Stratigraphic Significance of Organic Matter Variations--Example from Mancos Shale and Tocito Sandstone, San Juan Basin, New Mexico: ABSTRACT

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Investigation of the Upper Cretaceous (Coniacian) Mancos Shale and Tocito Sandstone has shown that both the amount and type of preserved organic matter varies in a predictable way according to depositional systems. Core and outcrop samples from the Mancos Shale above and below the Tocito Sandstone as well as mudstone interbeds within the Tocito were subjected to organic petrology and programmed pyrolysis measurements. Sedimentological studies have suggested that the Tocito was deposited as a transgressive shelf sand ridge. This analysis of the organic matter is consistent with that observation. The inferred transgressive shales above the Tocito Sandstone are much higher in both total organic carbon (TOC) and hydrogen index (HI) than regressive shales below. Kerogen in these transgressive shales is dominantly marine algal in origin (fluorescent amorphous protistoclasts), while the regressive shales below the Tocito possess considerable amounts of terrestrially derived kerogen (phytoclasts). Mudstone interbeds in the Tocito contain a mixed kerogen assemblage of mostly amorphous protistoclasts with minor amounts of amorphous structured and poorly preserved phytoclasts. This assemblage is a consequence of partial incorporation of underlying regressive shelf deposits into the transgressive ridge complex. Because the regressive Mancos Shale below the Tocito exhibits low TOC values (0.49-1.41%), low HImore » (159-255 mg/g C), and sparse fluorescence, little hydrocarbon source potential is expected. Transgressive shales above the Tocito possess sufficiently high TOC (2.14-5.29%), HI (353-623 mg/g C), and fluorescence to be considered potential hydrocarbon source rocks.« less