Sedimentology and Hydrocarbon Potential of the Paparoa Coal Measure Lacustrine Mudstones

Potential lacustrine source rocks have been recognised in several Cretaceous syn-rift basins including the producing Taranaki Basin, but have not been officially recognised from drill core and seismic data. The late-Cretaceous Paparoa Coal Measures contain three lacustrine mudstone formations which outcrop in several localities and have been extensively drilled for coal mining. These formations are considered to be an easily accessible analogue for late-Cretaceous lacustrine source rocks in New Zealand and also provide valuable information regarding syn-depositional tectonics and basin formation during the late-Cretaceous. Stratigraphic columns and isopach maps were constructed from field work and drill hole descriptions and results showed variations in lithofacies across the basin. The western side of the basin is characterised by sandy lithofacies, abundant proximal turbidites and debris flows. The transition to a sub-aerial environment is marked by thick conglomerate and meter wide rip-up clasts. The central and eastern sections of the basin show massive mudstone, distal turbidites, low energy fluvial sandstones and thin, discontinuous coal. Isopach maps constructed from drill hole data identified three NNE – SSW oriented lakes with lacustrine sediment of up to 180m thick truncated by the eastern Roa – Mt Buckley Fault Zone. It was determined fault control during deposition was to the west and the basin extended further than its current location. Revisions to isopach models highlighted a lack of change in basin orientation during deposition of the Paparoa sediments. Plate reconstructions combined with direct evidence from the basin indicate formation of the Paparoa Coal Measures could have occurred in either a rift or transtensional basin. The mudstones were geochemically assessed for hydrocarbon potential using a Source Rock Analyser (SRA). Preliminary analysis of the three mudstones has shown TOC values ranging from 1.0 to 4.5 wt.%, HI values ranging from 68 to 552 mHC/gTOC and Tmax results show the mudstones to range in maturity from immature to late – mature. A sample from the Waiomo Formation has excellent potential for oil generation and the low maturity results for the Goldlight Formation make it a potential shale gas resource. These results have shown the potential for hydrocarbon bearing lacustrine source rocks to exist in the Greymouth Coalfield. In addition, revisions have been made to basin formation which should be considered. Due to the availability of data from the Paparoa lacustrine source rocks, they should be used as an accessible analogue for Taranaki and other Late Cretaceous basins