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

Abstract

The giant, offshore Maui gas condensate field covers an area of 160 km 2 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 95% of 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 interftuve 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.

Sequence stratigraphy and controls on reservoir sandstone distribution in an Eocene marginal marine-coastal plain fairway, Taranaki Basin, New Zealand

“…This interpretation is supported by magnetostratigraphic data from the Maui Field, which show channel sandstones from this interval with different magnetic polarity to the underlying beds and hence recording a break in sedimentation (Turner and Bryant, 1995).…”

Late Paleocene to middle Eocene foraminiferal biostratigraphy of the Hampden Beach section, eastern South Island, New Zealand

“…This is significantly older than the previous date for this boundary, which was 37.0 Ma (Morgans et al 2004), based on the FaD of the nannofossil Chiasmolithus oamaruensis. It is also older than the correlation of Bortonian strata with magnetochrons C17 and C18 in the Maui-6, 7, and a1 petroleum exploration wells in the Taranaki Basin (Turner & Bryant 1995). Re-examination of the biostratigraphic data from Maui-6 (Pocknall 1991), and the magnetostratigraphic study on cores from Maui-6, 7, and a1 (Turner & Bryant 1995), indicated that a Bortonian age applied only to the lower C1 sands.…”

“…2006). Bortonian strata in offshore Taranaki Basin (Maui-6, 7, a1), correlated with magnetochrons C18/C17 by Turner & Bryant (1995), provide the only magnetostratigraphy published on late Paleocene to earliest late Eocene time in New Zealand. all other dating is mainly by correlation of key planktonic foraminiferal, calcareous nannofossils, and dinoflagellate datums (Fig.…”

“…Strata from 2821.41 to 2798.93 m contain no restricted Bortonian taxa, and the presence of the dinoflagellate Deflandrea heterophycta and an assigned MH2 Miospore Zone (Pocknall 1991) indicate that the upper C1 sands have a late Bortonian-Kaiatan age. The C17/C18 magnetochron boundary is placed at c. 2812.5 m (Turner & Bryant 1995), is dated at 38.5 Ma by Cande & Kent (1992, and overlies the highest confirmed Bortonian strata. The upper C1 sands and the overlying C1 shales in the Maui-6, 7, and a1 wells are late Bortonian-Kaiatan and have a magnetostratigraphic age of 38.5 Ma.…”

Integrated 3D facies modeling of the Mangahewa Formation, Maui Gas Field, Taranaki Basin, New Zealand

“…In general, these include 6 sandstone-dominated lithofacies, 4 siltstone-dominated lithofacies and 2 mudstone-dominated lithofacies. It is to be noted that lithofacies analysis was one of the major indicators for depositional facies interpretation (Walker et al 2016;Miall 2016;Turner and Bryant 1995;Seybold Transgressive shelf/reworked bar/ offshore barrier sands Low, blocky gamma response, generally above relatively thin upward-cleaning log profile Clean to slightly argillaceous, micaceous, coarse to medium-grained sandstone Cross-lamination et al 1996). Cored sections are interpreted based on the well log response, depositional facies variability and historical reports.…”