Opportunity Abounds in Unconventional Reservoir Source Rocks

With the conversion to unconventional resource development, drilling and evaluating a horizontal shale (gas/oil) well, has become a very expensive proposition; requiring a better understanding of the Rock your production will come from.

When you read reports on a given play, you may see statements much like this: "Organic rich Type II Kerogen shale, with up to 7.2% TOC – total organic carbon (averaging 3.8% TOC). The rock is a mature source rock (in the condensate rich gas zone), with an average of %Ro = 1.11."


What does this mean, and how does it correlate in evaluating your play? Let’s have a look at the following, before proceeding any further.

If you are in a hydrocarbon productive basin, there will be a mature Source Rock someplace in the section. The terms Source Rock, Kerogen, and TOC (total organic carbon), are often referred to in the study of gas shales. To truly understand unconventional resource development {gas shales}, we need to consider what a shale is, what constitutes its makeup, and what distinguishes a gas/oil shale.

Shales are:

  • Fine grained sedimentary rock compiled of clay minerals, and microscopic fragments of other minerals such as quartz, dolomite, and calcite and varying amounts of dispersed organic matter.

  • Shales can appear as laminations, parallel to the bedding.

Gas/Oil Shales are distinguished by:

  • Gas shale’s contain adsorbed gas, and the adsorbed gas is proportional to the organic content of the shale.

  • Free gas is contained in the porosity, proportional to the effective porosity and gas saturation in the pores.

  • Higher radioactivity readings, due to uranium enrichment.

  • Clays that hold the bound water, while the Kerogen holds the absorbed gas.

Organic rich shales have been Source Rocks, providing other rock with oil and gas over time. These were formed as part of the depositional process of sedimentary rocks, and organic matter. Of prime interest in North America, are the marine - lipid and protein rich organic matter, contained within Type II Kerogen source rocks. A good example of this is the prolific Duvernay shale, which provided hydrocarbon to the Leduc and Nisku formations.

Kerogen is defined as the fraction of large chemical aggregates in sedimentary organic matter that is insoluble in solvent. Kerogen is classified by looking at the source material, using Pyrolysis, Vitrinite Reflectance (%Ro), or other lab procedures. Kerogen is the main source of TOC, and is radioactive due to Uranium salts.

Kerogen Type and Source:

  • Type I-algae

  • Type II-primarily plankton, minor algae.

  • Type III-higher plants

Kerogen Amount %, Type of Deposit:

  • Type I-II, ~ 1%, oil source rock

  • Type I-II, <50%, oil shale

  • Type III, >50%, coal

TOC (Total Organic Carbon) is the source of the hydrocarbons, and also takes up space. A geological model would show the conventional porosity holding the free gas and irreducible water, the clay holding the clay bound water, and Kerogen holding the adsorbed gas. TOC’s are measured in weight%.

Thermal Maturity is defined as the degree of heating of a source rock in the process of transforming Kerogen into hydrocarbon, and measured by Pyrolysis or Vitrinite reflectance.

With the above knowledge, we can better complete our homework when evaluating the play. By the use of: Logs, Cores, Geological Mud Logs, Gas data, and cutting descriptions, we can develop the ‘Big Picture’. These will be necessary for:

  • Understanding the geology within the region

  • Determining porosity (ø) and TOC (offsetting logs, Spectral Gamma Ray)

  • Geochemistry (cuttings and cores in the area)

  • Rock Mechanics (Sonic and Caliper logs)

These Unconventional plays will be expensive depending on depth and local. However, they can be within reach of a restrained budget, if the following criterion is met:

  • Initially, partner up; this may be the best initial move towards development

  • Drill a single vertical, ‘proof of concept well’

  • Determine the presence of mobile hydrocarbons, hydrocarbons in place, and level of production after the fracturing process

Unconventional Resource plays are risky; however, with accurate knowledge and understanding of the rock to be produced, the upside may be a prolific discovery well.