The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Sunday, May 11, 2014

May 11. Marcellus Shale and natural gas




In the early part of the Middle Devonian, between about 385 and 390 million years ago, what’s now western New York and Pennsylvania, much of West Virginia and some of Ohio were under water. There was a relatively deep and relatively restricted sea out there, where generally only the finest grains of sediment could reach –mud and clay particles much smaller than a grain of sand, together with fragments of organic material that might get carried that far from the land that lay to the east.

Click to enlarge
Because the sea was constrained between rising land on the east and a low region above sea level, in the Cincinnati Arch area to the west, it was a relatively stagnant sea. It must have been something like the Black Sea today, though somewhat smaller. The fine mud and organic material accumulated to form a black shale called the Marcellus Shale, for its outcrops near the town of Marcellus, New York – but it and similar rocks extend over a region from New York into parts of Kentucky and Tennessee on the south and into Ontario on the north. The map shows the extent of the Marcellus Shale.  

It’s possible that the anoxia, the stagnant, low-oxygen sea, was related to a global event and not just to a restriction here in this part of the world. Evidence for a period of anoxia has been found at many places in Europe, as well as in Morocco, China, and Australia.

The organic material in the Marcellus Shale ranges from 1% to 11% of the rock. Anything more than 1 or 2% is an incredibly rich hydrocarbon source rock. Some of the best source rocks in the world are around 5% organic content. So the Marcellus should have generated lots of oil and gas that migrated into appropriate reservoirs somewhere to make great resources, right?

Not right. The problem is the fine-grained shale has only tiny pore spaces to hold oil or gas, and the spaces, such as they are, are very poorly interconnected – we say that the rock has very low permeability. But the oil or gas is still there, stuck in the shale.

If you drill a well vertically through a rock like this, the well bore only touches the rock where it goes through it, maybe a few hundred feet at best. That’s not much exposure to get at those tiny pores with oil or gas in them, and there’s no way to suck the oil or gas from further away, since there are no connections – no permeability.

Two relatively modern hydrocarbon exploration technologies have turned the Marcellus into a huge target for natural gas. First, there’s horizontal drilling. The drill goes straight down, maybe several thousand feet until it hits the target in the Marcellus, then the drill string is steered to turn it 90 degrees, to near horizontal. Then you can navigate through the formation, along its length, so to speak – and go through thousands of feet of the good stuff rather than just punching a hole through it. You’ve exposed the well to a lot more gas-bearing rock.

But you still can’t get at anything away from the hole. Enter hydraulic fracturing. If there are no connections to suck the gas through, make some by breaking the rock. There are variations, but basically it’s a system of forcing water containing chemicals and sand into the rock, creating fractures and propping them open so the natural gas can flow into the well.

This is, as you probably know, a very big deal. The need for natural gas in the populous states of the northeastern United States is great. But there are environmental concerns associated with hydraulic fracturing – fracking in the vernacular usage. There may be introduction of unwanted chemicals, and the potential does exist for polluting drinking water aquifers, although most of the fracking is thousands of feet below any such aquifers. Fracking has been used for more than 60 years with minimal impact. I’m definitely not saying it’s 100% safe – nothing is, and there is evidence that some problems can be related to it. Even some possible generation of earthquakes. As with all resource extraction, these things have to be evaluated carefully – is the need and desire for cheap natural gas worth whatever the trade-off may be, in terms of the environment? I’m sure many residents of Pennsylvania might say no, it’s not.

This is an ongoing controversy, and the concerns are not limited to environmental issues. It’s also not at all clear how sustainable these wells may be. Calculating the impact, and the trade-off, depends on a certain life time for natural gas production – but if it’s much less than projected, the decision might be different. There are many things in play, and if you expect me to come down solidly for or against fracking, sorry, I’m not going to do that. There are good arguments on both sides, and it’s not a simple choice. The Marcellus Shale is today the largest aggregate producer of natural gas in the U.S. and still growing. Almost all this production has been established since 2008, and given our incredibly consumptive lifestyles, that’s not something to discard easily.

—Richard I. Gibson
Resources:
U.S. Geological Survey Open-File Report 2005-1268
Fracking debate

2 comments:

  1. Your comment about the content of hydrocarbon bearing rock at 5% being among the best producing, raised my eyebrows. Am I to understand that producing oil wells in Texas and elsewhere which are pumping 10 to 50 barrels a day, are doing so even though the rock contains less than 5% hydrocarbons? This is going to cause me an adjustment of how I viewed oil in the ground. I'd imagined subterranean oil being just liquefied coal... i.e., literally huge pools of the black gold being held in place by a "cap" of some geological formation.

    ReplyDelete
    Replies
    1. Hello - yes, 5% total organic carbon is a very good source rock. That's not the same thing as the reservoir that contains the oil generated from that source rock though. Still, it is definitely not correct to think of the oil as lakes of liquid. With very few exceptions, oil reservoirs are very solid rock. The oil is in whatever pore spaces may exist in that rock. A sandstone has pores between the sand grains - depending on how tightly cemented they are together, the pore spaces might amount to a fraction of one percent of the whole rock, or as much as 20% or even more. I would say that a rock with 10% porosity would be an excellent reservoir. Even then, if you could hold a sample of the rock, with some exceptions, it would not be 'dripping' oil. Mostly it just sits there, flowing slowly or being pushed or sucked into a well. Here is a reference for some more info - http://www.geomore.com/porosity-and-permeability-2/

      Delete