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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "H1: Petroleum Engineering".

Deadline for manuscript submissions: closed (20 April 2022) | Viewed by 17526

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Prof. Dr. Xianming Xiao

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Guest Editor

School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
Interests: deep oil and gas accumulation process; unconventional oil and gas evaluation and exploration

Prof. Dr. Shangbin Chen

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Guest Editor

School of Resources and Geoscience, China University of Mining and Technology, Xuzhou 221116, China
Interests: unconventional oil and gas geology; geothermal and urban geology

Prof. Dr. Hui Tian

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Guest Editor

Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
Interests: deep oil and gas; unconventional oil and gas

Special Issue Information

Dear Colleagues,

Marine shale oil and gas are becoming increasingly important as a world energy resource, and their exploration and development have experienced great success in the past decade, with successive breakthroughs in production. However, in considering how to further extend shale oil and gas exploration and development, there are still challenges regarding the accurate assessment of shale reserves as well as the efficient prediction of sweet spots, especially for deeply burial (>3500 m) and/or overmature marine shale gas, terrestrial shale oil and gas, and marine–terrestrial transitional facies shale gas. These problems have become new hot topics in recent years. Many scholars have made outstanding progress in geological and geochemical investigations related to these fields, which will be beneficial in further promoting shale oil and gas exploration and development. We are therefore calling for submissions to be included as part of a special collection covering the new advances and challenges in shale oil and gas exploration, and novel original research articles and reviews are all welcome. Potential topics include but are not limited to the following:

Depositional, mineralogical and diagenesis, and other geological characteristics of shale oil and gas reservoirs.
Petroleum generation and evolution kinetics of shales and the mechanism of oil and gas retention in shales.
Occurrence and distribution of pore water in shales and its influence on shale adsorption capacity.
Formation and evolution mechanism of nanopores in shales and their constraints on shale oil and gas.
Origins of non-hydrocarbon gases in shales and thermal stability of methane in geological conditions.
Advances in characterizing shale organic and mineral properties, and assessing and predicting sweet spots of shale oil and gas.

Prof. Dr. Xianming Xiao
Prof. Dr. Shangbin Chen
Prof. Dr. Hui Tian
Guest Editors

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Keywords

shale oil and gas
oil and gas retention
nanopore
maturity
pore water
methane
adsorption
non-hydrocarbon gas
competition adsorption
sweet spot

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Published Papers (9 papers)

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Research

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12 pages, 12607 KiB

Open AccessArticle

Effect of Methane Cracking on Carbon Isotope Reversal and the Production of Over-Mature Shale Gas

by Jingkui Mi, Wei Wu, Di Zhu and Ziqi Feng

Energies 2022, 15(17), 6285; https://doi.org/10.3390/en15176285 - 29 Aug 2022

Cited by 1 | Viewed by 1510

Abstract

The geochemical statistics indicate that the wetness (C₂~C₅/C₁~C₅) of over-mature shale gas with carbon isotope reversal is less than 1.8%. The magnitude of carbon isotope reversal (δ¹³C₁–δ¹³C₂) increases with decreasing wetness within a wetness range of 0.9~1.8% and then decreases at wetness <0.9%. The experimental result demonstrates that CH₄ polymerization proceeding to CH₄ substantial cracking is an important factor involved in isotope reversal of over-mature shale gas. Moreover, δ¹³C₁–δ¹³C₂ decreases with an increase in experimental temperature prior to CH₄ substantial cracking. The values of δ¹³C₁ and δ¹³C₂ tend to equalize during CH₄ substantial cracking. The δ¹³C₁–δ¹³C₂ of mud gas investigated at different depths during shale gas drilling in the Sichuan Basin increases initially, then decreases with further increase in the depth, and finally tends to zero, with only a trace hydrocarbon gas being detectable. Thus, the approximately equal value between δ¹³C₁ and δ¹³C₂ for over-mature shale gas and very low wetness could potentially serve as useful criteria to screen CH₄ substantial cracking. Two geochemical indices to indicate CH₄ substantial cracking in a geological setting are proposed according to the variation production data with the geochemistry of over-mature shale gas in the Sichuan Basin, China. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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18 pages, 4310 KiB

