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ABSTRACT: Cycle architecture of Miocene deep-water hydrocarbon reservoir units, East Coast, New Zealand

Field, Brad, Steve Edbrooke, Greg Browne, and Rob Funnell , Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand

Advances in horizontal drilling have meant that thinner reservoirs, or parts of reservoirs, can be targeted for exploration. Understanding what controls vertical variations in the properties of thin-bedded reservoir units enables better decisions on where to perforate or side-track to improve flow rates. Cyclostratigraphy can predict vertical variations in architecture.

Miocene deep-water turbidite units of the East Coast North Island show cyclic variations in clay content. Time series analysis of well logs shows the cycles have frequencies in the Milankovitch band (and higher).

The Makaretu Formation, the reservoir for a recent gas discovery, has cycles of wavelengths 100, 30 & 14 m. These thicknesses are inferred to reflect orbitally-controlled climatic signals of c 123, 41 & 20 thousand years respectively. The cycles are detectable in the Vshale log for the Ruakituri-1 well.

Miocene turbidites in Titihaoa-1 have cycles inferred to reflect 410, 123-95, 41 & 23-19 Ka periodicity, as well as higher frequency cycles of wavelengths 5 m & 1-1.7 m thick (possibly 8 and 2-3 Ka respectively). The longer wavelength cycles are detectable in gamma ray logs and smaller cycles also show up in the FMI log.

Turbidites and mudstones of the Whakataki Formation show good lateral continuity: about 80% of beds over a 3.5 m thick interval are traceable laterally over 470 m. Packets (?cycles) of beds are also laterally continuous.

Recognising cyclic, predictable variations in reservoir parameters should help optimise extraction techniques.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia