High-resolution Sequence Stratigraphy of the Southwestern Llanos Basin, Colombia: An integrated analysis
Creators: Valderrama Lopez, Yahir A.
Publisher: name: University of Alberta Library
Year: 2024

<p>This research aims to enhance the understanding of the Upper Cretaceous sequence in the southwestern area of the Llanos Foreland Basin, Colombia. The core of this study revolves around the analysis of seven cored locations, which cover nearly the entire Cretaceous sequence. In addition to core data, gamma-ray logs, palynological data, micro-resistivity image logs, and formation pressure data were utilized. These diverse datasets were analyzed using an integrated approach that combines sedimentology, stratigraphy, palynology, and ichnology, ultimately leading to the development of a robust sequence stratigraphic framework.</p>

<p>The Upper Cretaceous wedge in the southwestern Llanos Basin is composed of three recognized formations: Une, Chipaque, and a portion of the Lower Guadalupe. The Upper Cretaceous sequence is differentially bounded by several subaerial unconformities primarily related to the evolution of the Cretaceous back-arc basin and to the compressional phases during the Cenozoic period. The relative chronostratigraphic framework (from palynology) indicates that the Cretaceous wedge sequence in the studied area corresponds to the Middle Cenomanian through the Middle Campanian.</p>

<p>The Une and Lower Guadalupe formations correspond to classic terrigenous coarse-grained reservoirs, while the Chipaque Formation is primarily a fine-grained source rock. However, at the base of the Chipaque Formation, there are some coarse-grained bodies embedded within a fine-grained matrix. In general, the rock record exhibits a wide variety of facies. The Une Formation is primarily composed of tabular and trough cross-bedded sandstone beds. In contrast, the Chipaque Formation displays a more pronounced facies variability, characterized by heterolithic rocks, dark gray mudrocks, and sandstone beds with variable stratification. The Lower Guadalupe Formation shows similar sand-to-mud ratios as the Une Formation; however, the monotonous presence of tabular and trough cross-bedding is no longer observed, instead additional internal sedimentary structures are found.</p>

<p>Fluctuations in the ratio of continental to marine palynomorphs suggest variations in proximity to, and/or influence from, continental or marine environments within the three main formations. For the Une Formation, palynological data suggest a continental to continental marine-influenced setting. While the Chipaque Formation is interpreted as a marine to marginal marine setting, though some minor intervals can be seen as continental marine-influenced. For the Lower Guadalupe Fm., a continental marine-influenced to marginal marine setting is interpreted.</p>

<p>The integration of palynology, sedimentology, stratigraphy, basic ichnology, and sequence stratigraphy allows us to propose the following depositional settings for the studied units: an alluvial setting for the sparse deposits at the base of the sequence; a lower and upper delta plain for the Une Formation; a transgressive brackish water tidally-modulated setting for the lower section of the Chipaque Formation; a regressive proximal offshore/embayed area setting for the upper section of the Chipaque Formation; and a lower delta plain to upper delta front for the portion of Lower Guadalupe Formation.</p>

<p>Bioturbation within Chipaque Formation primarily occurred as an opportunistic behavior, characterized by diminutive forms and low diversity. The intensity of bioturbation increased seaward (toward the west and northwest) and decreased landward (toward the east and southeast).</p>

<p>Paleocurrent measurements from Une, Chipaque, and Lower Guadalupe formations supported by core data, indicate that the main sediment transport directions were toward the west and northwest, this means that the depositional settings toward the east and southeast became increasingly continental-influenced or dominated, while those to the west and northwest were more marine or marine-influenced. Inherently, the paleocurrent data suggest that paleocoastlines were likely oriented (NE-SW, NNE-SSW) perpendicular to the sediment transport directions.</p>

<p>In terms of sequence stratigraphy, the basal lowstand systems tract represents the sedimentation’s onset of the Upper Cretaceous, followed by a transgressive systems tract corresponding to the lower section of the Chipaque Formation, and a highstand systems tract associated with the upper section of the Chipaque Formation. The intra-Campanian subaerial unconformity, occurring landward of the proto Guaicaramo fault system, marks the presence of a falling-stage systems tract. Subsequently, after a period of sediment bypass and erosion, sedimentation resumes to the area with a lowstand systems tract represented by the Lower Guadalupe Formation. The proposed sequence stratigraphic framework from this effort, explains the geological processes before, during, and after the sedimentation of the three formations that comprise the Upper Cretaceous wedge, and also predicts reservoir spatial variations, including the occurrence of forced regressive deposits westward of the proto Guaicaramo fault system.</p>