Diagénesis y estratigrafía secuencial de la arenisca «U» de la Fm Napo del Oriente, Ecuador

Abstract

The Napo Formation of Cretaceous age in the Oriente basin, Ecuador, is an important sandstone reservoir. The «U» sandstone is one oil interval within the Napo Formation. It is buried at a depth of 1500 m in the eastern part of the basin and down to 3100 m in the western part. This sandstone display higher porosity values (av. 20%) than other reservoirs in the region. The sandstone was deposited in fluvial, transitional and marine environments, and their correlation with the sequence stratigraphy is related to HST, LST, TS. «U» sandstone is fine to medium grained quartzarenite and subordinate subarkose. The principal cements are carbonates, quartz overgrowth and kaolin, with scarce amounts of pyrite and chlorite. Carbonate cements include: Eogenetic siderite (S1), mesogenetic and post-compactional calcite, Fe-dolomite, ankerite and siderite (S2). Early siderite helped to retain porosity by supporting the sandstone framework against compaction. Dissolution of feldspars and mesogenetic carbonate cements is the main mechanism for secondary porosity development during mesodiagenesis. The stable isotope composition of the S1 siderite are consistent with precipitation from meteoric waters. The anomalous low ä18O‰ values of some of these carbonate phases could reflect the replacement and recristalization from S1 to S2 siderite at deep burial and high temperature. However, due to this higher Mg content, siderite S2 could have precipitated as a result of the thermal descarboxilation of the Mg rich organic matter. The last carbonate cements to precipitate were dolomite/ankerite. The negative ä18O‰ in these cements is related to the continued precipitation at higher temperature and burial depth