Distribution and Controlling Factors of Paleogene Overpressure Top Surface in Dongying Depression,Eastern China

Overpressure is a non-equilibrium state of underground fluid, and the morphology of its top surface is important for dividing overpressure type and studying fluid flow in overpressure system. Underlain by Paleozoic bedrock, Dongying Depression of Jiyang Basin is a rift lacustrine basin developed through multistage tectonic movements. Observed pressure reveals that overpressure prevails in Paleogene formation and occurs throughout the whole depression except the basin margin and boundary faults. The study shows that overpressure top surface is not a plane,and its burial depth is from 2000m to 3400m, that means the top surface is not isobathic but is bounded by some strata. The overpressure top surface mainly distributes between T4 and T6 reflection surface. Sedimentation and tectonic activities controlled the occurrence of the top surface. The rock types in certain depth range below the surface are dominately dark and thick claystone, sandy mudstone and calcareous mudstone. The Paleogene had been in a rapid and sustained subsidence before the end of Dongying Formation deposition in the study area. For example, when the overlying layer (Upper Sha3 sub-member) of Fan 4 well began to deposit, the deposition rate was greater than 600m/Ma, much higher than the lower limit for uncompaction generation which is merely 200m/Ma. The effect of tectonic movement on overpressure top surface is different in each stage. Early tectonic activity controlled the formation of overpressure body, but the later controlled the burial depth of the overpressure top surface.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California