A Sedimentologic, Sequence Stratigraphic, and Ichnologic Characterization of the Cretaceous U and M2 Member, Napo Formation, Oriente Basin of Ecuador

Please join us for a graduate student seminar this Friday November 1 at 3:30 pm in rm 155 Geology presented by Mayra Zuniga, MSc candidate:

A Sedimentologic, Sequence Stratigraphic, and Ichnologic Characterization of the Cretaceous U and M2 Member, Napo Formation, Oriente Basin of Ecuador

The M2 and U members of the Cretaceous Napo Formation are prolific producers of hydrocarbon in the Oriente Basin, Ecuador. To understand the depositional origin of this reservoir rocks, a detailed sedimentological, stratigraphical, and ichnological study using 490 ft (~149 m) of conventional core from six wells was done. This study reveals eleven detailed lithofacies: (1) Fine-to medium-grained massive sandstone (Proximal Delta Front), (2) Trough cross-bedded, coarse-grained sandstone (Fluvial channel fills), (3) Calcareous sandstone (Transgressive deposits), (4) Bioturbated, very fine-grained silty sandstone and mudstone (Tide-influenced Prodelta), (5) Bioturbated, medium to coarse-grained sandstone (Distal Delta Front), (6) Cross-stratified glauconitic medium-grained sandstone (Tide-influenced distributary bars, (7) Medium to coarse-grained cross-stratified sandstone with mudstone drapes (Tidal channels and bars), (8) Wavy and lenticular bedded mudstone and silty sandstone (Floodplain), (9) Interbedded medium-to coarse-grained sandstone and mudstone (Tide-influenced mouth bar), (10) Mudstone (Muddy Prodelta to Muddy Offshore), and (11) Bioclastic wackestone (Transgressive deposit).

The presence of mudstone drapes on bedform, facies, double mudstone layers, and thick-and-thin rhythmic alternations of silt and clay layers suggests tidal influence. The beds with reverse to normal grading indicate waxing and waning periods of discharge, reflecting typical deposits of hyperpycnite flows and indicating the influence of fluvial processes. Moreover, intrasequence erosional contacts reflect the abrupt deltaic-lobule direction changes during the deposition of the M2 Member. In contrast, wave-generated structures are conspicuously absent. The low ichnodiversity, sporadic substrate colonization, reduced trace fossil size, of suspension feeders, reduction in bioturbation intensity, and presence of opportunistic trophic generalists indicates brackish-water conditions. Specifically, a mixed tide- and deltaic environment is proposed for the M2 Member.