Energy & Fuels 1990,4,644-646
644
Controls on Porphyrin Concentrations of Pennsylvanian Organic-Rich Shales, Western U.S.A. J. L. Clayton* and G . E. Michael US.Geological Survey, Box 25046, M S 977, Denver Federal Center, Denver, Colorado 80225 Received April 27, 1990. Revised Manuscript Received October 9, 1990
Organic-rich black shales of Middle Pennsylvanian (Desmoinesian) age occur over much of the central U S . and as far west as the northern Denver and southeastern Powder River basins. Total organic carbon contents (C0J are commonly greater than 10 wt %. Porphyrin concentrations (vanadyl nickel) are as high as 40OOO ppm relative to extractable bitumen. In bulk, the organic matter contained in the shales is mostly type I1 and I11 (Rock-Eva1 hydrogen indexes 200-400 mg of hydrocarbons/g of Cow). The finding of high porphyrin concentrations in type I11 organic matter is unusual but can be explained by a depositional model wherein high preservation of primary organic production (water column photosynthesis) is combined with substantial input of allochthonous organic matter. The allochthonous organic matter (low porphyrin concentration) may come from erosion during advance of the sea across the area or from fluvial transport from shore.
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Introduction This report summarizes the preliminary findings of our investigation of the relation between depositional setting (Le., "paleoenvironment") and organic geochemistry, particularly porphyrin geochemistry, of organic-rich black shales or marls of Desmoinesian age (ca. 290-301 Ma) in the western United States (Figure 1). During Desmoinesian time, a repetitive series of sediments was deposted over much of the midcontinent region extending as far west as the present-day Denver and Powder River basins. Each individual sequence of lithologies, or cyclothem, contains black, frequently organic-rich shale or marl. The shales are generally about 0.5-1.0 m thick and the cumulative thickness is typically less than about 7-8 m, although cumulative thicknesses as much as about 30 m have been reported in the western Nebraska panhandle.' During Desmoinesian time, a topographic high located in southwestern Nebraska (Figure 1) impeded oceanic water circulation between the northern part of the Denver basin and the large epicontinental sea to the south and east. Consequently, at low water levels caused by periodic retreat of the sea or climatic changes? this northernmost arm of the sea became sufficiently saline for precipitation of evaporite minerals. For example, carbonate rocks and both bedded and nodular anhydrite are present in the cyclothems of the northern Denver basin and southeastern Powder River basin. The present-day location of the central Powder River basin and southwestern South Dakota were emergent during Desmoinesian time. In the epicontinental sea to the south, water salinities were never sufficiently high for precipitation of evaporite minerals ("open marine" in Figure 1). Experimental Section
Table I. Summary of Porphyrin Concentrations and VOP/(VOP Nip) Ratios for Desmoinesian Organic-Rich, Black Shales" VOP/ VOP NIP (VOP t Nip) N Denver and SE Powder 50-39700 T-600 >0.9
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River basins epicontinental sea (SE Colorado, Kansas, Oklahoma, Missouri)
180-11800 T-2200
0.2->0.9
OVOP and NiP in ppm relative to extractable bitumen; T = trace amount.
Thermal maturity of the Desmoinesian interval is about the same throughout the study area. Vitrinite reflectance ( %Ro) or equivalent vitrinite reflectance determined from other thermal maturation parameters is in the range 0 . 5 4 6 % . The Desmoinesian interval has apparently experienced a fairly low temperature history so that only limited thermal maturation has taken place in the Desmoinesian black shales, despite the advanced age and fairly deep burial (up to about 3000 m prior to erosion) of the rocks. Rock samples were pulverized to approximately 100-mesh particle size, and a split was removed for determination of Cow C,, values were determined by using a Delsi Model I1 Rock-Eval instrument equipped with a C, module. Bitumen was obtained by extraction for 24 h with C8C13 in a Soxhlet apparatus. Asphaltenes were removed from the bitumens obtained above by CHC13 elution of the maltene fraction (i.e., the bitumen remaining after removal of asphaltenes) from a 250-rm macroporous silica gel column prepared in cyclohexane? The maltene fraction was taken to dryness by use of a rotary evaporator, and nickel and vanadyl porphyrins were isolated by successive elution with 0.35% ethyl acetate in cyclohexane and 8.0% ethyl acetate in
cy~lohexane.~ Quantitation of the two porphyrin fractions was accomplished with an HP 8542 UV/visible diode array spectro-
photometer.
