Statistical classification of log response as an indicator of facies variation during changes in sea level: IODP Exp 313

Petrophysical measurements and images allow the characterization of the lithology and physical properties of subsurface successions, thus making a powerful contribution to the evaluation of facies and sediment composition and to the recognition of key intervals and surfaces in siliciclastic successions. The mission-specific IODP Expedition 313 (May-July 2009) cored and logged sequences deposited on the New Jersey continental margin during the post-Eocene icehouse world. Sedimentary successions show heterogeneities at several scales (micro meter to tens of meters) that can be interpreted in terms of sequence stratigraphy (including depositional units, erosional surfaces, paleoenvironment), fluid content or diagenetic features. Such heterogeneities are often associated with changes in the physical properties of the sediment and can therefore be identified from downhole geophysical log responses, in borehole wall images and from core petrophysics measurements. Multivariate statistical analysis (iterative non-hierarchical cluster analysis) of the downhole logs provides an objective assessment of the location of both subtle and major changes within and between intervals. The analysis is concentrated into two groupings: firstly using those logs that are primarily influenced by lithology and secondly an analysis incorporating parameters influenced by the degree of lithification and by pore fluid salinity. The characteristic petrophysical responses in the first analysis can be used to infer the lithology of the sequences and can be compared to the sedimentological classifications. In places where there is poor or no core recovery this can aid the placement of boundaries between sequences in addition to providing detail of petrophysically defined intervals that can be compared between sequences. The second analysis provides an objective analysis incorporating sonic and conductivity logs and so reflecting pore fluid composition and the varying degree of cementation of the sediments in addition to lithology. Some of the petrophysical characteristics correlate with the depositional sequences predicted from seismic reflection profile analysis. The calibration of these datasets with the excellent core recovery in key intervals of each borehole aids interpretation in intervals with incomplete core recovery, thereby contributing to one of the Expedition 313 aims to evaluate sequence stratigraphic facies models that predict depositional environments, sediment compositions, and stratal geometries in response to sea-level change.