Abstract:

Gulong depression in Songliao basin is a major hydrocarbon generating area, where the mudstones have well- developed fractures and abnormally high fluid pressure. Some of these fracture reservoirs have produced commercial oil and gas. Drilling results reflect that the lateral distribution of these mudstone reservoirs in Cretaceous Qingshankou Formation is neither related directly to growth of faults, nor related to the fold curvature ; the vertical distribution of these mudstone fracture reservoirs is not corresponding to the stress in the formation, too. These features indicate that fractures would not be the result of tectonic force. After modeling geological processes concerning the sedimentary burial history and the fluid generation, it is concluded that these mudstone fractures in Gulong depression are non-tectonic fractures,including the shrinking rocks caused by clay mineral de-hydration and pressure boost fracturing caused by heating of hydrocarbon. And these fractures have continuously been developed and expanded further by fluid overpressuring during Mingshui period, and some new fractures have also been produced in this period.

Key words: fluid overpressure, non-tectonic fracture, Gulong depression, Qingshankou Formation