2 Naturally Fractured Reservoirs I

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14th Week Lecture:

SEDIMENTARY ROCKS AND MINERALSIntroduction

Sedimentary rocks and minerals cover most of the surface of the Earth. The processes that produce them are more variable than those that produce igneous rocks and minerals, but the number of common sedimentary minerals is small.

Sediments and sedimentary rocks cover about 80% of the Earth's surface but are less than 1 % of the volume of the Earth's crust. They are, in effect, a thin blanket on top of igneous and metamorphic basement rocks (Figure -50). Sediments, and thus sedimentary rocks, are mostly recycled materials derived from preexisting igneous, metamorphic, or sedimentary rocks. Fig. 50 Conceptual cross section of the earth crust, showing sedimentary rocks over basement. In some parts of the earth crust layers of sediment-ary rocks are thousands of meters thick. In other places, notably Precambrian shields, the layers of sedimentary rocks are completely missing.

Once exposed to the Earths surface, all rocks are subjected to processes of erosion, transportation and deposition. Thus sediment becomes sedimentary rock. Petrologists usually divide sedimentary rocks into two main groups: Detrital rocks based on size of the detrital particles (e.g., sandstone, siltstone, mudstone)

Chemical rocks based on the chemical composition (limestone, chert, evaporates); sometimes.

Biogenic rocks are classified separately.

Detrital (clastic) sediments involve erosion, transportation and deposition by moving water. Requires energy thresholds to transport particles of different sizes, therefore water-transported detrital rocks are often well sorted by grain size. Table 16 summarizes Detrital Sedimentary Rocks, based on grain size.

Table 16 Classification scheme for Detrital Sedimentary Rocks.

Detrital sedimentary rocks are formed by compaction and cementation of clastic sediments composed of individual mineral grains or lithic fragments (pieces of rock). The mineral grains and lithic fragments vary greatly; we call them collectively clasts (derived from Greek word klastos, which means "broken"), so detrital sedimentary rocks are often called clastic rocks. Because their mineralogy varies so much, we generally classify detrital sedimentary rocks based on grain size rather than composition. Conglomerate "contains large rounded clasts (>2 mm in longest dimension) separated by a fine-grained material called matrix. Sandstone contains sand-sized (0.062 to 2 mm in longest dimension) quartz or feldspar grains, and sometimes lithic fragments. Mud-stone and shale primarily contain microscopic (< 0.062 mm in longest dimension) clay and quartz grains. Sedimentary Petrologists use the term clay to refer to clastic grains smaller than 0.004 mm in longest dimension.In the coarser-grained sedimentary rocks, the compositions of lithic fragments give clues to the origin of the sediment. In the finer-grained rocks, mineralogical composition is often difficult to determine and interpret. Chemical sedimentary rocks are formed by precipitation of minerals from water, or by alteration of already precipitated material. Many limestones, dolostones, evaporites, and cherts form this way. Petrologists name chemical sedimentary rocks based on chemical composition. Chemical sedimentary rocks usually include only one or a few minerals because the chemical processes that form them tend to isolate certain elements. The most common precipitated minerals consist of elements of high solubility (for example, Na or K) or elements of great abundance (for example, Si).

Some limestones cherts and other rocks are formed largely from biogenic (organic) debris. Petrologists often classify them separately from chemical and detrital sedimentary rocks. For sack of simplicity we will not, however, consider them separately here. Much overlap exists between chemical, detrital, and organic sedimentary rocks. Many chemical sedimentary rocks contain clastic material, and many detrital sedimentary rocks are held together by chemical cements precipitated from water. Both chemical' and detrital rocks may contain biogenic components.

WEATHERING

Figure 51 shows the two parallel processes that result in sedimentary rocks.

An original source rock (igneous, metamorphic, or sedimentary) is exposed to weathering. The weathering forces may be mechanical (water, wind, gravity, glaciers, waves, and frost) or chemical (dissolution by water, perhaps containing acids). Mechanical weathering, which produces clastic material called detritus, is of much less significance than chemical weathering. Even apparently dry climates have enough water to promote chemical weathering on exposed surfaces, although the weathering rate may be slow. Chemical weathering produces dissolved material, called the hydrolysate, and leftover rock and mineral detritus that did not dissolve. We sometimes call the undissolved material the resistate because it resists dissolution. More easily dissolved elements, especially the alkalis and alkaline earths, go into solution, while resistate remains to become sediment (Table 17). Typical resistates include quartz, clay, K-feldspar, garnet, zircon, rutile, or magnetite.

Chemical weathering often produces clay minerals, the most important being montmorillonite, illite, and kaolinite. Reactions that produce clays are complex, involving the reaction of water with previously existing minerals, such as feldspars, to produce clays and dissolved elements. We call such reactions hydrolysis reactions. Mechanical and chemical decomposition produces kaolinite and dissolved ions, including K+ and Si4+. The dissolved material is carried away and will eventually precipitate somewhere else. The residual kaolinite may remain where it forms, but water, gravity, or wind can transport clays produced by hydrolysis, just like any other detrital material

Fig. 51 Processes that form sedimentary rocks. The weathering products-rock debris and dissolved material-are transported and then deposited by gravity, chemical precipitation, or organic activity to produce sediments and eventually sedimentary rocks.

Table 17 Products of Weathering of Some Common Igneous Minerals

If we examine fresh outcrop in a road cut, rock often appears hard and shiny. Examination with a hand lens reveals that minerals have well-defined boundaries and jagged outlines and may show good cleavage or crystal faces. Minerals have their normal diagnostic colors: quartz is clear; feldspars are white or pink, muscovite is silvery and sparkly, magnetite appears metallic, and biotite and other mafic minerals appear black. The picture is not the same in outcrops exposed to weathering for a long time. After weathering, rock and most minerals have a dull or drab appearance. Grain boundaries and cleavages are obscured. Red, brown, and gray' material coats all surfaces, obscuring diagnostic minerals.

Goldich (1938) made such observations, publishing a well-known weathering series showing the way in which some common igneous minerals break down (Figure 52). He derived his series from studying the formation of clays on outcrops of granite, diabase, and amphibolite in the Minnesota River Valley. The keen reader will notice that Goldich's series is nearly identical to Bowen's reaction series (see Fig. 49). Minerals that crystallize from magma at high temperature-minerals poor in Si and O are generally less resistant to weathering than those that crystallize at low temperature. Fe-Mg silicates, such as olivine or pyroxene, calcic feldspars, and many minerals with high solubilities in water, break down relatively easily. Quartz, some feldspars, and some nonsilicate minerals are relatively resistant to weathering because they contain more Si-O bonds, which do not break easily. It should not be surprising that minerals stable in high-temperature igneous rocks, or those most often precipitated from water, are the first to decompose under Earth surface conditions

Fig. 52 A modified version of Goldich's weathering series describing the relative order in which minerals decompose due to weathering. Olivine and Ca-plagioclase weather most rapidly, while quartz is the most resistant.

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