Abstract:

Mingbulak oil field in Fergana Basin is an over deep oil and gas reservoir, which is characteristic with the prevalence of abnormal overpressure, pressure coefficient up to 2.0, and with the development of rock salt, gypsum salt bed, low porosity reservoir with abundant fracture. These characteristics have close relationship with the formation and existence of overpressure fluid compartments. By researching these properties, it is found that sediment undercompaction due to rapid subsidence and tectonic compression are main formation mechanisms of overpressure in Upper Tertiary formation，and the development of rock salt layers and its accumulated thickness over 200 m are important conditions for oil formations. Two overpressure fluid compartments can be identified in Tertiary formations; the pressure coefficient of the upper compartment is bigger than the lower one. The fluid properties of both compartments are clearly different, and they are two relatively independent accumulation systems; The upper compartment can further be divided into three sub-compartments separated by two sealed salt beds, and compose a ladder style complex compartment. Three sub-compartments are named as upper Ⅰ, upper Ⅱ and upper Ⅲ respectively from up to low. Overpressure, micro fracture and rock salt in compartments are the main reasons to cause drilling trouble such as well kick, circulation loss and drill pipe sticking, and so on. In addition to oil and gas pools found in the lower compartment and upper Ⅲ sub-compartment, the bottom of upper Ⅱ sub-compartment also has a certain exploration potential; Ultra deep formations accumulated high density oil is related to overpressure fluid compartment. Compartment^,s reservoirs developed abundant fractures due to high pressure, which is not only conductive to high oil production, but also easy to early water breakthrough.

Key words: Fergana Basin, Mingbulak oil field, overpressure, compartment, fractured reservoir