiguous siliceous shales, chert was characterized by a high content of aerobic zooplankton (e.g. sponges and pogonophoras) with low V/Cr ratios. This represented a sedimentary facies altered by hydrothermal fluid input in the marine environment. Organisms were well preserved in chert with delicate ornamentation on the surfaces of spherical acritarch cells, and to a lesser extent in silica-rich siliceous shales, but less in mudstones. This suggested that hydrothermal silica fluid with rapid sedimentation provided a good environment for organism preservation. These well preserved organisms could provide organic carbon content with little or no hydrocarbon generation in the rocks, because the morphology were not changed and no solid bitumen were found surrounding these organisms. The study provided a clue for careful consideration in the context of hydrocarbon generation evaluation in early Cambrian source rock.

Comparison of in-source solid bitumen with migrated solid bitumen from Ediacaran-Cambrian rocks in the Upper Yangtze region, China

In-source solid bitumen was found in some cracks within the calcareous concretion in an overmature (Rbitumen > 3.5%) black shale of the Lower Cambrian Niutitang Formation from Songtao, Guizhou, China. The solid bitumen (grain size >2 cm) concentrated in source rock, which was formed by the transformation of petroleum-like substances that have not been expelled from source rock. This provides a substitute for studying the physical and chemical properties of in-situ solid bitumen, which usually occurs as finely dispersed particles in black shale. On the other hand, Ediacaran-Cambrian carbonate rocks (which serve as reservoir in the area) contain pyrobitumen derived from petroleum that migrated from the black shale of the Niutitang Formation.A comparative petrological and geochemical analysis was conducted on the in-source solid bitumen from the Lower Cambrian Niutitang Formation and the migrated solid bitumen in the Ediacaran-Cambrian carbonate rocks. The in-source solid bitumen displays a relatively homogeneous optical structure with very weak anisotropy and is visibly different from the migrated bitumen in the Ediacaran-Cambrian carbonate reservoir. This migrated bitumen in the reservoir displays strong anisotropy and optical structure similar to that of coke. Considering a strata like the lower Palaeozoic in South China, which has no vitrinite and is highly over mature, the in-source solid bitumen is likely to be the most suitable organic component for measuring the reflectance and subsequently determining the thermal maturity of organic matter.Interestingly, the in-source solid bitumen displays some inheritance of its parent material. The stable carbon isotope ratios are very similar to those of kerogen in the black shales (approximately −34‰ to −33‰), and the distribution pattern of the trace elements and rare earth elements (REEs) is consistent with that of the Niutitang black shales. The distribution patterns of the trace elements and REEs in the migrated bitumen are basically consistent with those of in-source solid bitumen, however, their stable carbon isotope composition is slightly heavier (−33.6‰ to −31.2‰) than that in the source rocks. In other words, as a key link in the chain, the in-source solid bitumen could connect the migrated bitumen in the reservoir with its source rock. This provides direct support for tracing oil and gas sources in the area by using the element geochemical characteristics of migrated solid bitumen in reservoir.

The selection of outgas temperature for shale in low-pressure N2 adsorption: An evidence from thermogravimetry

Low-pressure N2 adsorption (LPNA) could provide quantitative data for characterizing the pores in gas shale. However, the inconsistencies of outgas temperature have caused significant deviations in LPNA experiments. To explore the effects of outgas temperature on pore characteristics, two shale samples of Lower Cambrian Niutitang formation from Northern Guizhou, China, were collected for LPNA experiments and thermogravimetry-fourier transform infrared (TG-FTIR) spectroscopy. The samples were outgassed at six temperatures: 80°C, 100°C, 150°C, 200°C, 250°C, 300°C. Larger adsorbed volumes were presented in the isotherms at higher outgas temperatures. Similar regularity is obtained from the relationship between specific surface area, micropore volume and outgas temperature. Comprehensive analysis of TG-FTIR and LPNA at different outgas temperature indicated that at lower outgas temperatures (from 80°C to 100°C), the free water was unlikely to be removed completely, and resulted in large amounts of micropores couldn’t be accessed. An excessive outgas temperature might expulse liquid hydrocarbons or decompose organic matter (from 200°C to 300°C), and could lead to the generation of micropores. When the sample were outgassed at 150°C, TG-FTIR analysis indicated that the sample composition unchanged and a better removal of free water happened. Therefore, 150°C should be a suitable outgas temperature for shale in LPNA experiments. The findings in this research not only provide reliable evidence for the selection of outgas procedure in LPNA for shale, but clarify the important effects of free water and volatile materials on pore accessibility in shale.

