Mechanisms of Petroleum Migration

Abstract

The mechanisms of petroleum migration are important for understanding the distribution of petroleum in the inland and coastal shelf basins. Spores and pollen found in crude oils are similar to those in petroleum source rocks in terms of species and color. This indicates that the spores and pollen in crude oils still retain their original exine morphology after migration. So, the spores and pollen in crude oils can bear witness to petroleum migration. Microfissures formed by abnormal high pressure and undercompaction during the process of diagenesis are common, and such microfissures are presumably available for initial migration and expulsion of petroleum and microfossils such as spores and pollen. Connective pores, interstratified openings, joints, and fissures in the carrier and petroleum reservoir beds are all available passages for secondary migration. Fault surfaces and unconformity interfaces provide larger passageways for secondary migration. The phase state of petroleum migration depends on the openings through which petroleum migrates. Because microfissures are wide enough for the passage of spores and pollen, the passageways are equally unimpeded for the passage of oil droplets. It follows that the migration of petroleum in the liquid phase is fully possible during the course of primary migration. Liquid phase migration is also common during the course of secondary migration, because the passageways are much wider than microfissures. It is well known that the migration of petroleum in gaseous phase has never met any resistance, because gas can penetrate through any pores. Spores and pollen in crude oils can also provide information about the direction and route of petroleum migration. The main direction of petroleum migration could be either vertical or lateral migration for different reservoir types and in different tectonic regimes.