NUMERICAL MODELING OF ABNORMAL FLUID PRESSURES IN THE NAVARIN BASIN, BERING SEA

The Navarin Basin, located in the Bering Sea, Alaska, is composed of t hree major units: (1) a thick Late Eocene and Oligocene overpressured shaly section; (2) a Miocene sandy section; and (3) a Late Miocene to Pliocene section, characterized by high porosity and a possible zone o f abnormal pressure due to the presence of diatomaceous shales. The ba sin was affected by folding during the early stages of basin filling, whereas strike-slip faulting occurred until mainly Miocene time. To mo del the fluid flow and compactional history of the basin, a cross-sect ion was used, controlled by four wells for which porosity and pressure data were available. A first approximation used a one-dimensional mod el to bracket the range of parameter values which govern the fluid flo w/compaction equations, using data from the Arco COST No. 1 well. A se nsitivity analysis then allowed: (1) determination of the best set of parameters of the model; (2) the behaviour of each parameter and its r elative importance to be assessed; and (3) the role of an erosion even t in influencing the evolutional history of the basin to be tested. Wi th a slight readjustment of the parameters to allow for lateral fluid flow, a two-dimensional model was then constructed. The model reproduc ed correctly the variation of porosity and pressure data with depth fo r the four wells. The main overpressure in the deeper part of the basi n arose during the earliest stages of basin filling by shale because o f strong undercompaction. In the shallower part of the basin, high por osity values were obtained in agreement with the data, whereas some ov erpressure was developed in association with the undercompaction and l ow permeability of the diatomaceous shales. Different assumptions have been investigated, such as the permeable or impermeable nature of fau lts, the role of a rock fracturing coefficient and the anisotropy of p ermeability but, because of the location of the wells on structural hi ghs, no major variations were observed relative to the well data, so t hat the impact of such different assumptions on the total dynamic hist ory of the basin cannot be determined from presently available data.