Microfractures in black shales and their transport properties

During erosion of continents and deposition of sediments and organic matter in shallow seas, sedimentary layers form, creating the so-called sedimentary basins. With burial of these sediments, their organic and non-organic grains are compacted and heated, leading to their transformation into solid rocks. With temperature increase, the organic matter, initially present into what geologist call a "source rock" layer, will mature and evolve into oil and gas. However, these source rocks are known to be very resistant to fluid transmission; they are impermeable. The question arises then on how oil and gas produced in such impermeable rocks can escape and migrate to reservoirs where they accumulate and can be exploited. This process is called primary migration of hydrocarbons. In this project, we test the hypothesis that during maturation of organic matter in source rocks, microfractures will nucleate, grow, then merge, to finally create a flow path allowing hydrocarbons to escape. For this, we have analyzed rock samples collected at several kilometers depths in the Norwegian Continental Shelf. We have also reproduced the process of organic matter maturation and rock fracturing in laboratory conditions. Finally, we have developed computer models of the coupling between organic matter maturation in source rocks, production of hydrocarbon, creation of microfractures, and escape of the produced oil and gas towards reservoirs.

The project has trained three early career researchers on topics related to shale maturation and primary migration hydrocarbon. Each of them has produced high quality articles in the best journals of Petroleum Sciences (e.g., Journal of Geophysical Research, Marine and Petroleum Geology, Fuel), and communicated results in both academics (EGU) and engineering (EAGE, SEG) conferences. An important outcome is the definition of a new rock physics template for organic-rich shales of the Norwegian Continental Shelf that can be used directly by the industry. The project has also imaged unique core samples from two boreholes drilled in Jurassic source rocks off-shore Norway using state-of-the-art synchrotron X-ray imaging. Finally, an important outcome of the project is that, based on the competences in synchrotron imaging, a highly competitive European Research Council Advanced Grant has been obtained by the principal investigator, to study rock deformation processes.

The aim of the PROMETHEUS project is to create novel structural and mechanical knowledge and geological understanding of microfracturing and dynamic permeability in black shales in order to provide a strong scientific foundation for considerably increasing hydrocarbon exploration success on the Norwegian continental shelf that contains shale formations that act as source rocks or the cap rocks of many reservoirs. Important source rocks include the Draupne and Tau formations in the North Sea, and the Hekkingen formation in the Barents Sea. Currently, exploration risks in these segments of the Norwegian margin are quite high because: (1) our mechanical understanding of black shales is limited, and (2) our knowledge of the structures of these rocks on short length scales is incomplete, because they can only be studied from seismic or borehole data with sub-meter to meter resolution; whereas their mechanical and transport properties depend on scales down to micrometers or even nanometers. The PROMETHEUS project aims to overcoming these challenges through state-of-the-art multidisciplinary research that integrates: (1) quantitative analyses of the structure and composition of shale rock samples obtained from outcrops and borehole cores, (2) cutting edge quantitative laboratory experiments using the latest technical developments at the University of Oslo, in collaboration with the European Synchrotron Radiation Facility, and (3) numerical modeling of shale deformation that includes discontinuities and multiphase flow. The results from the PROMETHEUS project will: (1) considerably improve our understanding of microfracturing and dynamic permeability in black shales, and (2) facilitate the assessment of cap rock integrity and hydrocarbon expulsion capacity of tight rocks. The project results thus have the potential to become strategic for substantially enhancing hydrocarbon exploration prospects in the as yet poorly explored Norwegian tight rocks.