Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

George F. Hart

Abstract

Study of Eocene-Oligocene sediments in Alabama and Upper Cretaceous strata in New Mexico shows that organic facies deposited on the shelf change noticeably at surfaces associated with transgression of the shoreline. Organic matter in sediments below transgressive surfaces is dominated by well preserved terrestrial organic matter (phytoclasts) whereas sediments deposited on the shelf during transgression contain sparse, highly degraded phytoclasts. Shelf deposits associated with transgressions generally contain more hydrogen-rich organic matter than those deposited during regression. Sediments associated with starvation surfaces contain abundant amorphous nonstructured protistoclasts and rare, highly degraded phytoclasts. Well preserved phytoclasts increase in abundance in sediments overlying the starved intervals. These results suggest that the type and preservation of organic matter is related to the rate of terrigenous sediment supply to the shelf. Variations in the amount and type of organic matter preserved in shelf sediments are predictable within a sequence stratigraphic framework. Each depositional systems tract has a distinctive depositional style that affects the amount of terrigenous influx and, consequently, the type and preservation of organic matter. 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, marine depositional systems of both the lowstand and highstand systems tracts contain less total organic carbon and yield less pyrolyzable hydrocarbons. Petrographic analysis of organic matter in these rocks reveals abundant macerals of terrestrial origin. Phytoclasts in the lowstand systems tract are especially well preserved. Integration of data from characterization of organic matter with sedimentologic and regional stratigraphic information enhances resolution in locating surfaces that bound systems tracts within the depositional sequence. Integration of these data not only improves systems tract identification in fine-grained, basinward intervals but also allows application of the predictive capabilities of sequence stratigraphy to marine petroleum source rocks. Results from a study of the Upper Cretaceous Mancos Shale indicate that optimum source rock potential is found in the transgressive systems tract below the downlap surface.

Pages

252

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