Niutitang Shale Research Articles

Paleo-environments and organic matter enrichment in the shales of the Cambrian Niutitang and Wunitang Formations, south China: Constraints from depositional environments and geochemistry

A comprehensive study of the lithology, mineralogy, major element and trace element geochemistry of the organic-rich shales in the Lower-Middle Cambrian Niutitang and Wunitang Formations in South China was performed to assess the depositional environments and the mechanisms for organic matter enrichment. Results show that the Lower Cambrian Niutitang shales were mainly deposited under anoxic conditions with high paleo-productivity, while the overlying Wunitang shales were deposited under dysoxic conditions with lower paleo-productivity. Both these shales were deposited in a moderately restricted deep-water basin. This paleogeographic setting was conducive to limited water interchange between the outer shelf and open ocean, resulting in a high flux of nutrients from shelf upwelling. These nutrients were produced from upwelling and hydrothermal vents and were carried by oceanic currents, enhancing blooms of primary producers within the surface waters. Organic matter enrichment of the Niutitang and Wunitang shales was mainly controlled by redox conditions and paleo-productivity. The intermittent renewal of connections to open ocean water during sea level rise weakened the restricted conditions in the depositional area, which significantly accelerated the consumption of organic matter.

Depositional environment and hydrothermal controls on organic matter enrichment in the lower Cambrian Niutitang shale, southern China

The purpose of this research was to examine paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Niutitang Formation shale gas reservoir in the eastern Xuefeng uplift and to determine factors affecting organic matter (OM) enrichment. In the studied borehole Xiangan 1 well in western Hunan Province, the Niutitang Formation can be subdivided into the Niu1, Niu2, and Niu3 Members based on geological and geochemical features, including trace element enrichment, lithology, and fossil content. Total organic carbon values of the Niutitang Formation are variable, averaging 1.5 wt. % in the Niu1 Member, 12.7 wt. % in the Niu2 Member, and 5.1 wt. % in the Niu3 Member. Paleoclimatic changes were responsible for changes in biota, which impacted patterns of OM enrichment. Climate proxy (chemical index of alteration) and productivity proxies (biogenic Ba, Cu/Al, and Ni/Al) consistently indicate higher paleoproductivity in the Niu2 Member. The Niu1 and Niu2 Members may be affected by hydrothermal events, whereas hydrothermal activity was absent during deposition of the Niu3 Member. Hydrothermal activity may provide nutrients and silica but may also enhance the reducing condition of the water column. In addition, hydrothermal events may have possibly influenced biological survival in the different environments, which in turn increased their reproduction within the early Cambrian ocean and affected OM production. Redox proxies (Mo and U enrichment factors) imply that the Niu1, Niu2, and Niu3 Members were deposited in suboxic, euxinic, and ferruginous environments, respectively. Redox conditions, strongly restricted water environments, and hydrothermal events were conducive to OM enrichment during the early Cambrian.

Constraints on the Organic Matter Accumulation of Lower Cambrian Niutitang Shales in the Middle Yangtze Region, South China

Abstract The lower Cambrian Niutitang shales, as one of target intervals with the greatest potential for shale gas exploration and development, have attracted much attention. Nevertheless, the organic matter enrichment mechanisms of the lower Cambrian Niutitang shales need further study, especially in the hydrothermal active zone. In this study, samples from ND1 well in western Hubei Province, middle Yangtze region, South China, were investigated for the controlling factors of organic matter accumulation of Lower Cambrian Niutitang shales by detailed petrographic, mineralogic, and geochemical proxies. The results show that hydrothermal activity and sea level fluctuation controlled the redox conditions and paleoproductivity of seawater and ultimately controlled the organic matter accumulation of Niutitang formation. In the Niu-1 member, the intense hydrothermal events lead to a suboxic to anoxic environment, which is conducive to the organic matter preservation. However, low sea level strengthens the restriction of water mass and reduced nutrient upwelling into the shelf, leading to decreased marine primary productivity, which was ultimately responsible for depleted organic matter accumulation in the Niu-1 member. In the Niu-2 member, the anoxic-euxinic environment and high paleoproductivity, driven by continuous hydrothermal activity and rising sea level, were the main factors controlling the enrichment of organic matter. In the Niu-3 member, the dysoxic to oxic condition plus low primary productivity, caused by the disappearance of hydrothermal activities and sea-level fall, resulted in the unfavorable organic matter accumulation. The results of this paper enrich the model of organic matter enrichment in the lower Cambrian black shale in the middle Yangtze region.

Slope Belts of Paleouplifts Control the Pore Structure of Organic Matter of Marine Shale: A Comparative Study of Lower Cambrian Rocks in the Sichuan Basin

Extensive exploration of the marine shale of the Niutitang Formation in south China has been conducted. However, exploration and development results have varied considerably in different areas. For example, the Niutitang shale in Jingyan City (Southwestern Sichuan Basin) produces a large amount of gas with a long period of stable production. In contrast, most development wells in the Niutitang shale in Chongqing City do not produce gas. Scanning electron microscopy images showed that the organic matter (OM) pore development in the Niutitang shale in Jingyan is abundant, large in size, and are well connected. In contrast, OM pores in the Niutitang shale in Chongqing are rarely observed. OM pore development of the Jingyan and Chongqing shales is mainly controlled by thermal maturity as shown by equivalent vitrine reflectance determinations. The moderate thermal maturity has resulted in the development of a large number of OM pores in the Niutitang shale in Jingyan, whereas the high thermal maturity of the Niutitang shale in Chongqing has led to the destruction of most of the OM pores. Due to the existence of ancient uplift, the shale was buried shallowly in the process of hydrocarbon generation evolution, and the shale avoided excessive thermal evolution and retained appropriate thermal maturity. In the Jingyan area, due to its location near the central uplift in the Sichuan Basin, the Niutitang shale deposited nearby avoided excessive evolution, and a large number of OM pores were retained in the reservoir.

A novel approach of evaluating favorable areas for shale gas exploration based on regional geological survey and remote sensing data

The periphery of the Huangling Massif is one of the principal prospecting regions of the Lower Cambrian shale gas resources in South China. The geological setting and ground surface conditions are complicated within this area, and the petroleum geological data is limited due to the low degree of exploration. A novel approach based on the regional geological survey and remote sensing data was proposed. The favorable areas for gas exploration targeting the Lower Cambrian Niutitang Shale within the periphery of the Huangling Massif were evaluated by using this approach. Seven key indicators, including the depositional facies and current burial state of the shale strata, density of the exposed faults, dip angle of exposed strata, elevation and slope of the current ground surface, and land utilization, were selected. An evaluation criterion of these indicators was established to facilitate the classification of the study area into four ratings: favorable, low-risk, high-risk, and unfavorable. The overall evaluation of the seven indicators suggested a favorable area of 386.02 km2, a low-risk area of 1784.81 km2, and a high-risk area of 1689.97 km2, accounting for 0.91%, 4.21%, and 3.99% of the whole area, respectively. These results achieved a high level of compliance with the actual exploration discoveries and eliminated over 90% of the total area unsuitable for exploration and production. It is demonstrated that this novel approach can be effectively adopted in the comprehensive evaluation of the favorable area for shale gas exploration with a low degree of prospecting.

