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Abstract: Petrophysical and Textural Characteristics of Thin-Bedded Turbidite Sands, Miocene Mt. Messenger Formation, Taranaki Peninsula, New Zealand

Douglas W. Jordan, Peter R. King, Greg H. Browne

The Mt. Messenger Formation exposed along the Taranaki coastline (North Island) and in nearby producing oil fields is 600 meters thick and consists of a turbidite sequence deposited in a foredeep trough adjacent to a relatively narrow shelf in lower to upper bathyal water depths.

Gamma-ray values of 'Thick-bedded' sands greater than 15 cm thick on the outcrop are less than 175 counts per second (cps). These sandstones have the highest permeability values (100-800 mD). 'Thin-bedded' sands (5-15 cm) have values greater than 175 cps and range from 200-600 mD. 'Very Thin-bedded' sands (less than 5 cm) range from 10-200 mD; gamma-ray values are impossible to resolve on the outcrop. Gamma-ray values in claystones exceed 250 cps.

In the subsurface, permeabilities are highest (ave. 225 mD) in uvfg sands in the lower part of Bouma sequences that are massive-appearing or contain climbing ripples and flat laminations or within sands reworked by bottom currents. Lower very fine to silty sands (upper part of sequence) have significantly lower permeabilities because of sorting and the presence of mica and clay. A typical cored section through a pay interval consists of an upper 10-meter interval with average pay thickness of 17.5 cm (44% of the beds are greater than 20 cm thick and permeabilities average 144 mD) and a lower 20 m thick interval containing thinner pay sands (average 12.5 cm and 22% of the beds are greater than 20 cm thick). Permeabilities average 130 mD. Thirty-seven percent of the interval contains pa sand and the remainder is claystone or non-pay sand.

Interbedded thin-bedded water and oil-bearing sands have similar permeabilities and capillary properties. Clean, wet sands have less clay or feldspar and a lower gamma ray count than sands containing oil. The cause for wet sands interbedded with oil sands in the subsurface appears to be isolation and early sealing (microcompartmentalization) of sands by claystones. These features are observed in the coastal exposures.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France