天然气的再分配及其成藏研究

【作者】 付晓文；

【导师】 王廷栋； 颜其彬；

【作者基本信息】 西南石油学院 ， 矿产普查与勘探， 2004， 博士

【摘要】 我国油气勘探实践中发现,在一些与老构造相邻的新构造中,有时会找到早期生成的天然气;这些天然气显然是从其原来的聚集地转移而来的。根据已建立的生烃模式,烃源岩在生烃演化的早、中期阶段将产出其大部分烃类产物。因此,若在一个探区找到了再分配的早期天然气(包括油的裂解气),则意味着该探区烃源岩所生油气的主体部分很可能得以保存,从而预示该探区有着良好的勘探前景。这种再分配天然气藏的发现与油气勘探前景之间的联系已在我国塔里木、四川、准噶尔等盆地的勘探中得到印证。但由于目前对再分配天然气成藏的机制和规律知之甚少,对再分配天然气的指认亦缺乏系统的方法,因此有目的的寻找此类天然气藏成了勘探家面临的一个难题。基于此,本文应用含油气系统理论的基本思想、天然气的生成模式及油气分段捕集原理,运用油气藏地球化学方法并结合传统地质学方法,首次对再分配天然气的成藏机制、再分配天然气的判识方法以及再分配天然气的成藏模式进行了较为系统的研究。 论文主要得出以下重要认识与成果: 1.再分配天然气藏是指由于某种地质变动(常常为构造变动)而导致古油气藏中的天然气发生转移、进而在新的圈闭中再次聚集形成的天然气藏。在具有古老烃源岩并经历过多次构造变动的盆地中,天然气的再分配是一个非常普遍的现象;同时,再分配天然气藏可以拥有可观的储量,因此,这种特殊方式聚集而成的天然气藏为天然气勘探提供了新的领域和方向。 2.对再分配天然气的判识关键有两点:a.现今所找到圈闭中的天然气的聚集阶段明显与烃源岩生烃史和圈闭形成时间不匹配:b.基于a的判断,结合构造研究,运用地球化学方法判识气的运移途径。 3.为了确认再分配的天然气,在天然气的研究中除了气源研究外,本论文还特别注意了天然气捕获阶段的研究,并建立了系统识别方法。除了烃组成和天然气碳同位素组成外,还采用了氮气和氦气来区分油的裂解气和晚期干酪根裂解气。从理论与实验及气藏实测结果出发建立了油的高更多还原

【Abstract】 It has been noted in the petroleum exploration practice in China that, in some cases, natural gases of early-generation stages were found in relatively newly formed traps. Apparently, these gases must have undergone re-migration from their original accumulation site to enter the newly formed traps. According to established models of hydrocarbon generation, a source rock will yield most of its hydrocarbons in the early to middle maturation stages; therefore, if re-distributed gases of the early- to middle-generation stages are found in an area, it would indicate that the main portion of the hydrocarbons generated by the source rock most possibly have been preserved, implying a good prospective in that area. Such a correlation between the finding of re-distributed gases and good prospective has been affirmed in the petroleum exploration in Tarim, Sichuan and Junggar basins in China. Our current knowledge about the mechanisms and regularities of the formation of re-distributed gas accumulations, however, is inadequate, and methods of recognizing such gas accumulations are in a short. As a result, purposely looking for such gas pools becomes a difficulty laying in front of the exploration geologists. In an effort to partially solve this problem, a relatively systematic research has been done, for the first time, by the author of this dissertation on the mechanisms and modes of pool formation, and methods of recognition of redistributed natural gases, with the application of the essential thoughts of petroleum system theory, models of natural gas generation, principles of fractionation trapping and methods of reservoir geochemistry, integrating with traditional geologic methods.Main conclusions and achievements of this research are as follows: 1、 Redistributed gas accumulations refer to those formed as a result of natural gas re-migrating from its original accumulation site into another trap, caused by geologic （in most cases tectonic） changes. In basins where source rocks are old and multiple-phase tectonic changes have taken place, natural gas redistribution is a fairly common phenomenon. Furthermore, reserves of some redistributed gas accumulations can be quite large. Therefore, such gas accumulations provide a new domain and new direction for natural gas exploration.2、 The key procedures in recognizing redistributed gas accumulations are: （a） to prove that there is a mismatch between the timing of accumulation of the gas found in the trap and the hydrocarbon generation history of the source rock and the timing of the trap formation; （b） if the mismatch is proved, geochemical methods integrated with structural analysis are then applied to trace the migration pathway of the gas.3、 In addition to gas source studies, special attentions are also paid to the study on the timing of gas entrapment in relation to stages of hydrocarbon generation. And a systematic set of methods has been established in the present work. Nitrogen gas and Helium are used to distinguish between oil cracked gases and late-stage kerogen cracked gases, coupled with the use of hydrocarbon composition and carbon isotope character of the natural gas. On the basis of theoretical analysis and measured data from gas fields, mathematical relationships of δC₁ and Q areestablished for oil cracked gases and late-stage kerogen cracked gases. And with these mathematical expressions, the individual amount of each in a mixed gas of the two can be estimated.Formation of gas accumulations sometimes spans a long time interval, which often requires the study of gas pool formation starting from the formation of oil pool. It is exactly the case for the gas pools （Weiyuan gas pool, Jilake gas condensate pool and gas condensate pools in the eastern section of the Tazhong Fracture Zone） dealt with in this research. Apart from study of the geochemical character of oil （often using the "whole hydrocarbon" method as proposed by Prof. Wang Tingdong ）, the study of reservoir bitumen is also important. With this study, the source of oil and gas, the direction of oil and gas migration, the timing of migration and the conditions of pool preservation can often be unveiled. In recognizing redistributed gas pools in the Sichuan and Tarim basins, reservoir bitumen studies have played an essential role. Especially, in basins where only dry gas pools are present, reservoir bitumen study is extremely useful. When Reservoir bitumen study is integrated with investigation of solid reservoir bitumen （in thin sections） and fluid enclusions, information on the stages of entrapment and types of effective reservoir pore spaces in each stage can be gained. Such information helps decision making in selecting exploration targets. Using the character of changes in the gas composition is often an effective way of tracing gas migration. For example, a study of the dryness of natural gases in Tazhong reveals that the gases in the Tazhong 4 and Tazhong 6 structures came from kerogen cracked gas within an eastern paleo-structure. A case study of successfully tracing gas migration with the use of waterwashing principles has been performed in this research, and conditions for the occurrence of waterwashing is discussed.Pre-existed oil/gas pools function as a "gas provider" for re-distributed gas pools. And tectonic activities in the geologic past are frequently the direct cause of natural gas re-distribution. On the basis of case studies, four formative modes of redistributed gas accumulations are proposed, which are: （1） redistribution caused by formation reversal and paleo-structure contraction; （2） redistribution caused by faulting of existing oil/gas pools; （3） redistribution caused by formation reversal, and （4） redistribution caused by activities of syn-depositional faulting.更多还原