Formation Pressure Evolution In A Compressional Tectonic Setting

The Kuqa Depression has become the origin of China’s west-east gas transmission project with the discovery of a number of large and medium-sized oil and gas fields.The Kuqa Depression has been experiencing varying overpressures.Previous studies on the overpressure mechanism in the Kuqa Depression are mainly qualitative speculation and much of controversies.There are great challenge and uncertainty on restoring paleopressure due to the complex tectonic-evolution,which restricts the understanding of hydrocarbon accumulation and exploration process in the study area.In this paper,the present-day formation pressure was presented based on the data of drilling,logging and seismic.The tectonic history during compression stage was restored,using geomechanics-based structural reconstruction.Pressure evolution was performed,combined with restored geometries and multiple overpressure causes.The contributions of overpressure mechanisms were evaluated quantitatively.Furthermore,the influence of tectonic evolution and fluid overpressure on hydrocarbon migration and accumulation is discussed.This study has achieved the following conclusions: 1)A method for pore pressure restoration coupling with multiple overpressure mechanisms was proposed,by which formation pressure evolution was reconstructed.This study tested the impacts of tectonic compression,disequilibrium compaction,opening and closing of faults,folding and salt-bearing seal on pore pressure.Overpressure was caused by disequilibrium compaction during the Paleogene time.A combination of lateral compression and vertical loading increased overpressure.The open faults had a distinct pressure-releasing effect,in which releasing influence depended on permeability of rocks.Pressure compartments were caused due to fault closing at present day.The groundwater-potential related overpressure developed within anticlines.2)Overpressures caused by lateral compression,gas generation,and salt seal were evaluated quantitatively.Through a combination of basin modelling and the newly-proposed mean stress method,the pressure increase caused by shortening can account for a maximum of more than 60% of overpressure.The contribution of compression in the Kelasu Structural Belt was greater than the Yiqikelike Structural Belt,the fold and thrust zones greater than sags,and the low-permeability rocks greater than permeable rocks.The overpressures in the Jurassic and Triassic source rocks resulting from gas generation reached 26～20 MPa,while it is up to only 4 MPa in the insitu Cretaceous reservoir.Hydrocarbon saturation and pore connection significantly affected the pressure generated by gas.The increase in salt has promoted the onset time and magnitude of overpressure in the underlying strata.Aquathermal pressuring in the study area was negligible.3)Based on tectonic and formation pressures evolution,the migration and accumulation of hydrocarbons were reconstructed: hydrocarbons driven by overpressure and buoyancy force firstly charged reservoir rocks since the Neogene,and then migrated along the tilted carrier bed into the paleo traps.Opening and closing of faults significantly affected the vertical migration and preservation of hydrocarbons,respectively.Tectonic compression triggered multiple adjustments of paleo traps until the formation of present-day accumulations.This study has an important theoretical significance for the understanding of overpressure mechanisms and evolution,and provides scientific evidence for searching favourable zones in a tectonic compression setting.