--> --> The Eagle Basin of Northwest Colorado: A Compartmentalized Evaporite Basin and Large Hydrocarbon Frontier

AAPG Rocky Mountain Section Meeting

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The Eagle Basin of Northwest Colorado: A Compartmentalized Evaporite Basin and Large Hydrocarbon Frontier

Abstract

During the past two decades we have witnessed significant changes regarding recognition of low-permeability sandstone, carbonate, and shale reservoirs as desirable exploration targets. The interplay of source-rock geochemistry, structural geology, burial history analyses, and reservoir diagenesis studies have revolutionized exploratory techniques in these tight rocks. The Eagle Basin, a Pennsylvanian (predominantly Desmoinesian aged) evaporite basin located in northwestern Colorado, contains widespread carbonate rocks and black shales that are exploration targets for oil and gas reserves. During Pennsylvanian time the Eagle Basin was part of the Central Colorado Trough, which was bounded by ancestral mountain-range uplifts. The thick sequence of Desmoinesian cyclically deposited evaporites, carbonates, and black shales, commonly referred to as the Minturn Formation and its equivalents, bears a striking resemblance to the coeval Paradox Formation of the Paradox basin. Normal marine and penesaline facies are found flanking hypersaline halite facies in both basins. Over the past decades, various interpretations regarding the source rocks for Minturn oil have been discussed. Recent studies reveal that the productive sections of the Minturn Formation consist of low porosity, self-sourced, and likely over pressured carbonate rocks. Geologists have assumed that the northwest portion of the Central Colorado Trough is one large evaporite depocenter. By using conventional 37 Wyoming Geological Association – September 15-18, 2019 well logs and subsurface rock samples, the Desmoinsian aged section of northwest Colorado has been subdivided into individual rock packages. The Eagle Basin is here interpreted to be a series of four or five smaller basins, each basin having served as a center for halite accumulation. Halite sub basin margins are attractive hydrocarbon targets where fault related fracturing and hydrothermal activity have enhanced reservoir permeability. Reserve estimates can be high with one specific prospect reserve estimate in excess of 400 million barrels of oil equivalent.