Effects of Mechanical Stratigraphy and Structural Position on Fracture Development as Observed in Core from Elk Hills Field, California

Gross, Michael R.¹, Tania C. Campbell², Jon R. Schwalbach³, Timothy L. Davis⁴ (1) Florida International University, Miami, FL (2) Occidental Oil and Gas, Tupman, CA (3) Aera Energy LLC, Bakersfield, (4) Occidental Oil and Gas Corporation, Houston, TX

We analyzed fractures in core taken from two vertical wells drilled into the main anticline at Elk Hills as part of a larger effort to build a conceptual fracture model for the field. The anticline is a south-verging, asymmetric fold whose axis trends ~WNW-ESE. The vertical cores, one from the crest and the other from the southern flank, display a wide variety of fractures owing to the fine-scale mechanical stratigraphy and intense brittle deformation produced by tectonic shortening. Brittle features observed in both vertical cores include small confined fractures restricted to individual cm-thick beds, multilayer fractures linked across numerous beds, microfault and multilayer fault planes at high angles to bedding, bed-parallel faults, and breccia/fault zones that span up to 60 cm in thickness. Fracture populations observed in the two wells exhibit several important similarities such as (1) lithologic controls on fracture style, with joints predominating in porcelanites and faults more abundant in clay-rich shales; (2) heterogeneous distribution of brittle strain characterized by envelopes of intense fracturing around discrete breccia/fault zones; and (3) evidence for a second phase of deformation as manifested by sub-horizontal striae on reactivated joints and faults. Bedding plane faults are greater in number and width in core from the flank, implying a flexural slip mechanism for folding. In contrast, multilayer fractures having greater lengths and apertures are more abundant in core from the crest, indicative of pure shear extension. Thus mechanisms for accommodating brittle strain differ markedly as a function of structural position, which in turn may influence the internal structure of the reservoir scale fracture network.