Characteristics and Evolution of Fluid-Rock System Near the Top Surface of Deep Overpressure, Central Junggar Basin, Northwest China

Zhi Yang
China University of Geosciences, Department of Petroleum Geology and Engineering, Faculty of Earth Resources, Wuhan, Hubei Province, P.R.China
[email protected]

This study is to make clear characteristics and evolution of fluid-rock system near top overpressure surface in central Junggar Basin and to better understand the processes of episodic hydrocarbon migration, fluid-rock interaction, secondary porosity formation and hydrocarbon accumulation controlled by overpressured system.

Junggar Basin is a late-Palaeozoic, Mesozoic and Cenozoic superimposed foreland basin, with present-day geothermal gradient 22/km. The thickness of sedimentary rock sequence in central basin is about 15,000m and consists of Carboniferous to Quaternary rocks that overlie a pre-Carboniferous metamorphic basement. Overpressured system exists large-scale and the vertical distribution of pore pressure is comprised of an upper hydrostatic pressured system and a deeper overpressured system in study area. The observed surplus pressures range from about 10 to 60 MPa in depth interval of about 4400-7000m. Overpressured system is one of the most important factors in controlling hydrocarbon distribution.

Assistant with numerical modeling, look for evidences from analysis on relative data near top overpressure surface, such as hydrocarbon-bearing fluid inclusions, carbon-oxygen isotope of calcite cement in sandstones, organic geochemistry characteristics, diagenesis characteristics especially the plagioclase dissolution and carbonate cementation and dissolution, crude oil property, formation water geochemistry, pressure-temperature field and so on.

Deep overpressure is typical and important geological phenomena in study area. Overpressure is probably hydrocarbon-originated and post-diagenesis-stage overpressure. Studying on characteristics and evolution of fluid-rock system near top overpressure surface, will help to understand the processes of geofluids activity related to overpressured system and to give some useful instructions for oil-gas exploration actions.

AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid