Temperature and Pressure Distribution and Evolutionary Characteristics of Permian Strata in the Yuanba-Longgang Area of Northeastern Sichuan

The Permian system in northeastern Sichuan is a key natural gas exploration area in the Sichuan Basin. The deeply buried Permian carbonate reservoirs have undergone multiple tectonic events, resulting in complex diagenesis and varying degrees of modification. Fluctuations in temperature and pressure in these deep reservoirs over geological history have shaped the current states of hydrocarbons, reservoir diagenesis, and porosity evolution, all playing significant roles in hydrocarbon migration, accumulation, and preservation.

This study aims to analyze the temperature and pressure distribution characteristics of Permian reservoirs in the Yuanba-Longgang area. It investigates the mechanisms behind overpressure generation and the evolutionary pathway of formation pressure in the Permian Changxing Formation, based on the historical development of the geothermal field.

Data from 54 wells in the Yuanba-Longgang area were analyzed, revealing formation pressures in the Changxing Formation range from 53 to 68 MPa in Longgang and 69 to 90 MPa in Yuanba. The pressure distribution shows that Yuanba has higher formation pressures than Longgang. Sealing layers, including the Middle and Lower Jurassic formations, exhibit normal hydrostatic pressures, while the Lower Jurassic base and Upper Triassic Xujiahe Formation constitute the first overpressure system, followed by the Lower Triassic Feixianguan Formation and Upper Mesozoic Permian as the third. Temperatures in the Changxing Formation range from 145°C to 154°C, with Yuanba generally exhibiting higher temperatures than Longgang.

Paleo-heat flow in the Permian of northeastern Sichuan ranged from 55 to 70 mW/m². By analyzing burial and thermal histories, we reconstructed the temperature and source rock maturity evolution in typical wells in the Yuanba-Longgang area. In the Early Triassic, source rocks in the Longtan Formation exceeded the hydrocarbon generation threshold but stagnated. From the Early Jurassic onward, source rocks continued evolving, reaching high maturity by the Late Jurassic and overmaturity by the Early Cretaceous, with peak thermal evolution in the Middle Cretaceous. Oil generation began in the Early Triassic, transitioning to gas generation by the Middle Jurassic, while crude oil cracking started in the Late Jurassic and concluded in the Early Cretaceous.

Gas component analysis in the Feixianguan and Changxing formations shows that the region’s natural gas primarily originates from crude oil cracking. These formations contain abundant asphalt from thermal cracking, with gaseous hydrocarbons forming a significant component of the natural gas. This process also drives overpressure development in the Feixianguan Formation. Basin modeling, using paleopressure values restored from fluid inclusions and current measured pressures, shows that prior to 230 Ma, formation pressures were hydrostatic. From 230 to 91 Ma, overpressure developed, peaking at 91 Ma, and since then, formation and residual pressures have declined.