The subsurface geological complexities in Krishna Godavari Basin, India often result into discrete and challenging reservoirs. Limited geological correlation plus HP-HT regime down-hole further add up to the challenge to understand reservoir HC potential. In this context, application and meaningful interpretation of gas ratio analyses are discussed in reservoir sections of two exploratory wells (A & B of L. Cretaceous and L. Eocene) from KG Basin.

Formation Gas data obtained during drilling are commonly analyzed as well-known ratios of ‘Dryness’, ‘Wetness’, ‘Balance’, ‘Character’ & ‘Pixler ratios’ which are simple, quick and field based. It is a proxy that can save time and resource; by aiding in identification of reservoir fluids, fluid contacts & choosing testing intervals, thus enhancing confidence about reservoir. It can be practiced especially where log quality is poor and in cases where logging scope is limited due to complications or high thermal regime.

It is found that gas ratio studies indicate gas-condensate in Well-A while gas with light crude oil in Well-B reservoirs. Moreover it is capable of picking up top of reservoir facies, gas-water & gas-oil contacts and variation in reservoir tightness across reservoir sections as found in Well-A & B. The comparative approach of ratios also interprets about presence of several discrete yet closely spaced HC pools across the reservoir section of Well-B. These findings simply enhance understanding of otherwise complex reservoirs. Along with petrophysical analysis it adds confidence to propose precise testing intervals against HC fluid of interest.

Since gas-ratios are proxy of fluid contacts, their utility can be extended to drilling reservoir lateral wells, if compositional gas data can be obtained fast- in which case gas-ratio analysis can guide drilling effectively to avoid deviation to unwanted fluid zone although reservoir litho-facies variation is less. Gas ratios and electrologs are integrated to introduce to a practical concept of Oil Potential Parameter (OPP) to evaluate effective extent of arenaceous reservoir and reduce qualitative geological risk while planning wells to develop an oil field, as shown with a KG basin example.

This content is only available via PDF.
You can access this article if you purchase or spend a download.