Key issues and development direction of petroleum geology research of source rock strata in China

After more than 20 years of technological advancements, the novel field of oil and gas production from source rock strata, which comprise tight and shale oil and gas reservoirs, has become the major contributor to the increase in unconventional oil and gas reserves in China. Accordingly, this field has gradually entered a new stage of revolutionary development. The oil and gas production in China from source rock strata will achieve sustainable development in the future. Different types of source rock strata present distinct challenges and require diverse development paths. Based on the geological conditions of source rock strata in China, this study focuses on identifying the “sweet areas” among hydrocarbon accumulations. It specifically analyzes the key development issues of tight oil, tight gas, shale oil, shale gas, and coal-bed methane, while proposing potential solutions and identifying the possible directions for future development. This study aims to provide a reference for scientists concerned with the development of unconventional oil and gas reserves in China.Cited as: Li, J., Yang, Z., Wu, S., Pan, S. Key issues and development direction of petroleum geology research on source rock strata in China. Advances in Geo-Energy Research, 2021, 5(2): 121-126, doi: 10.46690/ager.2021.02.0

Longitudinal interference analysis of shale gas multi-stage fracturing horizontal wells upon high-precision pressure test

Shale gas plays a crucial role in the national energy supply. However, fast pressure drop, production decline, and water resources pollution caused by well interference and fracture hits become more severe in multi-layer mining shale gas fields. Such as, it is urgent to evaluate the interference of multi-stage fracturing horizontal wells (MFHWs) between the upper and lower gas layers in Chinese Jiaoshiba shale gas field. Therefore, we put forward a comprehensive method to analyze the MFHW interference in this paper. The method contains bottom-hole pressure response analysis (BHPRA) during neighboring well fracturing, BHPRA of well interference test, and production dynamic analysis. Our study indicates that longitudinal pressure interference exists between the Jiaoshiba upper and lower gas layers upon the apparent interference pressure response in a multi-well test. However, MFHW interferences occur in the corresponding fracturing stages with shorter distance, and the interference strength is related to both well distance and fracturing scales. The Jiaoshiba upper gas layers can be developed to increase the gas production performance, but it is necessary to maintain a reasonable well spacing to avoid severe interference during the development

COUPLED NUMERICAL SIMULATION OF FRACTURING MULTILAYER RESERVOIR FLOW WITH LEAN-STRATIFIED WATER INJECTION

Many multilayer sandstone reservoirs have reached extreme high water content and high recovery stage after long time waterflooding. These inner LPTOLs have been the main potential reserves. Lean-stratified water injection is one of the most important technologies to increase production and develop potential for multilayer oilfields with extreme high water content. However, traditional models cannot entirely solve the inner boundary conditions of lean-stratified water injection. Therefore, we established the injection wellbore constraint equations, which were coupled with the oil/water two-phase numerical reservoir models. Upon an embedded fracture model for simulating hydraulic fractures, a method to simulate the reservoir with horizontal fractures is shown. The influences of reservoir and fluid, horizontal fractures, and injection-production characteristics are analyzed for oil production and water-content dynamic. Considering the specific situation of lean-stratified water injection wells, the influences of different segments numbers, modes of combination in segment layers, and rhythm characteristics of remaining oil reserves and distribution are evaluated

Chemical stimulation for enhancing coal seam permeability: laboratory study into permeability variation and coal structure examination

Many potential coalbeds have sub-economic permeability for coal seam gas (CSG) extraction even though all the other key characteristics (gas content, thickness etc.) can meet the requirement for successful production. To enhance coal seam permeability various CSG stimulation techniques, including hydraulic fracturing, cavity well completions and horizontal wells, have been used, but none is ubiquitously successful. The potential of chemical stimulation including both acid (1% HCl) and oxidant (1% NaClO) stimulation are examined in this paper as an alternative method to enhance coal seam permeability. Acid stimulation targets the removal of certain minerals in coal cleats, while the oxidant dissolves the coal matrix, thereby etching pre-existing coal cleats and perhaps forming new ones. The permeability variation during stimulations are measured with core flooding tests and the corresponding coal structural changes are examined using X-ray microcomputed tomography (μCT) technique. The chemical reaction mechanisms are confirmed by elemental analysis via inductively coupled plasma optical emission spectroscopy (ICP-OES) and dissolved organic carbon (DOC) in the effluent. Acid stimulation was performed horizontally with cube samples cut from coal cores (Bowen Basin, Australia) and it exhibited a positive effect on coal permeability, which is proposed to result from demineralisation as shown by before and after CT scanning observations together with high calcium (Ca) content in the effluent. NaClO oxidation was able to etch pre-existing coal cleat surfaces, widen cleat apertures and generate new horizontal fractures and/or void space. These changes were caused primarily by chemical attack, confirmed by the high DOC concentration in the effluent. Oxidant stimulation caused a decrease in vertical permeability and an increase in horizontal permeability for some cores. The decrease in permeability is proposed to be due to a combination of increased void space weakening the coal associated with movement of the coal into newly created space against confining pressure. Minerals in the cleats appear to play a role keeping void space open and preventing collapse under confining pressure. Furthermore, the results for these samples indicate that NaClO oxidation appears to be lithotype independent and preferentially attacks coal sections that have more initial pores or fractures

