Physics and History of Biogenic Gas Systems in the Eastern Mediterranean ; the importance of PVT Conditions

Duncan Macgregor, MacGeology Ltd, Reading, UK,

The Eastern Mediterranean now constitutes the largest offshore and non-coal associated biogenic gas system in the world. The high productivity of the system may be related to a number of unusual observations, the main one being that reservoirs are generally older, deeper and hotter than elsewhere, lying close to or beyond the biogenic window that is conventionally thought to end at around 65-70

This investigation attempts to explain these observations and analyse whether conventional models relating to biogenic gas generation require any revision. The development of large fields at or below the limits of the conventional biogenic gas window (e.g. Tamar at 78 oC at their gas-water-contacts) requires either that a temperature extension to the window is developed or that the physics of the system allows earlier filled fields to be buried below the window without significant gas shrinkage or leakage. Biological data elsewhere, particularly from China, suggests that most biogenic gas is generated below 62 oC, hence the focus in this investigation lies on the physics of the systems, particularly PVT (Pressure-Volume-Temperature) conditions. The study relies on rather sporadic data released on some of the deepwater fields from Noble engineering papers and environmental impact studies, though a more substantial database exists on the shallow water fields. No direct data exists on Zohr at this stage, though inferences are made.

The region appears to have a remarkably consistent surface heat flow of around 50 mW/moC/km. The published PVT data has been used to construct a series of charts determining the variation in reservoir temperature, pressure and Gas Expansion Factor against time (e.g. Figure 2), allowing relationships to be observed with the evolution of key petroleum system elements such as seal compaction and trap formation. While it clear that the gas in the Zohr, Mari B and the Nile Pliocene fields was generated post-Messinian, this seems unlikely to have been the case for the older ‘Tamar Sand’ fields. The model proposed for these is that the gas pools were filled prior to the Messinian lowstand and then suffered de-pressuring and gas expansion followed by a rapid re-pressuring and compression. A lack of significant subsequent recharge due to the imposition of higher temperatures beyond the biogenic window is speculated to lead to these closures having only a partial gas fill in comparison to apparently higher degrees of fill in the Pliocene fields such as Mari B. The presence of small amounts of condensate and intermediate isotopic compositions in many pools (Figure 3) indicate a degree of mixing with thermogenic gas, particularly in the Nile system, though regional correlations between condensate yields and the isotope values seems to confirm that the deepwater fields are primarily biogenic, confirming the various operators reports.

The Eastern Mediterranean biogenic systems are optimised for the formation of large biogenic gas fields due to a number of favourable factors. Firstly traps of various types (Syrian Arc anticlines, carbonate buildups and sand Injectite mounds) are formed in time for the closures to be filled with gas generated throughout the biogenic temperature window. The deposition of all elements of the petroleum system in already highly pressured deepwater settings ensures that early gas fill at low temperatures is sufficiently compressed to enable gas preservation in traps in non-generative periods. A particularly key aspect here is a trend of flattening of the Gas Expansion Factor (Bg) against increased burial (e.g. Figure 2), which allows biogenic pools to survive burial below the actively generating window without suffering significant shrinkage. Effective seals must also form early, with mudstones sustaining columns of the order of 300m and evaporites over 600m at Zohr : the early pressurisation of the system likely plays a role as well here. These same physical controls are recognised in other recent deepwater biogenic discoveries, for example in Myanmar, and should form the basis for deliberate exploration for biogenic gas in frontier provinces worldwide.