In a previous blog on unconventionals, ”Conventional vs. Unconventional Shale: What is my Reservoir?,” Richard Day wrote about the nontrivial problem of classifying reservoirs as conventional or unconventional formations. I would like to continue this topic, as, in Europe, this issue has made it into the headlines of local newspapers. People in small villages have become “experts” in the field of geology, and believe they can determine whether exploration is for conventional or unconventional hydrocarbons, and whether it threatens their tranquillity. If they deem it so, from England to Poland, they voice their concerns.
Personally, if I had a choice, I would prefer to have unconventional drilling in my backyard rather than conventional. The high environmental standards and restrictive regulations give more guarantees that unconventional drilling is more secure and environmentally friendly than conventional drilling. But, sometimes, local people are afraid of whatever we call “shale.” Here, I would like to show examples of rocks that do not meet the definition of shale, but are still perceived as shale. Definitions can be misleading, and the nature of shale is more complex than people believe.
If you are looking to find the correct definition of “shale,” you are doomed to fail. There are many different definitions of shale in books and papers, and even more on the Internet. With so many definitions of what “shale” is, people have a tendency to choose the one that best fits their needs. If you are an operator, and your country has banned shale gas drilling, you can certainly find a definition that bypasses your type of shale. For instance, you could say, “My reservoir isn’t shale because it produces from sandstone intercalations.” We know that sandstone intercalations are quite common within producing shale gas plays in the U.S. – yet, no one is arguing to change the play classification from shale to conventional.
Recently, I was forced to change my presentation because I used the word “shale” for a rock containing over 45% clay minerals (as was reported in an X-ray diffraction (XRD) test and consistent with my petrophysical analysis), but the operator was wary of naming this rock as a “shale.” Shale can be defined as: ”Shale is laminated, indurated (consolidated) rock with > 67% clay-sized materials.” Jackson, J.A. (1997). Glossary of Geology, 4th Ed., American Geological Institute.
While it is always good to have reliable sources of knowledge, please take a look at the mineral composition of known shale gas plays in the U.S., as presented in Fig. 1 – which shows that almost none of the U.S. shale gas plays meet the criteria of the definition given above. According to this definition, there are no shale gas plays in the U.S. “Houston, we have a problem …”
Who knows more about rocks than petrophysicists? We are armed with the tools and methods to calculate rock properties. Today our triple-combo logging, Chi Modeling® Logging Simulation Software Service, tells me more about rocks than old definitions and out-of-date technologies.
Even having such great weapons as modern petrophysics with wireline openhole logs, cased-hole logs, and lab tests, the question is still: ”Can we define shale at all?“ Hmm … My professional answer is that I doubt it, because there are plenty of shales. Some of them are purely quarzitic, while some are purely calcareous in their characteristics – but the average geologist can get a pretty clear idea of what a shale is composed of, just from an outcrop and a core.
Now it’s time for a simple quiz. The example from Fig. 2 shows a ShaleXpertSM Integrated Analysis Solution interpretation of a certain well. Surely, you will be able to recognize the conventional reservoir. As for the second one, how would you classify it? Be brave in your assessment.
Congratulations! Yes, you found conventional sandstone with a nice porosity of about 24% (last track, purple curve) saturated by gas (before last track, blue curve) in the bottom of the well. While this analysis was relatively easy, please take a look at the uranium track. High uranium content points to the existence of organic matter in the source rock, which we call kerogen.
In this particular example, as the kerogen is immature, the gas must come from other sources – so perhaps the gas is biogenic and not thermogenic? The situation is getting complex, and the conventional sandstone is not conventional anymore. But really challenging is the zone in the middle of the picture. This thick zone has a shale volume of about 45% and porosity around 15%, as well as some component of an immature organic matter. This zone is 50% saturated by gas, and the rest is 50% water (irreducible water, we wish). Moreover, because of fine grain and high clay mineral contents, this zone needs to be stimulated to enhance production. So I would call this rock – shale! Fortunately, geology can help resolve this impasse that is so inconvenient for petrophysicists.
In essence, defining unconventionals is quite intricate work. As seen in the example above, the conventional sandstone can be defined as unconventional because of the presence of immature organic matter and can therefore be stimulated. At the same time, it can be characterized as conventional due to its ductility; you can’t frac it at all because all the energy you will pump into a wellbore to do hydraulic fracturing will turn into a plastic deformation of the rock. No fracture will be created. These intricacies are what make the study of geology so exciting!