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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 977

Last Page: 977

Title: Hypothesis Combining Dilation, Natural Hydraulic Fracturing, and Dolomitization to Explain Petroleum Reservoirs in Monterey Shale, Santa Maria Area, California: ABSTRACT

Author(s): Lowell Redwine

Article Type: Meeting abstract


Fractured reservoirs in the generally siliceous Monterey Shale of the Santa Maria area represent an anomalous lithology and type of fracturing. Some, perhaps all, are not fractured chert but parts of the Monterey embrittled by dolomitization. Reservoir fractures, unlike ubiquitous Monterey fractures, are mostly abundant, disordered, open extension fractures that commonly produce epigenetic, dolomitic breccias. These dolomite-cemented breccias commonly contain open voids, many of which are 15 cm across or larger. Breccias locally have an exploded appearance and contain some matched fragments separated by vein-like or dike-like matrix, which apparently was an injected slurry of water and oil containing fragments of dolomite and dolomitic Monterey Shale.

The highly organic Monterey also served as the source rock and probably originated as a rich diatomaceous slope sediment beneath an oxygen-minimum zone. The depositional site was much larger than the Santa Maria area and unconfined to silled basins. Local dolomitization may have been due, at least in part, to rising solutions and injected slurries.

The reservoirs are explained by a hypothesis involving repeated episodes of rock dilation followed by natural hydraulic fracturing, all produced by episodic but continued tectonic compression of the region (principal, maximum, effective stress oriented northeastward). Increasing fluid pressures enlarged underpressured dilation microfractures into macrofractures and produced breccias by hydraulic fracturing. Viscous oil expressed from indurated Monterey was pumped into voids as part of overpressured slurries whose fragments were propping agents. Dolomite precipitated from slurries, on pressure release by fracturing, partly healing the fractures. Repetition of these events fractured additional rock, so that the reservoirs grew outward and somewhat upward.

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