Interactions Between Fracture Formation and Burial Diagenesis in the Monterey Formation and Other Tight Matrix Reservoirs: Implications for Unconventional Reservoir Development and Production

Eichhubl, Peter; Fall, Andràs; Gale, Julia F.; Laubach, Stephen E.
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The Monterey Formation of coastal California is a well-studied fractured reservoir, with natural fractures providing connectivity between matrix pores and the wellbore. Of particular interest to fracture research and prediction is the interaction between fracture formation and burial diagenesis, and the effects of diagenesis on fracture opening, fracture cementation, and fracture spatial distribution. It is generally recognized that open fractures are most abundant in the quartz diagenetic state which is usually attributed to the brittleness of quartz chert but, perhaps more importantly, also attributable to the diagenetically stable host rock lithology that tends to preserve fracture porosity. Prior to the quartz diagenetic stage, fractures form concurrently with opal-A, opal-CT, and dolomite diagenesis. Unlike late quartz-stage fractures, earlier formed fractures tend to be partially or completely cemented and thus unavailable for later hydrocarbon migration and production. Timing of fracture formation relative to the burial diagenetic evolution and the burial diagenetic state of the host rock are thus essential for fracture formation and their preservation, fluid migration, and production.

These insights are now successfully integrated into the understanding of natural fracture formation and preservation in unconventional shale and sandstone reservoirs. Using fluid inclusion microthermometry on fracture cements in conjunction with SEM-cathodoluminescence textural imaging, we demonstrate that fractures form during maximum burial and early exhumation in the studied reservoirs over time spans of 10-40 m.y.. We infer that these rates are comparable to rates of diagenetic dissolution-precipitation reactions in the host rock, and of mass transfer between host rock and fracture. It is thus suggested that dissolution-precipitation creep is a dominant deformation mechanism allowing accommodation of permanent fracture strain in these diagenetically reactive rock types.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013