Provenance, geochronology and sedimentary characteristics of the Campanian M1 Sandstone, Napo Formation, Oriente Basin, Ecuador

Lin, Sinong
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The Campanian-Maastrichtian M1 Sandstone is one of the major oil reservoir units of the Napo Formation in the Oriente Basin of Ecuador. Heterogeneous reservoir behavior suggests that the depositional environments of the M1 Sandstone were laterally variable, such that they generated complex reservoir compartmentalization. This complexity also makes it difficult to determine whether multiple sandstone units of 5-20 m thickness represent only the M1 Sandstone or a composite stratigraphic interval that includes both the M1 Sandstone and the basal Tena Sandstone. A combined provenance and geochronological study was undertaken to complement sedimentological observations and to understand the regional depositional setting of the M1 Sandstone. Detrital zircon U-Pb dating was applied to 24 core samples from across the entire Oriente Basin; most samples were from accepted M1 Sandstone horizons, but some were taken from levels thought to be the basal Tena Sandstone. Probability density plot (PDP) curves depicting the U-Pb age distributions of the samples strongly suggest that Napo sediments are mainly derived from the Amazonian Craton, with the largest contribution from Rio Negro-Juruena Province (1.78-1.55 Ga) and Rondonian-San Ignacio Province (1.55-1.30 Ga). Moreover, different reservoir compartments classified as the Main M1 and Upper M1 units display distinct local signatures dominated by either the Rio Negro-Juruena or Rondonian-San Ignacio province. An important point is that the U-Pb zircon results indicate that all M1 Sandstone samples in both the central and western Oriente Basin were derived from the craton, albeit likely reworked within the basin by tidal currents. This result is surprising because previous stratigraphic models have interpreted some of the uppermost sandstone units, especially in the westernmost and central areas of the basin, to be the basal fluvial Tena Sandstone, a Maastrichtian-Paleocene unit that unconformably overlies the M1 Sandstone. Further analysis of the contrasting age signatures for the M1 and Tena sandstones, together with well log data (from about 500 wells), core descriptions and core interpretations allow a reconstruction of sediment transport patterns and a new M1 Sandstone depositional model that recognizes a landward-offset stacking of both estuarine (transgressive) and deltaic (regressive) intervals