Pore characterization and FE-SEM analysis of the Niobrara Formation in the Aristocrat Angus PC H11-07 core, Wattenberg field, Denver Basin, Colorado

Abstract

The Niobrara Formation in the Denver Basin is a prolific unconventional, self-sourced oil and gas play in Wattenberg Field, Colorado. It is divided into two members: the Fort Hays Limestone at the base and the overlying Smoky Hill Member. The Smoky Hill Member is further divided into seven chalk-rich and marl-rich intervals. The organic-rich marls within the Niobrara Formation are source rocks while the chalks are reservoirs. The Wattenberg Field in the Denver Basin has been productive for 50 years. Due to the little research that has been performed over the last 50 years, this project aims to give a comprehensive account of the pore system and storage capacity in the Niobrara Formation. Qualitative data was collected by eld emission scanning electron microscope (FE-SEM) analysis, while quantitative data was collected by quantitative image, nitrogen gas adsorption, mercury injection capillary pressure (MICP) analyses, and liquid saturation and immersion experiments from the samples of the Aristocrat Angus PC H11-07 core. FE-SEM analysis was performed on sixteen representative samples of the Niobrara Formation in the studied core. After a detailed FE-SEM analysis of the samples from the chalk and marl intervals, matrix-related interparticle (interP) and intraparticle (intraP) pores, and organic matter (OM) pores were the most common pore types observed in this research. Fracture-related pores were only identied in the B and C Chalks of the studied core. Additionally, the FE-SEM analysis results show that the transition from euhedral to anhedral micrite moving deeper in the Smoky Hill Member implies the the porosity decrease from shallow to deep within the Smoky Hill Member. The quantitative image analysis data indicates that the pore sizes ranged from 0.085 m - 368.600 m in the chalks and 0.166 m - 221.600 m in the marls. Porosity values reported from MICP analysis agree with those determined by liquid saturation and immersion experiments. This indicates that using kerosene as the immersion liquid is more suitable than DI water on the Niobrara Formation samples. Furthermore, permeability values obtained from MICP analysis show a high correlation with porosity values obtained by the same analysis. Pore size distributions were determined by MICP, and nitrogen gas adsorption analyses in order to capture micro- and nano-pore distributions. According to the nitrogen gas adsorption results, a tri-modal pore size distribution is present within the chalks and marls of the Niobrara Formation with a major peak at 10-20 nm, a small peak around 6 nm, and a big peak at 30-40 nm. In addition, the most abundant pore type observed by the MICP analysis is nanopores (1nm-1m). This suggests that the nanometer size pores are the main contributors to the porosity of the Niobrara Formation in the studied core. Summing up the results, it can be concluded that the A Chalk is the best reservoir, while the A and C Marls are the best source rocks in the Niobrara Formation of the studied core.