We used only subsurface samples obtained from cores except Results one well where only cuttings were available (eight samples). Cuttings samples were hand-picked to minimize the possibility Porphyrin concentration (VOP Nip) of the black of contamination by caving or from mixing of organic-poor lishales from the northern Denver basin (squares in Figure thologies such as limestone and dolomite that occur strati2) is generally greater than 10000 ppm relative to exgraphically adjacent to the thin, organic-rich black shales. The samples from Oklahoma are from a previously published s t ~ d y . ~
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(1) Wilson, J. M. Energy Resources of the Denver Basin; Rocky Mountain Association of Geologists: Denver, CO, 1978; pp 129-140. (2) Cecil, C. B. Geology 1990, 18, 533-536.
(3) Wenger, L. M.; Baker, D. R. Org. ~ o c h e m ~ 1 9 8 6 , l O 85-92. . (4) Freeman, D. H.; Angeles, R. M.; Freeman, K. H.; Hoering, T. C.; Flynn, J. S.;Lango, T. A.; Homonay-Preyer, C. T. Metal Complexes tn Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 402-422.
This article not subject to U S . Copyright. Published 1990 by the American Chemical Society
Energy & Fuels, Vol. 4, No. 6,1990 645
Porphyrin Concentrations of Pennsylvanian Shales
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Figure 2. C, (wt %) versus total porphyrin concentration (VOP + Nip) in ppm relative to bitumen. Squares indicate restricted basin shales (Nebraska panhandle and adjacent areas north of Transcontinental Arch; Figure 1); +'s indicate open marine shales from northeastern Colorado near the restricted basin; X's indicate open marine samples from Kansas, Oklahoma, and Missouri.
tractable bitumen, and one sample contains nearly 40000 ppm. Similarly high porphyrin concentrations in geological samples have been reported only rarely."8 In contrast, black shales from the epicontinental sea in southeastern Colorado, Kansas, Oklahoma, and Missouri contain lower porphyrin concentrations (X's and +'s in Figure 2). With one exception, these latter samples all contain less than 10000 ppm and most contain less than 5000 ppm. VOP/(NiP VOP) ratios of all organic-rich black shale or marl samples from the northern Denver and southeastern Powder River basins (restricted basin north of the Transcontinental Arch in Figure 1)are greater than 0.9 but are as low as 0.2 for samples from southern Colorado, Kansas, Oklahoma, and Missouri (Table I). Corqvalues range from about 2 to 30 wt % and do not exhibit any appreciable systematic variation with geographic location within the study area (Figure 2). All of the data shown in Figure 2 are from the black shale or marl beds contained within the cyclothems. We analyzed se-
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(5) Didyk, B. M.; Alturtki, Y. I. A.; Pillinger, C. T.; Eglinton, G. Nature 1975,2256, 563-565. (6) Boreham, C. J.; Powell, T. G. Org. Geochem. 1981, 11, 433-449. (7) Eckardt, C. B.; Wolf, M.; Maxwell, J. R. Org. Geochem. 1989, 24, 659-666. (8) Van Berkel, G.J.; Quirke, J. M. E.; Filby, R. H. Org. Geochem. 1989, 14, 119-128.
(9) Desmaison, G.J.; Moore, G. T. A m . Assoc. Pet. Geol. Bull. 1980, 64, 1179-1209. (10) Parrish, J. T. Am. Assoc. Pet. Geol. Bull. 1982, 66, 750-774. (11) Parrish, J. T.; Curtis, R. L. Paleogeog. Paleoclim. Paleoecol. 1982, 40, 31-66. (12) Calvert, S. E. Marine Petroleum Source Rocks; Brooks, J., Fleet, A. J., Eds.; Blackwell Scientific: London, 1987; pp 137-151. (13) Pederson, T. F.; Calvert, S. E. Am. Assoc. Pet. Geol. Bull. 1990, 74, 454-466. (14) Orr, W. L.; Emery, K. D.; Grady, J. R. Am. Assoc. Pet. Geol. Bull. 1958,42,925-962. (15) Lewan, M. D.; Maynard, J. B. Geochim. Cosmochim. Acta 1982, 46, 2547-2560. (16) Baker, E. W.; Louda, J. W. Biological Markers in the Sedimentary Record; Johns, R. B., Ed.; Elsevier: Oxford, 1986; pp 125-225.
