The Petrophysics Of The Nita Formation, Central Canning Basin, Western Australia.
Hillock, Peter Mackintosh
Honours Degree, 1988
University of Adelaide
The Ordovician Nita Formation in the Canning Basin is a primary exploration target. This formation, Late Kundan to Lasnamagian in age, is often associated with hydrocarbon shows. It is informally subdivided into the Nita Formation Main Unit and the Nita Formation Transition Unit. The former, where dolomitised, is recognised as a potential reservoir rock, since dolomitisation is associated with the development of porosity and permeability within this unit. It has an ideal stratigraphic location, occurring above the Goldwyer Formation source rock and below evaporitic seals within the Carribuddy Group.
In a fully dolomitised section of the Nita Formation Main Unit in Acacia #2, six lithofacies have been identified: LFl basal pyritic packstone-grainstone, LF2 bioturbated-laminar wackestone, LF3 bioturbated mudstone, LF4 massive to laminar mudstone, LF5 argillaceous mudstone and LF6 finely laminated shale. These were deposited as a series of cycles with a range in thickness of 1 to 3m, each cycle consisting of up to 5 lithofacies (LFl-LF5), with the sixth (LF6) having no recognised position but occurring in localised topographic depressions. Lithofacies were deposited in a shallow-water tidal environment on a carbonate ramp of low depositional slope in zones of specific hydraulic energy parallel to the strand line, influenced by the overall regression of the Ordovician epeiric sea and punctuated by brief transgressions.
LFl contains significant quantities of the oil-prone alga Gloeocapsamorpha prisca, as well as other organic material. This is a potential source rock for the porous and permeable lithofacies which overlie it. Good porosity and permeability are restricted to LF3, mainly the result of the development of sucrosic dolomite in coarser burrow infill. Vugular porosity is also significant. This lithofacies is subdivided into a burrow-mottled mudstone (LF3B) and a stylonodular mudstone (LF3A). The former is of reservoir quality with low irreducible water saturations, low displacement pressures and good pore throat sorting. Although volumetrically less than intercrystalline pore space, vugular pore space has a significant effect on the cementation exponent M. The reservoir lithofacies (LF3B) has an M of 2.11, but this tends to 2.52 as vugular porosity increases. Each cycle is capped by a potential sealing lithofacies (LF5).
A porosity and permeability relationship has been defined for this formation which can be utilised for determining a potential range of permeability in drill cuttings and core plugs. Two curves have been constructed for calibrating in situ porosity and permeability, from values measured at air-ambient pressure.
A model is proposed for hydrocarbon accumulation in this formation in that, instead of the Carribuddy Group evaporite horizons forming the seal, individual cycles within the Nita Formation possess their own intra-formational seal (LF5). Each cycle is therefore a potential micro source-reservoir-seal unit. The vertical stratigraphy of the cycles results in a layered reservoir system composed of intra-formational seals (LF5), source rocks (LF1) and reservoir rock (LF3B). Although the thickness of each cycle is small (<3m) their lateral extent suggests good potential for hydrocarbon generation, accumulation and trapping.