Changes

^a_{The H/C values define the organic facies. Hydrogen index (HI) and oxygen index (Ol) values from Rock-Eval may vary slightly from indicated values for specific organic facies. (After Jones.<ref name=Jns1987 />)}

^a_{The H/C values define the organic facies. Hydrogen index (HI) and oxygen index (Ol) values from Rock-Eval may vary slightly from indicated values for specific organic facies. (After Jones.<ref name=Jns1987 />)}

An example of extreme weathering occurs in numerous thin (millimeters to a few centimeters thick), thermally immature to marginally mature chocolate brown oil shales of the Galena Group (Middle Ordovician) of the North American Mid-Continent. In shallow cores (<1000 ft, or ~300 m, deep) from east-central Iowa,<ref>Hatch, J. R., S. R. Jacobson, B. J. Witzke, J. B. Risatti, D. E. Anders, W. L. Watney, K. D. Newell, and A. K. Vuletich, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0011/1300/1342.htm Possible late Middle Ordovician carbon isotope excursion: Evidence from Ordovician oils and hydrocarbon source rocks, mid-continent and east-central United States]: American Association Petroleum Geologists Bulletin, v. 71, p. 1342-1354.</ref><ref>Jacobson, S. R., J. R. Hatch, S. C. Teerman, and R. A. Askin, 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0009/1050/1090.htm Geologic note: Middle Ordovician organic matter assemblages and their effect on Ordovician-derived oils]: American Association Petroleum Geologists Bulletin, v. 72, p. 1090-1100.</ref> these thin brown shales have TOC values occasionally exceeding 40 wt. % and Rock-Eval hydrogen indices (HI) near 1000. These thin layers contain more than 85% organic matter (mainly carbon, hydrogen, oxygen,  sulfur and nitrogen) by volume. Along the Mississippi River at Guttenberg and Specht's Ferry, Iowa, these chocolate brown shales have selectively weathered away completely, leaving recesses between the more resistant carbonate layers. Preferential weathering of most fine-grained sedimentary rocks occurs whether or not rocks are organic rich and whether or not the fine-grained sediments are well cemented. Weathering often penetrates beyond the observable changes in color and induration and reduces TOC and HI values. It appears that the quantity and quality of all are diminished when exposed.  

An example of extreme weathering occurs in numerous thin (millimeters to a few centimeters thick), thermally immature to marginally mature chocolate brown [[oil shale]]s of the Galena Group (Middle Ordovician) of the North American Mid-Continent. In shallow cores (<1000 ft, or ~300 m, deep) from east-central Iowa,<ref>Hatch, J. R., S. R. Jacobson, B. J. Witzke, J. B. Risatti, D. E. Anders, W. L. Watney, K. D. Newell, and A. K. Vuletich, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0011/1300/1342.htm Possible late Middle Ordovician carbon isotope excursion: Evidence from Ordovician oils and hydrocarbon source rocks, mid-continent and east-central United States]: American Association Petroleum Geologists Bulletin, v. 71, p. 1342-1354.</ref><ref>Jacobson, S. R., J. R. Hatch, S. C. Teerman, and R. A. Askin, 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0009/1050/1090.htm Geologic note: Middle Ordovician organic matter assemblages and their effect on Ordovician-derived oils]: American Association Petroleum Geologists Bulletin, v. 72, p. 1090-1100.</ref> these thin brown shales have TOC values occasionally exceeding 40 wt. % and Rock-Eval hydrogen indices (HI) near 1000. These thin layers contain more than 85% organic matter (mainly carbon, hydrogen, oxygen,  sulfur and nitrogen) by volume. Along the Mississippi River at Guttenberg and Specht's Ferry, Iowa, these chocolate brown shales have selectively weathered away completely, leaving recesses between the more resistant carbonate layers. Preferential weathering of most fine-grained sedimentary rocks occurs whether or not rocks are organic rich and whether or not the fine-grained sediments are well cemented. Weathering often penetrates beyond the observable changes in color and induration and reduces TOC and HI values. It appears that the quantity and quality of all are diminished when exposed.  

Generating and expelling petroleum reduces organic matter quantity and quality of source rocks (e.g., Daly and Edman<ref>Daly, A. R., and J. D. Edman, 1987, Loss of organic carbon from source rocks during thermal [[maturation]]: American Association Petroleum Geologists Bulletin, v. 71, p. 546</ref>). Therefore, organic facies maps must reflect the original organic constituents and changes caused by weathering and thermal maturation. Experienced organic geochemists can often reconstruct organic facies for thermally mature samples by tracking the kerogen types back to thermally immature values along pathways of Van Krevelen diagrams.

Generating and expelling petroleum reduces organic matter quantity and quality of source rocks (e.g., Daly and Edman<ref>Daly, A. R., and J. D. Edman, 1987, Loss of organic carbon from source rocks during thermal [[maturation]]: American Association Petroleum Geologists Bulletin, v. 71, p. 546</ref>). Therefore, organic facies maps must reflect the original organic constituents and changes caused by weathering and thermal maturation. Experienced organic geochemists can often reconstruct organic facies for thermally mature samples by tracking the kerogen types back to thermally immature values along pathways of Van Krevelen diagrams.