Formation and “sweet area” evaluation of liquid-rich hydrocarbons in shale strata

Liquid-rich hydrocarbons in shale strata include two kinds of resources, i.e. tight oil and shale oil. Based on the exploration and research progress of liquid-rich hydrocarbons in shale at home and abroad, their formation condition, accumulation mechanism, classification, and differences between lacustrine and marine shale systems are examined, and “sweet areas” are evaluated further. Analysis on the geological characteristics of the liquid-rich hydrocarbons in the shale strata in North America and China shows the liquid-rich hydrocarbons have two basic features: large-scale continuous distribution and no stable industrial production. The massive accumulation of the liquid-rich hydrocarbons needs four fundamental formation conditions: stable tectonic background, widespread high quality source rocks, large-scale tight reservoirs with massive reservoir space, and co-existence of source and reservoir. The study reveals the formation mechanisms of the liquid-rich hydrocarbons: source-reservoir coupling and porosity decrease during the diagenetic tightness; and identifies 24 kinds in 6 categories of the liquid-rich hydrocarbons. It is concluded that the geological conditions of the lacustrine shales in China are characterized by lower thermal gradient and stronger heterogeneity than those of North America, so large scale “sweet areas” have to be picked out to push up industrial production steadily. “Sweet areas” evaluation should consider the three aspects of geology, engineering and economics comprehensively, and the maturity of source rocks is first and foremost factor controlling the “sweet area” distribution. In China, prospective shale areas should meet the following conditions: the Ro between 0.8% and 1.3%, TOC higher than 2%, laminated shales or tight porous reservoirs, higher porosity (more than 8% for tight oil, and more than 3% for shale oil), higher content of brittle minerals (more than 70% for tight oil, and more than 40% for shale oil), oil saturation of 50%−90%, lower crude oil viscosity or higher formation pressure, and rich natural fractures. Liquid-rich hydrocarbons in shale strata are huge in resource scale, so deepening the geological understanding on the formation and distribution of liquid-rich hydrocarbons in marine and lacustrine shales constantly is of great significance for exploration and development of this important field. Key words: tight oil, shale oil, sweet area evaluation, unconventional oil and gas, fine grain sediment, tight reservoi

A new technology of basin fluid geochronology: In-situ U-Pb dating of calcite

Basin fluid is the most active geological agent in sedimentary basins, having a close relationship with the generation, migration and accumulation of hydrocarbon resources. Accurate determination of fluid flow history has been a challenging and frontier research topic. In general, the previous studies of basin fluids mainly rely on the analysis of fluid inclusions, which is difficult to successfully reconstruct the events of basin fluids. More seriously, this method is unable to determine the timing of fluid flow events. Authigenic calcite is the direct product of basin fluids. Thus, accurate dating of authigenic calcite provides a new approach to determine the history of fluid flow events. In the field of calcite geochronology, the most widely used dating method was the isotope dilution U-Pb dating approach. However, this approach is time-consuming, and has a low success rate. In recent years, laser ablation technology has greatly facilitated U-Pb dating of accessory minerals (including calcite) because of its high spatial resolution and rapid data acquisition. It has been confirmed that the newly-developed in-situ U-Pb dating method is able to accurately determine the age of calcite with U content less than 10×10 . This method has successfully reconstructed the history of fluid flow events in the sedimentary basins, suggesting that it has a good application prospect in the field of basin fluid geochronology. In the future, it can be expected that the application of in-situ U-Pb dating of calcite together with C-O isotope and rare earth element analysis will be a significant development direction in the field of basin fluid studies. It is worth noting that the determination of stage of authigenic calcite through systematic microscopic identification and diagenetic observation is the premise of application success

