Abstract

Tight gas reservoirs have become popular targets in petroleum exploration in recent years, due largely to the increasing market demand for gas and also to technology advances used in extraction. Reservoir quality is typically poor due to deep-burial diagenesis, resulting in significant compaction, cementation, and illitization. However, analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality, and these results can potentially be used to help predict reservoir quality at other sites.

The use of integrated analysis of diagenetic and burial history is here shown for the Eocene, K3E Kapuni Group reservoir in Cardiff-1, Taranaki Basin, New Zealand. The K3E belongs to a deep gas play that was considered uncommercial in the early 1990's due to poor reservoir quality associated with extensive authigenic illite. Potassium-argon dating has shown that destruction of the reservoir through illitization occurred in the Pliocene, associated with a late-stage heating and/or fluid-flow event.

Timing of illite authigenesis in the K3E reservoir postdates the likely circulation of CO2-rich fluids that caused reaction of feldspar to form kaolinite and quartz. Homogenization temperatures from two-phase aqueous fluid inclusions within quartz cements, together with 1D basin models of CO2 expulsion from coaly source rocks, indicate that feldspar reactions may have started in the mid Miocene. The CO2-rich fluids are thought to have been generated by thermal decarboxylation of intraformational Paleocene–Eocene coals. At the same time, oil was being expelled from older, Cretaceous coals and migrated into the mid-late Eocene reservoirs; evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil-bearing fluid inclusions. The products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar, suggesting a relatively "closed" diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved.

Petrographic evidence of reservoir sandstones at Cardiff-1 demonstrates the presence of some illité-free secondary macropores lined by residual oil. This local preservation of porosity may be replicating the original distribution of oil-saturated pores in the Pliocene, at the time when authigenic kaolinite underwent reaction to form illite. Remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c. 5 Ma.

The data presented in this study are consistent with redistributed liquid petroleum into up-dip traps and represent the potential for a new exploration play. The Eocene reservoir at Cardiff-1 is currently gas-charged, with the pore system composed of approximately one third large macropores connected by a tortuous, microporous network. However, it is possible that, with the currently buoyant gas market and modern hydraulic fracturing techniques, it may now be economic to flow gas from the locally large pores identified in the K3E reservoir.

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