unknown sedimentary rock

The particles in this rock cannot be seen with the naked eye. Even use of a hand lens doesn’t reveal the particles. To the touch the entirety of this rock feels very smooth. One notable feature is that this rock tends to form in a flat sheet unlike others. This rock also has a variety of others beyond what is shown in the sample. This rock does not dissolve with acid. This rock does not taste salty. On a rare occasion darker colored rocks of this type in reality smell like petroleum or tar. There are no fossils present in this particular rock; however, at times other rocks of this exact type contain very nicely preserved fossils. rocksandminerals CC BY(4.0) via sketchfab [https://creativecommons.org/licenses/by/4.0/legalcode]