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Geological Society, London, Special Publications; 1995; v. 98; p. 205-221;
DOI: 10.1144/GSL.SP.1995.098.01.12
© 1995 Geological Society of London

Magnetostratigraphic Applications

Application of a palaeomagnetic reversal stratigraphy to constrain well correlation and sequence stratigraphic interpretation of the Eocene C1 Sands, Maui Field, New Zealand

Gillian M. Turner1 & Ian D. Bryant2,3

1 Institute of Geophysics, Victoria University of Wellington, PO Box 600, Wellington, New Zealand
2 Shell Todd Oil Services, New Plymouth, New Zealand
3 Schlumberger-Doll Research, Old Quarry Road, Ridgefield, CT 06877-4108, USA

The giant, offshore Maui gas condensate field covers an area of 160 km2 that straddled NE-SW oriented shorelines during the deposition of the main reservoir interval (C1 Sands) in the mid-late Eocene. These sands were deposited as highstand shoreline deposits and lowstand channel-fills that are vertically partitioned by thin, tight transgressive deposits. Correlation of shelf mudstones basinward of the shoreline with equivalent, tight coastal sands is important to define the distribution of barriers to vertical permeability within this approximately 100 m thick interval, but is ambiguous using a lithostratigraphic approach alone.

A palaeomagnetic study of 273 samples of shales, siltstones and sandstones was undertaken from three cored wells, Maui-6, -7 and -A1, to place further constraints on well correlation. A variety of magnetic behaviour was observed: from strong, stable primary remanent magnetization, to weak magnetization carrying secondary overprints which sometimes could not be removed completely. Primary polarities were retrieved from about 950f samples.

Maui-6, the most basinward well, contains a significant proportion of marine shales and is considered to provide the most complete record. The upper and lowermost parts of the magnetostratigraphic records from the other two wells correlate well with that from Maui-6. Incision of a channel system in the area of Maui-A1 is interpreted to have removed significant parts of the record from the middle C1 interval of this well. The palaeomagnetic data support the correlation of the high permeability, tidally influenced fill of this valley with time-equivalent, tight transgressive sands in interfluve areas.

The composite magnetostratigraphic record shows a high frequency of reversals: it is of predominantly normal polarity and contains 10 intervals of reversed polarity. Five of these reversed intervals are apparently less than about 30 000 years in duration. On the basis of biostratigraphic evidence that the interval studied is Bortonian (Mid-Eocene) in age, the magnetostratigraphic record is correlated with Chrons 17 and 18 of the global polarity time-scale.