Lack of firm knowledge about major petrophysical properties of organic rich shales, despite their recent importance in production from self-resourcing accumulations has been a great challenge to our industry. There is no universal definition for the term " Shale" as it is used today: Shales as they are used nowadays encompass a wide range of compositions. This inconsistency arises mostly due to the lack of a more general term. Presence of clay minerals and organic matter combined with submicron grain sizes and nano scale pore diameters has demanded new techniques and technologies for exploration, reservoir characterization and production from these resources. Many of shale formations are the source rocks for conventional accumulations which store considerable volume of non-expelled hydrocarbon. Target zones in self-resourcing petroleum systems differ depending on the organic content, level of organic maturity, and kerogen type. At high TOC and low maturity levels, source rocks have very small effective porosity and sub-economical permeability. Hydrocarbon production from these resources requires in situ retort techniques. At high maturity level (gas condensate and dry gas window) economic hydrocarbon production occurs mostly as a result of higher effective porosity and smaller hydrocarbon molecular sizes.
The process of thermal maturation results in alteration of the physical properties of the kerogen and consequently the petrophysical properties of organic rich shale hosting the kerogen. Thermal maturation controls the storage capacities and hydrocarbon expulsion of the rocks. In order to use seismic and log measurements to locate organic rich rocks and to determine thermal maturity, we must understand organic richness and effect of maturation byproducts on petrophysical properties and seismic response of these sediments.
In this paper, we present our observations and analysis of some major petrophysical properties of organic rich rocks at different maturity levels. We found significant textural differences and mineral re-orientations in more mature rocks in comparison with immature shales. Laminated kerogen particles and clay minerals appear to be load bearing in immature state; they become more isolated, scattered and randomly oriented at higher maturity levels which decreases the anisotropy. Volume changes in kerogen also cause significant differences in bulk mechanical properties of the rock. Bitumen is produced at the early stages of maturation and significantly affects effective porosity. Bitumen occupies most of the porosity between clay minerals and intraparticle pores inside organic matter at early stages of oil generation. Lack of an exact cut off between fluid/movable organic matter and solid/part of the matrix organic components makes defining porosity a very challenging and subjective task.
The textural observations were integrated with analysis of a composite data set from the Bakken petroleum system that consists of well logs, RockEval, and mineralogy. These analyses help to understand how organic rich sediments evolve with burial and maturation and how hydrocarbons are stored and transported to sustain large storage, even at high overburden stresses.