Heterogeneous distribution of pyrobitumen attributable to oil cracking and its effect on carbonate reservoirs: Feixianguan Formation in the Jiannan gas field, China

摘要

Few previous studies have focused on solid bitumen distribution and its effect on gas reservoir quality during oil cracking. Solid bitumen is commonly found in both gas and nongas reservoirs of the Triassic Feixianguan Formation (T(1)f) in the Jiannan gas field. The T(1)f natural gases are mainly secondary cracking gases of oil generated from source rock of the Permian Wujiaping Formation (P(2)w), and the reservoir experienced temperatures above 150 degrees C (302 degrees F) for about 35 m.y. A relatively narrow range of ln(C-1/C-2) values and a wide range of ln(C-2/C-3) values and wide-spread solid bitumen indicate that oil cracking took place in the gas field. Low concentrations of H2S (commonly <0.81%) suggest that high-reflectance (2.57%-3.07%) solid bitumens are pyrobitumens, which would have been mainly derived from oil cracking. Gases preferentially occupy larger pore spaces, and oil is displaced into small pores and throats by overpressure during oil cracking. In this way, pyrobitumens can reduce the magnitude of porosity in relatively tight reservoirs. Moderate-quality oil reservoirs (paleoporosity 2.2%-8.0%) are between or adjacent to high-quality oil reservoirs and are probably poor-quality or nongas reservoirs after oil cracking. Carbonate reservoirs (paleoporosity >8.0%) can be high-quality gas reservoirs after oil cracking and should be favorable targets for future gas exploration in the northeastern Sichuan Basin and adjacent areas.