Geologic Hazards And Constraints Of Offshore Northern And Central California

Abstract

Abstract Existing and potential geologic hazards and constraints were indentified by interpretation of high-re-solution seismic profiles from five offshore areas in northern and central California. For purposes of this study, geologic hazards are defined as any natural geologic features or processes, existing or penitential, that would inhibit the development of petroleum resources. Geologic hazards identified are (1) areas of high incidence of seismic activity; (2) active faults; (3) mass movement; (4) steep slopes (> 10 °) and steep-walled submarine channels. Geologic features that are considered hazardous in their present state but whose effects can be feasibly lessened through existing technology are referred to as constraints. Constraints identified are (1) filled or shallow buried channels; (2) hydrocarbon seeps, seep mounds, and gas craters; (3) gas-charged sediments; and (4) pressurized shallow gas zones. Introduction A recently completed study has assessed the existing and potential geologic hazards and constraints which could adversely affect oil and gas resource development in five offshore areas of northern and central California. The areas studied encompassed 243 tracts (aggregating 532,588 hectares) tentatively selected by the Department of the Interior for inclusion in the proposed Federal Outer Continental Shelf (OCS) Oil and Gas Lease Sale 53. The boundaries of the regular tracts are 4,800m on a side and each complete tract contains 2,304 hectares. The five offshore areas are between the Oregon border and Point Conception (fig. 1): Eel River, Point Arena, Bodega, Santa Cruz, and Santa Maria (figs. 2-6). Each area corresponds to a Neogene-aged, fault-bounded stratigraphic basin (fig. 1). According to Silver16 and Blake and others,1 the basins along the California coast were formed by extension associated with the San Andreas fault and other related strike-slip fault systems. Based on the age of the oldest basinal sediments, the basins probably originated about late middle Miocene time. Geologic Hazards And Constraints Geologic hazards are any geologic features or processes, existing or potential, that would inhibit the development of petroleum resources. Geologic hazards that are recognized on the northern and central California continental margin are (1) areas of high incidence of seismic activity; (2) active faults; (3) mass movement of unconsolidated to semiconsolidated sediments; and (4) steep slopes (< 10 ° and steep-walled submarine channels. Geologic features that are hazardous in their present state, but whose effects can be feasibly lessened through existing technology and design, are considered constraints to development. Constraints indentified offshore northern and central California are (1) filled or shallow-buried channels; (2) hydrocarbon (gas) seeps, seep mounds, and gas crates; (3) gas-charged sediments; and (4) pressurized shallow gas zones. Data Collection And Analysis Assessment of geologic hazards was based on the interpretation of high-resolution seismic reflection profiles collected by Fairfield Industries under contract to the U.S. Geological Survey. Data were collected aboard the R/VPeacock and R/VWidgeon from July 1979 through June 1980. About 10,500 line-km of nonproprietary, multisensory high-resolution data were collected on an approximate 1.0 × 2.0-km grid over the surveyed area. A total of 35-45 line-km of geophysical data were obtained from each complete tract.