Amgad I. Younes1, Atilla Aydin2
(1) Stanford University, Stanford, CA
(2) Stanford University
We examined the relationship between fault offset, shale smear, and fault seal potential along several reservoir-scale faults cutting a single, 50m thick layer of shale, and one 10 km -long fault cutting 100m inter-bedded shales and sandstones. For single layers, the shale rotates until it parallels the fault consistently thinning with fault offset, regardless of the fault size. Because fault slip increases towards the center of the fault, shale smear is thickest at fault tips and thinnest near the fault's center, both in the strike and dip directions. Therefore, fault centers are potentially riskier than fault tips.
In contrast, multiple layers of incompetent rock causes a fault to segment in the dip direction. Each segment has its local slip distribution and cuts a different stratigraphic section. Unless the segments have merged, the contribution of incompetent units to fault sealing depends on the segment's offset to source shale thickness ratio (SSF) not an overall SSF. Competent rocks (sandstone or limestone) only boudinage forming discontinuous bodies. Consequently, a segmented fault may leak at different locations depending on the local slip/shale thickness distribution.
In both single and multiple layers, however, the shale smear thickness approaches zero as fault offset to source shale thickness reaches 4 - 6. But, multiple shale layers promote fault segmentation eventually merging into a composite zone associated with a wider and taller fault zone. It is essential to understand the 3D geometry of a fault and the process of fault rock formation prior to assessing its seal potential.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado