Early calcite cementation in a metre-scale delta front: changes in reservoir quality

Abstract

The Roda Sandstone corresponds to a lower Eocene delta located at the northern margin of the Graus-Tremp Basin in the South-Pyrenean foreland. The distribution of the different diagenetic products affecting a decameter-thick detrital delta-front regressive unit and the under and overlying transgressive carbonates are studied. The lower part of the sandy wedge contains nodules formed by precipitation of high magnesium calcite (HMC) during an early cementation stage which are absent in the upper part of the wedge. In the topmost part, there is a continuous zone cemented by low magnesium calcite (LMC). The δ18O of the early calcite cement is interpreted as related to meteoric water input during regressive periods. Transgressive carbonate units are dominated by HMC, mostly from the bioclasts. Herein the early diagenetic processes, mostly precipitation of calcite cement, are the fundamental control on the main petrophysical properties of the sandstones, instead of the internal primary depositional architecture of the sand body. Furthermore, the presence or absence of this early calcite cement has been proven to be basic in controlling later diagenetic processes such as cracking, pressure-solution between bioclasts and millimetric fracturing. All of them highly affect the quality of the sandstone reservoir.