This paper summarizes some significant achievements in natural gas exploration, such as “theory of hydrocarbon generation from polynary types of organic matter”,“theory of natural gas late formation” and “theory of lithologic and stratigraphic natural gas formation”, and great breakthroughs that are achieved by the application of these theories. It is pointed out that the difficulties of geological research on natural gas being faced with now are mainly complexity of geologic framework of Chinese basins and the current geological research on natural gas is mainly focused on those basins which were formed during Mesozoic and Cenozoic. This paper also indicates that more discoveries of giant gas fields need geologists to expand the thoughts on exploration, develop the existing theories and technologies, and study the natural gas geology based on mobilistic theory, palaeo\|plate thoughts and discovery of giant gas fileds.
Based on the analysis of gas samples of the Tarim basin，the gas geochemical characteristics of the platform basin region are summarized systematically.The hydrocarbon composition of natural gases is mainly wet gas in the platform basin region of the Tarim basin，and dry gas in some areas.The wet gas is distributed in central Tarim basin，Mandong-Yingjisu，Yingmaili，Donghetang，Harderson and Jiefangqudong-Jilake. And Bachu and Lunnan Sangtamu have both wet gas and dry gas.As for nonhydrocarbon components，nitrogen gas and carbon dioxide are dominant.Most areas in Bachu，central Tarim，Mandong Yingjsu，Donghetang Yingmaili，Harderson are characterized by the medium high content of N2，and natural gases in Hetian River and Donghetang have high content of CO2.The carbon isotope distribution of natural gas is characterized by light at the central part and heavier in the peripheral areas.The carbon isotope value of ethane is lower than -40‰ in central Tarim，Donghetang\|Yingmaili and Harderson，and higher than -40‰ in Hetian River，Mandong Yingjili，Lunnan Jilake.The carbon isotope value of ethane is generally higher than -30‰.
Natural gases of the T3x2 reservoir in Qiongxi structure have the same or similar geochemical characteristics because of the same source, but due to different time of gas accumulation, some subtle geochemical differences in the hydrocarbon fingerprint characteristics can still be detected. Based on the comprehensive analysis of gas composition, C3-C8 light hydrocarbon fingerprint, carbon isotope monomer composition and biomarkers of reservoir extract, this paper identifies the fluid geochemical characteristics of different wells and examines the reservoir connectivity. The results show that the T3x2 reservoir consists of four poor-connectivity blocks, including uplift tectonic West Wing (Qiongxi 5 well), horst fault block in the North, fault block in the South and Qiongxi 12 well. The North horst fault block and the fault block in the South have better internal connectivity. They are poorly connected to the uplift tectonic West Wing and the Qiongxi 12 well.
Natural gases of the Tazhong uplift were mainly accumulated along No.1 structural belt. This paper has discussed the character and origin of Ordovician natural gases in the No. 1 structural belt. It shows that all the gases are mainly composed of hydrocarbons, and have high dryness ranging from 0.78 to 0.99. Carbon isotope ratios of methane and ethane range from -54.4‰~-37.7‰ and -42.1‰~-30.8‰, respectively. All the Ordovician gases belong to thermogenic origin except that gases from Wells Tazhong 45 and Tazhong 451 have the character of biogenic origin owing to the good preservation conditions of western Tazhong uplift. Study of Lower Paleozoic strata evolution and oil source correlation suggests that gases from the eastern part of No.1 structural belt originated mainly from higher matured Precambrian source rocks, and gases from western end of No.1 structural belt were mainly from early matured Ordovician source rocks. At the same time, thermogenic gases were mainly produced by primary cracking of kerogen.
Steam soak and steam flood are popular thermal recovery techniques for viscous oil. But when they are used for production, some H2S will form for heating oil sand by steam. The formation of H2S is complex when geologic conditions and recovery methods are varied. The H2S formed during thermal recovery are secondary. Through research we think that the reasons for the formation of secondary H2S are thermal decomposition of sulfides (TDS) and thermo\|chemical sulphate reduction (TSR). Examples of thermal recovery for viscous oil abroad have proved that the quantity of H2S formed is in direct proportion to sulfur content of crude oil. Moreover, if there are sulfate ions in the layer, the high temperature will favor the TSR. In fact, the formation of H2S has something to do with both reasons above, and the reasons are connected to each other. Since H2S is harmful, something must be done to prevent H2S from forming.
Chuanxi area is located in the Sichuan basin, with abundant gas, complex structures. The bed of interest, carbonate reservoir, is deep in this area, with strong heterogeneity. Images of poststack time migration are poor in the Chuanxi area; however, results show that the high accuracy prestack time migration technique can effectively improve the image precision of seismic data. The result sections are characterized by clearer faults, reliable breakpoints. The results allowed for precise structural interpretation. This paper introduces the basic theory, main steps and application effects of the high accuracy prestack time migration.
