Mechanisms of Generating Overpressure and Its Influences on Super-Low Permeability Reservoirs of Upper Es4 Member, Chexi Depression

Abstract By means of analyzing super-low permeability reservoir of upper Es4 member in Chexi depression into sedimentary microfacies, diagenesis, reservoir porosity and permeability and testing geochemical data, the mechanisms of generating overpressure and its influences were discussed. The results show that:The reservoir, being evaluated medium-low porosity and super-low permeability, is disadvantageous to oil & gas exploration and development, as lithology is argillaceous fine sand, porosity is 10% ~ 20% and permeability is less than 10×10-3µm2 in general;The principal mechanism of generating overpressure is disequilibrium compaction, and the secondary is mineral diagenesis;The overpressure can effectively inhibited formation temperature to increase and diagenesis process;In overpressure wells section, the secondary porosities and fractures are preserved during mesodiagenesis stage and telodiagenesis stage, which are favorable to oil and gas exploration and development;In study area, it is not a positive linear relationship between oil production capacity and formation pressure coefficient, as high production capacity corresponds to the pressure coefficient of 1.2~1.3. With increasing production capacity of the global oil and gas and exploring most oil and gas reservoirs year by year, the influencing factors of super-low permeability reservoirs was also more and more attention [1–3]. Since the 1950s, through the geologic simulations and the typical overpressure basin studies, many scholars analyzed that overpressure could preserve the primary and secondary porosity to improve reservoir preformation in two kinds of mechanisms with physical and biochemical actions [4–6]. In development process of overpressure reservoirs, the overpressure fluid will change rock stress to make the elastic and plastic deformation, so as to reduce the porosity and permeability [7–8]. Therefore, overpressure constructive and devastating effects on reservoir properties still need further clarification.