Fractured reservoirs in the siliceous Monterey Shale of the Santa Maria area represent anomalous lithology and anomalous type of fracturing. Some, perhaps all, reservoirs are not fractured chert but chert embrittled by dolomitization. Abundant reservoir extension fractures are disordered and open, commonly containing epigenetic dolomite breccias. These fractures are partly dolomite-cemented but contain common open voids, many 15 cm across, some larger. Breccias locally have an exploded appearance and contain matched fragments separated by veins, which apparently were injected as a slurry of water, oil, and fragments of dolomite and dolomitic Monterey Shale.

The highly organic Monterey served also as source rock and probably originated as richly diatomaceous slope sediment beneath an oxygen-minimum zone at a depositional site much larger than the Santa Maria area, and not confined to a specific silled basin. Local dolomitization may have been due, at least in part, to rising solutions and injected slurries.

These reservoirs are explained by an hypothesis involving repeated episodes of rock dilation followed by natural hydraulic fracturing, all produced by continued episodic tectonic compression of the region (principal, maximum, effective stress oriented northeastward). High fluid pressures enlarged underpressured dilation microfractures into macrofractures and produced breccias by hydraulic fracturing. Viscous oil derived from the Monterey Shale was forced into voids as part of overpressured slurries whose breccia fragments were

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propping agents. Dolomite precipitated from slurries partly healed the fractures.

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