The features of seismic signals are different between time and frequency domains. We can make reservoir prediction better with seismic signals in frequency domain. The features of seismic data in frequency domain are studied with mathematical tools, such as Fourier transform and wavelet transform. And many reservoir prediction technologies in frequency domain are developed, such as the technology of time\|frequency analysis in wavelet domain, frequency division seismic facies, frequency spectrum correlation, reservoir imaging of frequency spectrum decomposition. These technologies are applied to practical data and have a good effect.
It is important to predict a favorable fractured reservoir and its main azimuth of fractures during the exploration and exploitation of petroleum. When fractures occur in the reservoir, a nature interface can be formed by the huge difference of physical properties between fluids and compacted rocks, and it changes the characteristics of seismic reflection waves of the reservoir. These are the important information of the fractures existence in the seismic sections. AVO can detect fractures in formation, especially those distributed in certain orientation under structural stress. Theoretically, the influence of fractures to seismic attributes may reach maximum in its dip direction, and may reach minimum in its strike direction. And the influence is the same to AVO parameters. Therefore, applying the multi\|azimuthal AVO method to fractured reservoirs and its vector difference analysis can determine the fracture intensity and density of the reservoirs. The authors discuss in detail various azimuthal AVO responses of fractured carbonate reservoirs to various incidence angles, azimuth and fracture dips through theoretical formula, and find the relation between the AVO characteristics of seismic wave and the fracture strike and dip, especially for the tilt fractures, which provides the foundation for the prediction of favorable fractured reservoirs. The theory is successfully applied to the analyzation of the favorable fractured zone of Tarim.
Classification on natural gas hydrate deposits occurrence is summarized, and a dynamic theory of gas hydrate reservoir formation is put forward. According to this theory, there are three types of reservoirs: growing type (including seeping type and diffusing type), mature type and regressive type. This theory is demonstrated by three examples: the natural gas blowout when drilling in the permafrost, the hydrate cropouts in the seafloor and the gas bubbles in abyssal plumes. Then, several conclusions are drawn: there maybe exist the seeping and diffusing types of gas hydrate reservoirs which may still be growing in the Xisha trough, Dongsha continental slope of the South China Sea, west and south of Taiwan basin, and Bijianan basin; whereas in the Qiangtang basin of the Qinghai Tibet plateau may exist the regressive type gas hydrate deposit. Furthermore, the phenomenon of hydrocarbon gas leaking in the Qilian mountain areas and Junger basin indicates that these areas may ensconce gas hydrate reservoirs.
Gas hydrate crystallization dissolution are exothermic/endothermic reactions, and gas venting from higher temperatures to the lower produces capacity heat, which results in the change bottom water temperature and consequently affects gas hydrate accumulation. We use the compositional kinetic model to analyze how hydrate latent heat and water capacity heat affect the gas hydrate stability in the Bush Hill. We take filter loss of venting gas as 1.8kg/(m 2·a) and there is 10% vent gas deposited as gas hydrate in the subsurface. Modeling shows that gas hydrate latent heat and water capacity heat within 10ka can perturb the temperature field in the subsurface of the Bush Hill, and thermal gradient increases 3 ℃/km at the surface of the seafloor and decreases ~1.4 ℃/km at 1km deep. The most obvious temperature variation, which is about 0.4℃,occurs in the middle part of the gas hydrate stability zone(400 mbsf deep), resulting in the reduction of hydrate stability zone by 12m and the dissolution of 0.06kg/m 2 of gas hydrate.
Jiyang depression is located in the southeast of Bohai Bay basin and is an important oil gas basin in China. The Indo Chinese movement and Yanshan movement were two important tectonic movements in the Mesozoic era. NW trending reversed faults were well developed in the IndoChinese epoch. The early stage of Yanshan is a transitional stage from reversed faults to normal faults. The reversal tectonic activity took place in the medium late stage of Yanshan among the NW trending reversed faults, and the reversed faults were translated into normal faults. The stratigraphic sequences of Paleozoic were reformed largely after the Indo Chinese movement and the stratigraphic feature of Mesozoic was affected after the Yanshan movement. The hydrocarbon resources of Mesozoic were also constrained greatly by these two tectonic movements.
According to the achievements of sequence stratigraphical study in the Bohai Bay basin in the east of China, the structure\|sequence analysis in continental basins should pay attention to three aspects: (1) the controlling function of the basin\|marginal faults on the sequence architecture; (2) the controlling function of the transverse transfer zone on the provenance orientation; (3) the controlling function of paleostructure and paleotopograhy on the distribution of sandstone bodies.
