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2021 Vol. 39, No. 1

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2021, 39(1)
Preface: New advances in unconventional petroleum sedimentology in China
ZOU CaiNeng, QIU Zhen
2021, 39(1): 1-9. doi: 10.14027/j.issn.1000-0550.2021.001
Significant progress has been gradually made on the exploration and development of unconventional petroleum on land in China over the past 20 years. In 2019, unconventional petroleum production in China accounted for about 23% of the total oil and gas production that year and reached 70 million tons of oil equivalent in 2020, indicating China’s petroleum industry has entered a new stage of unconventional petroleum development. Unconventional petroleum sedimentology as an important part of unconventional petroleum geology, and the theoretical system has been gained more and more attention over recent years, leading to important studies, such as “Depositional model of gravity flows, sandy debris in a deep lacustrine basin”, “Depositional model of marine and/or lacustrine organic-rich shale”, “Abundant micro- and nano- pore throat systems developed in fine-grained sedimentary rocks”, and “Sweet-spot areas (intervals) of unconventional petroleum resulted from coupling the sedimentology of several geological events”. This special issue systematically assembles new advances made in the unconventional petroleum sedimentology of China by Chinese unconventional petroleum sedimentologists in recent years. These studies refer to about 30 sets of unconventional petroleum strata (intervals) from the Proterozoic to Cenozoic in more than 50 basins, providing an important theoretical basis and technical support for unconventional petroleum exploration and development. Unconventional petroleum sedimentology would focus on deposition and formation of“sweet⁃spot box” and “sweet⁃spot group ”developed in unconventional petroleum industry in future, and further enhance efficiency of unconventional petroleum exploration and development.
Oil Shale Mineralization and Geological Events in China
LIU Rong, ZHANG Kun, LIU ZhaoJun, YAN Xu, YU JiaQi
2021, 39(1): 10-28. doi: 10.14027/j.issn.1000-0550.2020.104
Research findings increasingly indicate that geological events such as volcanic action, hydrothermal fluids, ocean anoxia, climate mutation, transgression, gravity flow and so on are frequently evident in the sedimentary sequences of oil shale (a type of fine-grained sedimentary rock rich in organic matter). High paleolake productivity and a stable reducing environment are important considerations for oil shale mineralization. A summary of the genesis and distribution behavior of oil shale in 50 basins (95 mining areas) in China reveals that the geological events listed above have led to various degrees of algal and microbial blooms which changed the water conditions in ancient lakes and formed the stable layered lacustrine environments that are important for oil shale mineralization. However, frequent volcanic eruptions release large amounts of gas and ash, and intermittent hydrothermal fluids at high temperature and pressure. Also, frequent gravity flow is not conducive to oil shale enrichment. The present study focuses only on a single event. In future it will be necessary to promote interdisciplinary studies of sedimentology, geochemistry and microbiology from the perspective of earth system science to reveal the ecological cyclic accumulation processes of unconventional oil and gas resources and major geological environment mutation. This will add to our understanding of the combined effects of various geological events on oil shale mineralization, and enrich the theory of unconventional oil and gas sedimentology.
Progress on sedimentology of transitional facies shales and new discoveries of shale gas
DONG DaZhong, QIU Zhen, ZHANG LeiFu, LI ShuXin, ZHANG Qin, LI XingTao, ZHANG SuRong, LIU HanLin, WANG YuMan
2021, 39(1): 29-45. doi: 10.14027/j.issn.1000-0550.2021.002
The successful scale-beneficial exploitation of marine shale gas in the Sichuan Basin has made China's shale gas step into the fast development lane,and establish the important strategic position in China's energy security. China's marine-continental transitional facies shale gas resources are abundant, which are expected to be the real field in terms of increasing the reserves and enhancing the production of China's shale gas. To date,the exploration and development of transitional shale gas and the study of its geological evaluation are still in the initial stage,and studies regarding unconventional petroleum sedimentology such as the development characteristics and distribution pattern of organic-rich shales, the sedimentary model of transitional shales, and organic matter enrichment mechanism, require further investigation. By defining the relevant concepts and characteristics of transition facies,typical transitional shale gas layers at home and abroad are reviewed, and the research status and new progress of transitional shale sedimentology are summarized, revealing the favorable environment for transitional facies organic-rich shales, sedimentary model, and the fundamental shale gas reservoir characteristics. The breakthroughs and new discoveries of transitional shale gas in several basins including the Ordos Basin,Sichuan Basin,Bohai Bay Basin and so on are systematically discussed,demonstrating the good prospects of the transitional facies shale gas exploration and development. Considering the transitional facies shales are often interlayered with coals,it is indicated that shale gas,coalbed methane could be developed in a collective manner,the “volume development” of transitional facies layers could be applied,so as to effectively promote the high quality development of the natural gas industry in China.
