CONSTRAINTS ON HYDROCARBON MIGRATION FROM THE QINGSHAHOU SOURCE-ROCK IN THE WEST OF THE NORTH SHONGLIAO BASIN, CHINA

Many small gas fields have been discovered in the west of North Shongl iao Basin of China while drilling for oil. Applications and results of DTIT and GEOPET II codes indicate that the formation and distribution of these gas fields are controlled by hydrocarbon expulsion and migra tion from tile major Qingshahou source rock. This generated most of th e oil and gas at 88-50 Ma and 50-0 Ma, respectively; the ratio of oil to gas is about 70:30. Most of the separate-phase oil was expelled at 35-15 Ma, but no separate-phase gas was expelled because of total solu tion of generated gas in oil and in connate water. Gas dissolved in wa ter tended to migrate from the deep source rock centre to the West Slo pe (shallower formations) of the basin through sandstone reservoirs an d normal faults, gradually being released from solution and accumulate d in traps along the migration route. Gas dissolved in oil tended to m igrate to traps around the source rock centre, and was released and ac cumulated at the tops of oil reservoirs. The best area for prospecting for gas is between 500 m to 2000 m.