ABSTRACT: Applications and limitations of mudlogging gas data in formation evaluation and hydrocarbon detection

Ten Haven, H. L.¹, B. S. Simon¹, and J. P Le Cann²
(1) Totalfina, Paris La Defense, France 
(2) Géoservices, le Blanc-Mesnil, France

Mudlogging gas data are a source of information always available during the drilling of a well, but hardly ever used in the interpretation of well results. Current utilisation is restricted to a number of gas ratios, generally considered not to be very reliable. These data can, however, provide essential information for well evaluation, such as the type of hydrocarbon fluid (gas/oil), and net pay computation in horizontal drains as well as in multi thin-layered stacked reservoirs. A new ratio has been developed based on the total gas and individual gas components, and defined as: (C3-C5) * Total Gas / (C1-C5). A volumetric normalisation of gas data is needed prior to calculation. The inclusion of the total gas in this ratio potentially has a three fold effect: (1) The difference between oil and gas is amplified in hydrocarbon zones as compounds heavier than C5 (and more abundant in oil than in gas) are measured by the total gas. (2) Water-bearing zones have low total gas values, resulting in a low ratio. (3) Problems with recycled gas are partly eliminated. This ratio provides information of the type of fluid in the reservoir (gas/oil/water). Furthermore reservoir properties, such as water saturation and porosity, can also be determined from mudlogging gas data, albeit with an error generally greater than calculations obtained from electric logs. A joint interpretation of electric logs and mudlogging gas data leads to an enhanced understanding of well results, which on its turn can be used to tune future logging and testing.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia