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Petroleum: a primer for Kansas, Page 3 of 15
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Geology of petroleum

Sedimentary rocks Petroleum may occur in any porous rock, but it is usually found in sedimentary rocks such as sandstone or limestone. Sedimentary rocks are grouped into three major classes: clastic, carbonate, and evaporitic.

Clastic rocks are those that are formed by the accumulation and cementation of sedimentary particles derived from weathered fragments of preexisting rocks. Weathering processes, such as freezing and thawing, rain, wind, and other similar events, break down the parent rock into small particles that can then be transported by wind and rain runoff. Streams carry the mud, sand, and gravel from the source area down to its final resting place, be that a stream channel, floodplain, lake, or ultimately the sea. There it accumulates, is buried and compacted by later-arriving sediments, and cemented to form sedimentary rocks. The mud compacts to shale or mudstone, the sands are cemented by silica or calcite to form sandstones, and the gravels become conglomerates. Sandstones, because of the inherent porosity between their grains, often become excellent reservoirs for oil or natural gas. In oil-field terminology, any potentially productive sandstone is called a "sand" (fig. 2a).

Figure 2a--A greatly magnified image of a sandstone as seen in a thin section of the rock under the microscope. The scale is equal to one millimeter. The rock sample was injected with blue-colored epoxy that is seen here filling pores which are interconnected (permeable). After plastic is injected and solidified, the rock sample is cut and polished on a glass slide to a thickness of 35 thousandths-of-an-inch. The "thin section" of the rock is thin enough to permit light to be transmitted through it as in this photomicrograph.

This particular sandstone contains grains of quartz (white), calcite, and feldspar (shades as brown). The grains originally came from other rocks that had been eroded. The sample is exceedingly porous and permeable. Thegrains are loosely packed and there is very little cement filling the space between the grains. The arrow indicates possible pathways for fluid movement.

large number of interconnected pores means oil will move easily through it

Carbonate rocks are limestones and dolomites. They usually form in warm seawater at shallow depths, ankle deep to about 20 ft (6 m), where various plants and animals thrive. The hard, usually calcareous parts of the organisms pile up on the seafloor over time, forming beds of lime particles. Algae, simple plants, are one of the greatest contributors of lime particles, but any shelled animal may contribute whole or fragmented shells to the pile. Reefs, banks of lime mud, and lime sand bars are commonly found preserved in rocks (figs. 2b and c).

Figure 2b--A thin-section photomicrograph of a Pennsylvanian limestone taken from a core sample of a producing zone in Victory field, haskell County, Kansas. This particular sample comes from an interval that is not a good reservoir rock. Circular grains composed of calcite (finely crystalline, reddish-stained areas in a grain) and dolomite (clear, coarse crystals) are completely cemented by medium crystalline calcite. No porosity is visible.

this rock, with very little pore space, is not a good reservoir

Figure 2c--A thin-section photomicrograph of a Pennsylvanian limestone. The scale bar equals one millimeter. These well-rounded carbinate grains consist of broken shells and other skeletal remains of marine organisms. The rounding is produced by coatings of dense, dark, finely crystalline calcite which formed rims around the particles as they were agitatated by current action in a shallow marine setting. The rounded particles are completely cemented together with finely crystalline calcium carbonate (calcite), and consequently no porosity is present.

calcite cement completely fills pores

Carbonate sediments are subject to many processes in becoming a rock. If lime mud is exposed to the air, it simply dries out and almost overnight becomes a natural "concrete," and lime sands become cemented by the evaporation of lime-rich seawater to form "beach rock." Rainwater, however, begins to destroy the rock and forms porosity in the process. If not exposed to the air, lime sediments continue to accumulate, perhaps to great thicknesses. Under these conditions, they compact under the weight of new sediments and eventually are cemented to form limestone. These rocks are rarely porous unless other processes become involved.

Limestone is composed of calcium carbonate (calcite or aragonite), thus the general rock term "carbonate" is used. Magnesium, a common element in seawater, can replace some of the calcium within the crystal structure of calcite. This often happens by various processes that are not fully understood; the resulting rock is known as "dolomite" (calcium-magnesium carbonate). The process of changing limestone to dolomite produces somewhat smaller crystals, so the resulting rock has tiny pores between the new crystals. This kind of porosity, much like that in sandstone, often contains oil and natural gas (fig. 2d).

Figure 2d--This thin-section photomicrograph came from a sample of a core taken 1 ft (.3 m) below the one shown in figure 2b. The scale bar equals one millimeter. This sample is representative of an excellent carbonate reservoir rock with pore space contributing nearly one-third of the total volume of the rock. In fact, this zone is one of the main reservoirs of Victory field. The carbonate grains artificially stained red on this thin section are poorly preserved remains of coated fossil fragments. Permeable porosity (indicated again by the blue epoxy) occurs within these particles as well as between them. The arrows trace possible avenues of migration for fluid such as oil and gas that now are found in this reservoir rock.

calcite that used to fill pores has been replaced and permeability has increased

Evaporites are formed by the direct precipitation of minerals by evaporation of seawater. Resulting rocks are ordinary salt (halite-sodium chloride), gypsum (calcium sulfate with some water), and various forms of potash salts. When gypsum is buried to considerable depths, the water is expelled from the crystals and "anhydrite" (meaning simply "without water"), a harder crystalline rock, results. Evaporites are not porous, although they are readily dissolved by water and are not source rocks for petroleum. However, they may be formed in highly stagnant water where black mud, rich in organic matter, may also be deposited, and so are commonly associated with good source rocks. Because they are impermeable, evaporates often form seals on other reservoir rocks. Evaporites of the Sumner Group form the upper seal of Chase Group carbonate reservoirs in the giant Hugoton gas area in southwestern Kansas.

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Kansas Geological Survey, Education
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