MIGRATION AND ACCUMULATION MECHANISMS OF “TIGHT OIL” IN THE JURASSIC RESERVOIRS, SICHUAN BASIN

Sichuan Basin is well-known for its natural gas accumulation, while it also has abundant oil resources. Approximately 4.8 million tons of oil has been produced by the end of 2010. The porosity of the Jurassic reservoir is only 0.2~5%, while the average permeability is less than 0.1 mD, which is typical "tight oil".

It is generally believed that "tight oil" is characterized as continuous reservoir laterally, and the hydrocarbon haven’t undergone large-scale secondary migration. Thus,the expelled oil can be regarded as potential petroleum resource when evaluated the resources. But after the study of oil distribution in the Jurassic reservoirs, we come to conclude that “tight oil” has also experienced migration and accumulation processes, similar to that of conventional oil.

Like most tight oil basins, the oil occurrences in the Jurassic reservoirs are quite heterogeneous. Vertically, there are five oil layers, while 80% of the oil has been produced from the Daanzhai(DAZ) Section. Laterally, the production differs a lot between different wells. Statistics shows that the high-productive 150 wells, 16% of the total oil wells, produced 3.3 million tons of oil and contributed 70% of the total production. Obviously, these high-productive wells are exploration “sweet spots”.

As to the DAZ Section, the productive layers are the Dayi and the Dasan subsections limestone, neither of which contains source rocks, and oil comes from the dark shale occurred in the middle of the DAZ Section. In this case, the oils need to migrate 5~30 meters. The migration direction can be both upwards and downwards. In some cases, hydrocarbons can also migrate laterally.

During the process of oil migration, it must accumulate in relatively low static energy potential region. In general, the low static energy regions are relatively high-porosity and high- permeability areas, where the infilling resistance is low. Similar characters are shown in other “tight oil” basins.

Oil migration is the basis for oil accumulation. This is also suitable for “tight oil”. Like conventional reservoirs, the relatively high-porosity & high-permeability “sweet spots” are still key exploration targets. When evaluate oils resources potential, the accumulation efficiency cannot be over looked.