Xenocomformity: A stratigraphic surface representing fundamental and abrupt paleoenvironmental change
GAO Yuan, CARROLL Alan R., WANG ChengShan
2021, 39(1): 46-57. doi: 10.14027/j.issn.1000-0550.2020.128
A xenoconformity is a stratigraphic surface or gradational interval that records a fundamental, abrupt, and persistent change in sedimentary facies across basinal to global scales. Different from traditional concepts of stratigraphic surfaces which are on the premise that the depositional environments either have not changed appreciably through time, or else have changed very slowly, xenoconformities emphasize relatively rapid, comprehensive paleoenvironmental changes, and therefore do not follow the classic “Walther’s law”. Continental environments are of great varieties and of high-amplitude changes, and favor the formation of xenoconformities. Examples of continental xenoconformities include those in Paleogene Greenriver Formation, in Cretaceous Songliao Basin and in Permian Junggar Basin. Marine xenoconformities represent paleoclimatic and paleoenvironmental events at global scale, including those at Paleocene-Eocene boundary, at Cretaceous-Paleogene boundary and at Cryogenian-Ediacaran boundary. We expect researches on xenoconformities could be of help on interpreting major paleoenvironmental tipping points and their responses in stratigraphic records. Furthermore, as paleoenvironmental changes control processes of organic matter burial, researches on xenoconformities could also help petroleum exploration.
Mudstone diagenesis: Research advances and prospects
ZHAO JianHua, JIN ZhiJun
2021, 39(1): 58-72. doi: 10.14027/j.issn.1000-0550.2020.133
Driven by the shale oil and gas revolution,important advances have been made in mudstone diagenesis, which is a frontier in sedimentology and petroleum geology. Mudstone diagenesis of not only controls the generation and migration of oil and gas, but also has important influence on the composition, microstructure, reservoir physical and mechanical properties. The research status and prospects of the study were documented. At present, the research advances of mudstone diagenesis is mainly reflected in 4 aspects: (1)diagenetic evolution of inorganic minerals; (2)diagenetic evolution of organic matter and the development of organic matter-hosted pores; (3)driving mechanism and physical response of mudstone diagenesis; (4)effects of diagenesis on mechanical properties of mudstone. The study of mudstone diagenesis will continue to complement and improve the basin diagenesis system, and provide theoretical support for the exploration and development of unconventional oil and gas resources. The study of mudstone diagenesis system at different scales under the control of multi-field coupling indicates a profound and extensive development prospect.
