Abstract

The Eel River basin, in Northern Coastal California, is comprised of Neogene marine clastic sediments in excess of 11,000 feet thick. The present basin shape is that of an elongate trough opening to the west. Structurally the area is a west plunging synclinorium with upturned and exposed strata on the south limb and with thrust faulting along the north and northeast margins. East-west trending folds reflect compression associated with recent tectonics. Generally the stratigraphic section displays a regressive basin filling sequence overlying strata that were deposited in an overall transgressive marine environment. Several unconformities of varying extent are present. Of particular interest to the petroleum geologist are rocks comprising the lower portion of the Wildcat Group. These rocks contain both source beds and reservoir facies and have produced over 85 Bcf of high btu gas plus small amounts of condensate. Encompassing the marine Pullen, Eel River and Rio Dell formations and the predominately nonmarine Scotia Bluffs and Carlotta formations, the Wildcat Group represents deposition from Late Miocene time through early Pleistocene time. In addition, Middle Miocene strata of the Bear River Beds are considered to be prospective within the basin. In the subsurface several gross genetic sedimentary sequences can be recognized including turbidites, slope shales and shelf facies. Although hydrocarbon production to date has been from predominately structural traps, subsurface data in this lightly explored region suggests a potential for large stratigraphic traps in several areas of the basin.