GAS DISPLACEMENT: AN IMPORTANT CONTROL ON OIL AND GAS DISTRIBUTION IN THE TIMOR SEA?

Abstract

A study of gas fields in the Timor Sea has shown that these traps often contain palaeo-oil columns, with a high abundance of oil bearing fluid inclusions recorded in sands which are presently gas saturated. These palaeo- oil columns are substantial, suggesting the volume of liquid hydrocarbons that has been redistributed is significant. This remobilised oil in effect constitutes an oil charge of known volume for nearby structures, with seismic mapping confidently allowing the recognition of remigration fairways into up-dip traps. For example, at Oliver-1, a 115 m relict oil column, equating to original oil in place of over 166 MMBBL, has been identified. However, estimates of current oil in place account for less than 45 MMBBL, suggesting that more than 120 MMBBL of oil has been displaced across the spill point of the Oliver structure. Remigration of this oil can be mapped into an adjacent fault block, representing a new exploration play. Significantly, the likelihood of this untested trap containing liquids is supported by the presence of an oil leg at Oliver-1, which shows that only oil has been displaced from the Oliver trap.

At Keeling-1, a smaller palaeo-oil accumulation has been detected which equates to about 14 MMBBL originally in place. However, the mechanisms responsible for the passage from palaeo-oil accumulation to the present gaseous hydrocarbon phase are often complex and decisions regarding a potential oil leg need be made judiciously. For example, at Keeling-1, oil shows and hydrocarbon related diagenesis in the overlying section provide compelling evidence that oil was lost due to fault breach rather than gas displacement.

In contrast, a lack of evidence for palaeo-oil accumulation at Sunrise-1 and Troubadour-1 is also significant, as it removes the potential for an oil leg displaced either down-dip within the same structure or as a remigrated oil charge to adjacent up-dip structures. Significantly, these fields presently contain condensate and would have existed as separate oil and gas legs if charge occurred before dew point pressure was reached. The absence of a palaeo-oil column implies that the traps were charged after formation pressures exceeded the dew point of the reservoired gas. At Troubadoirr-1, this observation constrains the time of charge to the last 5-10 Ma, when pressures within the trap are estimated to have reached dew point.