Open AccessArticle

Characterization of the Lower Cretaceous Shale in Lishu Fault Depression, Southeastern Songliao Basin: Implications for Shale Gas Resources Potential

by Qilai Xie, Hao Xu and Shuang Yu

Energies 2022, 15(14), 5156; https://doi.org/10.3390/en15145156 - 15 Jul 2022

Viewed by 1612

Abstract

Large thickness of shales over 180.0 m was developed in the source rocks of the Shahezi and Yingcheng formations in the Lishu Fault Depression. Moreover, the high amount of gas content and the total hydrocarbon value of gas logging in several boreholes illustrate that there is a great potential of shale gas resources in this region. Therefore, an integrated characterization of shales from the lower Cretaceous Shahezi and Yingcheng formations was provided to evaluate shale gas resources potential. The measurement results illustrated that the organic-rich shale samples with kerogen type Ⅱ during high to over thermal maturity had a higher content of brittle minerals (>50%) and clay mineral dominated by illite. The shales had a total porosity of 3.11–4.70%, a permeability of 1.24 × 10⁻³–1.52 × 10⁻³ μm², and possessed pore types including dissolution pores, inter-layer pores of clay minerals, micro-fractures, intra-granular pores, and organic pores, which were dominated by micropores and mesopores (0.5–1.7 nm, 2.2–34.3 nm) with a significant contribution from OM and clay minerals. According to the N₂ adsorption isotherms, the pore volume was comprised primarily of mesopores with mean widths of 4.314–6.989 nm, while the surface area was comprised primarily of micropores with widths in ranges of 0.5–0.8 nm and 1.0–1.7 nm. Thus, the shales have a suitable porosity and permeability, indicating that fine storage capacity and favorable gas flow capacity occur in the Shahezi and Yingcheng formations, which exhibit a good reservoir quality and excellent exploration potential since the considerable thickness of shales could form a closed reservoir and served as cap rocks for in situ gas generation and accumulation. Especially, according to the measured CH₄ excess adsorption amount and the calculated maximum absolute adsorption capacities of CH₄ based on the Langmuir adsorption model, the estimated GIP values (1.388–3.307 m³/t) of the shales happened to be in a sampling depth under geological hydrostatic pressure and temperature conditions. This means that the shale storage capacity and high gas content from well site desorption completely met the standard of industrial exploitation when synthetically considering the GIP model. As a consequence, shales in the Shahezi and Yingcheng formations in the Lishu Fault Depression could be potential targets for shale gas exploration. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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26 pages, 8170 KiB

Open AccessArticle

Oil Retention and Its Main Controlling Factors in Lacustrine Shales from the Dongying Sag, Bohai Bay Basin, Eastern China

