Shale Story: Shale is the most widely distributed sedimentary rock in the Earth’s crust

The success of the large-scale development of shale oil and gas in the United States has led to a world-renowned “shale revolution”, which suddenly made the shale that has been sleeping underground for hundreds of millions of years famous all over the world. Now in various media, and on the cover of the American “Time” Weekly. One can’t help but ask what kind of rock is shale? A detailed introduction will be made below.

There are all kinds of rocks on the earth, which can be described as various types and strange. According to their origins, they can be divided into three categories. One is magmatic rocks formed by underground lava, also known as igneous rocks, such as granites that form strange peaks and rocks in Huangshan Mountain, Anhui; It is a metamorphic rock formed from the original magmatic rock, sedimentary rock or early metamorphic rock through metamorphism, such as marble (also called marble) named after it was produced in Dali, Yunnan; the third type is the rock family sedimentary rock to which shale belongs.

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Sedimentary rock is formed by the fragmentation or decomposition of parent rock under the action of physics, chemistry and biology, and re-deposited diagenesis under the transportation of wind, running water, glaciers and other media. The sedimentary rock family is the most widespread on the Earth’s surface, with about 70% of all rocks being sedimentary.

Shale is a fine-grained sedimentary rock belonging to the clay rock group. Clayrock is a sedimentary rock mainly composed of particles with a particle size of <0.0039 mm and containing a large amount of clay minerals (kaolinite, montmorillonite, hydromica, etc.), but it is mixed with quartz, feldspar debris and other chemicals. These clay-solidified rocks are called mudstones or shale. The difference between shale and mudstone is their sedimentary structure. Mudstone has no obvious laminar bedding and tends to be massive. Shale has the characteristic book-like or sheet-like bedding, from which the shale name is named. Clay rock with neither mudstone nor shale characteristics is obvious, it belongs to the transitional rock type between mudstone and shale, and the imperfectly developed lamellar bedding can be seen, which is usually called mud shale. Shale also has various colors, including organic matter in gray-black and black. The iron-containing ones are brown-red, brown-red and other colors, as well as yellow, green and other colors.

In the sedimentary rock family, shale has many brothers, such as conglomerate, sandstone, siltstone, limestone, dolomite, etc. Among them, shale is the most widely distributed sedimentary rock, accounting for about 60% of the sedimentary rock. However, due to the weak ability of shale to resist weathering, unlike some sedimentary rock brothers (such as limestone and sandstone) that can form landforms on the surface, limestone can form picturesque stone forests, red sandstone can form colorful Danxia landforms, shale The weathering and erosion of the exposed surface often forms low mountains and valleys on the terrain, and it is easy to be covered by vegetation, so shale was not so widely known before.

Shale is formed in a still water environment. After a long time of sedimentation and diagenesis, shale usually develops in sedimentary environments such as oceans, lakes, and river deltas. Shale also often contains a large number of fossils of animals and plants. A good recorder of biological evolution.

Shale can be subdivided into: black shale, carbonaceous shale, calcareous shale, iron shale, siliceous shale, etc.

1) Black shale. Contains more organic matter and finely dispersed pyrite and phosphite, with organic matter (TOC) content of 3-10% or higher, similar in appearance to carbonaceous shale, the difference is that black shale does not stain hands Abundant and anoxic still water environment (occluded bays, deep water Lu Peng, deep lakes, lagoons, etc.). It is the main rock for the formation of shale oil and gas. Black shale usually contains trace amounts of uranium, which is associated with the organic matter in the shale. The presence of naturally occurring radioactive material (NORM) responds on gamma geophysical logs, a signature used to identify subsurface hydrocarbon-bearing shale.

2) Carbonaceous shale. It contains a large amount of carbonized organic matter, which can pollute hands and contains high ash content, so it is not easy to burn. It is often formed in lakes and swamp environments and coexists with coal seams.

3) Calcareous shale. A shale that is rich in CaCO3, but contains no more than 25% CaCO3, if it exceeds 25%, it belongs to marl. Commonly found in continental red strata or marine calcareous argillaceous rocks.

4) Iron shale. Usually contains a small amount of iron oxides, hydroxides, carbonate minerals (siderite) and iron silicate minerals (oolitic chlorite, chlorite) and so on. Usually red or gray-green. It is more common in red beds and coal measure strata.

5) Siliceous shale. It contains more chalcedony, opal, etc., and the SiO2 content is above 85%. Its hardness is greater than that of ordinary shale, and it is often symbiotic with iron rock, manganese rock, phosphorous rock and flint. There are biological, volcanic and chemical causes.

It should be pointed out that not all shale produces shale oil and gas, so the “shale” referred to in shale oil and gas does not include all shale. It usually refers to gray-black and black shale that is rich in organic matter and has generated oil and gas, such as many media. The Barnett shale, Bakken shale, Eagle Ford shale, and Marcellus are often mentioned as proven source rocks.

In addition to pure shale, the “shale” mentioned in shale oil and gas is not limited to pure shale. Etc., the Bakken shale is a set of strata in which the upper and lower sections are dominated by shale and the middle section dominated by dolomite and siltstone is in a “sandwich” structure.