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The Napo Formation of Lower-Middle Cretaceous age in the Oriente basin, Ecuador, is an important sandstone
reservoir. The formation is buried at a depth of 1,500 m in the eastern part of the basin and down to 3,100 m in
the western part. The sandstones display higher porosity values (av. 20%) than other reservoirs in the region.
These sandstones were deposited in fluvial, transitional and marine environments, and they are fine to medium
grained quartzarenites and subarkoses. The principal cements are carbonates, quartz overgrowth and kaolin,
with scarce amounts of pyrite-pyrrhotite and chlorite. Carbonate cements include: Eogenetic siderite (S1),
mesogenetic and post-compactional calcite, Fe-dolomite, ankerite and siderite (S2). Early siderite and chlorite
helped to retain porosity by supporting the sandstone framework against compaction. Dissolution of feldspars
and carbonate cements are the main mechanism for secondary porosity development during mesodiagenesis.
The high intergranular volume (IGV) of the sandstones indicates that cementation is the predominant contributor
to porosity loss in the reservoir and that the precipitation of the carbonate cement occurred in early and late
diagenetic stages. The stable–isotope composition of the S1 siderite is consistent with precipitation from meteoric
waters in fluvial sandstones, and from mixed meteoric and marine waters in transitional sandstones. The
low δ18O‰ values of some of these carbonate phases reflect the replacement and recristalization from S1 to S2
siderite at deep burial and high temperature. Textural evidence, together with a low Sr content, also suggests
that siderite (S1) in fluvial environment is an early cement that precipitated from meteoric waters, near the sediment/
water interphase, followed by the generation of calcite with a higher Fe and Mg content. 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 progressive decrease in δ18O values in all carbonate cements could be related to the
continued precipitation at different temperatures and burial depth. |
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