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ABSTRACT

Heavy petroleum is found in limited deposits as tar-saturated sandstone within the western Santa Cruz Mountains of central California. The largest accumulation of petroleum is in the middle and upper Miocene Santa Margarita Sandstone. The Santa Margarita Sandstone rests disconformably on pre-Tertiary Salinian basement rocks and unconformably on the Monterey Formation (middle Miocene) and older Tertiary rocks.

The Santa Cruz Mudstone (upper Miocene), which conformably overlies the Santa Margarita Sandstone, provided an initial seal for petroleum entrapment. The present distribution of petroleum apparently is related to the depositional environment of the Santa Margarita Sandstone and to later structural development.

The Santa Margarita Sandstone is a tidally-dominated marine shelf deposit. An 8- to 10-km (5-6 mi) wide, northeast-trending facies of unidirectional, large-scale cross-strata of uncemented sand and gravel represents a zone of intense tidal currents; this facies provided a conduit for initial petroleum migration. West-dipping homoclinal folding, including northeasttrending and southwest-plunging compaction anticlines, formed structures for initial petroleum entrapment within the cross-bedded facies.

Strike-slip movement on the San Gregorio fault system formed a postdepositional conjugate fault-and-fracture system in the western Santa Cruz Mountains. Where petroleum had previously accumulated on structures, faulting and fracturing caused intrusion of the petroleum-saturated sand into the overlying siliceous mudstone; these intrusions range in width from a few centimeters (inches) wide to complex injection zones more than 200 m wide (650 ft). Faulting and associated clastic intrusions also formed partial seals across structures within the Santa Margarita Sandstone. The maximum thickness, 30 m (100 ft), of tar-saturated sandstone occurs in fault traps downdip on southwest-plunging anticlines within the cross-bedded facies. Petroleum accumulations also occur in stratigraphic traps where sand pinches out on paleotopographic highs, in structural traps within fault-bounded blocks on homoclinal folds, and in possible diagenetic traps where sandstone lies on marble.

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