Carbonate source rocks

Oils C (Boscan), D (Cerro Negro), E (Athabasca) - from the class of asphaltenic-aromatic crude oils, have a high sulfur content. The first two oils coming from carbonate source rocks contain polar compounds consisting of very stable polycyclic aromatics. On the other hand, the last oil contains aromatics which are less condensed and more reactive. 

Claypool, G. E. Mancini, E. A. 1989. Geochemical relationships of petroleum in Mesozoic reservoirs to carbonate source rocks of Jurassic Smackover Formation, southwest Alabama. American Association of Petroleum Geologists Bulletin, 73, 904-924. 

Figure 9. Comparison of Cl5 chemical group type distributions of expelled oil and retained bitumens for typical Type II (marine shale) and Type IIS (carbonate) source rocks (middle of the the oil window).

These deposits would result in carbonate rock (e.g., limestone). A third source rock possibility would be evaporite rocks (e.g., salt, gypsum, anhydrite), which often contain large organic concentrations when originally deposited [26-29]. 

S. Source rock potential of shale A relation exists between the uranium-to-potassium ratio and the organic carbon content. The source rock potential of shale can thus be evaluated. 

The composition of the particles is related to that of the source rocks. Quartz sand [composed of silica (silicon dioxide)], which makes up the most common variety of silica sand, is derived from quartz rocks. Pure quartz is usually almost free of impurities and therefore almost colorless (white). The coloration of some silica sand is due to chemical impurities within the structure of the quartz. The common buff, brown, or gray, for example, is caused by small amounts of metallic oxides iron oxide makes the sand buff or brown, whereas manganese dioxide makes it gray. Other minerals that often also occur as sand are calcite, feldspar and obsidian Calcite (composed of calcium carbonate), is generally derived from weathered limestone or broken shells or coral feldspar is an igneous rock of complex composition, and obsidian is a natural glass derived from the lava erupting from volcanoes see Chapter 2. 

Petroleum is formed under the earth s surface by the decomposition of organic material. The remains of tiny organisms that lived in the sea and, to a lesser extent, those of land organisms were carried down to the sea in rivers along with plants that grow on the ocean bottoms combined with the fine sands and silts in calm sea basins. These deposits, which are rich in organic materials, become the source rocks for the formation of carbon and hydrogen, i.e., natural gas and crude oil. 

Considerable geographic variability exists in the distribution of the source rocks contributing salts to river and groundwaters. As shown in Table 21.3, most of the evaporites, which are the dominant natural source of Na and Cl in river water, lie in marginal and endorheic (internal) seas. Some of these subsurfece evaporite deposits dissolve into groundwaters, which eventually carry Na and Cl into the ocean. Carbonates are the prevalent rock type between 15°N and 65°N. Precambrian-age crustal rocks and meta-morphic minerals predominate between 25°S and 15°N and north of 55°N. Shales and sandstones represent on average 16% of the terrestrial surfece lithology. 

Rather than using the isotopic composition of methane alone James (1983, 1990) and others have demonstrated that carbon isotope fractionations between the hydrocarbon components (particularly propane, iso-butane and normal butane) within a natural gas can be used with distinct advantages to determine maturity, gas-source rock and gas-gas correlations. With increasing molecular weight, from Ci to C4, a enrichment is observed which approaches the carbon isotope composition of the source. 

Hughes, W. B. In Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks. AAPG Studies in Geology 18 Palacas, J. G., Ed. AAPG Press Tulsa, 1984 181-196. 

The importance of these depositional environments makes it desirable that studies concerned with the reconstruction of palaeoenvironments from sediments or source rocks of oils also establish molecular parameters for palaeohypersalinity. Recently, ten Haven et al. (7-9) have summarized a number of "organic geochemical phenomena" related to hypersaline depositional environments. In addition to previously known parameters, such as an even-over-odd carbon number predominance of n-alkanes and a low pristane/ phytane ratio (<0.5), several new parameters were suggested. These parameters, however, are mainly based on empirical relations. 

Schlanger S. O., Arthur M. A., Jenkyns H. C., and SchoUe P. A. (1987) The Cenomanian-Turonian oceanic anoxic events I. Stratigraphy and distribution of organic carbon-rich beds and the marine C excursion. In Marine Petroleum Source Rocks, Geological Society of London, Special Publication 26 (eds. J. Brooks and A. J. Elect), pp. 371-399. 

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