Vanadium isotope evidence for widespread marine oxygenation from the late Ediacaran to early Cambrian

Early animals experienced multiple-phase radiations and extinctions from the late Ediacaran to early Cambrian. Oxygen likely played an important role in these evolutionary events, but detailed marine redox evolution during this period remains highly debated. The emerging vanadium (V) isotope system can better capture short-term perturbations to global ocean redox conditions. In this study, we analyzed V isotope compositions (δ51V) of organic-rich cherts and black shales deposited from the late Ediacaran to early Cambrian (ca. 560–518 Ma) in the Yangtze Block, South China. The robust positive correlation between the δ51V values and previously reported δ98Mo values validates V isotope system as a paleo-oxybarometer. The continuously temporal open-ocean seawater δ51V variation is reconstructed from the sedimentary δ51V records. The results suggest that the ocean experienced a rapid transition from expansive euxinia at ca. 560–553 Ma to widespread oxygenation likely reaching the modern level at ca. 552–551 Ma, and kept extensively oxygenated approaching the modern oxygenation level from ca. 551 Ma to 521 Ma and reaching the modern level again at ca. 521–518 Ma. The prolonged and widespread oceanic oxygenation may have been beneficial to the ecological radiation of early animals from the late Ediacaran to early Cambrian, ultimately leading up to the “Cambrian Explosion”

Timing and characterization of multiple fluid flow events in the Beibuwan Basin, northern South China Sea: implications for hydrocarbon maturation

The history of thermal and fluid flow events is of crucial importance to understand the mechanism of hydrocarbon maturation. The northern South China Sea (SCS) contains abundant hydrocarbon resources with high heat flow anomalies, but the mechanisms responsible for modification of thermal regimes are poorly understood. Here we conducted the first high-precision Rb-Sr isochron dating and geochemical analyses of authigenic illites separated from the Palaeogene sandstones in the Beibuwan Basin, northern SCS. Eight high-precision Rb-Sr isochron ages reveal three major thermal and fluid flow events in the Beibuwan Basin, northern SCS. The 34.5 ± 0.9 Ma and 23.6 ± 0.8 Ma events occurred in the western area are probably related to the Eocene-Oligocene transition characterised by a significant regional unconformity resulted from intensive uplift of Hainan Uplift and Oligocene-Miocene transition marked by a post-rift unconformity, respectively. By contrast, the 31.2 ± 0.6 Ma event affecting the eastern area appears to be closely associated with the igneous intrusion dated by zircon U/Pb to 32.3 ± 0.7 Ma. REE and stable isotope characteristics indicate that the illites from the western area formed within a meteoric-hydrothermal system, whilst those from the eastern area were likely precipitated from fluids heated by the large-scale igneous intrusion at higher water/rock ratios. Moreover, our data in conjunction with palaeo-temperature reconstructions show that the fluid flow events had profound effects on the rapid organic maturation in the northern SCS

Global energy transition revolution and the connotation and pathway of the green and intelligent energy system

The essence of energy system transition is the “energy revolution”. The development of the “resource-dominated” energy system with fossil energy as the mainstay has promoted human progress, but it has also triggered energy crisis and ecological environment crisis, which is not compatible with the new demands of the new round of scientific and technological revolution, industrial transformation, and sustainable human development. It is in urgent need to research and develop a new-type energy system in the context of carbon neutrality. In the framework of “technique-dominated” new green and intelligent energy system with “three new” of new energy, new power and new energy storage as the mainstay, the “super energy basin” concepts with the Ordos Basin, NW China as a representative will reshape the concept and model of future energy exploration and development. In view of the “six inequalities” in global energy and the resource conditions of “abundant coal, insufficient oil and gas and infinite new energy” in China, it is suggested to deeply boost “China energy revolution”, sticking to the six principles of independent energy production, green energy supply, secure energy reserve, efficient energy consumption, intelligent energy management, economical energy cost; enhance “energy scientific and technological innovation” by implementing technique-dominated “four major science and technology innovation projects”, namely, clean coal project, oil production stabilization and gas production increasing project, new energy acceleration project, and green-intelligent energy project; implement “energy transition” by accelerating the green-dominated “four-modernization development”, namely, fossil energy cleaning, large-scale new energy, coordinated centralized energy distribution, intelligent multi-energy management, so as to promote the exchange of “two 80%s” in China's energy structure and construct the new green and intelligent energy system