Characteristics and Formation Conditions of Oil/Gas Fields (Pools) in Abnormal Formation Pressure Environments in China

Abstract

Abnormal pressure oil/gas fields (pools) are widely distributed in China, which with the less than 0. 9 and greater than 1. 1 pore fluid pressure coefficient account for 42% of total oil/gas fields (pools), 35% of which are overpressured ones and 7%, subpressured ones. The abnormally low pressured gas fields (pools) are relatively less in numbers and each has less than 5X 109m3 in reserves; the number of super-pressured gas fields (pools) is more than that of the former, but decrease with the increasing of pore fluid pressure coefficient, and reserves range from 2X109m3 to 15X105m3. The spatial and temporal distribution, the abnormally pressured oil/gas fields (pools) are chiefly located at the formations with composite lithological-structural traps and lithological traps from the Paleozoic to the Cenozoic. Their stages of accumulation were generally in the Mesozoic and the Tertiary. The subpressured oil/gas (pools) were buried at a depth of less than 2000m, and the overpressured ones at a depth of more than 3500m. The more the depth is, the more the number of overpressured oil/gas fields (pools) becomes obviously. The sedimentary-type overpressured oil/gas fields (pools) are mainly located in the eastern area of China, and controlled by the under-compaction, aquathermal, hydrocarbon generation and dehydration of clay minerals which all are the contributors to the pore fluid pressures. But in the central area of China, the structural-type overpressured oil/gas fields (pools) were mainly influenced by the hydrocarbon generation and the tectonic compression in the late stages. The subpressured oil/gas fields (pools) in northeast area of China are chiefly controlled by the tectonic elevation and erosion in the late stages. Our investigation shows that the very favorable conditions to form abnormally high pressure oil/gas fields (pools) in China include the abundant soure rocks, the matching of hydrocarbon generation with the timing of traps, the effective regional seals and the pore fluid pressure coefficient between 1. 1~1. 3.