IMPROVING PALYNOSTRATIGRAPHY TO REDUCE EXPLORATORY RISK: A CASE STUDY FROM THE LATE CRETACEOUS UMIR FORMATION IN COLOMBIA

The Middle Magdalena Valley Basin (MMVB) is one of the most productive oil basins in Colombia. Ongoing oil exploration in the region focuses on complex structural areas and non-conventional reservoirs. In MMVB Petroleum System, the Umir formation is a highly effective regional seal, especially for structural traps involving reverse faults and Cenozoic reservoirs. The upper Umir sandstones have recently been considered as a reservoir in the Central Eastern Basin. But because of both large structural complexities and the presence of a major unconformity known as the Middle Magdalena Valley Unconformity (MMVU), the regional distribution of these oil-bearing sandstones is not well-known, and exploratory risk is high. This research focuses on developing a biostratigraphic framework for the Late Cretaceous Umir Formation using palynology. Major biostratigraphic events will be used to develop regional correlations, and serve as a key tool for on-site, real-time biostratigraphic control of exploratory wells. The Umir formation is composed of a series of dark grey to black shales, siltstones and coal beds, with cross stratified sandstones and intraformational conglomerates in the upper section. The depositional environment was a bay or extensive coastal plain. The study area is located on the western side of the Los Andes syncline, in Central Eastern MMVB. Eighty-two samples were selected from four cores, and are being analyzed for palynological content. The palynological samples were processed in the biostratigraphy Laboratory of the Colombian Petroleum Institute (ICP). Standard palynological analyses are being conducted at Louisiana State University (LSU) using a BX41 Olympus light microscope under a 60x oil immersion objective. A minimum of 300 grains is tabulated per slide. Thus far, some of the key Maastrichtian terrestrial palynomorphs recovered include Echimonocolpites protofranciscoi, Proteacidites dehaani, Buttinia andreevi, Echitriporites trianguliformis, Echitriporites suescae, Bacumorphomonocolpites taussae, and Monocolpites grandispiniger. Two layers with a marked marine influence have been recorded as well. The excellent recovery of abundant and diverse species shows potential to find new markers and events to improve the Maastrichtian biostratigraphy of Northern South America.