Ichthysガスコンデンセートフィールド・Brewster Memberの貯留岩性状分布と続成過程に関する考察

抄録

The Ichthys gas-condensate field is located in the Browse Basin, North West Shelf of Australia and it is currently under development.

The Brewster Member, one of the main reservoirs of Ichthys field consists of submarine channel and lobe complex sandstones and minor mudstones, approximately 200 m thick, and has a high net sand-to-gross thickness ratio (over 90%).

Although massive sand facies dominates and porosity is relatively stable at around 10%, the permeability distribution is complicated. Core measurement indicates that permeability has a large variation against porosity. In addition, an abrupt reduction of permeability with depth is observed in all wells in the field. The depth of the permeability reduction in each well can be mapped as a gently dipping almost planar surface. The surface, which is a potential diagenetic boundary separating good and poor permeability reservoirs, is called the Diagenetic Limit (DL).

To investigate the cause of DL, detailed petrographic analysis and Illite morphology observation were conducted.

The results of these analyses indicate that:

· “Quartz cement volume per Intergranular volume” and Illite volume have negative correlations with permeability.

· Illite volume increases with reservoir depth.

· Dominant Illite morphology changes from “flaky” to “fibrous” with reservoir depth. It becomes fibrous dominant in all observed samples below the DL.

Based on these results, two ideas for the cause of the DL have been developed:

1) Differential diagenesis occurred due to the existence of an initial hydrocarbon accumulation, which may have inhibited or reduced the level of diagenesis in the interval above the paleo-FWL.

2) Differential diagenesis was inherited from differences in texture and mineralogy of primary deposits.

Both hypotheses still have issues to be verified. Since the prediction of permeability distribution is essential to maximize the hydrocarbon recovery from the field, further investigation will be conducted to clarify the cause of the DL.