Abstract

An analysis of the petrophysical and diagenetic effects of the emplacement of Cretaceous basaltic lava flows (Serra Geral Formation) on aeolian sandstones (Botucatu Formation) has been undertaken on core samples from the Paraná Basin, Brazil. Between 0.1 to 1 m from the contact zone, acoustic wave velocities and porosities in sandstones show a significantly wider scatter than those located >1m away from the lava contact. Higher P-wave values (av. 3759.3 ms-1) occur between 0.1 to 1 m from the lava contact in contrast to those areas > 1 m away (av. 3376.8 ms-1), whilst the average porosity is 6.5% near the contact (0.1 to 1 m), and 10.7% away from the contact (>1 m). Petrographic evaluation reveals two diagenetic pathways responsible for modification of the petrophysical properties: early hydrothermal Mg-rich authigenesis (Type 1) and early chemical dissolution (Type 2). Type 3 diagenesis occurs away from the lava-sediment contact (>1 m) with the appearance of poikilitic calcite and smectite. The sandstone samples associated with Types 1 and 2 diagenesis display a decrease in porosity and increased acoustic velocities in relation to Type 3, while Type 3 samples show little or no variation in reservoir properties. The lava-induced diagenetic effects at the sandstone-lava contacts (0.1 to 1 m) may form a baffle or seal to fluids around the margins of the sandstone bodies. Therefore, whilst diagenesis associated with lava emplacement may hinder reservoir quality around the margins, the original reservoir properties are preserved within these large sandstone bodies.

Supplementary material: Petrophysical and petrographic data is available as annex files. https://doi.org/10.6084/m9.figshare.c.5244473

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