Clayton and Michael
646 Energy & Fuels, Vol. 4, No. 6, 1990
preceding reasoning. All of the samples with porphyrin concentrations greater than about 12O00 ppm are from the northern Denver and southeastern Powder River basins, north of the Transcontinental Arch of Figure 1 (see Figure 2, square data points). As noted previously, the cyclothem sequences in this northern area contain evaporite deposits. These data suggest that this northern area was a restricted basin, a t times highly saline, and that water circulation between the northern area and the epicontinental sea to the south was limited. A density-stratified water column with anoxic conditions in the lower water layer would be consistent both with the geology of the area and the occurrence of high porphyrin-to-bitumen ratios, particularly if the water was fairly shallow. Geological studies support the interpretation that shallow water prevailed throughout Desmoinesian time in the western Nebraska panhandle." We suggest that high primary organic productivity fostered by high nutrient levels derived from the oceanic transgression across a mature soil profile coupled with warm, shallow water resulted in high Corgflux to the lower, anoxic water layer. The upper, oxic water layer must have been sufficiently thin that tetrapyrroles reached the lower, anoxic water layer before complete degradation could occur. This process could account for the very high porphyrin/bitumen ratios observed in the black shales of the northern part of the study area. Eventually during deposition of each cyclothem, the water became sufficiently saline (regressive phase) that precipitation of carbonate and sulfate (gypsum) evaporite minerals occurred. Although primary organic productivity may have been high during these times of higher water salinities (ca. 15%),la preservation of organic matter was low. Corgcontent of the carbonate (limestone, dolomite) and anhydrite rocks averages less than about 0.2%.19 We did not determine porphyrin concentrations of these organic-lean lithologies. Compared to the restricted basin in the northern part of the study area, the epicontinental sea to the south in southern Colorado, Kansas, Oklahoma, and Missouri contains consistently lower porphyrin concentrations even though Corgvalues are quite high (Figure 2). Geological data indicate that water depths were as shallow as 50 m during deposition of the black shales in this area.2o Relatively low porphyrin concentrations (compared to the Nebraska panhandle) in organic-rich rocks deposited in shallow water could be explained by an oxic water column. Water depths of about 50 m can result in substantial depletion of the tetrapyrrole content of sedimentary organic matter if oxygen is present.14 However, reducing conditions must have been present in the sediments themselves because the porphyrin concentrations are fairly high compared to shales in general worldwide and VOP/(VOP + Nip) ratios are greater than 0.9 in some samples (Table 11.15 (17) Doyle, J. D.Ph.D. Thesis, Colorado School of Mines, 1987, 309
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(18) Hite, R.J.;Anders, D. E. Deuelopments in Sedimentology Series; Melvin, J. L., Ed., Elsevier: Oxford; in press. (19) Clayton, J. L.Unpublished data. (20) Heckel, P. H. Am. Assoc. Pet. Geol. Bull. 1977,61, 1045-1068.
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Water column conditions and both organic productivity and preservation undoubtedly varied dramatically during deposition of the Desmoinesian shales resulting in the observed wide range of porphyrin concentrations and HI values (Figures 2 and 3), even in the restricted portion of the sea in the Nebraska panhandle and adjacent parts of Wyoming and South Dakota. However, based on the data of Figure 2, it is clear that high preservation of porphyrins, and by inference primary organic matter in general, occurred during deposition of most of the organic-rich, black shales in the restricted basin area, but not in the epicontinental sea to the south and east. Despite high preservation of primary organic matter implied by high porphyrin concentrations (relative to bitumen), types I1 and I11 organic matter are predominant in these rocks. This suggests dilution of the primary organic matter (water-column photosynthate) by relatively nonreactive organic matter (i.e., Corg with low HI). Moreover, rather high amounts of this secondary C,, are required to account for the exceptionally high C,, contents observed. Two possible sources of this secondary C, are recycling of organic matter via erosion as the sea advanced across mature soil profiles and swamps developed during the regressive phase of each cyclothem sequence? or fluvial transport of terrestrial organic matter into the sea. Insufficient geological or geochemical data are currently available to fully resolve this issue. Conclusions Our results illustrate the potential usefulness of porphyrin geochemistry for interpretation of sedimentological processes, particularly for interpreting water-column conditions and relative degrees of organic matter preservation in organic-rich rocks. The porphyrin content of sedimentary organic matter, normalized to bitumen, can provide a relative scale of preservation of primary organic matter (i.e., organic matter derived from the photic zone of the water column). However, the porphyrin content of the bulk sedimentary organic matter (i.e., normalized to Cor,, is relatively low in the Desmoinesian black, organic-rich shales owing to dilution by transported, or secondary, organic matter and would be misleading as an indicator of degree of preservation of the primary photosynthate. Moreover, these results emphasize the potential for ambiguities in interpretations of sources of sedimentary organic matter unless a multiparameter approach is used. Bulk characterization, e.g., pyrolysis, of the organic matter contained in the Desmoinesian shales would lead to the expectation that the soluble organic matter (bitumen) contained in the shale would be characteristic of terrestrial and/or highly oxidized organic matter. Whereas, the high porphyrin content of the bitumen indicates that much of the bitumen was derived from the overlying water column and was preserved under strongly reducing conditions. Acknowledgment. We gratefully acknowledge D. H. Freeman for help developing our procedure for porphyrin analysis and T. A. Daws for Rock-Eval analyses. We thank J. R. Hatch for providing shale samples from the midcontinent.
