New Zealand Journal of Geology and Geophysics abstracts
Cretaceous carbonaceous rocks from the Norfolk Ridge system,
Southwest Pacific: implications for regional petroleum potential
R. H. HERZER
R. SYKES
S. D. KILLOPS
R. H. FUNNELL
D. R. BURGGRAF*
J. TOWNEND+
J. I. RAINE
G. J. WILSON
Institute of Geological & Nuclear Sciences
Gracefield Research Centre
P. O. Box 30 368
Lower Hutt, New Zealand
*Conoco Inc.
P. O. Box 2197
Houston, TX 77252-2197
USA
+Present address: Department of Geophysics, Stanford University,
Stanford, CA 94305-2215, USA.
Abstract Late Cretaceous carbonaceous rocks have been dredged
from two sites on the Norfolk Ridge system northwest of New Zealand. On the
West Norfolk Ridge, Raukumara Series (late Cenomanian to late Coniacian,
95-86.5 Ma) coal measure sandstones and mudstones contain dispersed,
immature (Ro 0.37%), terrestrial (type III) organic matter and have poor
petroleum generative potential (TOC 2.2-2.8%, S2 1.2-1.6 mg HC/g rock).
However, the in situ coal measure sequence may contain abundant coal
seams with excellent generative potential, and thus constitutes a potential
source rock formation for the adjacent New Caledonia and Reinga Basins that
have thick sections of sediments. The coal measures probably formed in a
coastal plain environment subjected to episodic marine incursions. At the
second dredge site, near the junction of the Norfolk and Reinga Ridges, a late
Piripauan to early Haumurian (Santonian-Campanian, 85-75 Ma) marine shale
contains sparse, mixed marine and terrestrial (type II/III) organic matter.
Hopane and sterane parameters imply a maturity equivalent to at least 0.6% Ro,
but the analysed shale has poor generative potential (TOC 0.9%, S2 1.4 mg
HC/g rock). Biomarkers suggest that the terrestrial contribution to bitumen in
the shale is minor compared with the marine, but kinetic parameters are more
consistent with a type III kerogen than the type II/III kerogen identified. A
likely explanation of the discrepancy is that early oil generation has occurred
in the sample, possibly associated with sulphur incorporated into the kerogen
during diagenesis. In addition, a high saturated:aromatic hydrocarbon ratio
suggests that mature bitumen may have migrated into the shale. Given greater
TOC contents and sufficient volume, the shale could be a potential source rock
within the Reinga Basin and Norfolk and Three Kings Ridges. A simple thermal
model at a pseudo-well site in the Reinga Basin shows that at depths
>4.5 km below seabed, potential source rocks with the kerogen kinetic
parameters of the dredged rocks should be generating hydrocarbons and
approaching the threshold of oil expulsion. The model does not explain the
presence of migrated hydrocarbons in the shale sample unless similar source
rocks are more deeply buried nearby or thermal conditions were greater than
those modelled. In comparison, a predominantly marine unit like the Waipawa
Shale would be in the oil window at 3.0 km. Maturity levels may well have
been enhanced by increased heat flow associated with the latest Oligocene to
early Miocene rifting and opening of the adjacent Norfolk Basin. If equivalent
rocks with sufficient organic richness exist in the Northland Basin (part of
the Reinga Basin adjacent to New Zealand), where sediment thickness commonly
exceeds 5 km and locally exceeds 7 km, they will have generated and
expelled hydrocarbons.
Keywords petroleum; source rocks; organic geochemistry; Late
Cretaceous; Norfolk Ridge; New Zealand
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42: 57-73
0028-8306/4201-0057 $7.00/0 (c) The Royal Society of New Zealand 1999
PDF file of entire paper: medium quality (2881K); (scanned from paper original: notes about this process)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
