A SEDIMENTOLOGIC, SEQUENCE STRATIGRAPHIC, AND ICHNOLOGIC CHARACTERIZATION OF THE CRETACEOUS U AND M2 SANDSTONE MEMBERS, NAPO FORMATION, ORIENTE BASIN OF ECUADOR
Zuniga Albuja, Mayra Alejandra
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The Upper Cretaceous M2 and U Sandstone members of the Napo Formation are prolific hydrocarbon producers in the Oriente Basin, Ecuador. To understand the depositional origin of these reservoirs, a detailed sedimentologic, sequence-stratigraphic, and ichnologic study was performed, using 490 ft (ca. 149 m) of conventional core from six wells. Sedimentary facies, stratal stacking pattern, discontinuity surfaces, and trace fossils were documented. Nine lithofacies, two depositional sequences in each member, and depauperate and fully marine ichnofacies were identified. Both members present evidence of tidal (e.g. mudstone drapes on bedforms, double mudstone layers, flaser, wavy, and lenticular bedding, and thick-and- thin alternations of siltstone and claystone layers) and river (e.g. hyperpycnal flow deposits) influence. The shoreline was trending northeast-southwest within the study area and the predominant sediment source came from cratonic areas located to the east. In the study area, the U Sandstone Member represents three main broad environments: fluvial, estuarine, and deltaic. The base of the U Sandstone Member marks the base of depositional sequence 1 (DSU1), representing a subaerial unconformity formed as a result of valley incision during a relative fall of sea level. DSU1 comprises the lower and middle intervals and the lower part of the upper interval. DSU1 consists of lowstand moderate-sinuosity fluvial deposits, followed by transgressive estuarine deposits and highstand mixed tide- and river-influenced deltaic deposits. A subaerial unconformity marks the base of depositional sequence 2 (DSU2), which was followed by renewed lowstand fluvial deposition within an incised valley in the more proximal areas and transgressive estuarine sedimentation. DSU2 is recorded in the upper part of the upper interval. Trace fossils in the U Sandstone Member are recorded in the estuarine and deltaic deposits; fluvial deposits present sparse bioturbation. The depauperate Skolithos and Cruziana ichnofacies are commonly present in the estuarine and deltaic deposits, recording brackish-water conditions. The M2 Sandstone Member records sedimentation in a mixed tide- and river-influenced deltaic environment, encompassing delta front and prodeltaic subenvironments, as well as transgressive deposits signaling deltaic abandonment. Two depositional sequences have been recognized (DSM1 and DSM2). The underlying A Limestone Member, which pinches out towards the east, most likely represents the transgressive systems tract (TST) of DSM1. Deposition may have been controlled by an interplay of eustatic changes, tectonism, and active volcanism. This member consists of discrete thickening- and coarsening-upward packages that may represent either parasequences or intervals recording delta lobe switching. The parasequence sets exhibit progradational-stacking (seaward) patterns and have clinoformal geometry that exhibit both vertical and lateral facies changes. Various degrees of biogenic reworking are recorded (BI 1-6), commonly in the sandstone-dominated facies, generally representing the depauperate Cruziana Ichnofacies, indicative of brackish-water conditions. Integration of ichnology, sequence stratigraphy, and sedimentology was fundamental in order to provide detailed paleoenvironmental models for the U and M2 Sandstone members. This study represents the first detailed ichnologic study in Ecuador. It is expected that this research will encourage geoscientists in the country to adopt these conceptual and methodological tools in reservoir characterization.
DegreeMaster of Science (M.Sc.)
CommitteeMángano, Maria Gabriela; McBeth, Joyce; Michaud, Kain; Ansdell, Kevin
Copyright DateAugust 2020