A Review on Sedimentary Processes of Marine Mudstones and Shales
LI YiFan, WEI XiaoJie, FAN TaiLiang
2021, 39(1): 73-87. doi: 10.14027/j.issn.1000-0550.2020.131
With the development of global unconventional resources, research on sedimentary features and processes have received more attention. Significant progresses have been achieved on classification of shale facies, interpretation of sedimentary processes, and reconstruction of sequence stratigraphy. Shale facies have been classified at centimeter to millimeter scale. Nomenclature of marine mudstones and shales should be based on their texture (grain size), bedding and composition. Regarding texture, proportion of sands, coarse muds and fine muds should be considered. Regarding bedding, geometry, continuity, and shape should be described. About mineral composition, components of clay minerals, quartz, and carbonate minerals would be compared. Sedimentary processes of marine mudstones and shales are various. Main transport and sedimentary processes of fine-grained sediments in in the marginal shelf sea include eolian input, hypopycnal plumes, gravity driven transport processes (prodelta turbidity currents, hyperpycnal flows, and wave and current enhanced sediment gravity flow), and storm induced currents. In the extensive, flat, and shallow epicontinental seas, bottom current driven by seasonal wind or tide is the dominated transport process. A few types of graded bed would be produced by gravity driven transport processes as well as storm driven bottom current, whereas offshore bottom current would result in laminated shale. Detailed description of sedimentary characteristics promotes establishment of sequence stratigraphy in fine-grained sedimentary rocks. Recognition of sequence boundaries is to find comparable main erosional surfaces, while division of parasequence in a depositional sequence depends on recognition of flooding surfaces (secondary erosional surfaces) and quantitative analysis of sedimentary features. Combining with flume experiments, detailed description of sedimentary features and quantitative analysis are main research directions for studies on sedimentary processes of marine mudstones and shales.
Discussion on Research of Deep⁃water Gravity Flow Deposition in Lacustrine Basin
YANG Tian, CAO YingChang, TIAN JingChun
2021, 39(1): 88-111. doi: 10.14027/j.issn.1000-0550.2020.037
In the last few years, great progress has been achieved in the understanding of gravity⁃flow deposits in marine basins, but the study of gravity⁃flow deposits in lacustrine basins lags far behind. Research on gravity⁃flow deposits in China and globally are summarized here to clearly illustrate the shortcomings of gravity⁃flow deposit research for lacustrine basins. Clarifying meanings and relationships between different terminologies, as well as detailed interpretation, are reliable ways of reducing terminology confusion. Gravity⁃flow transformation, flow⁃state transformation, and lubrication are mainly dynamic processes of gravity flow. Transportation, hindered settling, turbulence damping, and traction carpet are dynamic processes of gravity flow, whereas settling, sediment re⁃transportation, sustained supply from flooding rivers, and the settling of buoyant plumes, are the main mechanisms of gravity flow in a lacustrine basin. Comprehensive analysis of detailed facies⁃tract types, analysis of the internal structure of massive sandstone, and analysis of the characteristics of inverse⁃then⁃normal sequences are all important means of revealing the genesis of gravity⁃flow deposits. Gravity⁃flow channels are formed by erosion due to supercritically turbid currents. The material composition and source of gravity flow are dominated by factors external to the basin, while the effective segregation of gravity flow types is dominated by internal basin factors. The integration of internal and external basin factors are termed “source⁃to⁃sink” systems, covering the overall gravity⁃flow evolutionary processes. The depositional model of gravity flow caused by sustained supply from flooding rivers comprises gravity⁃depositional elements, which include channel⁃levée deposits, channel and lobe transition zones, and lobe deposits. Depositional models of gravity flow due to sediment re⁃transportation contain gravity⁃depositional elements, such as sediment failure at the delta front, chaotic deposits, and lobe deposits. Gravity⁃flow deposits are the main reservoirs for unconventional oil and gas in a lacustrine basin. The fine⁃grained deposits resulting from flow transformation have the potential for oil and gas generation and enrichment — and, at the same time, they are appropriate for hydraulic fracturing, which makes them the foremost lithofacies association for sweet⁃spot development in shale oil and gas exploration.
