MODELING ABNORMAL PRESSURE DEVELOPMENT IN SANDSTONE SHALE BASINS

Overpressure development in a sedimentary basin is directly related to the rate of fluid escape from the sediments. The model used here for fluid pressure evolution is a two-dimensional model (GEOPETII, develop ed at the University of South Carolina), which includes a dynamic indi cator inversion method so that present day indicators of dynamic evolu tion with depth, such as porosity, permeability, formation thickness a nd fluid pressure, can be used to evaluate the parameters controlling the temporal behaviour of geological processes as well as those in int rinsic equations of state. In general, the dominant factors influencin g fluid pressure evolution are the lithology, faulting/fracturing of r ocks and sedimentation rate; other factors, such as fluid thermal expa nsion, dewatering of clays, as well as hydrocarbon generation, also co ntribute to the abnormal fluid pressure, but are relatively less impor tant. A shale-dominated section can lead to overpressure as high as 30 0 atm at about 2000+/-500m sub-mudline depth with a sedimentation rate of 50 m/Ma, whereas in a section dominated by sand and sandy shale, l ow to zero overpressure obtains. Hydraulically open faults modify the fluid pressure significantly in shale sections. The high permeability zone created by the faults provides a pathway for fluid escape and the refore bleeds off pressure within or close to the fault zone, which co nsequently lowers the rock porosity. In contrast, a hydraulically clos ed fault has little impact on fluid pressure. Thus the hydraulic natur e (open or closed) of a fault is an important factor in basin evaluati on processes. Salt sheets in basins also play an important part in pre ssure development because of both the low permeability and high mobili ty of salt in the subsurface. Significant overpressure build-up beneat h a salt sheet, and an area of normal or smaller overpressure above a salt sheet, were obtained in the modelling procedure, as observed. An example of an abnormally pressured basin from the Navarin Basin, Alask a is also presented here. Prediction of present day formation pressure with depth is made with four controlling wells. The main overpressure , deeper than about 1500+/-200 m, reaches a maximum of about 800 atm i n the central graben with a sedimentation rate of about 160 m/Ma, whic h is probably the primary factor driving fluid movement in the basin t owards the shallower and margin-ward directions. The rapid deposition of low permeability shale appears to be the main cause of the extensiv e overpressure in the Navarin Basin. In addition, the sensitivity of p arameter values is shown, as is the association of predicted and obser ved quantities at the four wells, indicating the resolution obtainable .