Sequence Stratigraphic Significance of Organic Matter Variations<subtitle>Example from the Upper Cretaceous Mancos Shale of the San Juan Basin, New Mexico</subtitle>

Abstract

Study of a portion of the Upper Cretaceous Mancos Shale in the San Juan Basin of New Mexico shows that organic facies deposited on the shelf change noticeably at surfaces that have sequence stratigraphic significance. Shelf sediments below transgressive surfaces contain abundant, well-preserved terrestrial organic matter (phytoclasts) whereas sediments above transgressive surfaces contain sparse and highly degraded phytoclasts and more hydrogen-rich organic matter. Shelf sediments associated with the maximum flooding surface typically contain the least terrestrial organic matter. These results indicate that the type and preservation of organic matter is related to both the rate of terrigenous sediment supply to the shelf and the bottom water oxygen conditions present on t e shelf. Variations in the amount and type of organic matter (organic facies) preserved in shelf sediments are predictable within a sequence stratigraphic framework. Each systems tract has a distinctive depositional style that affects the amount of terrigenous sediment influx to the shelf and, consequently, the type and preservation of organic matter that is deposited on the shelf. Fine-grained marine sediments in transgressive systems tracts possess high total organic carbon and yield relatively high amounts of hydrocarbons during pyrolysis. Petrographically, this organic matter is composed primarily of amorphous nonstructured protistoclasts. Phytoclasts in the transgressive systems tract are highly degraded. In contrast, progradational marine depositional systems of both the lowstand and hig stand systems tracts contain End_Page 221------------------------ less total organic carbon and less pyrolyzable hydrocarbons. Petrographic analysis of organic matter in these rocks reveals abundant macerals of terrestrial origin. Phytoclasts are especially well preserved in the lowstand systems tract. Integration of data from the characterization of organic matter with sedimentologic and regional stratigraphic information provides greater precision in locating surfaces that bound systems tracts within the depositional sequence. An example of this approach is presented for a part of the Upper Cretaceous Mancos Shale of the San Juan Basin, New Mexico. Organic matter data not only improve systems tract identification in fine-grained, basinward facies but also demonstrate that the predictive capabilities of sequence stratigraphy are applicable to marine petroleum source rocks. These results indicate that optimum source rock potential is found in the transgressive systems tract below the condensed section facies that contains the downlap surface.