Print this pageAnother Tale of Two "Shales": How the Original Composition of Mud Determined Very Different Petroleum Source Rock Quality
Joe H. S. Macquaker1, David A. Polya1, Kevin G.
Taylor2, and Margaret A. Keller3
1 Manchester University, Manchester, United Kingdom
2 Manchester Metropolitan University, United
Kingdom
3 USGS Menlo Park, Menlo Park, CA
"Shale" is problematic as a general term for fine-grained fissile sedimentary rocks because it is commonly used indiscriminately to describe both rocks composed predominantly of clay and silt as well as rocks dominated by diatoms and coccoliths. Distinguishing between these rock types is important as initial compositional differences may drive mudstones down very different diagenetic pathways and effect composition of hydrocarbons generated.
In order to investigate the control exerted by initial mineralogy on "shale" diagenesis the Kimmeridge Clay Formation (Dorset, UK) and the Monterey Formation (Santa Barbara coast) have been investigated utilizing optical, electron optical and geochemical methods. Analyses reveal that while both contain Type II marine kerogen assemblages, their mineralogies are very different. The Kimmeridgian "shales" are dominated by detrital clay and silt, some diagenetic pyrite and carbonate, sulfur-poor organic matter, and small fractions of calcareous nannoplankton debris. In contrast, the Monterey "shales" are dominated by siliceous nannoplankton debris, sulfur-rich organic matter, and diagenetic phosphate with only small fractions of clay, silt, and pyrite.
Different starting sediment compositions significantly impact geochemical evolution of porewaters. Scarcity of detrital iron in the Monterey Formation during sulfate reduction resulted in a) low concentrations of Fe(II) which limited pyrite precipitation and b) restricted Fe-reduction which led to porewaters becoming acidic. Acidic porewaters coupled to high porewater bicarbonate and reduced sulfur concentrations caused early phosphate rather than early carbonate authigenesis, and sulfur to be incorporated into organic matter rather than pyrite. Differences in starting sediment composition may thus control final petroleum source rock quality.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005