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Occurrence, Migration, and Accumulation Mechanisms of Hydrocarbons in Unconventional Reservoirs

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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "H: Geo-Energy".

Deadline for manuscript submissions: closed (15 August 2023) | Viewed by 10449

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Dr. Feng Yang

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Guest Editor

Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, China
Interests: petrophysics; fluid transport; unconventional reservoir
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Prof. Dr. Shang Xu

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Guest Editor

School of Geosciences, China University of Petroleum, Qingdao 266580, China
Interests: hydrocarbon accumulation mechanism; unconventional petroleum geology
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Dr. Qiulei Guo

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Guest Editor

School of Earth and Space Sciences, Peking University, Beijing 100871, China
Interests: unconventional reservoir geology and engineering

Special Issue Information

Dear Colleagues,

Unconventional shale reservoirs have attracted considerable attention because of their emergence as both source rocks and reservoirs. With the successful extraction of hydrocarbons from unconventional shale reservoirs, the USA has recently become the largest oil productive country in the world. There are also abundant shale oil and gas resources in other countries, e.g., China, Canada, Brazil, et al. However, the total production from shale reservoirs of the latter is relatively limited. Knowledge of the occurrence, migration, and accumulation of hydrocarbons in unconventional reservoirs is critical for the accurate evaluation of hydrocarbons storage capacity and production performance. Though there are many theories and hypotheses regarding the mechanisms of hydrocarbon migration and accumulation, and some insight has been gained recently by experimental measurement and numerical simulation, there is still a considerable lack of observational data concerning the physical and chemical effects of hydrocarbon migration and retention in shale reservoirs.

The aim of this Special Issue is to collect original research articles and review articles to better understand the occurrence, migration, and accumulation mechanisms of hydrocarbons in unconventional reservoirs. Papers about the physical and chemical changes in both hydrocarbon and source rocks during migration are highly encouraged. We are also interested in articles that explore novel methods to determine the physical state of hydrocarbons at different scales; for example, pore-scale modelling and upscaled experiments performed on core samples about hydrocarbon migration and accumulation.

Potential topics include but are not limited to the following:

Physical and chemical variation in hydrocarbons during migration;
Oil migration and expulsion in source rocks;
Pore structure and fluid phase behavior in shales;
Experimental and numerical simulation of hydrocarbons migration in shale and tight rocks;
Qualitative assessment of oil retention and accumulation;
Novel and effective methods to characterize physical properties of hydrocarbon and source rocks;

Dr. Feng Yang
Prof. Dr. Shang Xu
Dr. Qiulei Guo
Guest Editors

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Published Papers (5 papers)

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Research

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30 pages, 18888 KiB

Open AccessArticle

Study on the Tight Gas Accumulation Process and Model in the Transition Zone at the Margin of the Basin: A Case Study on the Permian Lower Shihezi Formation, Duguijiahan Block, Ordos Basin, Northern China

by Hanwen Yu, Jiaren Ye, Qiang Cao, Yiming Liu and Wei Zhang

Energies 2023, 16(3), 1493; https://doi.org/10.3390/en16031493 - 2 Feb 2023

Viewed by 2005

Abstract

Recent discoveries of oil and gas have principally been located in the central part of the Ordos Basin, which is a petroliferous basin with the largest discovered reserves and annual production of tight sandstone gas in China. For tight sandstone gas reservoirs in the transition zone of the basin margin, the process of natural gas accumulation has remained relatively vaguely understood, because of the transitional accumulation of geological conditions such as structure, sedimentation, and preservation. In this study, thin-section identification and scanning electron microscopic observations of the reservoir core, measurement of the physical properties of the reservoir, microscopic petrography research and measurement of the homogenization temperature of fluid inclusions, digital simulations, and laser Raman spectroscopy analysis were combined to analyze the process of natural gas accumulation of the Permian Lower Shihezi Formation in Duguijiahan block, Hangjinqi area, northern Ordos Basin. The results showed that the Lower Shihezi Formation reservoir in the Duguijiahan block began gas charging in the southern part as early as the Early Cretaceous (130–128 Ma), and then gradually charged in the northern part. Three stages were identified in the digital simulations of gas charging, i.e., the breakthrough, rapid, and fully saturated stages. The initial porosity of the Lower Shihezi Formation reservoir ranged between 28% and 40%. Later, because of strong compaction and interstitial filling during burial, the sandstone porosity decreased rapidly, and densification (porosity < 10%) occurred in the mid–late Jurassic. This late tectonic uplift caused a continuous reduction in ground temperature, and diagenesis had a weak effect on pore transformation. The present porosity of the Lower Shihezi Formation reservoir basically inherited its characteristics in the late Early Cretaceous. The current average porosity of the reservoir is 8.58%, and the average permeability is 0.88 mD, and it can thus be characterized as a tight reservoir. The gas accumulation process of the Lower Shihezi Formation has three stages: (1) the depositional stage (C–P), corresponding to the depositional stage of the source-reservoir-cap combination in gas reservoir; (2) the natural gas accumulation stage (T–K₁), corresponding to the period of rapid source rock maturation and natural gas charging step-by-step; and (3) the gas reservoir adjustment stage (K₂–present), corresponding to the period of uplift and natural gas charging in the early stage that gradually migrated and accumulated northward along the fracture zone. Finally, the gas accumulation model in the transition zone at the margin of basin was established. Full article