A new set of oi- and gas-bearing reservoirs between Cretaceous and Jurassic have been found in the Honglian oilfield, Tuha basin. But there are different opinions on its classification, which has negative effect on the naming, correlation of the stratum, and the evaluation of oil and gas exploration potential as well as the making of development technology policies. The unconformity surface between the Cretaceous and Jurassic resulted from the second episode of Yanshanian tectonic movement. This paper presents the correlation and demonstration of several opinions on the new oil and gas-bearing reservoir classification based on the integration of multi disciplinary information such as core, logging and seismic feature, etc. It is concluded that the new oil and gas bearing reservoirs discovered in Well Hongnan9-12 should be classified to the Lower Cretaceous, for it is located above the unconformity surface between the Cretaceous and Jurassic.
Qianmiqiao buried hill is a complex condensate reservoir. There are many paradoxes in the process of exploration and production. The main purpose of this paper is to discuss the hydrocarbon accumulation pattern within this buried hill. After analyzing the well log information, the authors believe that the buried hill is a weathering crust reservoir, and the effective reservoirs are within the 300m interval from the top of weathering crust. The authors establish a double reservoir system based on well testing and production data, and investigate the performance of the occlusion water system. This pattern indicates that perforated completion, especially acid treatment, is the main cause of producing water in the wells. If the occlusion water is ignored, the reservoir in the Qianmiqiao buried hill could be considered to be a “layered pure condensate pool”.
To analyze the petroleum condition and hydrocarbon accumulation in the Leting sag of Bohai Bay basin, the Lower Tertiary petroleum system is studied. It is shown that there is plenty of mature source rock and large scale distribution of dark mudstone which has the ability of hydrocarbon-generation. Both the middle-lower Es3 and lower Es1 argillaceous rocks have excellent seal ability. There are two reservoir patterns: source below and reservoir above, self source-reservoir. In this sag, four traps are found: fault block or fault nose trap, lithologic trap, stratigraphic trap and complex trap. Channels for oil migration are as follows: sandstone, fault and unconformity surface. The formation of the Lower Tertiary petroleum system experienced three main stages, including uplifting and folding, faulting, and sedimentation. The Lower Tertiary petroleum system is evaluated based on the parameter of source rock potential. It suggests that the “self source\|reservoir” by lateral migration has better oil accumulation conditions than the “source below and reservoir above” by vertical migration.
The Yaha fault structure belt is located in the north of the central Luntai anticline of the northern uplift of Tarim basin. Influenced by several tectonic movements, faults are widely developed and fault system is the most important factor that controls the development of Cambrian dolomite fracture reservoirs. In this paper, we analyze the developing time, space distribution and evolution history of the fault system which will contribute to oil and gas accumulation research and exploration of the Cambrian buried hill in this area.
The Bannan Oilfield is on its later stages of development, new discovery of oil potential is inevitably the most reliable way in sustaining its historical importance of production. A refined technique portfolio is developed to achieve a better understanding of reservoirs featuring heavily faulted, low relief, thin beds with narrow lateral extension. The problems arising from the recognition of micro-faults, tiny blocks, and microstructures are tackled in this paper through a series of techniques as: seismic aided genetic correlation, slant well trajectory calibration, 3D seismic well tying, wavelet domain coherence and true three-dimensional seismic based structural modeling. The spatial extension of individual sand bodies is delineated based on micro facies analysis and lithological inversion. The study shows that the distribution of oil and gas accumulations is governed by both structural features and lithological variations. Sub-bed group structures are well developed. These micro structures, together with the fault proximity regions and the previous undiscovered oil bearing sand bodies contribute to the major sustaining potential. The promising oil and gas flows of the 3 wells drilled based on this study are prudently advertising the remarkable value of this technical practice in extending the life of developed oil fields.
Multiple intra-Craton rift and passive marginal sedimentary basins are developed in eastern Africa. Since the exploration degree was very low, only several natural gas fields were discovered. With the increasing demand for hydrocarbon all over the world, eastern Africa will be the new important area for petroleum exploration and development. Based on studies of regional geology and petroleum geology in eastern Africa, this paper concludes that these basins are composite basins that were developed on the Cambrian base during Tertiary. Several basic characteristics of petroleum geology are generating gas in the Karoo group of Paleozoic, generating oil in rock of Mesozoic and poor preservation in the basins of Cenozoic. The potential exploration and evaluation targets are reservoir groups below the Karoo Group, middle Cenozoic basins along the seashore of Somalia Mozambique, Tertiary rift basins, and sedimentary basins bordering on Sudan and Red Sea.
