Grain size, composition, porosity and permeability contrasts within cross-bedded sandstones in Tertiary fluvial deposits, central Spain

Abstract

Permeability measured with a portable probe pcrmcflmeter on outcrops of cross·bcdded sandstones ranges betwecn 0·9 and 19 O. The highest pcrmeability (2-19 0 with an avcrage of8·5 D) occurs in thc coarsest grained foresct laminae (CFL). intermediate values (2-120 with an average of 5·3 D) occur in fincr grained foreset laminac (FFL) and thc lowest values (0·9-1 0 0 with un avcrage of 4·8 D) occur in boltomsct laycrs (BL). In the cross·bcds the uverage grain sizc rangcs from medium graincd sand in the CFL to finc grained s,lnd in thc FFL and BL. In all three subf'lcies. the average size of the primary pores is approximately It/> unit smaller than the averagc grain size. The abundance of unstablc carbonatc clasts correlatcs with incrcasing avcrage grain size, micritic clasts being most abundant in the CFL. Converscly. quartz content incrcascs with decreasing grain size and is highest in the FFL and BL. Oiagenetic destruction of primary porosity by compact ion and cementation. as well �IS generation of sccondary porosity through dissolution, were controlled by the original mineralogical composition of the sand. Contrasts in grain size detcrmine the primary pore size contrasts and differences in composition bctwecn CFL. FFL and BL. Permeability contrasts reflect variations in averagc primary pore size rather than differenccs in total porosity. Probe permeability contrasts between eFL. FFL and BL depend on contrasts in llverage pore size and contrasts in mineralogical composition between the subfacies.