Analysis on Evolution of Abnormal High Formation Pressure in Gas Fields of Upper Triassic Xujiahe Formation, Central Sichuan Basin

Key words: 
• pressure evolution

Abstract: The characteristics of present abnormal high formation pressure (AHFP) in Xujiahe Formation of central Sichuan Basin were obtained based on the measured formation pressure. The formation pressure of main reservoirs obviously shows high formation pressure or AHFP at present. Bajiaochang gas field is a typical AHFP gas field with pressure coefficient between 1.7 and 2.0, while the others belong to high formation pressure gas fields with pressure coefficient between 1.2 and 1.6. This paper use coupled fluid flow compaction method to reconstruct the pressure evolution of the major reservoirs in Xujiahe Formation. This method could perfectly consider the changes of porosity, permeability and fluid volume in the reservoirs, which could perfectly reconstruct the pressure evolution history. Thermal history is a key parameter during reconstructing the pressure evolution, so this paper reconstructs thermal history by using vitrinite reflectance based on Easy% Ro model. The pressure evolution history of the major reservoirs were reconstructed according to relevant parameters and constrained in measured formation pressure. The evolution of formation pressure in Xujiahe Formation can be divided into three stages: (1) normal pressure before 150 Ma; (2) pressure increase and AHFP accumulation between 150 Ma and 100 Ma. The formation pressure and pressure coefficient gradually increased during this period. The formation pressure got to maximum with pressure coefficient between 1.1 and 1.6 at about 100 Ma; (3) pressure release but AHFP accumulation after 100 Ma. Both the formation pressure and the hydrostatic pressure decreased, but the hydrostatic pressure decreased more than formation pressure, so the pressure coefficient still increase and finally reach between 1.4 and 2.0. The origin of abnormal pressure was analyzed based on the reconstructed formation pressure incorporated burial history and maturity evolution history. It was found that the hydrocarbon charging and the hydrostatic pressure decrease during strata erosion were the main origin of AHFP, while the temperature decrease during strata erosion was main origin for the decrease of formation pressure at later period. AHFP is conducive to the hydrocarbon migration and accumulation in this area. With the characteristics of strong heterogeneity in the reservoirs, the development of AHFP was main force for hydrocarbon migration and accumulation through the dense barriers. The fluid migration is nondarcy flow in low permeable reservoir which must overcome restart pressure. In this case, the overpressure was necessary for hydrocarbon migration. The decrease of formation pressure at later period was beneficial to desolution of dissolved gas and desorption of coal bed methane in Xujiahe Formation, which enhanced the gas accumulation efficiency. To a certain extent, it reduces the requirements for gas kitchen, forming a series of large gas fields under the condition of source rock deficiency of Xujiahe Formation in central Sichuan Basin