Redistribution Of Natural Gas And Hydrocarbon Accumulation Study

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.