Abstract

A palynological investigation was conducted to determine the paleoenvironmental setting and kerogen type of 20 core samples from the Cretaceous Mancos Shale taken from the 148M SJ 28-6 Unit in the San Juan Basin of northwestern New Mexico. The Mancos Shale Formation was deposited along the Cretaceous Western Interior Seaway and ranges in thickness between 450–600 meters (1,500 – 2,000 feet) across the San Juan Basin. The Mancos Shale remains untapped due to the shadow of coalbed methane in the San Juan Basin, but with new shale technology and advanced hydraulic fracturing techniques, the unconventional target has sparked interest among several oil and gas companies. A detailed palynological study on the Mancos Shale can assist in understanding the potential of the formation in the San Juan Basin. Three main groups of palynological elements were identified: phytoclasts, palynomorphs, and amorphous marine organic matter (AMOM). Two marine palynofacies were recognized within the studied interval. Palynofacies 1 (oldest in age), is primarily composed of phytoclasts (average abundance 53.7%), AMOM (40.3%), and palynomorphs (6.0%). The paleodepositional environment for this palynofacies is interpreted to be dysoxic, outer shelf with a pronounced terrestrial influence. Palynofacies 2 is dominantly made up of both well-preserved and poorly preserved phytoclasts (average abundance 60.6%), opaques (28.8%), AMOM (9.4%), and palynomorphs (1.2%). An environmental transition to a proximal, shallow marine environment with greater terrestrial influx and more oxic conditions is apparent in this palynofacies. The deposition of the Mancos Shale within the studied interval displays an overall progradational trend with a brief transgressional occurrence. The organic matter constituents in Palynofacies 1 suggest type-II/III kerogen (mostly oil prone), while those of Palynofacies 2 indicate type-III kerogen (gas prone). Palynomorph coloration within the entire studied interval lead to an interpretation of thermally mature organic matter. Geochemical analyses performed on 10 samples confirm palynological data and provide additional evidence for possible hydrocarbon production.