Pore Systems of the Tow Creek Bench of the Niobrara Member of the Mancos Shale Group, Piceance Basin, Colorado

Burt, Connor Charles

Graduate Thesis Or Dissertation

Pore Systems of the Tow Creek Bench of the Niobrara Member of the Mancos Shale Group, Piceance Basin, Colorado Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/wm117p28d

Abstract

The Niobrara Member of the Mancos Shale is an unconventional gas reservoir in the Piceance Basin, western Colorado. The lower Tow Creek bench within the Niobrara member is a carbonate rich zone that is commonly targeted for production. This study focuses on three main questions: (1) what are the different pore types present in the Tow Creek landing zone, (2) do the varied lithologies, microfacies, geochemical proxies, and fabric elements influence pore types in that zone, and (3) do pore types vary laterally as a function of thermal history? .From all 54 available samples, eight samples (one in each well and a second in two wells), which best represent the full range of all geologic attributes, were selected for FIB-SEM analysis. All FIB-SEM images were segmented for pores and related materials based on grey scale values in Avizo Firetm. Measures of size, shape, and area of all segmented objects were extracted for use in data analysis. Pore types observed are divided in to two groups: mineral-associated pores and organic matter (OM) associated pores. Mineral pores have median equivalent circular diameters (ECD) that range from 79 to 353 nm, with a small number (6-12) of large pores (ECD > 500 nm) typically comprising 20% of pore volume. Matrix pores are more elongate than peloid pores, reflecting the presence of clay-associated pores in the matrix. No correlation between thermal history, microfacies, lithofacies, or geochemical facies was found with respect to mineral-associated pores. Peloid porosity average four times the porosity in the matrix, suggesting that peloid-rich microfacies offer the best potential pore systems. Samples originally had up to 9.5% open mineral-associated porosity, however, up to 80% of that pore space has been filled with residual hydrocarbons (RHC). OM pores occur within the RHC but the contribution of OM pores to total porosity is quite small (0 to 1.6%). OM pores have median ECDs that range from 29 to 63 nm at the fine scale, with no correlation in size to thermal history. The abundance of fine scale OM pores is lowest in the lowest maturity wells but no other trend is present.

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