Abstract:
The Eocene C1 Sands reservoir contains the majority of the gas-condensate reserves in the giant offshore Maui Gas Field. Taranaki Basin, New Zealand. The main sequence of the C1 Sands comprises interbedded cross-bedded sandstones (Sc), heterogeneous sandstones (Ss), burrowed and argillaceous .sandstones (Sb) and mudstones (M), which were deposited in lower coastal plain to Inner-shelf environments during the middle Eocene. Regression and transgression of the shoreline during the deposition of the sequence imparted a strong vertical layering that forms the basis for a geologically-meaningful subdivision of the reservoir by correlating flooding surfaces. Using both flooding surfaces and paleomagnetic reversal stratigraphy, the C1 Sands can be subdivided into nine reservoir layers within Maui Field.
The application of high resolution sequence stratigraphy to sub-surface reservoir-scale architecture in this study has been based on the recognition of key stratigraphic surfaces (sequence and flooding surfaces) that are interpreted to reflect fluctuations in relative sea-level. The C1 interval constitutes two sequences, each of which represents a high-frequency cycle at approximately fourth-order. Detailed petrographic mapping, combined with sedimentologic description and sequence stratigraphic interpretation, has led to the recognition of carbonate-cemented horizons being frequently associated with maximum flooding surfaces, sequence boundaries and flooding surfaces.
Diagenesis has dramatically modified the primary mineralogy and texture of sandstones in the C1 Sands interval. Significant diagenetic events, in paragenetic order, are: (1) mechanical compaction. (2) carbonate cementation, and (3) dissolution of feldspar (mainly plagioclase) associated with precipitation of kaolinite. Diagenetic variation in different parts of the C1 Sands is closely related to primary mineralogy and texture as influenced by the depositional environments, and four different diagenetic patterns, each with distinct petrographic and petrophysical characteristics, have been recognised by the detailed petrographic study.
The direct comparison of petrographic and petrophysical data for the C1 Sands indicates that both the porosity and permeability are determined by a combination of depositional and diagenetic effects, and the variation in the diagenetic patterns observed in different lithofacies of the C1 Sands is closely related to primary mineralogy and texture which were controlled by depositional environments. This result suggests that depositional variables are the primary controls on reservoir porosity and permeability in the C1 Sands.
