Study On Pore-fluid Pressure Of The Ordovician Carbonate Reservoir In Shuntuoguole Low-uplift,Tarim Basin

The marine carbonate reservoirs in Tarim Basin have become popular worldwide within the oil-gas industry and a research focus.The study of pressure field in sedimentary basins is a key issue for hydrocarbon migration and accumulation mechanism.However,for the complex ancient marine strata,the pore-fluid pressure often lacks effective means.Based on the Ordovician current pore-fluid pressure,we analyze the overpressure mechanism by using the Bowers curve and the quantitative model of crude oil-cracking gas and gas charging in this paper.The differential stress method was used to recover the paleo-pressure during the period of deformation that affected sedimentary strata in basins.With the aid of fluid inclusion and basin modelling,the pore-fluid pressure evolution of middle-upper Ordovician in the study area has been reconstructed.Through comparative analysis,we discuss the influence factors for pressure evolution and mechanism differences among different secondrary tectonic units.The current pore-fluid pressure in Tarim Basin can be divided into five pressure systems in the longitudinal direction.The overpressure in Ordovician is laterally controlled by the structural units.Since the seals of pressure system near the faults and fractures are destroyed,it appears as a normal pressure-weak overpressure.And the overpressure-strong overpressure is developed away from the fault development zones.Based on the comprehensive analysis of Bowers curve and logging curves,fluid expansion and tectonic compression are the main causes of overpressure formation in the study area.For fluid expansion,we established a quantitative modle for evaluating overpressure caused by crude oil-cracking gas and gas charging in Ordovician.Considering the data from crude oil-cracking gas kinetics experiments,the contribution of two factors is calculated.The results show that gas charging is the main factor of the overpressure formation with the contribution rate up to 94%.For tectonic compression,we explore the application of dfferential stress method to recover the pore-fluid pressure in Shunnan Area,taking the microscopic deformation characteristics of twinned calcite as a breakthrough.The pressure coefficients of middle-upper Ordovician in this area are 1.15-1.19 and 1.35-1.41 in Caledonian Orogeny and Hercynian Orogeny,respectively.Constrained by the current pore-fluid pressure,overpressure mechanism and paleo-pressure from fluid inclusion method and dfferential stress method,the pore-fluid pressure evolution of the Ordovician carbonate reservoir in Shuntuoguole Low-uplift is reconstructed with combination of basin modelling method.The evolution trend in each secondary structural unit is similar,while there is a significant difference between the overpressure mechanism and magnitude.The research shows that the pressure evolution of the middle-upper Ordovician has experienced normal pressure,weak overpressure,pressure relief,strong overpressure,pressure relief and overpressure/normal pressure successively.The Shuntuo-Shunnan Area develops overpressure again during the Himalayan Orogeny,while the Shunbei Area showed normal pressure.The target layer of the Shunbei Area is overpressured by hydrocarbon charging and formation temperature in the early stage,and is normal pressure due to low geothermal gradient and tectonic activity in the later stage.The overpressure occurred in the Shuntuo Area combines various factors,such as hydrocarbon charging,fluid phase change and tetonic compression.In contrast,the overpressure in the Shunnan Area is mainly affected by early tectonic compression and late natural gas charging.During the Himalayan Orogeny,the marine carbonate strata in the Shuntuo-Shunnan Area are stable deposited and the pore-fluid pressure is preserved,bringing about an overpressure environment in the area.The causes of the differences in the overpressure mechanism are mainly related to the differences in thermal mechanisms and tectonic movements between different structural units.Correspondingly,there are significant differences in hydrocarbon accumulation periods,types and intensities from each other.