Palynostratigraphy of the Maastrichtian Umir Formation, Colombia: An Approach to Reduce Exploratory Risk

The Middle Magdalena Valley Basin (MMVB) petroleum system is one of the most productive systems in Colombia. Although oil exploration in the area generally targets reservoirs associated with Cenozoic clastic sequences, ongoing exploration focuses on Cretaceous non-traditional reservoirs. Recent new targets are located in cross-stratified sandstones in the upper Maastricthian Umir Formation, a unit generally considered to be a highly effective regional seal. Structural complexities and the presence of a large regional unconformity obscure the regional distribution of the oil-bearing sandstones and the exploratory risks are high. The depositional environment for the Umir formation is interpreted as a bay or extensive coastal plain where terrestrial (pollen and spores) and marine palynomorphs (dinoflagellate cysts and acritarchs) are abundant; hence palynology is the best biostratigraphic proxy to be used. Here we present the organic-walled microfossil biozonation that we are developing for the Maastrichtian Umir Formation. The study area is located in Central Eastern MMVB, on the western side of the Los Andes syncline. Ninety-nine samples from four cores covering a stratigraphic thickness of 2990 feet are being analyzed for palynological content. The samples were prepared in the Colombian Petroleum Institute, and standard palynological analyses are being conducted at Louisiana State University. Thus far, the palynological assemblage is characterized by the occurrence of key Maastrichtian markers from Northern South America such as Echimonocolpites protofranciscoi, Proteacidites dehaani, Buttinia andreevi, Echitriporites trianguliformis, Crusafontites grandiosus, Bacumorphomonocolpites tausae, Syndemicolpites typicus, Tetradites umirensis, Ulmoideipites krempii and Monocolpites grandispiniger. A predominant coastal plain environment is suggested by the abundance of several species of Echimonocolpites, Gemmamonocolpites, Monocolpites, Psilamonocolpites and Spinizonocolpites genera, and a constant and abundant record of Psilatriletes group. Spikes in abundance of dinocysts and foram linings evidence six marine-influenced events. The excellent recovery of palynomorphs shows potential to find new events for Maastrichtian biostratigraphy of Northern South America. Major biostratigraphic events will be used to improve geological models, reduce the exploratory risk and will serve as a key tool for on-site biostratigraphic control of exploratory wells.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California