Abstract: Structural Analysis of an Inverted Half-Graben: Implications for Fracture Distribution and Reservoir Quality in Point Arguello Oil Field

Paul G. Genovese, John Suppe

Analysis of 3-D seismic and well data suggests that the present-day Point Arguello anticline contains part of a northward-elongate Miocene-Pliocene half graben that was compressionally inverted to a northwest-trending double-plunging anticline during the Pliocene-Pleistocene. Upper Sisquoc and Foxen sedimentation postdated the normal faulting event so the shape of the fold at these stratigraphic levels is relatively simple being the product of only the late, compressional event. Refolding of the earlier rift structure resulted, however, in a relatively complicated saddle-shaped anticline at lower Sisquoc and Monterey reservoir levels. Nevertheless, we observe that several distinct homoclinal-dipping panels of rock (kink bands) define the fold shape at reservoir level. We perform a str ctural analysis using fault-related fold theory and the new technique of Axial Surface Mapping (Shaw et al., MPG Bulletin, in press) to map kink bands at Monterey reservoir level. Although both folding events produced kink bands, open permeable fractures are associated with the late compression, whereas filled, crenulated fractures likely formed earlier by compaction of syntextensional sediments. Therefore, we predict a greater fracture density within kink bands formed during the late folding event. We also predict better reservoir quality within kink bands within the plunging ends of the structure which show structural thinning and more numerous small faults in seismic profiles than in non-thinned fold limbs. Qualitative predictions of fracture density and reservoir quality based on the fold history and geometry of individual kink bands compare favorably with fracture measurements in core and drill-stem test results.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994