Sedimentary Enrichment Mode and Effect Analysis of Exploration and Development: A case study of Fuyu reservoir tight oil in northern Songliao Basin
MENG QiAn, ZHAO Bo, CHEN ShuMin, LIN TieFeng, ZHOU YongBing, QIAO Wei
2021, 39(1): 112-125. doi: 10.14027/j.issn.1000-0550.2020.057
Taking the Fuyu reservoir in northern Songliao Basin as an example, this paper discusses the sedimentary enrichment mode and exploration and development effect for tight oil. The characteristics of the tight sandstone reservoir in the Fuyu oil layer (e.g., many longitudinal small layers, large interlayer span, plane sand body staggered distribution, thin single layer thickness, poor continuity and strong heterogeneity), the fine geological anatomy of the development test area was investigated using abundant drilling and three-dimensional seismic data. The development characteristics of the small-level stacked dense channel sandbody in the Fuyu oil layer, and a prediction technology for the stacked sandbodies, are proposed. This provides a scientific basis for large-scale geology and engineering of multiple “sweet spots” in tight oil. The results show that there are three types of superimposed sandbodies developed in the Fuyu oil layer: the main channel, the main thin channel, and the thin distributary channel. The pre-stack time migration technology applied to a viscoelastic medium achieved high-resolution interpretation of the thin layers in the FI and FII reservoir groups, and improved the vertical resolution of imaging. High-resolution seismic imaging and Z-inversion technology improves the ability to predict 3⁃5 m channel sandstone reservoirs. The plane distribution and vertical position of the superimposed sandbodies are clearly described, which effectively guides well location for exploration and development of terrestrial tight oil in the Fuyu reservoir. The development technology may be selected depending on the differences between the superimposed sandbodies and maximize the production rate of tight oil. Daqing Oilfield has achieved good exploration and development results over many years of developing greater geological understanding, sandbody identification and exploration technology.
Sedimentary system and sandstone reservoir petrology of a shallow water delta: Case study of the Shan-1 and He-8 members in the western Ordos Basin
ZHU ShiFa, CUI Hang, CHEN JiaHao, LUO GaoJun, WANG WenYu, YANG Yi, SHI Yi
2021, 39(1): 126-139. doi: 10.14027/j.issn.1000-0550.2020.115
The shallow water delta has become the focus of sedimentology research and oil and gas exploration. Abundant oil and gas resources have been found in conventional and unconventional sandstone reservoirs within shallow water deltas. Large scale shallow water delta deposits were developed during the Mesozoic to Paleozoic in the Ordos Basin. Based on the core, well logging, and geochemical data, the parent rock, sedimentary filling, sand body distribution, and reservoir characteristics of the shallow water deltas from the Shan-1 and He-8 Members in the western part of the basin are studied in this paper, which provides a geological basis for conventional sandstone reservoirs and tight oil-gas exploration in the study area. The results show that the heavy mineral assemblages of sandstone in the south and north regions of the study area are similar, mainly composed of zircon and white titanium. Using LA / Yb rare earth elements (REEs), Dickinson triangle diagrams, REE distribution models, and the analysis of detrital zircon dating, we find that the provenance in the north is mainly from Archean and Proterozoic granites, gneisses, and quartz sandstone of the Alashan ancient land in the northwest part of the basin and affects the northern part of the Longdong area southward. The provenance in the south part of the basin mainly comes from Archean and Proterozoic granites, gneisses, and schists in the North Qinling area and affects the central and southern Longdong area northward. Meandering river delta was developed in the Shan-1 Member, braided river delta was developed in the He-8 Member. Isolated, and vertical superimposed underwater branch channel sand bodies are the main sand body types. During the depositional period of the He-8 Member, the ancient lakes have shrunk; the delta lobes formed by the North-South provenance converged and mixed in the southeast part of Huanxian County in the central part of the study area. Sandstone types are mainly lithic quartz sandstone, quartz sandstone, and lithic sandstone, with medium to poor textural maturity. Sandstone has undergone a heavy diagenetic transformation and complex water-rock reaction. Compaction is the main cause of reservoir densification, and multi-stage cementation makes the pore throat structure of the sandstone very complex. The pores are micro-nano scale, and the main types are dissolution pores of rock debris and residual intergranular pores, as well as kaolinite intercrystal pores. The studied sandstone belongs to the unconventional tight sandstone reservoir as a whole. Sand body distribution and "sweet spot" prediction is the key to tight oil and gas exploration in the study area.
