The Esmeralda Basin: A Proven Intra-continental Lacustrine Rift Play along the Walker Lane Shear Zone, Western Nevada
Andrew Cullen and James H. Henderson
6100 N Western, Oklahoma City, OK
We demonstrate a widespread working petroleum system in western Nevada, which challenges conventional wisdom that, owing to complex tectonics and Cenozoic volcanism, the region's petroleum potential is marginal. Oil samples from a well in the Gabbs basin and an active oil seep in the Columbus basin share biomarkers signatures (sterane-hopane ratios, sterane-sterane ratios, gammacerane, and oleonane) indicating expulsion from a Tertiary algal source deposited under anoxic hypersaline conditions. We interpret these data as evidence for an organic-rich lacustrine facies in the Oligo-Miocene Esmeralda Formation. Although oil-prone source rocks in the Esmeralda Formation have not been reported, shallow bituminous coal seams (20ft thick) prove conditions favorable for preservation of organic-rich units. The Esmeralda coal Ro (0.54) is consistent with a high geothermal gradient measured in wells (60oC/km). The Columbus and Gabbs basins lie in the central Walker Lane shear zone. Gravity modeling indicates both basins have >12,000ft of sediment. Because Walker Lane was not active during deposition of the Esmeralda Formation, the presence of lacustrine-sourced oils in both basins (70mi apart), suggests a large deep stratified lake once existed in a single basin, the Esmeralda Basin. Subsequent development of the Walker Lane shear zone segmented the Esmeralda Basin into the currently configured sub-basins. The Esmeralda Formation is primarily filled with interbedded tuffaceous sandstone, shale, volcanics, and diatomite beds. In the Gigante-1, on the Cobble Questa anticline, strong sample and mudlog shows (up to C5) in several overpressured intervals (~10ppg MW) demonstrate the opportunity for stacked pays. The Esmeralda Basin represents an inverted intra-continental lacustrine rift play. Although play POS=1, commercial and prospect risks include trap size, reservoir quality, and producibility of waxy, high pour-point oils.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013