Organic Matter Variations in a Depositional Sequence: Implications for Use of Source Rock Data in Sequence Stratigraphy

Abstract

Variations in the amount and type of organic matter preserved in shelf sediments are predictable within a sequence stratigraphic framework. These variations can be documented using routine petroleum source rock evaluation techniques. The type and preservation of organic matter is related to the rate of allochthonous shelf sedimentation. Each depositional systems tract in a sequence has a distinct depositional style that affects changes in the amount of terrigenous influx. Therefore, integration of source rock data with sedimentologic and regional stratigraphic results provides greater resolution in locating the critical surfaces that bound depositional systems tracts within the depositional sequence. This integrated sequence stratigraphic approach has been applied to conventional cores of Upper Cretaceous strata in the San Juan basin of New Mexico. 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 macerals. Both the lower Juana Lopez and the Mancos Shale above the Tocito Sandstone provide examples of organic matter deposited in the transgressive systems tract. In contrast, marine depositional systems of both the lowstand and highstand systems tracts contain less total organic carbon and less pyrolyzable hydrocarbons. This methodologymore » not only improves systems tract identification in fine-grained, basinward intervals but also reveals those portions of the depositional sequence that are most favorable for marine petroleum source rock formation.« less