Factors Controlling Differential Enrichment of Organic Matter in Saline Lacustrine Rift Basin: A case study of third member Shahejie Fm in Dongpu Depression
HU Tao, PANG XiongQi, JIANG FuJie, WANG QiFeng, XU TianWu, WU GuanYun, CAI Zhe, YU JiWang
2021, 39(1): 140-152. doi: 10.14027/j.issn.1000-0550.2020.125
Saline shales are widely developed in the Mesozoic and Cenozoic continental saline lacustrine rift basins in China, containing abundant shale oil resources. The development of "sweet spots" in shale oil plays is closely related to the organic matter richness, but the key factors controlling differential enrichment of organic matter in saline shales are still unclear few and controversial. Taking the Dongpu Depression in the Bohai Bay Basin as an example, this study conducted the TOC, FE-SEM, energy spectrum, major, trace, and rare earth elements analyses on the Es3 (Third member of the Paleogene Shahejie Fm) Shales, exploring the correlations among the organic matter richness with paleoclimate, paleo-salinity, sedimentation rate, paleo-productivity, and redox condition. Results show that the organic matter enrichment degree is jointly controlled by paleo-productivity, paleo-salinity and sedimentation rate. The greater the paleo-productivity is, the greater the organic matter enrichment degree is. With the increasing paleo-salinity and sedimentation rate, the organic matter enrichment degree increases first and then decreases. Redox conditions have little effect on the enrichment of organic matter, which is mainly related to the generally strong reduction condition of the shale cores. The combination of high paleo-productivity, proper paleo-salinity and proper sedimentation rate is most favorable for the organic matter enrichment. Differential enrichment of organic matter within shales is one of the important contents for Unconventional Petroleum Sedimentology studies. Revealing the key factors controlling differential enrichment of organic matter is of great significance to the exploration and development of lacustrine shale oil in China.
Research Advances and Prospect of Mixed Deposition
LI QuanQuan, BAO ZhiDong, XIAO YuXiang, CHEN JianYang, LI ZhongCheng, WANG ZhenJun, LIU MinZhao, LI ZhuoLun, XU XiTing, CAO Fan
2021, 39(1): 153-167. doi: 10.14027/j.issn.1000-0550.2020.140
Mixed deposition has been in the limelight since the 1950s from a 'niche' type of sedimentation. After many scholars' researches, mixed sediments are widely distributed in marine, continental, transitional sedimentary and all kinds of environments, which indicates that the mixed sediments and siliciclastic rocks and carbonate rocks belong to the same continuum, and mixed deposition is a kind of widely existed sedimentary type. On the basis of previous studies, this paper summarizes the concepts related to mixed deposition and distinguishes the different types and scales of mixed deposition. In terms of the formation process of mixed deposition, four types of mixed deposition models proposed by Mount are discussed in detail. The four types include intermittent mixed deposition caused by high intensity events, gradual contact of phase boundaries, in-situ calcareous biological assemblages, denudation of tectonic boundaries. The formation of mixed deposition is controlled by climate, tectonics, the relative changes of sea (lake) plane, hydrodynamics, wind field and so on. The main control factors are different in all kinds of environments. There are still some problems need to be solved, the interaction between the components in the formation of mixed sediments, the differences between multicomponent mixed sediments and mixed siliciclastic-carbonate sediments, complex lithology logging identification of mixed sediments, sequence division of mixed fine sediments in deep water environment, applicability of mixed deposition models in different sedimentary environments, restoration of the evolutionary sequence of mixed sedimentary diagenesis. The studies of these aspects have great significance on improving the dynamic theory, improving the sequence theory of mixed fine sediments in deep water environment, restoring mixed sedimentary paleoenvironment, reconstructing the mixed deposition process, clarifying the distribution of mixed sedimentary systems, predicting the reservoir spatial distribution, guiding unconventional hydrocarbon exploration and development in mixed sedimentary reservoirs and so on.