Maturity Assessment of the Lower Cambrian and Sinian Shales Using Multiple Technical Approaches

The Lower Cambrian Niutitang and Sinian Doushantuo shales are the most important and widespread source rocks and target layers in South China. Reliable data of the thermal maturity of organic matter (OM) is widely used to assess hydrocarbon generation and is a key property used in determining the viability and hydrocarbon potential of these new shales. Nevertheless, traditional thermal maturity indicators are no longer suited to the vitrinite-lack marine shales. This study aims to combine high throughput Raman and infrared spectroscopy analysis to confirm and validate the thermal maturity in comparison with the bitumen reflectance (Rb). Raman parameters such as the differences between the positions of the two bands (VG-VD) are strong parameters for calculating the thermal maturity in a large vitrinite reflectance (Ro) ranging from 1.60% to 3.80%. The infrared spectroscopy analysis indicates that the aromatic C=C bands and CH2/CH3 aliphatic groups both are closely correlated with thermal maturity. The calculated Ro results from Raman and infrared spectroscopy are in strong coincidence with the Rb. The relationships between Rb and pore volumes/surface areas (calculated from N2 adsorption) indicate that the sample with Rb of 3.40% has the lowest pore volumes and surface areas. Focused ion beam scanning electron microscopy (FIB-SEM) observations of OM pores indicate that Ro of approximately 3.60% may be an upper limit for OM porosity development. Obviously, kerogen Raman and infrared spectroscopy can indicate methods for reducing the risk in assessing maturity with practical, low-cost accurate results. Exploration of shale gas in the high maturity (>3.40%–3.60%) region carries huge risks.

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.

Multi-Angle Investigation of the Fractal Characteristics of Nanoscale Pores in the Lower Cambrian Niutitang Shale and Their Implications for CH₄ Adsorption.

Shale gas has received widespread interest due to its successful commercial development in China. Pore structures in shale can directly control its gas storage and migration properties. In this study, field emission scanning electron microscopy (FE-SEM), low-pressure N₂/CO₂ adsorption and highpressure methane adsorption were used to investigate the nanoscale pore structures of the Lower Cambrian Niutitang Formation in the southeastern Upper Yangtze platform. The fractal parameters of the pore structures were also calculated using the Frenkel-Halsey-Hill (FHH) model. The relationships between the fractal dimensions and TOC content, mineral composition and pore structure parameters were also discussed. The results show that organic matter and clay minerals are primary factors affecting the nanoscale pore development. Slit-shaped pores and ink-bottle-shaped pores are the predominant pore types in the Niutitang shale. The Brunauer-Emmett-Teller (BET) surface areas vary from 4.91 m²/g to 34.33 m²/g, and the pore volumes range from 0.689 m³/100 g to 2.964 m³/100 g. Two fractal dimensions (D1 and D₂) of the Niutitang shale were obtained using the FHH model, with D1 ranging from 2.605 to 2.684, and D₂ ranging from 2.681 to 2.865. D1 adequately characterizes the surface roughness of the pore structures, while D₂ represents the complexity of the pore types. Inter-particle (InterP) pores commonly have greater shape complexities than OM pores and intra-particle (IntraP) pores, based on analyses using Image-Pro Plus software. In addition, the TOC content and clay minerals have great effects on the fractal dimension D1. Meanwhile, the fractal dimension D1 increases with increasing BET surface area, but there is no definite relationship between the fractal dimensions and pore volumes. Both the fractal dimensions D1 and D₂ are negatively correlated with pore sizes. Further investigation indicates that the fractal dimension D1 exhibits a strong positive relationship with the methane adsorption capacity indicating that Niutitang shales with greater values of the fractal dimension D1 have higher methane adsorption capacities.

