Implications of Solid Bitumen in Source-rock Reservoirs

Abstract

This work reviews the significance of solid bitumen with emphasis on source-rock reservoirs. Discrepancies with terminology will be discussed in the context of difficulties in conforming to chemical-, solubility-, or origin-related definitions. Based on these difficulties we suggest adapting the reflectance of solid hydrocarbon as a practical choice for placing the boundary between solid bitumen and pyrobitumen, and 1.50 % is proposed as the boundary value, with a possibility of shifting this boundary down to 1.3% for sulfur-rich kerogen. Recently, much progress has been made by combining imaging and physical adsorption techniques in porosity studies, and the porosity of solid bitumen will be given a special emphasis in this presentation. Comparing pore characteristics obtained from SEM versus those generated by gas adsorption, mercury intrusion, or neutron scattering techniques indicates that the SEM pore inventory fails to account for the smallest pores (<5 nm in size) mostly occurring in organic matter. Therefore, low-pressure CO₂ adsorption is still the most effective technique to assess microporosity (pores <2 nm in diameter) in shales. We conclude that combining observational in situ techniques with techniques based on physical principles is necessary to make progress toward a better understanding of porosity systems in organic matter, including the porosity of solid bitumen. We will review the implications of the abundance of solid bitumen on reservoir quality, porosity, permeability, and producibility, based on examples of selected sequences. One of the difficulties in predicting the influence of solid-bitumen-bearing horizons on reservoir quality arises from the problems with detecting organic phases using various logging techniques. The use of specialized techniques such as NMR logging that allows two-dimensional T₁ and T₂ measurements should be expanded, and other potential techniques need to be further researched and tested. Certain properties of solid bitumen that are not as well understood, such as its hydrocarbon generation potential or its role in hydrocarbon migration will also be discussed with the aim of identifying further research that could lead to a better understanding of the role that solid bitumen plays in unconventional reservoirs.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019