Facies Characteristics and Depositional Processes of Shelf Mudstones: Examples from the Late Cretaceous Western Interior Seaway of North America
LI ZhiYang
2021, 39(1): 168-180. doi: 10.14027/j.issn.1000-0550.2020.126
Conventionally, most fine-grained sedimentary rocks (mudstones or shales) were considered as deposited in relatively deep-water, low-energy environments. As the most common sedimentary rock type on the Earth’s surface, mudstones/shales contain valuable information for the interpretation of paleogeography and paleoclimate conditions through the geologic history. In addition to being effective source rocks, recent advances in drilling and completion technologies have also unlocked the enormous potential of mudstones/shales as important unconventional reservoirs. Over the past decade, significant research progress has been made in the flume experiments of fine-grained sediments, detailed facies analysis of mudstones, and close examinations of the petrographic characteristics. These studies collectively have provided novel insights into the depositional processes and depositional model of mudstones. Most mudstones (even many that are organic-rich) were indeed deposited under relatively shallow-water and energetic environments. Different depositional processes (bottom currents) dominantly control the facies variability, as well as the burial and accumulation of organic matter in mudstones. Taking the Late Cretaceous Western Interior Seaway of North America as an example, this paper summarizes the primary depositional processes, facies characteristics, and the potential for the accumulation of organic matter in mudstones deposited from different shelf environments ranging from proximal prodelta/lower shoreface to distal outer shelf environment. Because depositional processes and environments are the most important factor determining the heterogeneity in the composition, sedimentary facies, and petrophysical characteristics of mudstones, the summarized facies and depositional model for mudstones deposited in different shelf environments can aid the reconstruction of paleogeography and paleoclimate conditions from ancient mudstone successions, and provide guidance for evaluating the potential of shelf mudstones as effective source rocks or reservoir rocks.
Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development
SHI ZhenSheng, QIU Zhen
2021, 39(1): 181-196. doi: 10.14027/j.issn.1000-0550.2020.097
Bedding type of black marine fine-grained sediments is an important indicator for reconstructing the paleo-sedimentary environment, since it affects the quality and fracturability of shale in oil and gas reservoirs. After fine-grained materials are formed by biogenic, chemical or biochemical debris genesis, they are carried into the ocean in the form of single particles, flocs, muddy intraclasts, lithics, organo-mineralic aggregates, and fecal pellets by wind, low-density flow, gravity, and bottom currents. The fine-grained sediment is deposited via suspension settling and/or advective sediment transport processes close to the sediment-water interface. After the fine-grained materials settle, two types of laminae (clayey and silty), two types of laminaset (clayey and silty) and two types of bedding (graded and homogeneous beds) develop, forming five major bedding types (massive, graded, rhythmic and varve, horizontal, and cross-bedding). Of these, the massive bedding may be divided into bioturbation-formed type and homogeneous type. In addition, horizontal bedding occurs in four types: graded (claystone), siltstone-bearing, graded (siltstone to claystone), and interlaminated siltstone and claystone. In black marine fine-grained sediments, the clayey and silty laminae differ in material composition, pore type and pore structure, plane porosity, pore size distribution and microfracture type and density, resulting in different shale reservoir quality. In addition, bedding types vary in geomechanical properties and crack-propagation behavior due to the different development of the stratum (thickness, thickness difference, continuity, morphology and geometric relationship).