鄂西宜昌地区页岩气勘探发现对MVT铅锌矿成矿的指示意义

The coupling relationship between hydrocarbon accumulation and the involvement of organic matter in metal mineralization has been a frontier scientific question in recent years. On the basis of the petrography and Raman spectral analysis of fluid inclusions, high-density methane inclusions were discovered in the samples of quartz and calcite veins from Sinian Doushantuo shales, Lower Cambrian Niutitang shales, and MVT (Mississippi Valley type) lead-zinc deposit of Sinian Dengying Formation in Yichang area. The Raman scatter peak v of methane inclusions was applied to calculate the density of pure methane inclusions, and the ore-forming ages of MVT lead-zinc deposit were determined by using Rb-Sr and Sm-Nd isochron dating. The density of methane inclusions trapped in the quartz veins within the Doushantuo shales of well EYY 1 mainly ranges from 0.237 to 0.278 g/cm, and the density of methane inclusions trapped in the calcite veins within the Hejiaping MVT lead-zinc deposit mainly ranges from 0.213 to 0.271 g/cm, which signifies methane inclusions of high density. Paragenetic mineral association sphalerite and galena within the Hejiaping lead-zinc deposit have a Rb-Sr isochron age of 189.1±1.8 Ma, and calcite within the Hejiaping lead-zinc deposit yields a Sm-Nd isochron age of 189.9±2.0 Ma, which indicates is the mineralization of Hejiaping lead-zinc deposit was closely related to the Yanshanian tectonic compressions. The Sr/Sr values of the paragenetic mineral association (0.711 92) and calcite (0.712 03-0.712 27) indicate that the ore-forming fluids of the Hejiaping lead-zinc deposit were derived largely from shale sources. The high-density methane trapped in fluid inclusions within the Hejiaping lead-zinc deposit was most likely derived from the high-density overpressure methane generated by the Doushantuo shales and/or Niutitang shales. The discovery of high-density methane inclusion in the shale gas layer and MVT lead-zinc deposit, and the determination of ore-forming ages of MVT lead-zinc deposit provide new evidence for the study of organic matter participation in the mineralization of MVT lead-zinc deposit.

A comprehensive study on the impacts of rock fabric on hydrocarbon generation and pore structure evolution of shale under semi-confined condition

Semi-confined pyrolysis experiments were performed on core and powdered samples taken from type-II lacustrine mudstone to investigate the effects of sample rock fabrics on hydrocarbon generation and porosity changes when shale is subjected to pyrolysis. The generated hydrocarbons were analysed by organic geochemical methods to reflect differences in constituents and carbon isotopes. Gas (N2 and CO2) adsorption tests were applied to solid residues to characterize the formation and development of pores after the two series of pyrolysis experiments. Simple calculations indicate that hydrocarbons prefer to expulse as intact rock fabric is destroyed. Secondary cracking gas accounted for approximately 23–30% and 28–49% of all gaseous hydrocarbons in the powdered and core samples, respectively. Side reactions, such as mineral catalytic and recombination reactions, prevailed during the semi-closed pyrolysis of powdered sample at 300–500 °C, causing generated C3+ hydrocarbon yields to be lower than those of the core sample. Additionally, gaseous hydrocarbons generated from the powdered sample were characterized by more positive δ13C and higher iso-/n-C4,5 and C1/C1-5 values. Side reactions ceased at 500–550 °C, and corresponding ratios tended to equalize. For the powdered samples, a higher expulsive ratio of hydrocarbons led to better pore development, the mesopore volume increased with increasing temperatures, and micropore volume decreased above 450 °C. Comparatively, the mesopore volume decreased above 450 °C for the core sample, and the micropore volume decreased above 500 °C. These results are helpful for revealing the genetic characteristics of shale gas and for understanding the origins of organic-hosted pores of the Cambrian Niutitang shale in southern China. The study suggests that samples with intact rock fabric should be used in future laboratory pyrolysis tests to generate results that better reflect natural conditions.

Organic matter pores structure and evolution in shales based on the he ion microscopy (HIM): A case study from the Triassic Yanchang, Lower Silurian Longmaxi and Lower Cambrian Niutitang shales in China

