Autocyclic and Allocyclic Controls in the Fluvial Sedimentation in the Oligocene-Early Miocene Sequence, Middle Magdalena Valley Basin

Peralta-Vargas, Juan ¹; Rodriguez, Corina ¹; Suter, Andreas ¹; Gomez, Laura ³; Leal, Cesar A.²
(1) Schlumberger, Bogota, Colombia. (2) Ecopetrol, Bogota, Colombia. (3) Instituto Colombiano del Petróleo, Ecopetrol, Bucaramanga, Colombia.

Three fields located in the Middle Magdalena Valley Basin (MMVB) are a good example on how improved reservoir description and characterization can have an impact on production. Those fields have been studied intensively to understand reservoir architecture associated to fluvial deposition during Oligocene-Early Miocene. Those studies are restricted to this specific interval of sedimentation, even thought the sequence continues up to the Pliocene and Pleistocene with minor unconformities associated to tectonic events related to basin adjustments or accommodation. This paper shows the importance of including external process such as allocyclic controls, to construct and understand the architecture of the reservoir, in this case, modifying the fluvial sedimentation during the Oligocene-Early Miocene. Considering as well the autocyclic controls responsible of overprinting the sediments sequence. Examples about autocyclic controls can be observed in the modern fluvial deposits of the Magdalena River, along the MMVB, which is a reference for analogies and comparison effects. During the Oligocene, a large transgressive eustatic response has been recorded in the North of South America, reaching the maximum flooding level during Early Miocene that covered large areas in the Venezuela basins and also in Colombian basins such as Catatumbo, Llanos and Lower and Middle and Upper Magdalena. Even thought, local autocyclic process control parasequences deposition along the Oligocene-Middle Miocene, as channel abandonments, paleosoils generation, channel stacking and channel avulsion, governing the bed and bedset distribution in these three different fields, a very good example of sedimentation clearly controlled by allocyclic process, in connection with global events, corresponds to the early Miocene interval denominated La Cira Shale at the top of Colorado Formation in the MMVB. Integrated information such as core data, logs (conventional and special) and 3D seismic, has been the key to identify these processes, but also outcrops located nearby the fields in the same MMVB have allowed to recognize the different processes involved, that, environmentally, represent complex fluvial systems with a reservoir architecture controlled by accommodation space, sediment supply, and internal factors such as channel migration, in a large sequence with cyclicity. This fluvial sedimentation cycle, will finally be drowned by the allocyclic process associated to sea level change (rise).

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.