Major breakthroughs have been made for natural gas exploration of Middle Devonian and Middle Permian formations in Sichuan Basin in recent years, but its natural gas source is still not clear, affecting the next exploration deployment decision. A comprehensive study is conducted on the geochemical characteristics of Middle Devonian and Middle Permian natural gas, the biomarkers of reservoir bitumen and source rocks. The results show that these natural gases are secondary cracking dry gas, including more than 86% of methane content, a small amount of ethane, propane and non-hydrocarbon gas such as nitrogen, carbon dioxide and hydrogen sulfide. The natural gas maturity varies slightly in different regions. The values of natural gas δ13C1=-35.7‰ to -27.3‰, δ13C2=-38.7‰ to -26.6‰, δ13C3=-37‰ to -26.5‰, and
In this paper, saturated hydrocarbons in biodegraded crude oil from Wuxia area in Junggar Basin were analyzed by two dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC/TOFMS) and two dimensional gas chromatography coupled with flame ionization detector (GC×GC/FID), which increased understanding of the composition and genetic mechanism of biodegraded crude oil. The result shows: (1) the major components of complex mixtures in biodegraded heavy oil from Wuxia are cyclic compounds with different carbon number alkyl groups and isomers. These cyclic compounds mainly consist of six-membered ring as the basic unit, including six types: Monocycloalkanes (carotenoid), bicycloalkanes (deca-hydronaphthalene, de-A, B-ring-steranes), tricycloalkanes (perhydrophenanthrenes, tricyclicterpanes, seco-steranes) and adamantanes, tetracycloalkanes (steranes, secohopanes, tetracyclic terpanes) and pentacycloalkanes (hopanes, 25-norhopanes); (2) the total content of saturated hydrocarbons does not change much, and different families of saturated hydrocarbons show a change with the increase of biodegradation. In the stage of severe biodegradation, dicycloalkanes have a large number of homologues and isomers that are not detected in normal crude oil. These compounds may be newly formed by microbial action, and due to their strong biodegradability, bicycloalkanes become the dominant components of saturated hydrocarbons, accounting for about 50% of the total saturated hydrocarbons. Anti-biodegradation ability of polycyclic hydrocarbon compounds is relatively strong, resulting in an increase in content of polycyclic hydrocarbon compounds with degree of biodegradation.
A large number of Silurian reservoirs have been found in the structural belt of the Tazhong Uplift of the Tarim Basin. Previous studies have shown that hydrocarbons were mainly formed in Hercynian Period. In this paper, advanced two-dimensional gas chromatography-time of flight mass spectrometry was used to identify molecular compounds in crude oil of Well TZ117. Two distinct molecular compounds with different maturity were found, indicating at least two stages of hydrocarbon accumulation. Among them, n-alkanes of crude oil are well preserved (nC7-nC28), and terpanes are abundant. Based on methylphenanthrene series compounds, crude oil is the product during the period of source rock with RO of 0.7%-0.9%. In addition, abundant diamondoids are detected, including adamantanes, diadamantanes and triadamantanes, with a total content of 1 425×10-6. Corresponding to high methyladamantanes index, crude oil is the product during the high-maturity stage of source rock with RO>1.6%. The difference between the maturity of the two types of parameters indicates that the reservoir has experienced at least two stages of oil and gas from different maturity sources. Comprehensive study of petroleum geology shows that the Silurian System in Tazhong area underwent oil and gas charging in the Late Caledonian and Himalayan stages, respectively. Well TZ117 captured both stages of oil and gas, and fluid inclusions also confirmed the two stages of hydrocarbon accumulation.
Taking carbon dioxide and hydrogen as examples, this paper discusses the influence of several commonly used gas collection devices on carbon, hydrogen and oxygen isotopes in the process of collecting and storing non-hydrocarbon gases. Specific sampling devices include high-pressure cylinders, foil bag, and glass bottles. Glass bottle sampling can be divided into distilled water storage and saturated salt water storage. The test results within 96 h show that different devices and storage time have little effect on the determination of carbon and hydrogen isotopes. As for the oxygen isotope, sampling devices without medium including high-pressure cylinder and foil bag show constant oxygen isotope in the first 5 h. However, accurate oxygen isotope ratio cannot be obtained by using glass bottle device, due to the distilled water and saturated salt waters medium affecting the testing accuracy. The experimental results can provide reference for researchers to collect non-hydrocarbon gas samples in the future studies.
