Abstract: Hydrogeologic Controls on Petroleum Accumulations in the Great Basin

Robert F. Grabb

Great Basin hydrocarbon accumulations and groundwater flow are intrinsically related. Hydrocarbon migration is controlled by forces generated or transmitted by water. Groundwater entrained hydrocarbons converge upon and are trapped in fluid potential minima; water is identified as an important reservoir drive mechanism in most Great Basin oil fields. Numerous hydrogeologic indicators of hydrocarbon accumulation (surface hydrocarbon seepage, chemically reducing near-surface conditions, electromagnetic anomalies, positive geothermal anomalies, etc.) occur in the region's oil producing valleys. The addition of hydrogeologic methodology to conventional exploration practices should reduce exploration risk in the region.

In an attempt to explain the distribution of oil fields and hydrogeologic hydrocarbon indicators, a two layer hydrogeologic model of the eastern Great Basin was developed. A deeper, regional flow system, coincident with a thick sequence of Paleozoic carbonate rock, underlies eastern Nevada and western Utah. Widespread distribution and high transmissivity fosters interbasin flow within the regional system. Superimposed on the regional system is a relatively shallow and localized groundwater flow system; flow in this system is confined to valley-fill deposits and adjoining mountain ranges.

Petroleum accumulation incidence increases towards areas of regional fluid convergence, and all Nevada oil fields were found to be located within the discharge areas of deep, regional flow systems. Vertical seals develop where hot, relatively mineral laden waters of the regional flow system mix with the cooler, fresh waters the local flow system.

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