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ABSTRACT: The Application of Light Alkane Ratios in Papua New Guinea

DUNCAN, NICHOLAS I., EXLOG International, Inc., Singapore, and M. C. DANIELS, Chevron Niugini Pty. Ltd., Port Moresby, Papua New Guinea

Hydrocarbon reservoir fluids contain discrete gas elements including ethane through pentane. These occur in concentrations proportional to the nature of the fluid. These fluids are released during drilling and subsequently altered by changes in temperature and pressure during the rise to the surface. This makes quantitative analysis of the reservoir using these gases quite subjective. However, the use of gas ratios can overcome much of this problem and aid in the characterization of reservoir fluids.

Recent exploration in Papua New Guinea used a light alkane ratio method for a first-pass evaluation of the presence and characterization of the reservoir fluids. Results were encouraging. Although the absolute values of the gas ratio interpretation varied slightly from earlier light alkane ratio studies, a distinct range of (1) curve/ratio values, (2) curve crossover separations, and (3) curve/trend shapes were found to be directly correlative to the reservoir fluids.

Values of crossover separation below 10 were indicative of gas. Crossover values of 35-55 were found to be indicative of a liquid-phase hydrocarbon. Occurrences of residual oil were identified utilizing trend/curve shape analysis by looking for zones with an overall decrease in crossover separation with depth. This situation was only present with a water contact or transitional zone of increasing water saturation.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)