Research paperVariations in petrophysical properties along a mixed siliciclastic carbonate ramp (Upper Jurassic, Ricla, NE Spain)

- The spatial distribution of Kimmeridgian carbonate-siliciclastic ramp sediments in NE Spain (Ricla).
- The distribution of rock properties of the ramp system: porosity, mineralogy and elastic properties.
- Understanding which rock properties control the elastic behaviour.
- The translation from the ramp system to synthetic seismic profiles.
- How rock properties translate to the seismic profile.

A multi-scale petrophysical study was performed on an Upper Jurassic siliciclastic - carbonate ramp system near the village of Ricla (Spain). A combination of textural (field observations, thin section and grain-size analysis), chemical - mineralogical (thermo gravimetric analysis) and physical (porosity, gamma ray and velocity measurements) properties was used to capture the heterogeneity of this mixed sedimentary system. Subsequently, synthetic seismic sections with varying resolution were created to show the characteristics of the internal ramp-structures. The translation from the measured petrophysical properties to a synthetic seismic section helps to better understand subsurface seismic. Although upscaling problems are a recurring issue, the homogeneity of the samples and the continuity of the outcrop and facies reduce the number of uncertainties.Acoustic properties are generally controlled by the amount and type of pore spaces. Due to the low porosities (ϕ < 7.2%) of this mixed ramp system the influence on the acoustic properties is limited; therefore other controlling parameters come into play. One of the primary controlling factors is the mineral composition, thus the presence of clay and the ratio between siliciclastic and carbonate components. A dominant presence of carbonate relates to relative high P- and lower S-wave velocities, a high percentage of quartz relates to relative lower P- and higher S-wave velocities, the presence of clay reduces the overall velocities even more. Carbonate infill of intergranular space (micrite or cement) slightly influences the acoustic properties. In general, the infill of microporous micrite shows lower velocities compared to the cemented samples. Finally microcracks may significantly reduce the velocity, although these cannot be quantified, their presence can be estimated using elastic property models.The ramp geometry is characterized by an overall lateral homogeneity and vertical heterogeneity. Textural, sedimentological and stratigraphic transitions in the outcrop are related to the defined facies, each has its own unique mineralogical signature. Amplitude variations in the synthetic seismic profile are primarily caused by changes in mineral composition thus facies transitions.