Logging Evaluation of Solid Bitumen in Tight Carbonate in Deep-buried and Ultra-deep-buried Strata of the Central Sichuan Basin
SONG ZeZhang, LIU GuangDi, LUO Bing, ZENG QingCai, TIAN XingWang, DAI Xin, JIANG Ren, WANG YunLong, LI Qiang, ZHAO JunYi, ABULA Abide, ZANG JianPeng
2021, 39(1): 197-211. doi: 10.14027/j.issn.1000-0550.2020.127
The accumulation and evolution of unconventional oil and gas , locating sweet spots and quantitative evaluation of resource potential in ultra⁃deep, ultra⁃old strata are the major difficulties in unconventional petroleum sedimentology. The development of relevant quantitative evaluation techniques and theories is of practical significance to promote "two deep and one unconventional" petroleum exploration policy in China. Solid bitumen is direct evidence of the existence of paleo-reservoirs, which is of great theoretical and practical significance to explore the distribution, scale and petroleum accumulation mechanism of paleo-reservoirs, and to demonstrate the exploration potential of oil and gas in the study area. In this paper, the fourth member of the Dengying Formation of Upper Sinian in Central Sichuan was taken as the research object. Firstly, the solid bitumen was characterized by core observation, microscopic observation in thin section and image processing method. Then, a multi-mineral volumetric model was established, and the inversion of the solid bitumen was carried out with the aid of the conventional logging curve combination (DEN-RT-RXO-CNL-AC), which overcame the substantial heterogeneity of the distribution of solid bitumen and quantitatively evaluated the content and distribution of the solid bitumen in the fourth member of the Dengying Formation. The results show that: (1) Most of the Dengsi member bitumens exist in the pores in a half-filled state, and the solid content of the reservoir can reach 4.62%, with an average value of 2.20%; vertically, the content of the solid bitumen in the upper subsection of the fourth member of Dengying Formation is generally higher than that in the lower subsection. (2) The reef-flat facies in the platform margin of Gaoshiti-Moxi area, which closed to the Deyang-Anyue rift trough, are the desirable areas for the formation of paleo-oil-reservoirs. The content of solid bitumen in reef-flat facies is generally higher than that in the eastern gas-bearing area. (3) There is no significant difference in the content of solid bitumen in the north-south direction due to oil charging from west to East. The existence of solid bitumen in the fourth member of Dengying formation in the north slope of the paleo-uplift confirms the existence of the paleo oil reservoir in the north slope and also confirms the oil and gas resources potential of the north slope from the side. From the point of view of solid bitumen, it is necessary to continue to deepen the deep and ultra-deep oil and gas exploration in the north slope of the Central Sichuan Paleo-Uplift.
Research Advances on Characteristics of Pores and Fractures in Coal Seams
LIU ShiQi, WANG He, WANG Ran, GAO DeYi, Ashutosh Tripathy
2021, 39(1): 212-230. doi: 10.14027/j.issn.1000-0550.2020.064
The study of the characteristics and relationships of pores and fractures in coal seams is an important research in unconventional petroleum sedimentology, and is of great significance for understanding the micromechanisms of the existence, state, and mass transfer process of coal seam fluid, and for optimizing the exploitation of coalbed methane (CBM) where geologically appropriate. The pores and fractures in coal seams have complex genesis, with wide-ranging scale and strong heterogeneity. Their characteristics are the result of the combined actions of coalification, metamorphism types, tectonic evolution, coal components, and underground fluids. Coalification is the internal cause of pore and fracture characteristics, while tectonic stress is the major external cause. The combined actions of internal and external causes have formed the presently observed characteristics of pores and fractures in coal of different rank, in different coal-bearing basins, and in different tectonic locations. Moreover, the coal-forming materials, sedimentary environment, burial history and thermal history of coal-bearing basins each play important roles in determining the present pore and fracture characteristics. The study of pore and fracture characteristics is closely related to the efficient exploitation of CBM, which implies that considering pores and fractures in coal as an entire desorption–diffusion–seepage network in further research will be a sensible approach. The source and effectiveness of coal permeability is controlled by pore/fracture connected networks, nanoscale pore/fracture characteristics and their syntagmatic relations: interfacial properties of pores, fractures and coal seam fluids, together with multiscale pore/fracture characterization, all need to be further researched. The development and application of digital petrophysical characterization technology provides new methods and ideas for research into pore and fracture characteristics.