by Peng Cheng, Xianming Xiao, Qizhang Fan and Ping Gao

Energies 2022, 15(12), 4270; https://doi.org/10.3390/en15124270 - 10 Jun 2022

Cited by 5 | Viewed by 1557

Abstract

The investigation of the shale oil development potential of the lower third section–upper fourth section (ES₃³–ES₄¹) of the Eocene–Oligocene Shahejie Formation in the Dongying Sag, Bohai Bay Basin, eastern China, continues to be a scientific challenge. A total of 23 shale samples was collected from these strata, and the organic petrology, organic geochemistry, mineral composition, porosity, and pore structure of these samples and their relationships with the retained oils were investigated. The results indicated that these shales with type I–IIa kerogen are rich in lamalginite and its debris, and the Ro values of these shales range from 0.70% to 1.00%. The non-micropores (>2 nm) that are mainly developed from inorganic minerals are greater than the micropores (<2 nm) that largely contributed from the organic matter of the shale. The retained oil contents presented by the free hydrocarbons (S1) and extracted organic matter (EOM) exhibited significantly positive relationships with the total organic carbon (TOC) contents and micropore volumes, which may indicate that the retained oils are largely stored in organic matter micropores resulting from the volume swelling of kerogen. The total oils and their light compositions, as well as the S1/TOC and EOM/TOC values, increase with the burial depth of the shales, indicating that the content and mobility of the retained oils are largely controlled by the maturity of shales. This study predicts that the burial depth of favorable shale oil reservoirs in the Dongying Sag should be greater than 3500 m (Ro > 0.90%), and the siltstone or carbonate rock interlayer, especially with laminated or layered textures, will further control the sweet spot intervals of shale oil. This study provides new geological evidence for revealing the retention mechanism of shale oils and has practical significance for shale oil exploration and development in the Dongying Sag, Bohai Bay Basin. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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20 pages, 6347 KiB

Open AccessArticle

Pore Water and Its Influences on the Nanopore Structures of Deep Longmaxi Shales in the Luzhou Block of the Southern Sichuan Basin, China

by Haitao Gao, Peng Cheng, Wei Wu, Shenyang Liu, Chao Luo, Tengfei Li, Kesu Zhong and Hui Tian

Energies 2022, 15(11), 4053; https://doi.org/10.3390/en15114053 - 31 May 2022

Cited by 8 | Viewed by 1646

Abstract

In the Luzhou Block of the southern Sichuan Basin, the deep Longmaxi shales have become important exploration targets in recent years. However, the water-bearing properties of these shales are still unclear, which significantly limits evaluations of reservoir pore structures and gas-in-place (GIP) contents. In this study, twelve fresh shale core samples were collected at the well site, and the pore water (C_PW) and equilibrium water (C_EW) contents, as well as the pore structures of the shales, were analyzed under both as-received and dried conditions. The results indicate that the deep shales have low water-bearing extents with a pore water content (C_PW) of 3.82–16.67 mg/g, and that both the organic matter (OM) and inorganic matter (IM) pores can be used for pore water storage. The extent of influence of pore water on nonmicropores and IM pore structures is more significant than that on micropores and OM pore structures. Meanwhile, the pore water obviously reduces the retention effects of nanopores and may block nanopores with pore widths < 0.5 nm. An average of 40% of pore spaces were taken up by pore water in the studied deep shales in the Luzhou Block, and the residual pore surface area and pore volume of the shales were mainly contributed from micropores and nonmicropores, respectively. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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25 pages, 10109 KiB

Open AccessArticle

Pore Characteristics and Gas Preservation of the Lower Cambrian Shale in a Strongly Deformed Zone, Northern Chongqing, China

