Constraining the Dynamic Properties of Faults in Compartmentalized Oligocene Mid-Slope Turbidite Channels

Abstract

The reservoir development plan targets several pressure compartments which are cut by multiple small yet seismically resolved faults. These faults partially disconnect the channels between the planned injectors and producers and their dynamic behaviour is a key uncertainty for the development in the absence of production data. The purpose of this study was to review the data available to constrain the fault sealing and baffling capacity in the field. A review of the geological settings allowed a better understanding of the timing and mechanisms of deformation in the field. Faults formed at different times (Oligocene and Miocene extension, Late Miocene to present day compression) and are not expected to have the same flow characteristics. When deformation occurs at great depth of burial, stress and temperature conditions allow crushing and dissolving sand particles during displacement causing significant permeability reduction in the clay-free areas of the fault zone (where flow preferentially occurs). Fault seal analysis was performed integrating reservoir fluid pressures measurements, seismic DHI and structural/stratigraphic observations. Faults appear to leak over geological time in areas where they juxtapose thick sand units at the base of the channels. This explains the presence of a laterally extensive aquifer penetrated by 3 wells in different compartments. Elsewhere, faults throw and shale gauge lead to pressure disconnection of oil sands. Deformation features seen in cores were classified and measured, leading to the definition of three permeability/capillary entry pressure functions related to the fault clay percent. The different permeability/capillary entry pressure functions are utilised for faults of different ages. Early faults (formed at less than 1 km of burial depth) are open in clay free areas (as suggested by flatspot continuity across them). Large Miocene faults are baffling in clay-free areas and can hold minor pressure differences (10-40 psi). Faults related to the late contraction are strongly baffling in clay free areas and can hold major pressure difference between compartments (>100 psi). The fault permeability in clay free areas is thus constrained by the age of the fault and the fault seal analysis. Uncertainty remains on the estimation of fault shale gauge ratio which determines which areas of the faults are clay free and open, baffling (20 to 40% shale gauge ratio) or sealing (more than 40% shale gauge ratio).

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016