This is a full hole core of Serpentine silicate that I personally cut in 1993 in South Texas at a TMD of 3,800 feet. It is beautiful dark, molted green, hard as iron and, save the calcite filled natural fractures seen in the core, offers very little reservoir potential. The calcite was, however, HCL soluble.
Serpentine "plugs" in South Texas were actually late Cretaceous aged volcanic events, sort of like molten rock from a real volcano only it was extruded from a vent through basement rock into the sea where it cooled really fast and formed a big hump, or several humps. These subsurface humps could be found with areal magnetics in the late 1920s, were mapped and explored by old timers with big cajones.
Later in depositional time, after the plug was blown out, the sea moved back and forth over these big humps and sometimes good clean sediments sealed against the serpentine wall, not unlike sands pinched out against a salt pinnacle. These could offer traps for migrating hydrocarbons.
My well, above, was a "beach" sand test on the SE side of a serpentine plug. I was drilling dense shale then got a break in the penetration rate of the bit, circulated the well clean, came out of the hole and picked up a core barrel. We cored 40 feet of pretty clean sand, indicating an actual beach was there many tens of millions of years ago, but there was no seal and the sand was wet, or full of water. To see what the actual serpentine plug looked like under the beach sand, I cored 30 feet of it.
In some places in Central Texas the dense serpentine plug itself actually produced, like the great Humble Oil and Refining, Hilbig Field, southeast of Austin. That field made 7 MM BO from 2,500 feet and now acts as a gas storage facility for the Lower Colorado River Authority.
My well was a dry hole. I chased this stuff around a little bit and drilled four more dry holes on serpentine plugs in South and South Central Texas. Though many serpentine plugs were drilled in the 1930's and 1940's we thought as late as the 80's and 90's if we just got on the proper side of the plug, where all the wave's used to crash on the beach, we would find porous sand, hopefully with oil in it.
Never did. But man, it was sure fun. This is pretty close to true wildcatting, not quite, but close.
Torch Serpentine Plug, Zavala County, Texas. There is one beach sand well off to the west but this plug produced from the King Sand (Datum marker) draped over the top of the plug. Courtesy Baylor University. As you can clearly see, this was quite a "hump."
In the good 'ol days we stored all of these full hole cores, of whatever it was we were coring, in a core bank that was paid for by various Texas geological societies. If you belonged to, for instance, the South Texas Geological Society, like I did, you could get access to the bank and go look at cores whenever you wanted, indexed by dates, basins, regions, fields and formations. They were stored in long boxes.
I don't think anybody does much of this sort of thing anymore and I don't know where all those thousands of core boxes went. They are an insight into how the earth was made, I hope they are still available for people to see.