Abstract: Source Rocks, Thermal History and Oil in the Carson Sink and Buena Vista Valley, West Central Nevada

Charles E. Barker

Rock-Eval, gas chromatography, mass spectrometry, and thermal history reconstruction data from six wells suggest that Tertiary rocks in the Carson Sink and Buena Vista Valley areas are marginally mature to overmature with respect to hydrocarbon generation and have locally expelled oil. The lacustrine Tertiary calcareous mudstones and marls in these wells have a total organic carbon (TOC) range from 0.1 to 3 wt.-%, with modes at about 0.5 to 0.7, 1.5, and 2 to 3 wt-% TOC. However, in the Standard Amoco Carson Sink 1 well, some of these samples have up to 3 wt-% less TOC than reported by Hastings (1979) and these arc thought to represent drill cutting samples that have been depleted in more TOC rich rock chips. Even if the TOC data are biased, these TOC-depleted samples are still oil-prone rocks, with hydrogen indices commonly above 400 mg hydrocarbon/ g C, and some samples with TOC in the 2-3 wt.-% range.

Analysis of an oil show at Kyle Hot Springs in Buena Vista Valley revealed a wax-rich, low sulfur oil probably from a carbonate-rich, hypersaline lacustrine source rock. This oil could be generated from strata similar to those analyzed above.

Other Tertiary source rocks in the two valleys consist of lenses of humic coals that appear to be gas prone. Shows of biogenic(?) gas from shallow wells in Tertiary to Holocene lacustrine strata are common in the Carson Sink. Mesozoic rocks locally may have remaining hydrocarbon generation potential in the Stillwater Range which lie along the eastern margin of the Carson Sink. Published conodont alteration index data shows that the Paleozoic rocks are overmature.

Reconstructed thermal histories of the Carson Sink and Buena Vista Valley areas, indicate petroleum is presently being generated. Mechanisms for petroleum generation are rapid burial (140 m/m.y.) in a high geothermal gradient (45 to 110°C/km), and hydrothermal and contact metamorphism.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada