Radiometrics and High Resolution Magnetotellurics in the Exploration for Oil and Gas
Total count radiometrics has been used in oil and gas exploration since the early 1950s. Improved instrumentation has allowed the identification of the individual gamma emitting radioelements (Windowed Radiometrics)(WR). The increased concentration of uranium and the decreased concentration of potassium over a microseeping hydrocarbon accumulation at depth reflects the geochemistry of these two elements. Line processing of the WR data often shows a periodic mutual divergence of the concentrations of these two elements when compared to regional background (the potassium - uranium couplet: the K - U Couplet). This suggests some catalytic cracking of hydrocarbon liquids in the reservoir. This identifies the radiometric anomalies as surface oil and gas shows. Very similar to mud log shows in a vertical borehole. Aerial data can cover a large area quickly but is surface only. The correct call rate for radiometrics on previously mapped prospects is approximately 75% to 80% for positive and 60% to 80% for negative. The modern Controlled - Source ElectroMagnetic (CSEM) systems used offshore by ExxonMobil and others generate the input energy source at specified frequencies. CSEM data processing may'stack out' electromgnetic phase, thus possibly losing fluid content information. The input energy source for High Resolution Magnetotellurics (HRMT) is generated by the interaction of the solar wind with the earth's electric and magnetic fields. This input energy source contains all frequencies. The HRMT system must tune to the necessary frequencies. Both of these electromagntic systems investigate depth as a function of frequency. HRMT is point specific. The recorded HRMT data maintain frequency and phase. WR data can give location, approximate size and an indication of liquid hydrocarbons at depth (the K - U Couplet). HRMT data can give size, depth and thickness (frequency) as well as fluid content (phase: oil-gas-water). These two technologies together can provide the basic information for the initial economic analysis of the prospect without the cost of drilling a hole. The HRMT success rate is 49% for commercial production. The do-not-drill prediction rate is 100%.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013