Abstract

The Pannonian basin is characterized by low maturity gradients, frequent overpressuring, and organically lean rocks. This overall picture suggests that gas migration is heavily influenced by dissolution of methane.

The authors have developed a one dimensional computer model of gas migration to reconstruct the coupled compaction-gas generation histories. The aim of this reconstruction is: (1) to evaluate the volume of total HC-gases generated, both dissolved and free in the source rock, to that portion which has migrated out of the source; and (2), to assess the effect of some geological parameters on gas migration.

Application of this model to the Pannonian basin shows that in rocks above 3 km depth, the gas generation is significant and from 5 to 25% of the gas leaves the shale dissolved in squeezed-out pore water. During upward flow, a part of this gas exsolves to form free gas ready for trapping. At about 3 km depth the appearance of free methane in shale is likely, but the volume saturation caused by kerogen generated gas never exceeds the critical value needed for initiation of a bulk flow.

Because the known gas reserves in the Pannonian Basin can not be fully explained by the gas exsolved from compaction water, additional gas sources are needed. After oil generation has ceased, a large part of the oil remaining in the shale is cracked into gas during prolonged maturation. This “extra” gas is added into the free gas already present and can initiate the bulk flow. Older sedimentary rocks, underlying the Pannonian basin and now experiencing a second phase of maturation can also act as an additional source for gas. The heavy carbon isotopic composition of HC-gases and the high nitrogen content of several gas fields reinforce these ideas.