Modelling abnormal pressure development in sandstone/shale basins

Abstract

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, developed at the University of South Carolina), which includes a dynamic indicator inversion method so that present day indicators of dynamic evolution with depth, such as porosity, permeability, formation thickness and fluid pressure, can be used to evaluate the parameters controlling the temporal behaviour of geological processes as well as those in intrinsic equations of state. In general, the dominant factors influencing fluid pressure evolution are the lithology, faulting/fracturing of rocks and sedimentation rate; other factors, such as fluid thermal expansion, dewatering of clays, as well as hydrocarbon generation, also contribute to the abnormal fluid pressure, but are relatively less important. A shale-dominated section can lead to overpressure as high as 300 atm at about 2000±500 m sub-mudline depth with a sedimentation rate of 50 m/Ma, whereas in a section dominated by sand and sandy shale, low 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 therefore bleeds off pressure within or close to the fault zone, which consequently lowers the rock porosity. In contrast, a hydraulically closed fault has little impact on fluid pressure. Thus the hydraulic nature (open or closed) of a fault is an important factor in basin evaluation processes. Salt sheets in basins also play an important part in pressure development because of both the low permeability and high mobility of salt in the subsurface. Significant overpressure build-up beneath 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, Alaska 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 in the central graben with a sedimentation rate of about 160 m/Ma, which is probably the primary factor driving fluid movement in the basin towards the shallower and margin-ward directions. The rapid deposition of low permeability shale appears to be the main cause of the extensive overpressure in the Navarin Basin. In addition, the sensitivity of parameter values is shown, as is the association of predicted and observed quantities at the four wells, indicating the resolution obtainable.