The differential evolution of nanopores in discrete OM and organic-clay composites for shale: insights from stress manipulation

The organic carbon in shale has been proved to be a significant hydrocarbon source in two occurrences of discrete OM and organic-clay composites. Although the evolution of OM-hosted pores has been investigated by extensive research, the evolution of nanopores in discrete OM and organic-clay composites under additional stress is still unclear. In this study, two potential shales in China (Silurian Longmaxi (S1l) formation and Cambrian Niutitang (Є1n) formation) were selected to examine the nanopores characteristics developed in discrete OM and organic-clay composites using field emission scanning electron microscopy (FE-SEM) and statistical analyses. The microstructure observations of the two shales show that during the growth of nanopores, the expansion of nanopores in composites was restricted by the narrow space between clay layers, and then, the nanopores would be protected from collapse or compaction by clay layers with further maturity. The discrete OM particles constantly suffer squeezing from brittle minerals and lead to the nanopores thrown into compressive environment, resulting in a preferred orientation along the edge of mineral particles and narrowing shape, as well as diminishing the pore sizes. For organic-clay composites, the development of nanopores is determined by additional stress state. Under compressive stress, no pores could be observed, while the nanopores are widely distributed in tensile environment or stress-free state. Consequently, the nanopores development in discrete OM and organic-clay composites are significantly affected by additional stress. The research findings imply the potential hydrocarbon source in the nanopores located in organic-clay composites and would bring inspiration for the exploitation of shale gas.

Nitrogen geochemistry and abnormal mercury enrichment of shales from the lowermost Cambrian Niutitang Formation in South China: Implications for the marine redox conditions and hydrothermal activity

The Cambrian is an important period for life evolution during the geological history, and the basal Cambrian black shales with widespread occurrence worldwide record important paleoceanographic information during this critical period. The set of shales with stable distribution and thickness of about 200 m is also extensively developed in South China, which provides an important material for studying the marine redox environment, major geological events and biotic evolution. Here we conduct studies on the nitrogen cycle and mercury's abnormal enrichment based on mercury (Hg) concentrations and nitrogen (N) isotope compositions of the shales from the lower Cambrian Niutitang Formation in Tongren area of Guizhou Province, South China. The results show that both Hg and TOC exhibited abnormal enrichments at the bottom of the Niutitang Formation. Mo and U enrichment factors indicated a gradually oxidized redox evolution during the deposition of the Niutitang Formation, with two fluctuated redox cycles from reducing to oxic, and then from dysoxic to oxic conditions in the water column. And the abnormal enrichment of Hg during the early Cambrian may be related to the volcanic and hydrothermal activities. Nitrogen isotope (δ15N) has obvious negative shift at the bottom of the Niutitang Formation, which indicated that nitrogen fixation was enhanced under the anoxic condition in the water column, the organic carbon pool gradually reduced, and phosphorus (P) and other nutrient elements were available, leading to the enhancement of nitrogen fixation and an increase of bioavailable nitrogen in the water column. Nitrogen isotopes recorded the effects of early Cambrian biological evolution and marine redox environment on nutrient element circulation, with contemporaneous volcanic or hydrothermal fluid activities during this period.

Mercury record of intense hydrothermal activity during the early Cambrian, South China

The early Cambrian was an important interval in Earth history, marked by significant evolution of both life and the marine environments it inhabited. While enhanced hydrothermal activity has been proposed as a trigger for ecosystem perturbations in the early Cambrian, it remains unclear how intense and how long such perturbations may have been. To address this, we examined mercury (Hg) concentrations and mercury isotopes, as well as major and trace elements, of organic-rich black shales of the lower Cambrian Niutitang Formation, in the Tongren area of Guizhou Province, South China. Our data show that Hg in these sediments is hosted dominantly by organic matter. Elevated raw and normalized Hg contents provide direct evidence of intense hydrothermal activity during the early Cambrian in South China. We suggest that high metal flux (e.g., Mo, U, and Ba) into the seawater occurred through this intense submarine volcanism, altering seawater compositions. Intense hydrothermal activity was likely a significant trigger of environmental and biological evolution during the early Cambrian in South China.

