There are two places to gather rock samples: above ground and below ground. Above ground is referred to as outcrop and below ground rocks are in situ. When you think outcrop, think of rocks exposed at road cuts. Rocks in situ, in my world, contain oil and natural gas in their pore spaces.
For different reasons, in my doctoral research I use of both sources of rocks. The advantage of rocks found in outcrop is they are free. Provided I have permission of the land owner and I am not in a national park, I am free to pick up and take as many rock samples as I can carry. Prior to removing the rock mass I use a Brunton compass to determine my sample’s strike and dip. I also draw an arrow indicating which direction is up. Another outcrop advantage is easily seeing contact lines between formations, allowing rock samplers the ability to get exactly the formation they want for study.
The disadvantage of outcrop rocks is that they are typically weathered (increased friability, decreased competence), have been subjected to additional stresses as the formation moved above ground, and do not have the compressional stresses in situ rocks do. Among other things, compressional stress on in situ rocks helps maintain the fluid found in the rock pores (i.e. oil and natural gas). Despite the drawbacks, when studying the mechanical and elastic properties of a specific formation, rocks found in outcrop are a good start.
In situ rocks typically come to surface in the form of core. Occasionally drill cuttings brought to surface from formation are large enough to be studied, but this is rare and there is no possibility for sample orientation (which side faced up, how the bedding planes dipped, or the strike). Drilling for core is extremely expensive. It requires a drilling rig, a coring bit, and much more time than just trying to drill the wellbore.
Cores are fragile (4 inches or less in diameter), need to be taken slowly, and only 30 feet at a time. Once the core barrel is full, the barrel is brought to surface sometimes through thousands of feet of overburden. If the desired formation is 700 feet thick, will all of the formation be cored? If not, how much and which portion of the 700 feet? On occasion, the core is immediately moved to a pressurized, impermeable core holder to preserve the integrity of the specimen, adding to the expense. Core can even be oriented in the wellbore before it is brought to surface, an additional expense.
Core advantages are the reciprocal of outcrop disadvantages: until recently, the rock sample has been in it’s natural environment. Besides temperature, fluids, and stress remaining constant, the effects of weathering no longer add uncertainty.
How is an unfunded, impecunious graduate student to get her hands on the prized core for study? This morning I suited up and headed north with my mentor to make my pitch. My mentor knew of an energy company looking for a researcher who could both study the mechanics of the shale and produce technical publications based on their findings.
I will keep you posted.