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ExxonMobil Upstream Research Company, PO Box 2189, Houston, TX 77252
There has been a great deal of interest in mapping regional trends in stress orientation and their relationships to crustal deformation processes over the last quarter century. Less emphasis has been placed on explaining local variations in the stress field. However, it is variations in the local stress field which are most critical to characterizing hydrocarbon reservoirs. Fracture orientation, well stability, well orientation, and permeability anisotropy are all strongly affected by variations in the local stress field.
Using stress orientation data from a number of fields in different tectonic environments, we have tried to determine some of the tectonic and geological controls on variations in in situ stress orientation. We find that nearness to faults, fault structure, and magnitude of tectonic stress play primary roles in determining whether the regional stress field will be perturbed in a given reservoir and whether small-scale variations in the stress field can be expected.
We find that high tectonic stress environments (large horizontal differential stress) lead to a much more consistent local stress field than more tectonically quiescent areas. Faults can play a large role in rotating the local stress field. The smaller the difference between the maximum and minimum horizontal stress magnitude (i.e. lower tectonic stress), the larger a fault’s zone of influence is. Large-scale faulting with segmentation of the reservoir into discrete fault blocks can lead to significant stress orientation variations across the reservoir even in areas of large differential stress.
