High-Resolution Chemo- and Chronostratigraphy in Terrestrial to Transitional Marine Environments

Marilyn P. Segall and P. Sikora
Energy & Geoscience Institute Dept of Civil Engineering, The University of Utah, Salt Lake City, UT

Chemostratigraphic analyses are combined with chronostratigraphic evaluations of terrestrial to transitional marine environments. These systems typically are biostratigraphically challenged and difficult to assess in terms of depositional environments and tectonic processes. Our studies incorporate samples from wells in Trinidad, Libya and Georgia. Paleoenvironments include lacustrine, fluvial, fluvial-deltaic, intertidal, and shallow shelf. Deepwater sandstones are evaluated in instances where caustic fluid migration or low pH conditions removed biostratigraphic indicators. The chemostratigraphic and chronostratigraphic results are placed within a framework of electric logs, lithologic logs, and seismic, where available. Palynology also is used to aid environmental interpretations.

The major chemostratigraphic indicators are: i) quartz type (opal C-T; beta-quartz, alpha-quartz) and origin (biogenic, fluid enrichment, CO₂ enrichment and flushing); ii) zeolite assemblage (formation temperature; fluid cation saturation); and iii) illite and mixed-layer expandable minerals (formation temperature, source terrane discrimination, diagenetic evolution). Pattern recognition of key parameters allows us to distinguish among depositional and diagenetic horizons across well intervals, within a field and in some cases, within a basin. Thick rock sections with no biostratigraphic control and little log variability are subdivided solely on the basis of chemostratigraphic horizons. This technique is used successfully in Libyan wells to differentiate among the “upper sands” of the lower Cretaceous section and devise a paleogeographic reconstruction for the lower Aptian that includes prevailing winds, precipitation patterns, current flow and dominant depositional/erosional processes.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas