FRANCIS, ROBERT D., and LINDA M. CONLAN, California State University at Long Beach, Long Beach, CA
Biological markers in oils from Railroad Valley, Nevada, indicate a complex pattern of migration from at least two source facies to Paleozoic and Tertiary reservoirs beneath the unconformity at the base of the valley fill. Abundance patterns of biological markers (hopanes, tricyclic and tetracyclic terpanes, and steranes) are independent of the reservoir lithology or age, and seem to depend largely on geographic location. For example, Tertiary-reservoired oils in the west side of the valley (Trap Springs and Munson Ranch) are characterized by high abundances of tricyclic terpanes and low abundances are C31 to C35 hopanes and gammacerane. On the east side of the valley, oils at Eagle Springs (mostly in Tertiary rocks but some in Pennsylvanian Ely Limestone) have low tricyclics and high C31 to C35 hopanes and unusually high gammacerane, whereas Paleozoic reservoired Bacon Flats and Grant Canyon oils have low tricyclics and low C31 to C35 hopanes. Oil shows at 10,570 and 13,943 ft in the Meridian Spencer Federal 32-29 well are similar chemically to the Eagle Springs oils and Grant Canyon oils, respectively, suggesting that oil migrated from thermally mature source rocks in the center of the valley.
Competing structural models for Railroad Valley invoke such disparate mechanisms as block faulting, detachment faulting, and thrusting. Any model must account for source rock location and migration paths that led to the present distribution of oil types. Due partly to ambiguous seismic data (possibly resulting from structural complexity), interpretations of structure below the wells are speculative. Structural complexity could result from east-west ductile extension of shales and associated brittle extension of carbonates. Such attenuation could result in local absence of some source rocks, leading to the aforementioned geographic distribution of oil types. On the other hand, thrusting would tend to telescope source rocks, leading to a geographically homogeneous distribution of oil ty es.