--> Abstract: Chemostratigraphy: A Tool for the Correlation of Barren Sequences, by T. J. Pearce, D. K. Wright, S. Cope, R. T. J. Moody, and I. Jarvis; #90987 (1993).

Datapages, Inc.Print this page

PEARCE T. J., D. K. WRIGHT, and S. COPE, Kingston Geological Services, Kingston-upon-Thames, Surrey, UK; and R. T. J. MOODY and I. JARVIS, Department of Geological Services, Kingston University, Kingston-upon-Thames, Surrey, UK

ABSTRACT: Chemostratigraphy: A Tool for the Correlation of Barren Sequences

The geochemistry of sediments is highly variable, and is sensitive to subtle changes in composition. Apparently uniform successions 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. Diagenetic processes (addition of secondary carbonates, precipitation of diagenetic clays) may also produce marked changes in chemical composition.

Analysis by ICP-AES and ICP-MS enables the rapid acquisition of high quality data on approximately 30 elements for a large number of samples. These data are used to characterize a geochemical 'fingerprint' for individual sedimentary units and enable a chemostratigraphy to be established. This may be achieved in three ways: (1) the identification of progressive stratigraphic changes in geochemistry; (2) the recognition of geochemical fingerprints of sedimentological packages, and (3) the recognition of geochemical fingerprints for individual beds. Chemostratigraphic correlations are not solely constructed on the basis of bulk geochemical abundances, as these elements are invariably involved in diagenetic activity. 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. For example, trace elements such as Zr and Hf are concentrated in refractory grains which remain unaffected by diagenetic processes and preserve the detrital geochemical signature inherited from the source area.

The potential of chemostratigraphy has been demonstrated in a wide range of depositional environments. Geochemical data for late Quaternary tuibidites of the Madeira Abyssal Plain have been used to distinguish three compositional groups: calcareous, organic-rich and volcaniclastic turbidites. Statistically defined geochemical fingerprints for individual turbidites permit correlation over distances >250 km. Furthermore, lateral geochemical variations within individual turbidites are employed to model sediment dispersal patterns and infer palaeoflow pathways. In ancient sequences, chemostratigraphy has been successfully applied to barren continental sequences, correlating geochemically distinct fluvial sandstone and floodplain\lacustrine silt\claystone packages between wells over dis ances of about 5 km, correlations which previously were only partially identified by E-Logs. 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 #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.