The Sealing Ability of Overpressure Layer and Its Geological Factors — A Case Study of Dongying Depression, Eastern China

Sealing layers are important for maintaining overpressure especially in permeable strata. The sedimentary rock with low permeability is not only the most common seal in overpressure system,but also the primary caprock in traps. Observed pressure reveals that overpressure is widely developed below 2100m in Dongying Depression, Eastern China. It occurs throughout the whole depression except the margin and basal unconformities. In the study area, the sealing layers are mainly composed of dark grey mudstone with thickness among 500~900m. Moreover, carbonate and clay minerals are found in great abundance in the seals. X-ray diffraction shows that the diagenesis of mudstone is in the stage of quick transformation for illite and smectite, and corresponds to the strata that carbonate abounds in. This diagenesis evolution reduces the porosity and permeability of sealing layers effectively. Statistical analysis confirmed that overpressure sealing layers in Dongying Depression generally have higher displacement pressures (an average of 5.45MPa), and a good positive correlation is presented between the displacement pressure and the maximum pressure coefficient that can be sealed. However, the sealing layer with displacement pressure less than 4MPa can cover the abnormal high pressure with the coefficient greater than 1.7. This evidence demonstrates that capillary pressure sealing is not the single mechanism for the sealing layer. In fact, surpressure and (or) high hydrocarbon concentration sealing also have some impacts on the sealing ability of the sealing layer, and the thickness and diagenesis intensity are also the controlling factors for the sealing ability of the overpressure layers.

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