Abstract: Natural Fracturing Impacts Siliceous Shale Reservoir Development: Cymric Field, California, U.S.A.

JOHNSON, SCOTT J.,

Geophysical data show pronounced systematic variations in reservoir anisotropy orientation in a shallow (< 500 m) Monterey-equivalent reservoir in the Cymric Field of California. Variations occur over limited areal (< 125 m) and vertical (< 5 m) distances. Two main orientations are evident: north-northeast and west-northwest, perpendicular and parallel to anticlinal trends, respectively. Borehole image data indicate that north-northeast anisotropy is regional maximum horizontal in-situ stress and west northwest anisotropy is an oriented natural fracture system and/or a local stress related to natural fracturing. Andersonian fault theory accounts for local stress reorientation which is possible due to the small difference in maximum and minimum horizontal in-situ stress magnitudes.

Variations in reservoir anisotropy orientation and type result in significant areal and vertical variations in depletion geometry patterns. In relatively undeformed portions of the reservoir where fracture permeability is negligible and regional stress pervades, cyclic steam injection induces a fracture parallel to regional maximum horizontal in-situ stress. Where natural fracturing is well-developed, cyclic steam either induces a fracture of reduced width and height growth or is injected into the oriented natural fracture system. Oriented fracture systems in thin, widespread, stratum-bound zones focus injection and facilitate intrawell communication. Understanding depletion geometry patterns as they relate to natural fracturing is a key step in optimizing field development.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California