Application of chemostratigraphy to the Mungaroo Formation, NW Shelf K.T. Ratcliffe and A.M. Wright Chemostrat Inc, Houston, USA P. Montgomery, Chevron Energy Technology Company, Aberdeen, UK A.Palfrey, A. Vonk, J. Vermeulen and M. Barrett Chevron Australia Pty Ltd, Perth WA 6000

Paper Outline What is chemostratigraphy ? Sample preparation and analysis Background to study Mungaroo Formation in the Gorgon Field Why use chemostratigraphy on the Mungaroo Formation ? Chemostratigraphic characterisation Paleoclimate indicators Provenance indicators Chemostratigraphic correlation What this means for stratigraphy in the Mungaroo Formation

What is Chemostratigraphy ? The characterization, or ‘fingerprinting’ of sedimentary units using inorganic geochemical data This characterization enables the identification of chemostratigraphic units on the basis of elemental concentrations, ratios or systematic trends Inorganic geochemistry of sediments is highly variable and sensitive to subtle changes in source composition, facies, weathering or diagenesis. Therefore apparently uniform successions may show primary differences in the chemistry of constituent minerals, particularly in accessory phases The keys to a successful chemostratigraphic study is to understand the association of elemental chemistry and geology

Why use Chemostratigraphy in the Mungaroo? The Mungaroo Formation is a typical fluvial system, so tends to be poor in biostratigraphic and palynological markers Wireline log correlations can be inaccurate since the tendency is to make simple layer-cake correlations and tie sandstones that occur in similar stratigraphic positions and have similar e-log signatures Seismic correlations do not provide sufficient resolution and can often be ambiguous Chemostratigraphy offers the potential to refine biostratigraphic and palynological stratigraphies, can remove ambiguity in seismic interpretations and shows that the Mungaroo is not layercake

Location and stratigraphy

Stratigraphic correlation 3 Time lines 7 chemostratigraphic packages 22 geochemical units

How do we get the data ? 1. Surface contamination Removed - washing 2. Cuttings chips selected for analysis - picking 3. Sample powdered - grinding 4. Powder “dissolved” – Li metaborate fusion 5. Concentrations for 10 major elements 23 trace elements 14 Rare earth elements All reported in xls format 4. Solution analysed- ICP OES and ICP MS

Chemically recognising lithostrat units

First order chemostratigraphic divisions

Ga/Rb increases through time Al 2 O 3 /bases (CaO+MgO+K 2 O+Na 2 O) increases through time Notable steps that define the solid lines SO WHAT ???? First order chemostratigraphic divisions

Ga/Rb = kaolinite / illite in claystones – intensely weathered claystones tend to be more kaolinitic Al/bases = CIA (chemical index of alteration) which is a proxy for the degree of weathering Therefore blue = less intense weathering, red = more intense hydrolytic weathering First order chemostratigraphic divisions

Chemostratigraphic packages

Packages defined using variations in Nb, Cr and Na SO WHAT ???? Chemostratigraphic packages

Ga/Rb : Al/bases – change in paleoclimate through time Na/Al, Cr/Na and Nb/Al in provenance through time Chemostratigraphic packages

Sandstone data

Sandstone correlation

Each lithostratigraphic unit has a unique geochemical signature The Mungaroo Formation contains three chemostratigraphic surfaces that approximate to chronostratigraphic markers Seven chemostratigraphic packages and 22 units are identified and correlated in the Mungaroo Formation The paleoclimate during deposition of the Mungaroo Formation was becoming increasingly wet, resulting in more intense hydrolytic weathering and increased abundance of kaolinite in the claystone lithologies The older parts of the Mungaroo Formation contain common plagioclase feldspar, implying an intermediate igneous provenance. The intermediate provenance was replaced through time with greater influence of a mafic igneous provenance The Brigadier Formation contains an unknown Nb-rich heavy mineral species, implying that the provenance of this formation is different to that of the Mungaroo Formation In places, the chemostratigraphic sand unit correlation deviates from the lithostratigraphic sand correlation to an extent that may impact upon gas production. The variation between litho- and chemostratigraphic correlation are potentially vitally important for building reservoir models in the Mungaroo Formation of the Gorgon Field.

ACKNOWLEDGEMENTS Chemostrat would like to thank David Wray and Lorna Dyer of Greenwich University for carrying out the ICP OES MS analyses and Bill Ellington of Ellington and Associates for carrying out the XRD analyses. We are also grateful to Chemostrat and Chevron Australia for allowing us time and resources to prepare this manuscript. The authors are grateful to Eric Tenthorey, Andrew Jones and Ben Van Aarssen who reviewed the original manuscript and whose comments were invaluable to the final document. The authors would like to thank the Gorgon Joint Venture partners, Mobil Australia Resources and Shell Development (Australia) for allowing permission for this paper to be published.