To illustrate organic matter (OM) pores structure and evolution in shales with different maturities, the Triassic Yanchang shale (0.5% < Vitrinite reflectance (Ro) < 1.2%) in the Ordos Basin, the Lower Silurian Longmaxi shale (2.0% < Ro < 2.8%) and the Lower Cambrian Niutitang shale (3.0% < Ro < 4.2%) in the Yangtze Platform have been selected in this study. OM pores were analyzed by helium ion microscopy (HIM). The results show that OM pores are mainly developed in pyrobitumen. A large number of OM pores were developed in pyrobitumen of the Longmaxi shale samples. However, no OM pores were basically found in solid kerogen. OM pores number and size in pyrobitumen of the Yanchang shale samples are much smaller than that in the Longmaxi shale samples. OM pores are all basically undeveloped in pyrobitumen of the Niutitang shale samples. The structure that small size OM pores nested in large size OM pores is favorable to gas enrichment and seepage. Thermal maturity has more huge impact on OM pores structure than TOC content in shale. Low thermal maturity (Ro < 1.5%) will lead to insufficient thermal evolution of OM, resulting in small size and quantity of OM pores in shale. Excessive thermal maturity that ranges from 3.0% to 4.0% will leads to the disappearance of OM pores in pyrobitumen. Appropriate thermal maturity that ranges from 1.5% to 3.0% ensures a large number of OM pores in shale.

Characterization of methane adsorption on shale of a complex tectonic area in Northeast Guizhou, China: Experimental results and geological significance

Complex tectonic settings are generally considered to be unfavorable for shale gas accumulation, but a favorable desorption-gas content has been detected in Lower Cambrian Lujiaping Shales in Dabashan fold-thrust belt in northeastern Chongqing, Southern China, which suggests a good exploration potential. High-pressure (~30 MPa) and high-temperature (~120 °C) (HP-HT) methane adsorption isotherms were conducted on seven samples collected at 2665–2707 m below the ground surface from the Lower Cambrian Niutitang Shale (equivalent to the Lujiaping Shale) from northeast Guizhou Province, Southern China to assess methane adsorption capacity and the influence of total organic carbon (TOC) content, thermal maturity, mineralogy, and temperature-pressure conditions on methane adsorption in over-mature shales in complex tectonic areas. The samples contain high TOC contents, ranging from 5.1% to 8.4%, and equivalent vitrinite reflectance Ro values ranging from 3.62% to 3.85%. Over the experimental pressure range up to 30 MPa, Langmuir volumes correlate positively with TOC contents, which may be related to the increase in specific surface area and micropores (<2 nm) volume with an increasing TOC content. Organic matter (OM)-hosted micropores and nanometer-sized intergranular pores in clay-OM nanocomposites mostly contribute to the adsorption capacity of this over-mature Niutitang Shale. In addition, the different behaviors of other minerals (quartz, carbonate and illite) under intensive tectonic compression also affect the adsorption capacity of shales. Due to the low porosity of the Niutitang Shale, the free gas content in these samples is relatively low, and the adsorbed gas always makes up the majority of gas-in-place (GIP), hence the “equivalent point” of adsorbed gas and free gas does not exist at depths of <6 km. This suggests that the shallow (<3 km) Niutitang Shale should be primarily targeted for adsorbed gas under low-temperature and high-pressure geological conditions.

Effect of lithofacies on pore structure of the Cambrian organic‐rich shale in northern Guizhou, China

Taking the Cambrian organic‐rich shale in northern Guizhou for research object, total organic carbon (TOC), and X‐ray diffraction (XRD) were used to study shale lithofacies, and scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and nitrogen adsorption analyses were combined to reveal pore structure, in order to document the effect of shale lithofacies on pore structure. The results show that the Niutitang shale can be divided into six shale lithofacies, and three (siliceous shale lithofacies, carbonate‐rich siliceous shale lithofacies, and carbonaceous/argillaceous containing siliceous shale lithofacies) of them can be identified. Four types of pores (organic matter, interparticle, intraparticle, and dissolved pores) are present in the Niutitang shale. Organic matter and interparticle pores mainly occur in siliceous shales, and intraparticle and dissolved pores typically develop in carbonate‐rich siliceous shales and carbonaceous/argillaceous containing siliceous shales. Organic matter and brittle minerals related pores are two significant components of the pore volume in the Niutitang shales. The micropores and mesopores volumes promptly increase with the increase in organic matter content, indicating that organic matter is the primary contributor to micropores and mesopores. Clay content shows weakly negative relations with both the micropore and mesopore volumes. The development of macropores is associated with inorganic minerals (i.e., clay and carbonate). Organic matter and clay contribute the most to specific surface area, while the contribution of brittle minerals is negligible. Siliceous shales are the most favourable reservoir for shale gas due to their tremendous specific surface area and pore volume. In contrast, carbonate‐rich siliceous shales have the least storage capacity of shale gas because of their relatively smaller specific surface area and pore volume.

