MARTIN, KEN R., Consultant, Brisbane, Queensland; JULIAN C. BAKER, The University of Queensland, Brisbane, Queensland;

ABSTRACT: Oxygen Isotopic Evidence for Cross-Formational Porewater Flow in the Kupe South Field, Taranaki Basin, New Zealand

Hydrocarbon reservoirs in the Kupe South Field occur within the Paleocene, predominantly fluviatile Farewell Formation. This formation constitutes part of a 5000 m thick Cretaceous to Eocene siliciclastic sequence, and is bounded at the top by a regional unconformity of Oligocene age. Overlying the unconformity are thick, widespread marine mudrocks which constitute the reservoir seal.

Relatively early (shallow burial) authigenic chlorite-smectite in the non-marine reservoir sandstones has a (isotope){18}O<SMOW> value of between +11.2 and +12.6 o/oo indicating, given the likely low chlorite-smectite formation temperatures, that the chlorite-smectite formed from porewaters with a (isotope){18}O value close to that of seawater. On this basis, the source of these porewaters is likely to be marine mudrocks which overlie the Oligocene unconformity, implying that significant downward porewater expulsion into the section occurred during early burial.

Late (moderate to deep burial) authigenic kaolinite and ferroan carbonates have (isotope){18}O<SMOW> values ranging from +15.4 to +16.3 o/oo and +18.4 to +20.3 o/oo, respectively. These data indicate that, given geologically realistic kaolinite and ferroan carbonate formation temperatures of between 60 degrees and 80 degrees C, late-stage kaolinite and ferroan carbonate formed from porewater slightly depleted in {18}O relative to seawater. On this basis, porewaters involved in formation of the kaolinite and carbonate must have been partly meteoric.

The presence of meteoric water in the reservoir section during moderate to deep burial may be related either to an overlying regional Pliocene unconformity or upward expulsion of "connate meteoric" waters during compaction of the underlying, largely non-marine Cretaceous sequence. "Connate meteoric" waters appear to be the more likely source since the reservoir section was never exposed at the unconformity, implying that upward cross-formational porewater flow, perhaps via faults, occurred in the section during moderate to deep burial.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.