Case Studies - Oil & Gas Exploration

Field Example- Railroad Valley, Nevada

Overview

A combined GEOTEM and aeromagnetic survey was flown over several oilfields in the Railroad Valley, Nevada. Oil production is at 1800 metres from Devonian limestones and dolomites and sealed by Tertiary sediments. Many of the Railroad Valley fields produce from fault traps which may have been charged from the leakage of deeper hydrocarbon reservoirs.

The Trap Spring Field is located near the centre of the Railroad Valley. Cultural editing of the data was employed to remove obvious cultural spikes and the data were presented as aeromagnetic maps and GEOTEM decay constant (tau) maps.

The geophysical data are presented on a base map which shows productive wells as black dots. The Trap Spring Field is kidney-shaped and is located centrally on the survey grid. The total field aeromagnetic data were processed to enhance magnetic signal related to the alteration plume and the tectonics.

The horizontal gradient shows a magnetic signal which corresponds closely to the field. In this presentation the axis of the magnetic low and the axis of the magnetic high delineate the edges of the alteration plume.

The vertical gradient identifies contacts in the basement which indicates a wedge-shaped body. This reflects fault thrusts originating in the basement which structurally control the field.

The decay constant (tau) conductivity map shows a conductivity high (in red and purple) which corresponds to an alteration zone related to the hydrocarbon plume. In addition to an excellent correspondence with the field, there are two new leads suggested to the north and south-southwest of the Trap Spring Field.

Conclusions

GEOTEM electromagnetic data can effectively map surface conductivity contrasts related to hydrocarbon zones. High-resolution low-level aeromagnetics can identify micromagnetic anomalies associated with hydrocarbon alteration zones. The method is only useful in areas where there is a contrast in the conductivity and/or the magnetic susceptibility. A combined electromagnetic/ aeromagnetic program may be useful for regional evaluation, to identify leads, or for high-grading prospects in areas similar to Railroad Valley, Nevada.