Ichnology in Reservoir Characterization of Turbidites: Lessons Learned from Outcrop Studies

Ichnological methods have been successfully applied in the reconstruction of palaeoenvironments and their conditions (facies reconstruction), sequence stratigraphy (reconstruction of surfaces and trends), and in reservoir quality assessment (impact on heterogeneity and connectivity). This has been demonstrated in numerous studies of shallow-marine deposits, while the application of ichnology to reservoirs in deep-sea environments is still in its infancy. Although advanced concepts exist to differentiate architectural elements of fan deposits in outcrop, their application to subsurface cases is hampered because of contrasting kind of data: in outcrop, trace fossils preferentially occur parallel to bedding and can be observed in three-dimension, whilst core sections display two-dimensional ichnofabrics normal to bedding.

Eocene to Oligocene turbidites are exposed in the Grès d’Annot Basin in SE France and were investigated as an analogue to explain depositional trends of confined turbidites in Cretaceous reservoirs in the Norwegian Sea and elsewhere. The distribution of pre-turbidite versus post-turbidite trace fossils was studied in several outcrops along a proximal-distal transect. As the quality and quantity of shallow burrow preservation is related to the amount of erosion by the turbidity current, a semi-quantitative analysis of pre- versus post-turbidite trace fossils was performed in connection with detailed sedimentological investigations. In addition, the relationship of significant ichnofabrics to individual lithofacies types and assumed sub-environments was investigated.

Well core material from the Vøring Basin, offshore Norway, comprises intervals with moderate to intense bioturbation. Special attention was paid to the recognition and distribution of post-turbidite burrows as these are almost the only category of trace fossils visible in vertical core sections. Careful analysis of significant burrow features from outcrop enables the identification of such ichnotaxa in core and furthermore utilizes their value in the reconstruction of sedimentary environments.

This study illustrates the verification of ichnological methods in the analysis of deep-sea deposits as an integrated part of reservoir characterization. A careful selection of relevant data, together with their outcrop- to-core calibration, is necessary and promises high potential in future integrated studies.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California