Study On The Genesis Of Subnormal Formation Pressure In The Upper Paleozoic Strata Of Sulige Gasfield

So far, Sulige gas field has the most abundant reserves of natural gas in China and it belongs to lithologic gas reservoir. There is ample geological reserves of Sulige, but because of complex geologic setting and reservoir heterogeneity, the characteristics of Sulige gas field show low porosity, low permeability, low gas abundance, subnormal formation pressure, minor production and difficult to develop. To study the history of structural evolution, the history of source rock thermal evolution, the character of paleofluid evolution, recovery of formation temperature and formation pressure, recovery of denudation thickness, the character of gas migrating and accumulating, the relation between formation water and gas conserving and the structure of gas play will be important to realize the causation of subnormal formation pressure and exploration & development of Sulige gas field.This dissertation focuses on the main controlling factors of the causation of subnormal formation pressure in Sulige gas field. Through the analysis on structural evolution, sedimentation and lithogenesis, gas generating, migrating and accumulating, tectonic denudation, thermodynamics and laser Raman spectroscopy of fluid inclusion, the main controlling factors of the causation of subnormal formation pressure and the formation pressure evolutionary process of Sulige gas field are illustrated.Based on the above study, the following conclusions were drawn: firstly, most of the formation pressure of the upper Paleozoic is subnormal formation pressure; secondly, the closing conditions of Shihezi formation is preferable and it is advantageous to the development of subnormally pressured fluid compartment and the accumulation of natural gas; thirdly, the subnormally pressured fluid compartment is mainly in the middle-late period of uplifting half cycle in long-term and mid-term datum level, the main microfacies of which are distributary channel deposit, natural barrier deposit and mouth bar deposir etc; fourthly, the formation pressure of the upper Paleozoic in Sulige gas field underwent the following evolutionary process since mesozoic: from normal pressure(T) to normal pressure or subnormal pressure(J1) to abnormal high pressure or extra high pressure(J2~K1) to subnormal pressure or extra low pressure(K2~present); fifthly, tectonic uplifting, formation denudation and gas diffusionshy are the main controlling factors of the causation of subnormal formation pressure in Sulige gas field, the reduction of formation temperature resulted in about 23% decrease of formation pressure and gas diffusionshy resulted in about 15~25% decrease of formation pressure, at the same time, the structure of gas play, drochemistry activity of lithogenesis, the disequilibrium of gas loss and supply, gas column height and the difference between the density of gas and water did some work to the formation pressure.This thesis will be helpful to the study on subnormal formation pressure and it will be of great importance to the exploration and development of Sulige gas field and the similar gas fields.