Evolution of geopressure field in Lunnan Area and its significance on hydrocarbon accumulation

Using numerical simulation, seven profiles crossing Lunnan Area and two profiles along the sag (Caohu sag and Manjiaer sag) were selected to restitute and simulate the pressure field evolution. The result of the simulation, measured pressure data and drilling data showed there was weak overpressure in Carboniferous strata and upper Ordovician strata and prominent overpressure existed in the Cambrian strata. Analyzing the result of the simulation and measured data, in the main area of Lunnan Area, the overpressure in Carboniferous strata was formed by aquathermal pressuring and shale mineral transformation at the late Himalayan epoch, but in the sag, the overpressure was formed by under compaction at Late Hercynian. The overpressure in Carboniferous strata and upper Ordovician strata had the same characteristics such as forming time, development mechanism and overpressure features. The overpressure in Cambrian strata had three growth times in the sag, late Caledonian to early Hercynian, later Hercynian and Himalayan. They match up to three accumulation times in Lunnan Area and the strongest overpressure happened in Himalayan movement. In conclusion, weak overpressure in Carboniferous strata and upper Ordovician strata contributed a little to hydrocarbon accumulation dynamics but enhanced sealing ability of formations, so it had a great impact upon Himalayan gas accumulation. The overpressure in Cambrian provided dynamics for the primary migration of oil and gas.