The Sikeshu Sag, located in the west of the Junggar Basin, contains large oil and gas deposits. However, the patterns of source rocks and potentials of hydrocarbon generation in the Jurassic strata are still under debates. Taking into account of TOC, maturity and other organic geochemical indexes, hydrocarbon generation differences from various source rocks are speculated with the carbon isotopes and isopach map. Besides, thermal analysis and numerical modeling were conducted to establish the evolution of source rock and to evaluate the potential exploration prospects in the research area. The results show that the hydrocarbon generated from coal is much better than that of carbonaceous mudstone in the Jurassic strata of Sikeshu Sag. In addition, the gas is transported and gathered mainly in the south of the area. The source rocks in the Sikeshu Sag show high hydrocarbon potentials and explorational values.
The Mianzhu-Changning rift trough has great potential for the development of the Lower Cambrian source rock. Total organic carbon (TOC), mineral composition, major and trace elements of the Lower Cambrian shales in the central part of the rift trough were measured to reveal the relationship between the tectonic setting, terrigenous input, paleoclimate, primary productivity, redox conditions as well as the hydrothermal activity and the enrichment of organic matter during its development period. The results show that the study area is located in the passive continental margin sedimentary environment, with stable sedimentary rate, warm and humid climate alternated by intermittent climate cooling. The TOC values range from 5.96% to 23.15% with an average content of 11.64%. Nutrient elements of P, Ba, Cu and Ni reveal the high primary productivity during this period, which is due to the increased chemical weathering caused by warm and humid climate, thus importing large amounts of nutrients into the ocean. The index of redox sensitive elements and the content of pyrite indicate that the organic-rich sediments in the second and third stages are mainly developed in the bottom water environment of anaerobic sulfurization. The abnormal enrichment of trace metals may be related to the hydrothermal activity from the seabed under the stretching background in rift during the Early Cambrian, which not only provides material for the flourishing of anaerobic chemoautotrophs, but also facilitates the formation of anaerobic sulfurization environment in the bottom water and promotes the preservation of organic matter after mixing with seawater.
The Lower Cambrian Qiongzhusi Formation source rocks are one of the most important source rocks in Sichuan Basin and its surrounding areas. In order to clarify the distribution and formation environment of the high-quality source rocks in Qiongzhusi Formation in northeastern Sichuan Basin, data of well logging, organic geochemistry and major-trace element are obtained and studied in this research. Our results show that the high-quality source rocks of Qiongzhusi Formation were formed in warm and humid deep-water shelf and slope-basin environment with relatively weak hydrodynamic force. These source rocks are stored in reducing to weak-oxidation environment, which provides the favorable sedimentary environment for the generation and preservation of organic matters. The source rocks are mainly developed in deeper layers in the longitudinal direction and horizontally have three thickness centers in Chengkou-Kaixian rift area, Bazhong-Tongjiang area and Wufeng-Hefeng area. These conditions lay important foundation of oil and gas formation. Besides, a good source-reservoir allocation relationship can be found between the source rocks and the high-quality reservoir rocks in Longwangmiao and Dengying Formation. The abundant distribution of both the reservoir rocks and source rocks in Chengkou-Kaixian area makes it an attractive location to be prospected in the future work.
Based on the high resolution sequence stratigraphy theory, the sedimentary characteristics of the fan delta-beach dam system in the steep slope of the fault depression basin are identified by the comprehensive application of the core, logging, seismic and laboratory data. It discussed the controlling factors of the distribution of sand bodies in this area, and established the superposition model of sand bodies in the submember 3 upper of Shahejie Formation in Liuxi area. The results show that :(1) Underwater distributary channel, river mouse bar and beach bar sand bodies are the main sand bodies in the study area. (2) In the direction of the source, the width and depth of the underwater distributary channel in the fan delta are gradually reduced due to the slow topography. The variation of high frequency (level 4) lake level controls the progressive and regressive deposition of the fan delta and beach dam system, and the variation of higher frequency (level 5-6) lake level controls the superposition style and distribution rules of sand bodies in the sequence. (3) There are four types of vertical superposition modes of sand bodies in the study area, which are deep water channel superposition, river over bar superposition, isolated river superposition and river mouse bar-beach dam composite superposition.
Deep and ultra-deep tight sandstone gas reservoir is the key exploration and development object of unconventional oil and gas resources in Tarim Basin. In the process of drilling, completion and production, it often shows complex engineering behavior which is sensitive to the change of wellbore fluid column pressure or bottom hole flow pressure. In order to reveal the stress sensitive characteristics and main controlling factors of deep tight sandstone, taking the three gas reseruiroivs in Tarim Basin as examples, the stress sensitivity experiment of fracture and block samples are carried out under simulated increasing confining pressure. By scanning electron microscopy (SEM), the casting thin sections, X-ray diffraction (XRD), high pressure mercury injection, the effects of pore structure, mineral composition and fracture development on stress sensitivity of deep tight sandstone were analyzed. The research results show that the stress sensitivity coefficient of the block rock samples of deep tight sandstone reservoir in Tarim Basin ranges from 0.280 6 to 0.771 4, and the stress sensitivity degree is from moderately strong to strong, which is KS（0.771 4）>DB（0.654 0）>YM（0.579 6）. The stress sensitivity coefficient of the fractured samples of deep tight sandstone reservoir in Tarim Basin ranges from 0.532 3 to 0.806 9, and the stress sensitivity degree is from moderately strong to strong, which is YM（0.726 2）>KS（0.693 5）>DB（0.626 5）. The stress sensitivity of deep tight sandstone is controlled by a combination of factors including depth, pore structure, mineral composition and fracture development. The stress sensitivity of the block rock sample is positively correlated with the depth of the reservoir, the content of unstable mineral components and clay minerals, and negatively correlated with the quartz content, porosity, permeability and pore throat radius of the reservoir. The stress sensitivity of fracture samples is mainly controlled by the fracture width, which decreases with the increase of fracture width.
