Attenuation in Railroad Valley, Nye County, Eastern Nevada, and its Significance for Petroleum Exploration in the Eastern Great Basin

Charles T. Walker and Robert D. Francis

Data from over 100 wells suggest that the boundary between the Grant Range and Railroad Valley is a low angle detachment fault at or close to a metamorphic core complex. The White Pine Detachment, mapped previously in the White Pine and Grant Ranges, is penetrated by many wells because it occurs at a high structural level. A structure contour map based on both surface and subsurface data indicates that the detachment dips uniformly into the subsurface and is not displaced significantly by a high angle fault. In the Grant Range subparallel detachments are developed in the more ductile units. Because the White Pine Detachment is not displaced by high angle faults Railroad Valley cannot have formed by steep normal faulting but instead by attenuation along a series of subpara lel detachments.

Generation and accumulation of hydrocarbons in Railroad Valley and similar basins may be related to the attenuation that created the basins. For example, the extent to which attenuation juxtaposed potential source rocks and hot infrastructure rocks may influence maturity. Consequently, areas normally considered too shallow for petroleum generation should not necessarily be discounted. Furthermore, failure to find petroleum in a favorable structure may not mean that the basin lacks potential because, as in Railroad Valley, source rocks may be locally absent as a result of attenuation. Reservoir structures formed by attenuation, such as lenticular stretch structures and localized fracture zones in otherwise impermeable formations, may exist.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California