GC509QH Bleaker Shale Pit (Earthcache) in Prince Edward Island, Canada created by El Nimrod

GC509QH ▼

A cache by El Nimrod Message this owner

Hidden : 3/14/2014

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Size: Size: other (other)

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Geocache Description:

Shale (Figure 1) (visit link) is a fine-grained, clastic (formed from mechanical weathering debris) sedimentary rock (visit link) that forms from the compaction of silt and clay-size mineral particles (quartz, chert and feldspar) that we commonly call "mud". The mud is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. This composition places shale in a category of sedimentary rocks known as "mudstones". Shale is distinguished from other mudstones because it is fissile and laminated. "Laminated" means that the rock is made up of many thin layers. "Fissile" means that the rock readily splits into thin pieces along the laminations. Mudstones, on the other hand, are similar in composition but do not show the fissility.

Worldwide, sedimentary rocks cover approximately 75% of the worlds surface. Of this, approximately 50% are shales, 30% sandstone and 20% limestone. Shale is the Island's second most common sedimentary rock behind sandstone.

Island sedimentary rocks like shale and sandstone are considered to be of the Upper Carboniferous and Permian Ages and are referred to as Permo-Carboniferous rocks. Some of the formations are thought to be from the Triassic Age.

TEXTURE

Shale typically exhibits varying degrees of fissility breaking into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding plane because of parallel orientation of clay mineral flakes. Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystone (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.

COMPOSITION & COLOR

Shales are typically composed of variable amounts of clay minerals and quartz grains and the typical colour is grey. Addition of variable amounts of minor constituents alters the colour of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment. Black shale can also be referred to as black metal. Red, brown and green colours are indicative of ferric oxide (hematite – reds), iron hydroxide (goethite – browns and limonite – yellow), or micaceous minerals (chlorite, biotite and illite – greens).

Clays (visit link) are the major constituent of shales and other mud rocks. The clay minerals represented are largely kaolinite, montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites, whereas in older rocks, especially in mid to early Paleozoic shales, illites predominate. The transformation of smectite to illite produces silica, sodium, calcium, magnesium, iron and water. These released elements form authigenic quartz, chert, calcite, dolomite, ankerite, hematite and albite, all trace to minor (except quartz) minerals found in shales and other mud rocks.

Shales and mud rocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales which form in anoxic conditions contain reduced free carbon along with ferrous iron (Fe2+) and sulfur (S2-). Pyrite and amorphous iron sulfide along with carbon produce the black coloration and purple.

FORMATION

The process in the rock cycle which forms shale is called compaction. The fine particles that compose shale can remain suspended in water long after the larger particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water.

'Black shales' are dark, as a result of being especially rich in unoxidized carbon. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc. The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation.

Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally to gneiss.

FRACKING (visit link)

Black organic shales are the source rock for many of the world's most important oil and natural gas deposits. These black shales obtain their black colour from tiny particles of organic matter that were deposited with the mud from which the shale formed. As the mud was buried and warmed within the earth some of the organic material was transformed into oil and natural gas.

The oil and natural gas migrated out of the shale and upwards through the sediment mass because of their low density. The oil and gas were often trapped within the pore spaces of an overlying rock unit such as a sandstone. These types of oil and gas deposits are known as "conventional reservoirs" because the fluids can easily flow through the pores of the rock and into the extraction well (see Figure 2).

Although drilling can extract large amounts of oil and natural gas from the reservoir rock, much of it remains trapped within the shale. This oil and gas is very difficult to remove because it is trapped within tiny pore spaces or adsorbed onto clay mineral particles that make-up the shale.

Unconventional Oil and Natural Gas

In the late 1990s natural gas drilling companies developed new methods for liberating oil and natural gas that is trapped within the tiny pore spaces of shale. This discovery was significant because it unlocked some of the largest natural gas deposits in the world.

The Barnett Shale of Texas was the first major natural gas field developed in a shale reservoir rock. Producing gas from the Barnett Shale was a challenge. The pore spaces in shale are so tiny that the gas has difficulty moving through the shale and into the well. Drillers discovered that they could increase the permeability of the shale by pumping water down the well under pressure that was high enough to fracture the shale. These fractures liberated some of the gas from the pore spaces and allowed that gas to flow to the well. This technique is known as "hydraulic fracturing".

Drillers also learned how to drill down to the level of the shale and turn the well 90 degrees to drill horizontally through the shale rock unit. This produced a well with a very long "pay zone" through the reservoir rock. This method is known as "horizontal drilling" (see Figure 3).

Horizontal drilling and hydraulic fracturing revolutionized drilling technology and paved the way for developing several giant natural gas fields. These include the Marcellus Shale in the Appalachians, the Haynesville Shale in Louisiana and the Fayetteville Shale in Arkansas. These enormous shale reservoirs hold enough natural gas to serve all of the United States' needs for twenty years or more.

In 2010 it was estimated that 60% of all new oil and gas wells worldwide were being hydraulically fractured. As of 2012, 2.5 million hydraulic fracturing jobs have been performed on oil and gas wells worldwide, more than one million of them in the US.

Fracking has become a very contentious issue on PEI and the other Maritime provinces of Nova Scotia and New Brunswick. Proponents point to the economic benefits from the vast amounts of previously out of reach hydrocarbons. Opponents point to potential environmental effects, such as contamination of ground water, risks to air quality, the migration of gases and hydraulic fracturing chemicals to the surface, surface contamination from spills and flow back and the health effects of these.

NOTE:

This cache is down a red dirt road that is not maintained during the winter and will be muddy in the spring and after rain. Although it makes for a nice snowshoe or cross-country ski during the winter, the shale deposits at GZ will be hidden beneath any snow cover, thus rendering this EC "seasonal" in nature.

To claim this EarthCache you must:

E-mail me the answers to the following questions:

1. Based on the Mohs system of hardness (visit link) , what are 2 common minerals that shale could scratch?

2. What is the predominant colour of the shale at GZ and what gives it this colour?

Do NOT post your answers, encrypted or otherwise, on your log.

Do NOT log as a find until you have e-mailed the answers for this EC. Logs failing to answer required questions through e-mail will result in a log deletion without notice.

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