By the end of 2018, the Fuling shale gas field of Sichuan Basin had built more than 10 billion cubic meters of capacity and accumulated over 20 billion shale gas productions. It has become the first shale gas field in China to realize commercial development. Based on laboratory experiments, combined with field yield, a research is conducted on the main factors which affect the productivity of shale gas horizontal wells in Fuling shale gas field. The relationship between various factors and shale gas productivity is obtained. The results show that the influencing factors of shale gas wells in different development zones of Jiaoshiba block are different. The production effect factor which determined by variable importance in projection (VIP value) that calculated by partial-least-square (PLS) module large data analysis method, was consistent with the comprehensive evaluation result in zone production influence factor. It confirmed that the main influencing factors in different zone shale gas well production in Jiaoshiba first stage are different. The research results of this article have a certain guidance and reference significance for the study of the initial productivity affecting factor of marine shale gas in southern China.
At present, after initial fracturing in some gas fields, the effect of increasing production is not obvious, or after a period of production, the output is significantly reduced. In order to increase the production of gas wells, the gas wells can be repeatedly fractured. It is preferred to carry out repeated fracturing to obtain a better fracturing effect of the gas well. The factors affecting the effect of repeated fracturing of gas wells are analyzed. The influencing factors are divided into two parts: geological factors and engineering factors. Geological factors include porosity, permeability, skin coefficient, production layer thickness, gas saturation, formation pressure coefficient, remaining recoverable reserves and engineering factors include the success of the previous fracturing, the amount of fracturing fluid used in the previous fracturing, and the amount of sand added during the previous fracturing. Based on BP neural network theory, combined with the analysis of influencing factors of gas well repeated fracturing effect, the optimal model of re-fracturing well was established. The particle swarm optimization algorithm was used to optimize it. While increasing the convergence rate, it effectively prevented the occurrence of local optimal solutions, and predicted the daily gas production rate of the repeated fracturing wells. Based on this, the repeated fracturing wells were optimized. By predicting the effect of repeated fracturing in Zone C, it is better to repeat the fracturing well based on BP neural network to improve the accuracy of well selection.
Horizontal well spacing is one of the principal parameters for the economical and efficient development of shale gas reservoirs, which can not only influence the well and platform development performances, but also can directly determine the reserve producing degree and the economic performance of whole gas reservoir. An extensive investigation of the horizontal well spacing for shale gas reservoirs in North America was provided. Development tendency of the North America shale gas horizontal well was analyzed and the current research statusof well spacing optimization was illustrated. Conclusions were obtained as follows: (1) The horizontal well spacing of typical shale gas reservoirs in North America ranges from 180 m to 420 m, with two typical well spacing of 201 m and 402 m. (2) Average well spacing data decreased slightly during 2011 to 2017, while for some main shale gas reservoirs like Marcellus, Utica, Haynesville and Barnett the well spacing increased. (3) Due to the influence of inter-well interference and rapid production decline in small-spacing development pattern, the horizontal well spacing data in North America indicates a slight increase in recent years. (3) Horizontal well spacing optimization includes field pilot test and surveillance, production data analysis, numerical and analytical simulation. Latest researches give an optimal well spacing for the North America shale gas reservoirs that ranges from 300 m to 400 m.
The Shahezi Formation of Sujiatun block in Songliao Basin has developed multi-stage water diversion channel sand body, and the single body is only 3-5 m thick. It is difficult to perform effective reservoir prediction in the gas field, because of the inferior reservoir property and strong heterogeneity. In this paper, we present the virtual lithology and the error fusion technology based on Xu-White theory model, which optimizes the S-wave prediction process and improves the shear wave prediction accuracy. Combined with core sample test results, we build the fluid factor parameter of gas-bearing sandstone. Through pre-stack geological statistics inversion, we obtain the corresponding high resolution parameter bodies. The prediction results are in good agreement with the real drilling and achieve good application effect.
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