This is a full hole core of the Eagle Ford shale in E. Central Texas, likely from a depth of < 2,000 feet, not far from where the Eagle Ford outcrops on the surface. The core broke at a stress point because of the embedded fossil, the two halves match perfectly. Neato, uh?
Down regional dip, closer toward the Shelf Margin, where the Eagle Ford produces at 8,000-1o,000 feet TVD, it was more or less mud that washed down into the Gulf Coast basin some 80-90 million years ago and that mud was full of all kinds of rich, organic goodies, like this fossil, but all the other ingredients necessary to eventually make oil and natural gas. Then, in geological time, the overlying sea morphed itself (sic) into hard, dense carbonific limestone (Austin Chalk) that sealed over the mud. Millions of years of additional deposits on the sea floor put lots of overburden pressure on the mud, it got really hot down there, and the mud turned into dense shale. The Eagle Ford is the "kitchen" where all these organic goodies, heat, pressure and time, turned into hydrocarbons. Lots of those hydrocarbons then found a way to migrate up out of that Eagle Ford resource bed, to supply the overlying Austin Chalk, Taylor Sands (Olmos) and the thick Wilcox interval with oil and gas, some places anyway. A lot of that cooked oil and gas got left behind in the Eagle Ford, trapped. Now they frac the snot out of that dense, sometimes brittle mudstone and can wrench some grease out of it. Amazing, that. A tough way to make a living, if you are into that sort of thing, but nevertheless, amazing.