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Daqing oil field is one of the giant nonmarine oil fields. Taking Daqing oil field in the Songliao nonmarine sedimentary basin as an example, and on the basis of the study of organic geochemical and geologic conditions for the formation of the giant nonmarine oil field, the characteristics of formation of a giant oil field are discussed.
The formation of Daqing oil field, according to the analysis of depositional, structural, and geochemical conditions, can be characterized by the small area ratio of source rocks to reservoir rocks and the short distance of secondary migration of oil and gas. From three aspects of the reservoir characteristics of Daqing oil field--the geochemical conditions for generation, expulsion, migration, the accumulation of oil and gas; and the relationship between these conditions and structural growth--the process of formation of Daqing oil field is discussed by the writer.
It is considered that the kerogen in Daqing oil field is of "combined" sapropelic type. The source rocks in Daqing oil field have high efficiencies both in hydrocarbon generation and in hydrocarbon expulsion, thus forming good source rocks, indicating that even a relatively small hydrocarbon generation area can effect a giant oil field.
Because the sandy reservoir (parallel with the striplike oil source sags on both sides) is surrounded with source rocks, lateral secondary migration of oil and gas over a short distance is the main migration pattern of hydrocarbon during the formation of this giant nonmarine oil field. As a result, a typical model for a giant oil field in a large eutrophic-like basin is presented as follows.
1. A good reservoir composed of a huge delta complex, part of which directly extends as a carrier bed into source rocks; a large structural trap with a very thick sand body; and a large cap rock
overlying the trap are the factors necessary for the formation of a giant oil field. The five essential bodies--source bed, carrier bed, reservoir, trap, and cap rock--provided favorable geologic conditions that contributed to the formation of Daqing oil field.
2. Within the spatial assemblage, the favorable coordination among the geochemical conditions for the generation, expulsion, migration, and accumulation of oil and gas, and the stage of structure growth is obviously the key to the formation of a giant nonmarine oil field.
The vertical and lateral variations of properties of the crude oil in the main reservoir are discussed in detail. It has been determined that no differential accumulation ever occurred. Thus, it is possible for the huge space to receive great amounts of oil continuously and to form such a giant oilfield.
The formation processes of Daqing oil field provided an ideal model for the formation of a giant nonmarine oil field. The theoretical and practical investigation of this model and the analysis of the geologic and geochemical conditions for the formation of this kind of field, are of significance for understanding the formation regularities of giant nonmarine oil fields and are important for guiding exploration for oil and gas.
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