(This article belongs to the Special Issue Occurrence, Migration, and Accumulation Mechanisms of Hydrocarbons in Unconventional Reservoirs)

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15 pages, 3339 KiB

Open AccessArticle

Occurrence and Migration Mechanisms of Methane in Marine Shale Reservoirs

by Zhiming Hu, Ying Mu, Qiulei Guo, Wente Niu, Xianggang Duan, Jin Chang and Zhenkai Wu

Energies 2022, 15(23), 9043; https://doi.org/10.3390/en15239043 - 29 Nov 2022

Cited by 2 | Viewed by 1426

Abstract

The occurrence mechanism of methane is very important as evaluating the gas-bearing properties of marine shale reservoirs, and the evaluation of the development effect of shale gas wells need to focus on the migration mechanism of methane. In this study, LTNA technology and NMR technology were used to analyze the pores and methane of shale. The results show that inorganic pores have better connectivity, larger pore size, and micro–nano cracks between pores compared to organic pores. Most of the pores in shale are micropores and mesopores, which provide most of the specific surface area, but the contribution of macropores to pore volume cannot be ignored. Adsorbed gas volume depends on the pore surface area and gas pressure, while free gas volume depends on pore volume and gas pressure. The pore structure of micropores and mesopores is complex, and the specific surface area is large. The dispersion force between pore surface molecules and methane molecules is firm, which makes the pore wall an ideal enrichment space for adsorbed gas. Macropores have larger pore volumes and can store more free gas. In the process of gas well development, free gas is first discharged from pores under the action of the pressure gradient. As the pore pressure is lower than the critical desorption pressure, adsorbed gas begins to desorb in large quantities. It should be noted that the desorption process of adsorbed gas is slow and persistent, which makes it impossible for gas wells to achieve higher recovery in a shorter production cycle. Therefore, improving the recovery rate of adsorbed gas is the key to future research on shale gas development effects. This study is helpful in clarifying the occurrence and migration mechanism of methane in marine shale reservoirs and guiding the development of gas wells. Full article

(This article belongs to the Special Issue Occurrence, Migration, and Accumulation Mechanisms of Hydrocarbons in Unconventional Reservoirs)

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15 pages, 3010 KiB

Open AccessArticle

Adhesion Forces of Shale Oil Droplet on Mica Surface with Different Roughness: An Experimental Investigation Using Atomic Force Microscopy

by Ting’an Bai, Feng Yang, Huan Wang and He Zheng

Energies 2022, 15(17), 6460; https://doi.org/10.3390/en15176460 - 4 Sep 2022

Cited by 4 | Viewed by 2198

Abstract

In order to investigate the effect of rock surface roughness on the occurrence state of shale oil, muscovite mica was firstly characterized by performing atomic force microscopy (AFM). Two-dimensional (2D) images and the three-dimensional (3D) structure of the mica surface were obtained. Wettability of the micas was measured according to the sessile drop method using shale oil, collected from a lacustrine shale oil well drilling through the Yanchang Formation, Ordos Basin. Then, the adhesion forces between shale oil and mica surface with a different roughness were finely measured using AFM mounted with the shale oil modified probe tips. The adhesion force curves at the approaching and retract modes were obtained. The results show that the average roughness value of the mica samples was about 1 nm, while the maximum height was up to 4 nm. The contact angle between shale oil and mica ranged from 128.73° to 145.81°, and increased with increasing surface roughness, which can be described by the Wenzel model. The adhesion force between shale oil and mica also increased with an increasing contact area. Shale oil can fill the deep valleys on the rough surface of rocks and then form microscopic storage for oil droplets. The maximum adhesion force, reached at a distance of about 5–10 nm between shale oil droplets and micas, was between 14 and 30 nN. The adhesion force disappeared when the distance was larger than 40 nm. These indicate that shale oil in pores with a diameter of less than 10 nm was tightly adsorbed, and formed a layered accumulation pattern. Additional energy is needed to decrease the disjoining pressure and then separate shale oil from these tight pores. Shale oil is freely movable at pores with pore diameters of larger than 40 nm. This work provides a new insight about the interaction between shale oil and rock, and helps to understand the occurrence mechanism of shale oil. Full article

