There will undoubtedly be many new oil and gas discoveries as a result of the Eagle Ford shale drilling frenzy. These new discoveries will be made in geologic formations above where the Eagle Ford shale lies. I’m not making a bold prediction here, it is a fact. If you take an area that already has the geology for oil and gas, including stratigraphic and structural traps, (see below), and poke a few thousand more, very deep holes through it, across a wide area, you are bound to find more production. South Texas has produced much of the nation’s domestic oil and gas over the years, starting as far back as the 1930’s. Most of this came from traditional “traps” such as those pictured below.
How Most Oil Reservoirs Occur
Basically, ( and I am generalizing a lot here), layers of sedimentary rock are usually laid down in horizontal sections, over the course of millions of years. In South Texas, the Eagle Ford shale, though “only” 165 to 65 million years old, is one of the older rock formations bearing oil and gas. It lies beneath many younger rock formations which also bear oil and gas such as the Wilcox group. Only through geologic disturbances, such as faulting, salt domes intruding by forcing their way upward, tilting of rock formations, ancient river channels, reefs, etc, does the order and evenness of how sedimentary rocks are laid down get messed up.
Oil and Gas Traps
When this even layering of sedimentary rock gets messed up, it often forms an oil and gas “trap” in porous rocks such as sandstone and limestone. Denser rock layers such as shale, chert, etc, prevent the oil from migrating upward any farther. Oil and gas that has “cooked off” of organic rich shales underneath, fills up these traps. It’s not dinosaur bones that made the oil we use, it is the remains of trillions of dead plankton, algae, land based organic matter, and ocean creatures that once formed the very same kind of muck you wade through while fishing in Baffin Bay, or which is being deposited right now deep offshore of the Mississippi delta from sediment.
Through that method of “cooking off” of shale, and “trapping” in porous rocks above, do we get most of the oil reserves in the world. Shale oil plays, such as the Eagle Ford shale, are going directly to the source or the “mother rock”, where this oil and gas in the traps above came from in the first place.
Below is an illustration of the types of typical oil and gas “traps” that exist. This image is from the University of Wisconsin.
What will happen, (and is already happening to a degree), is that there will be many new oil and gas discoveries made as a result of drilling down to the Eagle Ford shale, which is among the deeper geologic formations in South Texas. Already there have been either “shows” (potentially productive zones found as drilling occurs ), or actual production brought online from the Olmos, Frio, Vicksburg, and other zones as a result of wells drilled into the Eagle Ford shale. These productive zones can eventually be perforated as Eagle Ford shale production declines, or simultaneously, to produce oil and gas in conjunction to that from the shale. The area where interest in the Eagle Ford shale is most intense right now also contains over a dozen geologic formations known to hold oil and gas. Years ago, many wells were drilled into the Austin Chalk, Olmos, Frio, Wilcox, Edwards limestone, Vicksburg and other formations. Aside from the Austin chalk boom, most of these wells took advantage of oil and gas trapped in one of the types of traps seen above. These are often tiny pockets of petroleum, in relation to the size of the entire geologic formation, such as the Wilcox group. Because shale formations are more uniform, oil and gas companies are not looking for “traps” per se. They are looking for the areas of the shale that have thickness, high organic content, greater permeability, (oil vs. natural gas right now), etc.
Note that below the Eagle Ford shale in many areas lies the Pearsall Shale. The Pearsall Shale is the subject of exploratory drilling by companies such as EOG Resources and Chesapeake, and could prove to be as valuable in the long run as the Pearsall Shale.
As hundreds at first, then thousands of Eagle Ford shale wells are drilled, many new finds will occur.
No Comprehensive 3D Seismic Survey Of South Texas Done Until Now
One reason that much “conventional” oil and gas remains to be discovered in South Texas is that a full, comprehensive seismic survey using modern technology referred to as “3D Seismic”, has never really been done. Primarily oil and gas companies have done these surveys on a hit and miss basis, in areas identified to have the most oil and gas traps. Along the Sligo and Stuart City trend, there have been more of these type seismic surveys. Now all of that is changing. EOG Resources, Pioneer Resources, and other companies, have completed the largest 3D seismic survey ever done of the areas of South and Southwest Texas where the Eagle Ford shale is present.
The Risk Of Exploratory Drilling In The Eagle Ford Shale
The entire nation has recently seen what destruction an oil and gas blowout can do after what happened to the B.P. well in the Gulf of Mexico. There is little damage to be done from oil spills in the Eagle Ford shale, since blowouts in South Texas have been historically of the natural gas variety, but loss of life and property must be avoided by proper safety measures and due diligence. A oil company may have years ago relieved the pressure of oil and gas in a trap, just across the fence, but a new well drilled a mile away may still puncture a pressurized zone. South Texas is no place to be complacent in either an exploration company’s mud program, correlation well studies, mudlogging, mud weight monitoring, etc.
Any time you drill down to one of the deepest rock formations in an area that is historically known for having high underground pressures, you face the risk of a kick or worse, a blowout. There are still many pockets of high pressure that have not been relieved through drilling. Over the years South Texas has seen blowouts of the magnitude that Red Adair used to deal with. Entire highways, such as US 281, were re-routed during the construction phase, as huge gas wells burned out of control for months. Many older South Texas residents can recall seeing the glow from one of these massive South Texas blowouts burning at night. That particular blowout occurred near Rachal Texas, south of Alice, TX. The chance of major blowouts is much less these days, with better BOP’s, mud programs, info from correlation wells, etc, but there is still a risk. Unlike the Barnett Shale, where blowouts and high pressure are a remote possibility, drilling for oil and gas, especially in the deeper fringes of the Eagle Ford shale to the south, will be subject to an element of that risk. What kind of new production will result as a consequence of Eagle Ford shale drilling? Only time will tell, but it’s going to be very interesting, to say the least.
Article by Nolan Hart, August 9, 2010.