There are 4 sets of potential reservoirs in the Kashan area in Iran, among them the carbonate reservoir of Qom Formation in Oligocene Miocene is the most favorable one, and high commercial oil and gas flow was yielded from it. According to drilling, logging, geology, geophysics etc, the reservoir is comprehensively studied and evaluated. Some knowledge is gained as followings: bioclastic limestone in the carbonate platform is the main rock type. Valid pores are solution pore and fracture, pore type is fracture+proe, having double medium feature and serious heterogeneity. According to sedimentary facies and diagenesis, reservoir physical characters are better in well Arn1, Arn2, Arn3 than in well Fkh1, for fractures are not developed and cemented by shale seriously in the latter. The sedimentary environment of the Qom Formation is turbid water carbonate platform. Grain shoal, bioclastic shoal and point reef are the most favorable locations for valid reservoirs. Sedimentary environment, diagenesis and tectonic movement are the controlling factors of reservoir development.
Base on the systematic analysis of geological conditions and the data of coalbed methane CBM exploration in China, this paper analyzes the size and density of CBM resources, reservoir pressure, permeability, depth, adsorption, and hydrogeological conditions. The results indicate that, for one area, one factor maybe very important for CBM development, but for most areas, the recoverability of CBM lies on the effective configuration of various geologic factors.
Ground stress is an important control factor of permeability, high productivity wells are distributed mainly in the relatively less minimum principal stress areas, the ground stress character has important guidance meaning to the prediction of high productivity zones. The authors analyze the control mechanism of the modern tectonic stress field on the physical properties of coal reservoirs in the research area. With principal stress difference increasing, permeability increases sharply in exponential form, and reservoir pressure rises in logarithmic form. Ground stress gradient in well test has correlation with permeability and reservoir pressure, but it disperses widely. Three favorable CBM reservoir-forming types are predicted: High principal stress difference Stagnant trap-Shallow buried CBM reservoir forming type in the Daning Panzhuang-Fanzhuang zone; Medium principal stress difference-Slow flow trap-Medium buried CBM reservoir forming type in the Qinahui zhengzhuang North Fanzhuang and Qinyuan-Anze zone; and Medium principal stress difference Slow flow trap Deep buried CBM reservoir-forming type in the North Zhengzhuang and East Qinyuan-East Anze zone.
With the development of drilling and petroleum engineering technology, low permeable and thin oil layers have been developed. So horizontal drilling technology has been emphasized and applied in fields. This paper analyzes the sensitivity parameter that influences the horizontal well productivity and optimized perforation parameters according to the data of the oilfield. A computer software is developed and it provides the theoretical basis for designing and applying horizontal wells.
Aiming at the problems in the process of the stabilized well test of gas condensate wells with high condensate content, the flexibility of stabilized test in this kind of wells is analyzed based on the withdrawal characteristics of gas condensate reservoirs. A well test method adapted to gas condensate wells with high condensate content is put forward, which takes the well production characteristics into consideration. The field conduct of the method shows that the modified isochronal testing method can eliminate the effects of retrograde condensation on test results and obtain precise testing results, providing the foundation for reasonable production proration.
Using the drilling, well logging, and all kinds of production data in different developing stages of the Tangjiahe oil field, this paper studies the changes of petrophysical properties of fluid units during the water flooding process, builds 3D fluid geologic models at different production times and gets a 4D geology model of the oil field. The 4D geology model is helpful in finding out distribution of residual oil, adjusting development plan, and improving ultimate production factor.
Chenghao-Nanliang area is one of the key blocks of exploration and evaluation in the Longdong oilfield in recent years. It is located in the most favorable oil generation area in the Ordos basin with Chang 6 sand bodies developed. Through the research of petrology, pore type and heterogeneity pattern on the 6 reservoir in the Chenghao Nanliang oilfield, we believe that the divergence of Chenghao Nanliang oilfield’s physical properties and petroliferous poperties is great, and the heterogeneity is strong. Underwater distributary channel deposit sandstones with good porosity and permeability are developed widely in the Chenghao Nanliang area, and are the favorable places for oil and gas accumulation.
The Three Gorges Project is famous in the world. The water head reached 135m above sea level on June 10, 2003, and the impounding water height is about 50m. The water head arrived at 156m on Oct. 27, 2006, and the total impounding water height is about 71m. In 2009, the water head will get to 175m with a total impounding water height of about 90m. The two gas wells, Luojia 16H well and Luojia 2 well, which belong to the China Oil and Gas Cooperation at Gaoqiaozheng of Kaixian county in Chongqing Municipality, triggered off two terrible gas hole blow accidents separately on Dec. 23, 2003 and on Mar. 25, 2006, and made a large loss. Apart from the factors with people’s operation, the two accidents make us think whether they are also related to geological conditions. This paper generally discusses the changing of local present hydrodynamics and geostress backgrounds caused by the Three Gorges reservoir impounding water, and suggestions are also given to prevent the third gas hole blow accident happening again.
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