Controls on acoustic properties of Upper Jurassic siliciclastic rocks (Boulonnais, northern France)

Abstract

More than 200 plugs from outcrop and a nearby borehole in a carbonaceous siliciclastic interval of Late Jurassic (Kimmeridgian to Tithonian) age were quantitatively analyzed for texture, mineralogy, and acoustic properties. Our primary goal was to study the effect of clay (fraction smaller than 8 μm), silt/sand (fraction larger than 8 μm), and carbonate on the acoustic properties. The quantitative nature and volume of the data made it possible to observe four-dimensional relationships in contoured ternary diagrams. Primary control on the acoustic properties is exerted by porosity, but the trend of this relationship significantly deviates from popular velocity transforms. The contribution of clay, silt/sand, and carbonate particular material and cement explains the remaining variation in acoustic properties. Although no clear linear thresholds are defined, a general trend is that clay and carbonate content have opposite and overlapping effects on acoustic properties, the influence of clay content progressively increases with decreasing carbonate content, and visa versa. With increasing carbonate content, the variation of acoustic velocity at a given porosity value increases to nearly twice of that in the clay-dominated sediment. Traditional classification boundaries are present but strongly overprinted by this interplay between clay and carbonate. This study may have important implications for porosity and lithofacies prediction from wireline logs in similar mixtures of sediment. In addition, the quantitative character of the textural and mineralogical data may provide a direct link from acoustic properties to the primary depositional system and sequence stratigraphy.