Multifractal Analysis of Acoustic Emissions during Hydraulic Fracturing Experiments under Uniaxial Loading Conditions: A Niutitang Shale Example

Fracture characterization is essential for estimating the stimulated reservoir volume and guiding subsequent hydraulic fracturing stimulations in shale reservoirs. Laboratory fracturing experiments can help provide theoretical and technical guidance for field operations. In this study, hydraulic fracturing experiments on the shale samples from Niutitang Formation in Hunan Province (China) under a uniaxial loading condition are conducted. The multifractal method is used to analyze the acoustic emission (AE) signals and characterize fracture initiation and propagation. The hydraulic fracturing process can be divided into three stages based on the characteristics of AE signals: the initial stage, the quite stage, and the fracturing stage. The multifractal analysis results showed that: (1) the value of the spectrum width, Δα, continues to increase as the energy accumulates until the fracturing stage starts; and (2) the difference in the multifractal spectrum values, Δf, reflects the relationship between small and large signal frequencies and can quantify the fracture scale, i.e., the lower the Δf, the larger the fracture scale and vice versa. The results were further verified using a time-frequency analysis of the AE signals and micro-CT scanning of the samples. This study demonstrates that the multifractal method is feasible for quantitatively characterizing hydraulic fractures and can aid field hydraulic fracturing operations.

Key reservoir parameter for effective exploration and development of high-over matured marine shales: A case study from the cambrian Niutitang formation and the silurian Longmaxi formation, south China

Success has been achieved in the exploration and development of marine shales of the Lower Silurian Longmaxi Formation in southern China, but there has been almost no commercial shale gas produced from the Lower Cambrian Niutitang Formation in much of the same area. Breakthroughs have been made with shale gas in the Niutitang Formation in Yichang City, south China, by the China Geological Survey (CGS). How to evaluate and exploit the Niutitang shale more quickly, accurately and efficiently remains a major question. This paper proposed that the key reservoir parameter for effective exploration and development of marine shales is based on the thermal maturity. Shale samples from Chongqing City of both the Longmaxi Formation and the Niutitang Formation were selected for the analysis of total organic carbon (TOC) content, mineral composition and equivalent vitrinite reflectance (Eq-Ro), as well as for the characterization of organic matter pores (OM pores) using focused ion beam scanning electron microscopy (FIB-SEM). Selected shale samples from Yichang City of the Niutitang Formation were also studied for comparison. Results show that shale samples from Chongqing present a large number of OM pores in the Longmaxi Formation, but almost no OM pores in the Niutitang Formation. While shale samples from Yichang present a large number of OM pores in the Niutitang Formation. Commercial gas has been produced from shale reservoirs with abundant OM pores, such as the Longmaxi shale in Chongqing and the Niutitang shale in Yichang. By contrast, there is no gas production from shale reservoirs with no OM pores as is the case for the Niutitang shale in Chongqing. Thus OM pores is an indicator for the evaluation of marine shale gas potential. Because these shales have similar TOC content, mineral composition and kerogen type, but different thermal maturity, OM porosity is mainly controlled by the thermal maturity and the key geological parameter for shale reservoir potential is thus a suitable thermal maturity. Marine shale reservoirs favorable for gas production generally have an Eq-Ro ranging from 2.0% to 3.0%, which is mainly controlled by ancient burial depth of reservoirs. This means exploration and development of highly over matured marine shales should be focused on less buried areas such as the edge of paleo-uplift.

Gas-bearing property of the Lower Cambrian Niutitang Formation shale and its influencing factors: A case study from the Cengong block, northern Guizhou Province, South China

To gain a better understanding of the gas-bearing property of Lower Cambrian Niutitang Formation shale and its influencing factors, the shale gas-bearing conditions, gas content, and composition in the Cengong block were investigated in this work based on wells CY1, TX1, and TM1. The Niutitang shale reservoir is characterized by large thickness, abundant organic matter with an average total organic carbon content of 4.74%, and high quartz content averaging 53.9%; therefore, it has good shale gas-bearing potential. The results of water immersion and ignition tests intuitively revealed the existence of shale gas based on the occurrence of continuous dense clusters of bubbles and long flames of 1–2 m. The Langmuir volumes of shale samples from well TM1, which ranged from 1.70 to 5.53 m3/t, were positively associated with TOC content and Brunauer–Emmett–Teller surface area, indicating a strong adsorption capacity and significant adsorbed gas potential. However, large differences were observed among the three wells with regard to gas-bearing properties; wells CY1 and TX1 had average total gas contents of 1.25 and 0.33 m3/t, respectively; the gas composition of well TM1 was dominated by nitrogen (N2), with contents generally exceeding 95%. Furthermore, the gas from well TX1 was mainly composed of methane with an average content exceeding 80%. Burial depth and TOC and quartz contents had significant control on the vertical distribution of gas content, and local tectonic preservation conditions resulted in differences of gas-bearing properties among various wells. The measured, lost, and total gas contents all presented positive correlations with TOC, quartz contents, and porosity, and were negatively related to clay content. The shale formed in deep-water shelf environments had better gas-bearing properties than that formed in shallow-water shelf environments. The broad axis of the box syncline in the Cengong block is a favorable location for shale gas accumulation, and fault development affects the shale gas plane distribution. Because of the influence of faults, the shale of well TM1 had low hydrocarbon content. Furthermore, based on the combined patterns of folds and faults as well as other special specific factors, a classification scheme of shale gas accumulation patterns in South China was developed.

Experimental Investigation of the Impacts of Fracturing Fluid on the Evolution of Fluid Composition and Shale Characteristics: A Case Study of the Niutitang Shale in Hunan Province, South China

Hydraulic fracturing is a widely used technique for oil and gas extraction from ultra-low porosity and permeability shale reservoirs. During the hydraulic fracturing process, large amounts of water along with specific chemical additives are injected into the shale reservoirs, causing a series of reactions the influence the fluid composition and shale characteristics. This paper is focused on the investigation of the geochemical reactions between shale and fracturing fluid by conducting comparative experiments on different samples at different time scales. By tracking the temporal changes of fluid composition and shale characteristics, we identify the key geochemical reactions during the experiments. The preliminary results show that the dissolution of the relatively unstable minerals in shale, including feldspar, pyrite and carbonate minerals, occurred quickly. During the process of mineral dissolution, a large number of metal elements, such as U, Pb, Ba, Sr, etc., are released, which makes the fluid highly polluted. The fluid–rock reactions also generate many pores, which are mainly caused by dissolution of feldspar and calcite, and potentially can enhance the extraction of shale gas. However, precipitation of secondary minerals like Fe-(oxy) hydroxides and CaSO4 were also observed in our experiments, which on the one hand can restrict the migration of metal elements by adsorption or co-precipitation and on the other hand can occlude the pores, therefore influencing the recovery of hydrocarbon. The different results between the experiments of different samples revealed that mineralogical texture and composition strongly affect the fluid-rock reactions. Therefore, the identification of the shale mineralogical characteristics is essential to formulate fracturing fluid with the lowest chemical reactivity to avoid the contamination released by flowback waters.