(This article belongs to the Special Issue Occurrence, Migration, and Accumulation Mechanisms of Hydrocarbons in Unconventional Reservoirs)

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17 pages, 10428 KiB

Open AccessArticle

Laminar Structure and Reservoir Quality of Shales with High Clay Mineral Content in the Qingshankou Formation, Songliao Basin

by Ganlin Hua, Songtao Wu, Jinyou Zhang, Rongchang Liu, Modi Guan, Yi Cai, Mengying Li and Surong Zhang

Energies 2022, 15(17), 6132; https://doi.org/10.3390/en15176132 - 24 Aug 2022

Cited by 7 | Viewed by 1708

Abstract

This paper investigates high-maturity organic matter-rich shales with high clay mineral contents in the Qingshankou Formation, in the Gulong Depression of the Songliao Basin, at a sub-millimeter scale, using a new laminar division method based on XRF data. The influence of laminar structure on reservoir quality is examined using a combination of geochemistry, mineralogy, and pore structures. Explanatory models are established. Three types of laminar units are distinguished in the study area based on differences in pore structure. These are clay mineral laminae (UA), clay mineral-Ostracod laminae (UB), and clay mineral-felsic laminae (UC). UA has illite intergranular pores, micro-fractures, and organic pores, with diameters of 0.5~2 μm. UB primarily contains Ostracod shell margin fractures, pyrite intergranular pores, and chlorite intragranular pores. UC contains albite and illite intergranular pores. Nitrogen adsorption tests show that UA has the highest clay content and the best specific pore volume and specific surface area, indicating that clay minerals are the main contributors to the pores in this type of unit. 2D–3D models of different laminae reveal that carbonate cement is widely developed in UB and UC, but dissolution pores are less developed, with the result that the porosity of UA is two to three times greater than that of UB or UC. It appears that intergranular pores and fractures, formed during the transformation of clay minerals during the advanced thermal evolution stage, are the main contributors to storage space and flow channels. Thermal evolution, clay mineral transformation, and carbonate cementation are the key factors causing differences between laminar units. In addition, clay mineral laminae (UA) are the most important laminar units for shale oil enrichment in the study area. This finding is of great significance for accurately predicting the distribution of shale “sweet spots” and guiding shale oil and gas exploration. Full article

(This article belongs to the Special Issue Occurrence, Migration, and Accumulation Mechanisms of Hydrocarbons in Unconventional Reservoirs)

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Review

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17 pages, 3028 KiB

Open AccessReview

The Importance of Laminae for China Lacustrine Shale Oil Enrichment: A Review

by Shang Xu and Qiyang Gou

Energies 2023, 16(4), 1661; https://doi.org/10.3390/en16041661 - 7 Feb 2023

Cited by 6 | Viewed by 2086

Abstract

The laminar structure of shale system has an important influence on the evaluation of hydrocarbon source rock quality, reservoir quality, and engineering quality, and it is receiving increasing attention. A systematic study of the lamina structure is not only of great scientific significance but also of vital practical importance for shale oil production. In this paper, the identification and description classification of shale laminae are first reviewed. Multiple scales and types indicate that a combination of different probe techniques is the basis for an accurate evaluation of shale laminar characteristics. The influence of laminae on shale reservoir, oil-bearing, mobility, and fracability properties is discussed systematically. A comparative analysis shows that shale systems with well-developed lamination facilitate the development of bedding fractures, thus improving the shale storage space. The average pore size and pore connectivity are also enhanced. These factors synergistically control the superior retention and flow capacity of shale oil in laminated shales. In such conditions, the high production of shale oil wells can still be achieved even if complex networks of fracturing cracks are difficult to form in shale systems with well-developed lamination. This work is helpful to reveal the enrichment mechanism of shale oil and clarify the high-yield law of hydrocarbons, so as to guide the selection of sweet spots. Full article

(This article belongs to the Special Issue Occurrence, Migration, and Accumulation Mechanisms of Hydrocarbons in Unconventional Reservoirs)

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