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Abstract

Thin-bedded turbidites are an important hydrocarbon reservoir facies worldwide. An example occurs in an exceptionally well exposed, laterally extensive shore platform outcrop of early Miocene Whakataki Formation, East Coast North Island, New Zealand. The most likely depositional setting is overbank, near base of slope.

The thin-bedded turbidites studied comprise approximately 360 sandstone beds interbedded with mudstone in a 32 m thick section. Beds show cyclicity in thickness with wavelengths of approximately 1 m and perhaps 14 m. Misleading estimates of net:gross can be made if cyclicity is ignored. Detailed study of a single bed comprising mainly Tb-Tc turbidite intervals indicates considerable variation at a decimeter and finer scale in porosity and permeability, both laterally and vertically within the bed. This suggests reservoir parameters derived from cores may not be reliable when extrapolated away from a well. An overall modeled average porosity of 14.5%, when combined with average net:gross of 74%, indicates bulk porosity for the unit of around 10%.

FMI image logs and the seismic character of gas-bearing thin beds drilled nearby in the offshore at Titihaoa-1 exploration well also show cyclicity of wavelength ~ 1 m and suggest the outcrop studied is a close analog, and hence, could provide a useful insight into likely reservoir architecture and characteristics at depth.

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