LORENSON, THOMAS D., L. MAGOON, K. KVENVOLDEN, USGS, Menlo Park, CA; M. L. DAVISSON, Lawrence Livermore National Laboratory, Livermore, CA; D. ORANGE, Univ. Calfornia, Santa Cruz, CA; P. BREWER, MBARI, Moss Landing, CA; and J. MARTIN, U. Florida, Gainesville, FL

Abstract: Similarity of Gases in the Cape Mendocino-Eel River Basin Area, Offshore and Onshore Northwestern California

The Eel River basin and adjacent areas underlain by melange of the Franciscan Complex of northwestern California have seeps and minor commercial production of natural gas, condensate, and light crude oil. Hydrocarbon gas geochemistry provides evidence that gases from onshore gas seeps and wells in Franciscan melanges as far south as Briceland are related to one another and to offshore gas seeps in the Eel River basin west of Eureka. Ten onshore gas occurrences and two submarine seeps (at water depths of 520 m and 41 m) were sampled. Each onshore occurrence produces wet gas. The onshore occurrences are clearly related by similar hydrocarbon gas composition and, in most cases, by carbon isotopic composition of the hydrocarbon gases methane through butane. Methane carbon isotopic compositions ranged from -53 to -31 ?.

The offshore seeps produce dry gas suggesting a microbial methane source, but trace amounts of thermogenic gases are present at both seeps. In addition, thermally sourced ethane carbon from the 520-m seep is isotopically identical to that observed nearby on land (-26 ?), and methane carbon isotopic compositions of the 520-m and 41-m seeps were -49? and -43 ?, respectively. The combined molecular and isotopic compositions suggest that a mixture of gases from thermal and microbial sources is escaping at the sea floor, and that the thermal component is similar to the onshore hydrocarbon occurrences.

Vitrinite reflectances of sampled outcrops with the best petroleum potential from the Wildcat Group and Bear River beds do not exceed 0.6 R[o]. Thus, these rocks have not generated oil and gas at or near the surface. Similar gas geochemistry demonstrate that these gases likely come from the same source rock, rocks that must occur at depth. This implies that these hydrocarbons likely originate from structurally interleaved and/or underthrust Wildcat, Bear River or similar forearc strata within or beneath the Miocene Franciscan Complex.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California