How Mobile Is Your Total Oil Saturation? SARA Analysis Implications for Bitumen Viscosity and UV Fluorescence in Niobrara Marl and Bakken Shale, Supported by FIB-SEM Observations of Kerogen, Bitumen and Residual Oil Saturations Within Niobrara Marls and Chalks

Abstract

Variations in UV fluorescence among a continuum of rock types from clean Niobrara Chalk through black Niobrara Marl led us to investigate the role of asphaltene% not only on quenching of UV fluorescence but, more importantly, on hydrocarbon viscosity. Stepwise reintroduction of separated asphaltenes into an asphaltene-free extract of Niobrara Marl demonstrates asphaltene quenching of UV fluorescence and associated progressive increases in viscosity, even when measured at full bottom-hole temperatures. This simple study raises questions about the validity of log and core-derived (solvent-based) So: is it mobile oil or highly viscous bitumen? Parallel nano-scale FIB-SEM investigations of depressurized/degassed core samples suggest that contextual and morphologic distinctions are possible among kerogen, bitumen, and residual oil saturations (Sor). Our earlier SARA extraction work on the Bakken Shales demonstrates extreme differentiation in Asphaltene% between extracts from Upper and Lower Bakken shales versus from the intervening Middle Bakken reservoir cores as well as from produced fluids, to the point that we began to strongly question the relevance of source rock “So” derived from log and core analyses because its producibility was highly questionable due to high viscosity. The Niobrara, by contrast, shows a complete continuum between clean chalk and source rock “marl”, also expressed by gradational attributes such as gray-scale (% carbonate and %TOC), UV fluorescence, and Asphaltene%. This led us to pursue more involved extraction experiments to determine whether Asphaltene% impacts viscosity to the degree that we must ask what percent of OOIP is really Mobile Oil in Place within Niobrara Marls? SEM imaging suggests that kerogen, bitumen, and residual (mobile) oil saturation (“Sor”) can and should be differentiated. Not only does this have implications for flow capacity from source rocks in the oil window, but it has important implications for “Mobile” Original Oil in Place (OOIP) Calculations, which in turn impact expected and observed Recovery Factors. We need to understand bitumen evolution every bit as much as kerogen evolution when exploring for and exploiting tight oil plays. At reservoir conditions, is produced oil in full solution with the bitumen we extract from source rocks and hybrid reservoir/source rocks or are they two distinct phases in the reservoir, with bitumen behaving more as an obstruction to flow that inconveniently calculates as So?

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016