A.S. Andrew, D.J. Whitford, and P.J. Hamilton, SPE, Australian Petroleum Co-operative Research Centre, CSIRO Division of Petroleum Resources; S. Scarano, Santos (BOL) Pty. Ltd.; and M. Buckley, CSIRO Division of Mathematics and Statistics

Abstract

The geochemistry of sedimentary rocks is a product of their provenance, maturity and diagenetic history. Chemostratigraphy relies on the variations in major and trace element abundances in sedimentary rocks as a correlation tool. It is particularly relevant in field appraisal where conventional correlation techniques such as seismic and biostratigraphic methods often have insufficient resolution. In the oil and gas province of the Surat Basin of Queensland, a series of Triassic multi-storey channel fills overlie Permian peat swamp and basin flood deposits. Correlation of the Triassic sands between wells is crucial to definition of the trap sequences.

The chemical data are assessed in two ways: using image analysis/statistical techniques and by normalization relative to elements which do not readily partition into natural waters.

A mathematical comparison of the elemental profiles with depth shows a good match between wells Link #1 (ca. 2116 – 2132 m) and Roswin North #1 (ca. 2143 – 2157 m). Such a correlation implies the absence of the Showground and upper Rewan Formations in Roswin North #1. In its present form, this approach cannot allow for significant amounts of missing section in either well.

Using an approach that relies on the geochemical characterization of unconformity-bounded units, it is possible to correlate between the upper unit in Roswin North #1(2143–2147 m) with the second top unit in Link #1(2108 – 2113 m), and between Roswin North #1(2151 – 2157 m) and Link #1 (2120 – 2124 m). This correlation is consistent with the correlation based on mathematical comparison of elemental profiles.

This work makes up part of a multi-basin test of the application of chemostratigraphy to reservoir definition in Australian Basins. The Surat Basin example presented here tests the technique when applied to stacked channel sands in a marginal marine environment.

Introduction

Chemostratigraphy involves the application of major- and trace-element geochemistry for the characterisation and subdivision of sedimentary sequences into geochemically distinct units, and correlation of strata in sedimentary basins. Sedimentary rocks are faithful records of the sensitive and subtle changes in provenance, environment of deposition and post-depositional changes. Such changes mean that apparently uniform successions may show primary differences in the chemistry of their constituent minerals, or in the proportions of accessory phases such as heavy minerals and clays, many of which have very distinctive trace-element contents. An ability to characterise these subtle geochemical heterogeneities enables apparently uniform thick successions to be subdivided and correlated between wells.

Chemostratigraphic correlation is particularly applicable to sequences that have very poor biostratigraphic control or to thick, rapidly deposited sequences that cannot be subdivided further by biostratigraphic data. Ambiguity and uncertainty often associated with more traditional methods of correlation such as lithostratigraphy, biostratigraphy and geophysical logging may be resolved. Geochemical correlation between wells may be established by the identification of similar geochemical characteristics. Such characterisation typically involves the abundance determination of approximately 50 elements.

Inter-well correlation may be achieved by the recognition of similar geochemical characteristics in adjacent sequences and is accomplished by:

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