Abstract: Electromagnetic Imaging of Oil and Gas Traps

George V. Keller, M. Tahsin Tasci, John M. (Jack) Jordan

Hydrocarbons are trapped in reservoirs due to structural and lithologic causes. Migration which is an ongoing process charges the reservoirs with hydrocarbons and also increases the hydrocarbon content in the rocks associated with the reservoirs. During the migration process large volumes of fluids pass through the trap resulting in diagenetic changes. Oil and gas fields are focal points of diagenesis. When pore fluids in the rocks are replaced by oil or gas, and when diagenetic changes take place thus altering the rock porosity and permeability, most physical properties of the rocks associated with the trap are modified with electrical resistivity most affected. Electrical resistivity depends directly on porosity, pore fluid resistivity, and saturation. According to Arch e's Law, a rock unit with a 90% oil saturation is 100 times more resistive than the same unit with only salt water in the pores. The resistivity of a large volume of rock closely associated with and including the reservoir is modified. A surface based, low-frequency, time domain, electromagnetic sounding method was used to determine the resistivity image of oil and gas fields. As a result of numerous lines across existing fields and prospects in different geological environments in the USA and abroad, it was determined that deep seated robust resistivity anomalies which closely image oil and gas reservoirs are very common if not universal.

Lines across two Walsortian mound fields near Dickinson, ND, a Morrow channel field and a new discovery in southwestern Kansas and a new, large, overseas discovery will be presented.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana