Offshore pore fluid salinity estimation from downhole logging and petrophysical measurements

The mission-specific IODP Expedition 313 successfully cored and logged three sites on the New Jersey shallow shelf in order to characterize Miocene sea-level changes and their relationship to sequence stratigraphy (EXPEDITION 313 SCIENTISTS, 2010). Another unexpected success of this expedition is related to the discovery of relatively fresh pore water 50 km offshore and more than 400 m below the sea floor (Mottl et al., 2009). Numerous questions have arisen concerning the age and origin of such fresh water, and more broadly, the amount of fresh water stored now on passive margins. To answer these questions and understand onshore/offshore fresh water transfers, it is critical to refine the spatial organization of the fresh/salt water intervals and the precise relationship between fluid salinity distribution, depositional environments, sediment compositions and stratal geometries. Because geochemical analyses are carried out at discrete horizons at the scale of a borehole, vertical changes cannot be determined in highly resolved spatial precision. In this context, the objectives are to refine the vertical variability of the salinity thanks to downhole geophysical logs and core petrophysics measurements and in particular, by analysing the conductivity signals. Indeed, in holes M0027A and M0028A, the conductivity log trends correlate closely with changes in salinity measured on cores. Here, we use the Archie equation, which relates the measured formation resistivity to its porosity, fluid resistivity and formation factor, in order to estimate the in-situ salinity. Because this law postulates that the rock matrix is non-conductive (clean shale-free formation), this assumption is in general no longer true for clay rich intervals due to the clay's structure. We use Cation Exchange Capacities measured from several core samples to correct for this effect. Despite the shale effect however, in holes M0027A and M0028A, the formation conductivity appears at the first order driven by water salinity and porosity. In the lower part of hole M0029A, the conductivity shows high variability which does not follow the trend of the chlorinity, remaining relatively high and constant. The shape of the conductivity log correlates with the porosity and sonic logs, thus suggesting that in this hole the signal is particularly sensitive to changes in pore water volumes. EXPEDITION 313 SCIENTISTS, 2010. New Jersey Shallow Shelf: shallow-water drilling of the New Jersey continental shelf: global sea level and architecture of passive margin sediments. IODP Prel. Rept., 313. doi:10.2204/iodp.pr.313.2010; Mottl M. J., Hayashi T. & EXP313 SCIENCE PARTY. Fresh and Salty: Chemistry of Sediment Pore Water from the New Jersey Shallow Shelf: IODP Exp313. AGU, Fall Meeting 2009, abstr. #PP31A-1293;