Fluid-rock interaction of the early Cambrian black shale in the South China Block: Implications for low-temperature mineralisation

Chalcophile elements hosted by black shales of the Lower Cambrian Niutitang Formation, South China Block, contribute to low-temperature mineralisation in southwest China. To study the mobilisation and migration of ore-forming elements, hydrothermal experiments were performed to simulate leaching of Au, As, Sb and Ag from an Au-rich black shale from the Kaiyang Phosphate Mine (KYPM), China. The experiments were designed to simulate low-temperature mineralisation at 150 °C, with solvent concentrations of ~5%, durations of 15 d and 30 d, and water/rock mass ratios of 13:1 and 26:1. Reagent-grade NaCl, NaHCO3 and Na2S were used to provide the common anions found in hydrothermal systems. The results show that solutions containing Cl− ions in oxidised acidic systems leached Au most effectively. Weakly alkaline systems containing HCO3− ions leached Au less effectively. Solutions containing HS− ions did not leach Au effectively, and Au precipitated within the HS−-bearing system. The results for Ag were similar to those for Au, and Ag-leaching was most effective in the oxidised acidic system containing Cl− ions. As-leaching does not occur in the Cl−-bearing system, but As is leached effectively by HS−-bearing and HCO3−-bearing solutions. Sb was not leached by our experiments. Oxidised fluids produced by deep-sourced magma during the Indosinian and Yanshanian tectonic events infiltrated black shales affected by extensional deformation through faults, joints and fractures. Fluid–rock interaction leached a range of elements in these structural settings. These experiments provide insights into migration of chalcophile elements, particularly Au and Ag, and the source of large-scale low-temperature mineralisation within the South China Block.

Study on the Microscopic Fracture Process and Acoustic Emission of Shale Based on Digital Image

The Niutitang Formation shale is often filled with calcite minerals, which significantly affects the physical and mechanical properties of shale reservoirs. To correctly understand the microscale fracture characteristics of the Niutitang Formation shale and the evolution of acoustic emission signals, this paper uses digital image processing technology to characterize the geometric characteristics and nonuniform distribution of calcite minerals in the shale at the microscale and then maps it to finite elements; uniaxial compression tests of different calcite vein inclination angles are carried out on a microscale. The results show that under the microscale structure, the changes in compressive strength and brittleness index of the Niutitang Formation shale with different calcite vein dip angles are all N-shaped. The calcite veins affect the distribution of the stress field, leading to significant differences in the shale fracture process and fracture mode. The shale fracture process can be divided into two types. The first type (0°, 15°, 30°, 45°) is that the shale matrix is destroyed first, and then, the calcite veins are destroyed; the second type (60°, 75°, 90°) is that the calcite veins are destroyed first, and then, the shale matrix is destroyed. Shale fracture modes can be divided into w-type, v-type, inverted v-type, and inverted z-type. The inclination angle of calcite veins has a significant influence on the AE evolution characteristics of the Niutitang Formation shale. According to the characteristics of the AE active period, it can be divided into two types: surge type and step type. The surge type has a short active period, the number of AE count surges is small, the AE peak is large, and the failure mode is relatively simple. The step type has a long active period, the number of AE count surges is large, and the AE peak is small, and the failure mode is relatively complicated. The research results provide important theoretical guidance for shale gas fracturing mining.

Organic matter accumulation of the early Cambrian black shales on the flank of Micangshan-Hannan Uplift, northern upper Yangtze Block, South China

The Micangshan-Hannan Uplift area is proved a new shale gas exploration area with massive potential of Niutitang Formation in the past three years. Nevertheless, the organic matter enrichment mechanisms of this set of organic-rich shales lack discussion. Here, eighty-two samples of a drilling well from the Micangshan-Hannan Uplift, Upper Yangtze Block, South China were investigated for the organic matter (OM) accumulation mechanisms by geochemical analysis. Specifically, paleoredox conditions are classified by V/Sc, U/Th, and Uau; primary productivity is evaluated by TOC, Cu/Al, Ni/Al and, P/Al; sediment flux is indicated by Ti/Al; and paleoclimate is inferred by CIA and Ga/Rb ratios. Vertically, the Kuanchuanpu dolomite, underlying the Niutitang black shales, with TOC contents from 0.34% to 1.18% (avg. 0.71%), was deposited in oxic and low productivity conditions under lower sea level and more humid climate. Upwards, the silica-dominated and clay-rich siliceous mudstone (Assemblage 1) of Niutitang Formation, characterized by high TOC contents from 1.31% to 4.66% (avg. 2.78%) was classified into three units. The OM enrichment of Unit 1 was controlled by both anoxic condition and high primary productivity resulting from rapid sea level rise and increasing aeolian flux under cold and arid climate. The OM enrichment of Unit 2 was manipulated by the dysoxic bottom water condition, although the primary productivity remained relatively high. Then, the OM enrichment of Unit 3 was dominated by increasing primary productivity of surface water due to higher aeolian input and the dysoxic with intermittent anoxic condition advantageous for OM preservation. At last, the mixed siliceous mudstone (Assemblage 2) of Niutitang Formation displayed much lower TOC contents (avg. 1.34%). Both of oxic condition as well as lower primary productivity due to the declining aeolian flux led to the less efficient OM accumulation. In sum, the favorable organic matter accumulation in Niutitang Formation was controlled by anoxic bottom water condition and high primary productivity, which was related to the Early Cambrian global transgression and increasing aeolian flux under cold and arid climate.

Signals of combined chromium–cadmium isotopes in basin waters of the Early Cambrian – Results from the Maoshi and Zhijin sections, Yangtze Platform, South China

The combination of authigenic chromium and cadmium stable isotopes in marine sedimentary archives enables reconstruction of the redox conditions and levels of primary productivity in past oceans and basins. The Ediacaran to Cambrian Nanhua Basin (Yangtze Platform, South China) and its sedimentary sequences have been studied intensively in the past with respect to investigating the effects of post-Snowball Earth glaciations on the oxygenation of Earth's atmosphere, and intimately related, on the consequences of climate changes on the evolution of the ocean environment and of life during the Precambrian-Cambrian transition. Following recent propositions of using cadmium isotopes as a novel proxy for reconstructing the extent of primary productivity in paleooceans, we here present results from a combined cadmium (Cd)‑chromium (Cr) isotope study on metalliferous black shale, black shale and phosphorite sediments deposited during the Early Cambrian Niutitang Formation at the Maoshi and Zhijin sections. Results point toward strongly oxidized and highly productive surface water layers in the Nanhua Basin during this period. The samples have strongly positively fractionated δ114Cd and δ53Cr values up to +0.6‰ and + 1.4‰, respectively, similar to such values from carbonaceous sediments in modern oceans. The association of Cd with sulfides in the sediments, together with high Cd/TOC ratios particularly in the metalliferous black shales, implies a sulfide-controlled enrichment of Cd under restricted euxinic conditions. The lower Cd/TOC values in phosphorites and barren black shales and negatively fractionated δ114Cd values down to −0.33‰ point to less effective precipitation of Cd under more suboxic conditions as likely prevailing in an open shallow shelf. The Cr content in the sediments is mostly close to average shale, and the δ53Cr values are little fractionated in the 0–0.2‰ range, corroborated by flat shale-like rare earth element + yttrium (REE + Y) patterns. However, some metal-rich shale samples have clearly authigenic Cr isotope signatures with up to +1.4‰ δ53Cr, and the authigenic Cr component in the phosphorite samples is around 1‰ δ53Cr, indicating oxic surface waters. The latter sediments also depict seawater-like, middle REE enriched REE + Y patterns with distinct negative Ce and positive Y anomalies indicative of an oxygenated surface water layer, and with positive Eu anomalies which they likely received during phosphatogenesis in the euxinic bottom water environment. In accord with previous studies reporting on extreme trace metal enrichments (Mo, Ni, V, Hg, etc.) in these Early Cambrian sediments, our Cd and Cr isotope signatures favor a scenario of a strongly stratified basin in which high primary productivity thrived in an oxidized surface water layer, potentially as a result of increased nutrient supply from a strongly weathering continental hinterland in the aftermath of the last Snowball Earth glaciations in the Ediacaran.

Controls of Distinct Mineral Compositions on Pore Structure in Over-Mature Shales: A Case Study of Lower Cambrian Niutitang Shales in South China

Investigating the impacts of rock composition on pore structure is of great significance to understand shale gas occurrence and gas accumulation mechanism. Shale samples from over-mature Niutitang formation of Lower Cambrian in south China were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption to elucidate the controls of distinct mineral composition on pore development. Two distinct lithofacies, namely siliceous shale and argillaceous shale, were ascertained based on their mineral composition. Due to the variability of mineral composition in different lithofacies, pore structure characteristics are not uniform. Pores in siliceous shales are dominated by interparticle pores and organic matter (OM) pores, among which the interparticle pores are mainly developed between authigenic quartz. Furthermore, most of these interparticle pores and cleavage-sheet intraparticle pores within clay minerals are usually filled by amorphous organic matter that is host to OM pores. Due to the lack of rigid minerals, argillaceous shale was cemented densely, resulting in few interparticle pores, while cleavage-sheet intraparticle pores within clay minerals are common. Comparing siliceous shales with argillaceous shales, specific surface areas and pore volumes are higher on the former than on the latter. The content of total organic carbon (TOC) and authigenic quartz have a great influence on micropore structures, but less on mesopore structure for siliceous shales. The rigid framework structure formed by authigenic quartz is believed to be able to prevent primary interparticle pores from mechanical compaction and facilitate the formation of organic matter-associated pores. In terms of argillaceous shales, due to the lack of authigenic quartz, interparticle pores were rarely developed and its pore structure is mainly controlled by illite content.

