Relationships used for estimating the magnitudes of horizontal stress which can exist in faulted sedimentary basins are discussed. The relationships are based on simple assumptions and take into account the tectonic stress component associated with active normal and thrust faults. Predictions based on the equations indicate that the orientation of the far field minimum stress can change once a normal or thrust fault has fonned. Stress-depletion measurements from normally faulted basins and stress measurements performed 'at depth' in compressive tectonic 'thrust' regions are presented and the predictions from passive basin and faulted basin based relationships are assessed.
A knowledge of the magnitudes and directions of horizontal stresses existing at depth is required for fracture gradient estimation in well casing design and for wellbore stability studies. Theoretical estimates of in situ horizontal stress are commonly based on the assumption of either a passive basin or an Andersonian fault stress regime existing at depth. The stress system existing within a rock mass in the presence of active faults may differ from that which led to the formation of the faults.
Horizontal stresses present at depth are normally calculated using equations based on a passive basin assumption, where no lateral strain occurs and where the two horizontal stresses are equal in magnitude. The horizontal stresses generated under this condition result from gravitational loading of the sediment or rock. The presence of sub-parallel faulting constitutes the 'normal' condition in a basin or reservoir, indicating the anisotropic nature of the three stresses at depth. Breakouts observed in vertical well bores indicate that horizontal stresses are unequal and need to be accounted for when calculating the magnitude of the minimum horizontal stress. The estimation of the minimum stress magnitude using 'passive basin' equations can produce significant errors when applied in tectonic regions (McLellan 1988).
Estimations of horizontal stress magnitudes for geological and geophysical studies are calculated using the constraints of Andersonian faulting, and shearing of the rocks described by the Mohr-Coulomb criterion (Hubbert and Rubey 1959). The horizontal stresses acting within fault blocks can be estimated using simple boundary conditions. In nature, a large range of boundary conditions will exist.