Structure, Distribution, and Silica Diagenesis of the Monterey Formation in the Offshore Point Arena Basin, Central California

Dame, Robert
[email protected]

The offshore Point Arena Basin is located approximately 100 miles northwest of San Francisco, bounded by the Mendocino Fracture Zone in the north, the San Andreas Fault to the east, and the continental shelf in the south and west. The Point Arena Basin is roughly 95 miles long, and while only about 13 miles at its widest point, is a major offshore depocenter with up to 25,000 feet of Tertiary sediment fill overlying Cretaceous basement.

Strata within the offshore Point Arena Basin have been subjected to a complex tectonic regime varying between strike-slip, wrench, and thrust motion associated with the interaction between the North American and Pacific plate boundaries since the Neogene. Previous investigations of the Point Arena Basin have indicated that Pliocene to recent compressional forces have resulted in a series of long, narrow, continuous folds and large thrust faults oriented parallel to the northwest-trending San Andreas Fault.

Offshore petroleum exploration near Point Arena was confined to three exploratory wells drilled in the 1960s and 2D seismic reflection data acquired in the 1970s and 1980s. Mineralogical data derived from exploratory well core samples was obtained by the Minerals Management Service in the 1990s. The geologic and geophysical data were integrated to regionally constrain the structure and stratigraphic nature of the Monterey Formation in the Point Arena Basin.

Offshore Point Arena, the Monterey Formation was found to generally be between 1000 to 2000 feet thick, with thicknesses up to 3000 feet observed in the axial portion of the basin. Burial depths typically ranged between 3000 to 9000 feet. Offsets up to 1000 feet were interpreted across several major thrust faults. The Monterey was interpreted to be comprised of successions of laterally-continuous shale, chert, and dolostone, with discontinuous clastic intervals originating from episodic turbidity deposits. Seismic reflection sequences originating within the Monterey Formation were typically parallel to subparallel, with some localized chaotic zones through the axes of the thrust belts.

Anomalous, cross-cutting reflectors interpreted as the opal-CT/quartz silica diagenetic boundary were observed on seismic reflection data in the southern part of the basin and were correlated to the available mineralogical data for well OCS-P 00030-01. The diagenetic boundary is significant since a dramatic increase in fracture porosity and reservoir quality is expected in folded quartz-phase siliceous sediments and also because the conclusion of the opal-CT/quartz diagenetic transition typically coincides with the onset of hydrocarbon generation within the Monterey Formation. The widespread presence of thick, deeply buried Monterey sediments indicates that highly prospective, thermally mature fractured chert reservoirs are likely to be encountered throughout the Point Arena Basin.

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