Understanding Dry Gas Accumulations: The Challenge

Pittion, Jean-Luc¹, B. Lambert¹, M. Sullivan¹, D. Dessort² (1) Total, Paris, France (2) Total, Pau, France

In the recent years, the interest of industry in gas discoveries has grown. Gas is no longer considered as a “risk” but, on the contrary, in many cases as an economic issue.

Until to now, dry gas accumulations were generally interpreted as biogenic gas ( i.e gas generated only by bacterial processes ). Nevertheless, over the past few years, new concepts have emerged involving other process to explain the specific characteristics of the dry gas accumulations. The most frequently invoked alternative mechanism is compositional frac­tionation undergone by the thermogenic hydrocarbons during migration. At the end, such a process would produce gas with similar characteristics to biogenic gas.

In order to distinguish between these different hypotheses, it is necessary to analyze the behaviour of the petroleum system and to demonstrate the existence (or the absence) of a thermogenic kitchen associated with the gas accumulation.

Several case histories from different parts of the world are presented and discussed. Obviously, the explanation of the origin of dry gas (biogenic or fractionated thermogenic) has an important impact on the exploration strategy in a basin.

Other subsequent aspects related to the gas generation such as the exact behaviour of gas prone source-rocks and their kinetic representation, the mode of trapping or the mod­elling of the biogenic gas generation and the most favourable context to find it are also addressed.