by Guangming Meng, Tengfei Li, Haifeng Gai and Xianming Xiao

Energies 2022, 15(8), 2956; https://doi.org/10.3390/en15082956 - 18 Apr 2022

Cited by 10 | Viewed by 2303

Abstract

The Lower Paleozoic marine shale in southern China has undergone several strong tectonic transformations in an extensive region outside the Sichuan Basin. Although some shale strata underwent strong deformation, they still contain a significant amount of shale gas. The gas preservation mechanism in the strongly deformed shale has become the focus of attention. In this paper, the Lower Cambrian gas-bearing shale samples with a strong deformation taken from an exploration well in northern Chongqing, China, were investigated on their pore types and structure, with the aim to reveal the reason for the gas preservation. The pore types of the Lower Cambrian shale are dominated by microfractures and interparticle (interP) pores occurring mainly between clay minerals and between organic matter (OM) and clay minerals, while pores within OM that can be observed by FE-SEM (field emission-scanning electron microscopy) are rare. The shale has a low porosity, with an average of 1.56%, which is mainly controlled by the clay mineral content. The adsorption experiments of low pressure N₂ (LPNA) and CO₂ (LPCA) indicate that the shale is rich in micropores and small mesopores (<2–3 nm) provided mainly by OM, but mesopores with a size range of 3–50 nm are underdeveloped. The shale, as revealed by LPNA data, has dominant slit-like or plate-like pores and an obvious low-pressure hysteresis (LPH), indicating a low gas diffusion. The deformed shale samples with a removal of OM by oxidation and their isolated kerogen further indicate that the LPH is completely related to OM, without any relationship with minerals, while an undeformed shale sample, taken from another well for a comparison, has no obvious LPH for both of its OM-removed sample and kerogen. Based on a comprehensive analysis of the relative data, it is suggested that the nanopores related to OM and clay minerals in the shale were significantly altered owing to the deformation, with a result of the pores being squeezed into the slit-like shape and converted into micropores. This extraordinary pore structure of the shale formed during the deformation process should be the main preservation mechanism of shale gas. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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17 pages, 2239 KiB

Open AccessArticle

Water Distribution in the Ultra-Deep Shale of the Wufeng–Longmaxi Formations from the Sichuan Basin

by Ping Gao, Xianming Xiao, Dongfeng Hu, Ruobing Liu, Yidong Cai, Tao Yuan and Guangming Meng

Energies 2022, 15(6), 2215; https://doi.org/10.3390/en15062215 - 17 Mar 2022

Cited by 6 | Viewed by 2229

Abstract

Recently, deep and ultra-deep shales (depth >3500 m) of the Lower Paleozoic Wufeng–Longmaxi formations (WF–LMX) have become attractive targets for shale gas exploration and development in China, and their gas contents may be influenced by the occurrence of water to some extent. However, the water content and its distribution in the different nanopores of the deep and ultra-deep shales have rarely been reported. In this study, a suite of the WF–LMX ultra-deep shale samples (5910–5965 m depth) from the Well PS1 was collected for water content measurements, and low-pressure CO₂ and N₂ adsorption experiments of both as-received and experimentally dried shale samples were carried out to investigate the distribution of water in the different nanopores. Since the studied ultra-deep shales are characterized by higher thermal maturity (equivalent vitrinite reflectance (EqVRo) > 2.5 %) and ultra-low water saturation, the pore water is generally dominated by irreducible water. The content of irreducible water of the studied shales varies from 1.57 to 13.66 mg/g, averaging 6.74 mg/g. Irreducible water may mainly occur in the clay-hosted pores, while it could also be hosted in parts of organic pores of organic-rich shales. Irreducible water is primarily distributed in non-micropores rather than in micropores of the studied shales, which mainly occurs in micopores with a diameter of 0.4–0.6 nm and mesopores with a diameter of 2–10 nm. Very low contents of irreducible water could reduce the specific surface area and volume of non-micropores of the shales to some extent, but the effect of irreducible water on the specific surface area of non-micropores was more significant than the volume of non-micropores, especially for organic-rich shale samples. The ultra-deep shale gas may be predominately composed of free gas, so low contents of irreducible water may play a limited role in its total gas contents. Overall, our findings can be helpful for a better understanding of water distribution in the highly-matured shales, and provide a scientific basis for ultra-deep shale gas exploration. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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23 pages, 9543 KiB

Open AccessArticle

Methane Storage Capacity of Permian Shales with Type III Kerogen in the Lower Yangtze Area, Eastern China