Characteristics and Controlling Factors of Nanopores of the Niutitang Formation Shale from Jiumen Outcrop, Guizhou Province.

This paper studies the characteristics and controlling factors of nanopores in organic-rich shale in the Niutitang Formation. Six samples were collected from the bottom of the formation at the Jiumen Outcrop, Guizhou Province. Experiments were conducted to investigate the pore structures of these high-maturity shale samples. The TOC contents vary between 4.81-17.51% with an average of 10.18%. The XRD data show that these samples are dominated by quartz (44%-71%), with a significant amount of clay minerals, such as illite, with a content of 8%-27.5%. Based on the low-pressure liquid N₂ sorption measurements, the pore structures can be divided into two groups. Group A including samples of N-2, N-3 and N-4, mainly develop slit-shaped pores, mesopores and macropores. Group B shown from samples N-1, N-5 and N-6, are mainly composed of narrow slitlike pores, which may provide more space for shale gas than slit-shaped pores. The mesopores, macropores, porosity and specific surface areas of group B are more developed than those of group A. With the comparison of pore structures in shales with various organic matter and mineral contents, the dissolution of quartz and feldspar can be the important factor controlling pore development. The evolution of diagenesis is closely related to pore evolution. This diagenesis has various types and complex effects on the pores, mainly including compaction, dissolution and cementation.

Elemental geochemistry proxies recover original hydrogen index values and total organic carbon contents of over-mature shales: Lower Cambrian South China

The determination of the original organic matter type and content of source rock in over-mature petroleum systems is critical for accurately evaluating the original hydrocarbon-generation capacity, but has challenged researchers for decades. Here we present a new method for such analyses using inorganic geochemistry, which was applied to a case study of over-mature shales of the Lower Cambrian Niutitang Formation (Upper Yangtze region, South China). Multiple redox proxies (e.g., VEF, UEF, MoEF, and Th/U ratio) indicate that the Lower and Upper members were deposited under anoxic bottom water and suboxic conditions, respectively. This implies different proxies suitable for recovering the original hydrogen index (HIo) values, i.e., CuXS and Porg for the Lower (anoxic) and Upper (suboxic) members, respectively. Then, the original total organic carbon (TOCo) content of the shales were calculated by using the formula TOCo = (TOCpd – (S1+S2) ×0.085 – HIo × 0.0004)/(1 – HIo/1177), of which TOCpd refers to the measured present-day TOC data and S1 and S2 are the Rock-Eval pyrolysis hydrocarbons. Excellent original hydrocarbon generation capacity was revealed in that HIo and TOCo are 600–893.4 mg hydrocarbon (HC)/g TOC (average = 758.4) and 0.2–32.2 wt% (average = 10.2), respectively. Combined with six lithofacies identified from mineralogy and petrology, heterogeneities of the hydrocarbon-generation potential in different lithofacies of the Niutitang Formation shales of this study were revealed. The laminated siliceous shale and laminated carbonaceous shale facies had higher HIo and TOCo than the other facies, and can thus be regarded as the lithofacies with the highest hydrocarbon-generation potential. Our study provides a new method using inorganic geochemistry to restore the original kerogen type and TOC content of over-mature shales with a high degree of accuracy, which can be used to robustly evaluate the hydrocarbon generation potential of conventional and unconventional hydrocarbon resources in over-mature source rocks.

Hyperspectral analysis of shale gas drilling in southern Huangling anticline

In this paper, the visible near infrared, short wave red, and thermal infrared spectral data and high-definition core images of three wells in Huangling anticline in Western Hubei are obtained by hylogger. Through comprehensive analysis, it can be seen that the thickness of Niutitang Formation is decreasing from west to East, However, the thickness of Doushantuo Formation is basically the same, ranging from 210m to 220m. Quartz, clay minerals, carbonate and limonite are the main minerals in Niutitang formation, while in Doushantuo Formation dolomite is the main mineral. Dolomite fracture is developed in this area, which is a favorable area for shale gas development.