Process⁃product Linkages and Feedback Mechanisms of Deepwater Source⁃to⁃sink Responses to Multi⁃scale Climate Changes
GONG ChengLin, QI Kun, XU Jie, LIU XiTing, WANG YingMin
2021, 39(1): 231-252. doi: 10.14027/j.issn.1000-0550.2020.035
The responses of source⁃to⁃sink (S2S) systems to climate change is the frontier area for new research directions in deepwater sedimentology and stratigraphy. The primary purposes of this research are to review deepwater depositional responses of outer shelf⁃to⁃deepwater basin S2S systems (deepwater S2S systems) to tectonic⁃ and historical⁃scale climate changes. This revealed two main process⁃product linkages (buffered and reactive) and feedback mechanisms of S2S systems to climate change. Buffered deepwater S2S systems have wide transfer zones and a long response timescale of ≥ 104 yr. Favorable geological settings forming this type of deep⁃water S2S systems are: wide shelf without channels reached at inner shelf and Icehouse climates. Sediment routing processes along buffered deepwater S2S systems are driven by accommodation (i.e., compatible with classical Exxon sequence stratigraphic models). There was vigorous turbidity⁃current activity and resulting sandy products during tectonic⁃to orbital⁃scale cooling periods, and by contrast, weak turbidity⁃current activity and resulting muddy deposits during tectonic⁃scale to orbital⁃scale warming periods. Suborbital⁃to historical⁃scale climatic fluctuations might be “masked” by the rising sea⁃level, and thus had little effect on sediment⁃routing processes in deep water. Reactive deepwater S2S systems, by contrast, have narrow transfer zones and a relatively short response time of ≤ 104 yr. Favorable geological settings forming this type of deepwater S2S system are: narrow shelf, greenhouse climates, close proximity of canyon heads to shorelines, lacustrine basins, shelf⁃margin deltas overreached at the shelf break. These were more sensitive to sediment supply, and thus they are supply⁃driven (i.e., incompatible with classical, Exxon sequence stratigraphic models). Along reactive deepwater S2S systems, any suborbital⁃scale to historical⁃scale climatic fluctuations are able to trigger the changes in sediment supply, regardless of sea⁃level conditions.
Laboratory Experiments on Sedimentation of Fine-grained Sediment: A prospect review
ZHOU ChuanMin, ZHANG ZhiJie, QIU Zhen, CHENG DaWei, YUAN XuanJun, LIU YinHe, HUANG Xiu
2021, 39(1): 253-267. doi: 10.14027/j.issn.1000-0550.2020.130
A laboratory experiment on sedimentation is an experiment conducted under highly controlled conditions to simulate the sedimentary processes in a wide variety of sedimentary environments. It is a useful method which has been widely used in many scientific disciplines and sub-disciplines including hydraulics engineering, environmental science, environmental engineering, oceanography, sedimentology, as well as petroleum geology. The recent drastic increase in unconventional oil and gas production requires new insights into fine-grained sediment, such as processes of organic-sediment accumulation. Many equations or theories have been proposed for uncohesive sediments including physical properties of sand/clay particle, sedimentary bedforms, sedimentary landscapes, sediment gravity flows. However, because of their small grain size, and their cohesive properties, cohesive sediments present more of a challenge in the experiment and have been relatively understudied. A review is given of laboratory experiments on sedimentation of fine-grained cohesive sediments, with emphasis on the aspect of physical properties of particles, processes of erosion and deposition, sedimentary bedforms, transport mechanics and facies models, as well as organic matter accumulation and preservation. This article also discusses the organic matter transportation and accumulation in recent Qinghai Lake and proposes a hypothesis of accumulation of organic-rich fine-grained sediments. Laboratory experiments on sedimentation of fine-grained sediment are useful for both paleoenvironment reconstruction and lithofacies prediction, which can be applied to genetic analyses and prediction of sweet spots/areas in unconventional shale plays. A successful research of fine-grained sediment requires the application of methodologies and results of a broad range of scientific disciplines, a combination of physical and computational simulations, and some large-scale and long-term flume experiments.