by Lei Pan, Ling Chen, Peng Cheng and Haifeng Gai

Energies 2022, 15(5), 1875; https://doi.org/10.3390/en15051875 - 3 Mar 2022

Cited by 4 | Viewed by 1643

Abstract

Marine–terrestrial transitional Permian shales occur throughout South China and have suitable geological and geochemical conditions for shale gas accumulation. However, the Permian shales have not made commercial exploitation, which causes uncertainly for future exploration. In this study, high-pressure methane (CH₄) adsorption experiments were carried out on the Permian shales in the Lower Yangtze area, and the influences of total organic carbon (TOC) content and temperature on adsorption parameters were investigated. The characteristics and main controlling factors of methane storage capacity (MSC) of the Permian shales are discussed. The results show that the maximum adsorption and the adsorbed phase density of these Permian samples are positively correlated with TOC contents but negatively correlated with temperatures. The pores of organic matter in shale, especially a large number of micropores and mesopores, can provide important sites for methane storage. Due to underdeveloped pore structure and poor connectivity, the methane adsorption capacities of the Permian shales are significantly lower than those of marine shales. Compared with the Longmaxi shales, the lower porosity and lower methane adsorption of the Permian shales are reasonable explanations for their lower gas-in-place (GIP) contents. It is not suitable to apply the index system of marine shales to the evaluation of marine–terrestrial transitional shales. The further exploration of Permian shales in the study area should be extended to overpressure stable reservoirs with high TOC contents (e.g., >5%), high porosity (e.g., >3%), and deep burial (e.g., >2000 m). Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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29 pages, 11466 KiB

Open AccessArticle

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

by Xiaoyan Zou, Xianqing Li, Jizhen Zhang, Huantong Li, Man Guo and Pei Zhao

Energies 2021, 14(22), 7603; https://doi.org/10.3390/en14227603 - 13 Nov 2021

Cited by 7 | Viewed by 1878

Abstract

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

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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Review

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26 pages, 6484 KiB

Open AccessReview

A Review of the Heterogeneity of Organic-Matter-Hosted Pores in Shale Reservoirs

by Yanming Zhao, Ping Gao, Qin Zhou, Xianming Xiao, Yijie Xing and Wei Liu

Energies 2022, 15(23), 8805; https://doi.org/10.3390/en15238805 - 22 Nov 2022

Cited by 5 | Viewed by 1690

Abstract

Organic-matter-hosted pores are fundamental pore spaces in shale reservoirs, which largely control the expulsion and storage of oil and gas. However, the strong heterogeneity of organic pores greatly increases the complexity of the pore network in shale reservoirs, which make shale reservoir evaluation challenging. The heterogeneity of organic pores in shale reservoirs has been
one of the hottest topics of recent years. In this review, the heterogeneity of organic pores in shale reservoirs and their controlling factors are systematically summarized. The formation and evolution of organic pores in shale reservoirs are directly linked to hydrocarbon generation and expulsion,
and the heterogeneity of organic pores is a result of various geological and geochemical factors. The development and heterogeneity of organic pores are basically controlled by the differences in kerogen types and maceral compositions of shale deposits, which are mainly attributed to the differences
in hydrocarbon generation capacity of different maceral compositions. Thermal maturation of organic matter is responsible for the formation and evolutionary mechanisms of organic pores and their heterogeneities. With increasing maturity, the increasing trend of pore volume and porosity diminishes. The reduction in macropore volume first appears, and the collapse of macropores could lead to an increase in micropore and mesopore volumes. An important turning point for the thermal evolution of OM is 3.5% Ro. At an Ro greater than 3.5%, the chemical structure of OM is transformed from amorphous carbon to crystalline graphite, and the hydrocarbon generation capacity of the OM has been exhausted, thus, resulting in the destruction of OM-hosted pores. The TOC content and mineral compositions of shale reservoirs affect the development and preservation of organic pores, but enhanced TOC content and brittle minerals may work against the development and preservation of organic porosity. Geological factors, e.g., compaction, diagenesis, pore fluid pressure and tectonic deformation, can also affect the organic pore structure of shale reservoirs to some extent, and their differences can enhance the heterogeneity of organic pores. On the basis of the above-mentioned understandings, this review also puts forward and discusses the problems existing in the current study of organic pore and its heterogeneity of shale reservoirs, and points out further research directions. Full article

(This article belongs to the Special Issue New Challenges in Shale Gas and Oil)

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