Developmental characteristics and controlling factors of natural fractures in the lower paleozoic marine shales of the upper Yangtze Platform, southern China

This work elucidates the developmental characteristics and controlling factors of natural fractures, and their effects on the shale gas content in the lower Paleozoic marine shales of the upper Yangtze Platform, southern China. These objectives have been achieved by systematic observations and descriptions of fractures in outcrops and drilling cores, as well as experimental tests on samples from the corresponding fractured intervals. The types of natural macro-fractures observed in both the Niutitang and Wufeng-Longmaxi shales mainly include vertical tension fractures, high-angle shear fractures, bed-parallel slip fractures and bedding fractures, with bedding fractures accounting for the largest proportion. In addition, Field Emission-Scanning Electron Microscopy (FE-SEM) images show that interlayer, inter-particle, intra-particle and organic matter-associated fractures are the primary types of micro-fractures. Tectonism is one of the dominant factors in fracture development, but other factors also have a strong influence, such as TOC content, mineral composition and shale-bed thickness. Under similar tectonic settings, the development of fractures is mainly influenced by the TOC content. For the Wufeng-Longmaxi shale, fracture abundance is positively correlated with the TOC content. However, the TOC content in Niutitang shale displays a positive correlation with fracture density only when the TOC content is less than 6%; when the TOC content is greater than 6%, the correlation is shown to be negative. Shale with higher quartz or lower clay contents exhibits greater fracture development. Furthermore, the shale-bed thickness has a negative influence on the fracture development, which can be essentially attributed to the change in mineral composition of shale. Natural fractures provide reservoir spaces for free gas and are favorable for the desorption of adsorbed gas; therefore, the better shale fractures are developed, the greater the gas content will likely be.

Laboratory evaluation of flow properties of Niutitang shale at reservoir conditions

Abstract Absolute and water saturation dependent gas flow under reservoir conditions is important for the exploration of deep energy resources. Steady-state and various unsteady-state argon permeability tests were conducted on dry Niutitang shale during the confinement loading and unloading cycles, then different gases were used for steady-state permeability tests at the final confinement of 20 MPa. Permeability decreased by 36% with the confinement increasing from 5 MPa to 60 MPa, and around 66% of the permeability reductions was recovered after the unloading cycles. The apparent steady-state permeabilities were 38% higher than those derived by unsteady-state methods due to the enhanced slip flow. Two cycles of water imbibition and gas drainage tests were conducted with the long water evaporation process completed in the second gas drainage process. Argon to carbon dioxide permeability ratio ranged from 1.2 to 2.1, while the water permeability was 30–45% of the gas intrinsic permeability. The argon breakthrough pressure of water-saturated Niutitang shale sample was 8 MPa, and the permeability increased by 10 times after the long-term evaporation process. Argon permeability decreased by 90% and increased by 21.6%–29.6% respectively after the first and second gas drainage processes compared with that under dry condition with the residual water saturation being 29.7% and 11.3% respectively, which were caused by the water blockage and cracking respectively. Influences of carbon dioxide adsorption and induced swelling reached a minimum at residual water saturation.

Effect of thermal maturity on shale pore structure: A combined study using extracted organic matter and bulk shale from Sichuan Basin, China

The thermal maturity is of great significance in the evolution of pores in gas shales. In this work, the comparison of pore structures between Wufeng-Longmaxi and Niutitang shales and its corresponding extracted organic matter (OM) is made. The Wufeng-Longmaxi shales have higher total pore volumes, surface areas and porosity than the Niutitang shales and OM pores are the main pore types in both shales. With the thermal maturity (Ro) increases, the OM porosity decreases. Combined with literature results, it is further proved that the evolution of OM pores was related to the gas generation with increasing thermal maturity. Compared with Niutitang shales, the lower thermal maturity of Wufeng-Longmaxi shales is beneficial for preserving OM pores and providing better connectivity. It can be concluded that the thermal maturity is the main controlling factor on the development of OM pores, which are important for the shale gas generation and preservation.