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Characterization of Subsurface Accumulations Through Geochemical Surveys

Devendra Vyas, Mallika Vaidyanathan, Bal Ram, Jolly Joseph, and Jitendra Prasad
Regional Lab, ONGC, Chennai, India

Geochemical surveys involving adsorbed gas study is a powerful tool to comprehend the nature of subsurface hydrocarbons. Geochemical prospecting studies in deep waters were carried out to establish nature of subsurface hydrocarbons and to delineate probable areas of hydrocarbon accumulations.

The paper deals with dissimilar hydrocarbon signatures observed in five areas of one deep water block of KG basin. Around 500 grab samples (Depth ranges from 800m to 2500 m) were collected from bottom of sea floor by gravity coring through offshore survey vessel. These samples were desorbed and analysed over gas chromatograph. Adsorbed gases show presence of hydrocarbons from methane to pentanes in ppm range. Anomaly maps of hydrocarbons show strong anomalies in the surveyed block. Hydrocarbons cross plots, hydrocarbon ratio cross plot, frequency distribution of ethane / propane and Pixler’s diagram are used to reveal the nature of subsurface hydrocarbon in survey areas of KG deep waters.

Presence of significant amount of hydrocarbons from ethane to pentane clearly indicates that seeped hydrocarbons are thermogenic in nature. Using different cross plots and hydrocarbon ratios suggest that these have been generated from different zones / accumulation inputs. Out of five areas under study, two areas are oil prone and one area is wet gas / condensate prone in nature. Fairly high concentrations of methane compared to other homologues are observed in remaining two areas. It is an indication of different inputs for methane in these two areas which resulted into mixing of hydrocarbons of different zones. BSR observed in these areas corroborate mixing of thermogenic hydrocarbons with possible hydrocarbons from gas hydrates.

Thus adsorbed gas study is very useful in establishing nature / characterization of subsurface accumulations for hydrocarbon exploration.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery