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Université Pierre et Marie Curie
About 70% of the total world petroleum resources are concentrated in the Tethyan realm, the Mesozoic deposits being the most prolific source rocks of these oil and gas reserves. To understand the depositional controls of these organic-rich facies at the scale of the Tethys is a challenging problem. A recent set of paleoenvironmental maps for the Tethyan realm allows integration of source-rock mapping with other mappable geologic information. This integrated approach is attempted here for three short time intervals of the Mesozoic: Toarcian, Kimmeridgian, and Cenomanian, all of which were periods of good source-rock deposition.
The source-rock distribution during the Toarcian shows a contrast between the western European and Tethyan realms. While there are high concentrations of organic matter corresponding to thick deposits in the western European realm, there are only lower concentrations within thin sedimentary sequences in the Tethyan realm. Although the organic facies are similar in both settings, widespread anoxia must have existed in western European epicontinental seas, while the preservation of organic matter in the Tethyan realm must be related to morphological factors. During the Kimmeridgian, preservation of marine organic matter was important in epicontinental platforms as well as in newly created margins. The Cenomanian is also clearly associated with good preservation of oil-prone source rocks, especially in low latitudes. During this interval, numerous organic-rich shale deposits are preserved, whatever the environment: on platforms as well as in basins. Whereas the northern shelves seem more favorable for organic concentration than the Tethyan margins during the Toarcian--and probably also during the Kimmeridgian--the reverse is true for the black shales preserved during the Cenomanian.
During these three intervals of enhanced marine organic-carbon preservation, the distribution of source rocks was controlled both by plate movements that influenced opening or closing of seaways, basin morphologies and their evolution; and by paleocurrents and paleoclimates.
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