Evolution of the Lower Tertiary Elko Formation, a potential lacustrine petroleum source rock in northeast Nevada

Abstract

The Eocene-Oligocene Elko Formation of northeastern Nevada was deposited in a structural and sedimentary basin containing a fresh to moderately saline lake (or a series of lakes) following a long period of fluvial deposition. The Elko Formation includes thick intervals of petroliferous mudrocks, some of which contain sufficient organic matter to have been mined as an oil shale resource in the early twentieth century. The organic-rich intervals are also presumed to be the source for oil deposits in northeastern Nevada. Lithology, mineralogy and organic geochemistry of the Elko Formation were studied in outcrop and core samples. The core and outcrop samples are all thermally immature that, with the exception of a few very organic-rich intervals in two cores, show moderate to good source rock potential. Generally, these outcrop and core samples have limited value as an oil shale resource in comparison to the lacustrine basins of the nearby and similarly aged Eocene Green River Formation. The majority of samples (62%) had total organic carbon contents between 1 and 5 weight % and most (78%) had hydrogen index (HI) values of 500 mg/g or higher. Oil yields determined by Fischer assay were generally between 1 and 4 weight % (3 to 11 gallons per ton), but the yields from many samples were less than 1 weight % (about 3 GPT). The organic matter in the Elko cores consists of varying mixtures of Type I and Type III kerogen. Results from X-ray diffraction indicate the Elko Formation contains abundant quartz and feldspar, alternating dolomite-rich and calcite-rich intervals, along with some intervals rich in opal-CT and clay minerals. The Elko Formation paleolake may have become hydrologically closed leading to increased salinity as widespread lacustrine conditions developed. This is indicated by the disappearance of freshwater mollusks just above the base of the lacustrine interval. Organic-rich laminated carbonates were deposited in offshore areas of the lake, contemporaneous with pebbly sandstones and mudstones in marginal areas. Scattered air-fall tuffs indicate volcanism in northeastern Nevada during deposition of the lower part of the organic-rich interval. Greater volcanism, indicated by thick, reworked tuffaceous intervals interspersed with organic-rich mudrocks, appears to have occurred just prior to deposition of the richest oil shale in the Elko Formation. Large volumes of volcanic material are found in the remaining Elko lacustrine beds due to a period of prolonged volcanism in northeastern Nevada during this time.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016