PEARCE, T. J., D. K. WRIGHT, S. COPE, and R. T. J. MOODY, Kingston Geological Services, Kingston Upon Thames, Surrey, England

ABSTRACT: Chemostratigraphy: A New Tool for the Correlation of Ancient Sequences

Chemostratigraphy involves the application of major and trace-element geochemistry to the characterization and correlation of strata in petroleum basins, particularly where biostratigraphic control is poor. The geochemistry of sediments is highly variable, and is sensitive to subtle changes in composition. Apparently, uniform succession may show primary differences in the chemistry of their constituent minerals or in the proportions of

accessory phases (e.g., heavy minerals and clays), many of which have very distinctive trace element contents.

Geochemical data are acquired by analysis using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS). These techniques enable the rapid acquisition of high-quality data on approximately 30 elements for a large number of samples. Data are used to characterize a geochemical "fingerprint" both for individual beds and sedimentary units and to recognize long-term geochemical trends. Geochemical fingerprinting concentrates on the distribution of trace and rare earth elements, which are commonly associated with accessory minerals such as heavy minerals and clays. Chemostratigraphic correlations between wells are established on the identification of similar geochemical characteristics.

Chemostratigraphy is a very useful tool, which can reduce the ambiguity and uncertainty often associated with more traditional methods of correlation such as lithostratigraphy, biostratigraphy, and geophysical logging. It 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. Geochemical data can differentiate between successions of sedimentary units, which have repetitious lithological and E-Log characteristics. Core-based chemostratigraphies have the potential for high-resolution fingerprinting and correlation of individual beds and small-scale sedimentary units (e.g., thicknesses <30 ft). This approach is suitable for detailed stratigraphic refinemen and correlation within reservoirs. Cuttings-based chemostratigraphies can be employed to differentiate between thick sedimentary units and to identify long-term geochemical trends. Such an approach can be used to establish basin-wide correlations; alternatively, geochemical data can define marker horizons, which can be utilized as a basis for E-Log correlations.

Other applications of geochemical data are (1) to determine changes in sediment provenance, which is important in modeling depositional systems and reservoir geometry, (2) to fingerprint and predict the extent of claystone permeability barriers in reservoirs, (3) to investigate the geochemical controls responsible for E-log signatures, therefore aiding research into interpretation of the responses of downhole geochemical tools, and (4) to investigate diagenetic histories.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.