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Sequence Stratigraphy of Organic-Rich Successions: A Multi-proxy Approach - La Luna Formation, Western Venezuela

Kertznus, Vanessa R.1; Lorente, Maria Antonieta; Zapata, Eglee; and Padron, Victor
1[email protected]

The recent revelation of vast quantities of potentially gas- and oil-productive mudrocks, has resulted in rapid advancements in the understanding of their unique geochemical and stratigraphic properties. Because these organic-rich intervals typically serve as the source, reservoir and seal of the hydrocarbons produced from them, their geological properties have a direct impact on their production properties. Although these formations may be hundreds of meters in gross thickness, and may appear largely homogeneous, the vertical variability in the organic richness, geochemical, mineralogical, sedimentological and faunal signals can vary on relatively short vertical scales (e.g. mm to 10’s cm). This vertical heterogeneity can be directly tied back to geologic and biotic conditions at the time of deposition

In this study, we apply a high-resolution multi-proxy analytical approach to the Late Cretaceous La Luna Formation through detailed analysis of sedimentologic, paleoecologic, and geochemical data acquired in four outcrop sections located along the Venezuelan Andes. Varied combinations of proxies prove to be most useful in delineating sequence development in anoxic-, dysoxic-, and oxic-dominated successions, and in determining the paleoenvironmental setting and depositional processes that allowed the input and preservation of organic material. This study has revealed the diachronic character of the deposition, and a generally common sequence stratigraphy, consisting of four regionally correlatable intervals, from the base upward: i) late Cenomanian to early Coniacian HST characterized by the shift from the final phase of anoxic and oligotrophic conditions associated to the OAE2, to eutrophic water masses experiencing anoxia-dysoxia fluctuations; ii) mid Coniacian to early-mid Santonian transgression when the most anoxic conditions within the basin evolution developed associated with the OAE3; iii) mid to late Santonian characterized by anoxic-dysoxic and oligotrophic-eutophic fluctuations resulting from an increase in upwelling under a HST; iv) late Santonian to early-mid Campanian characterized by more recurrent fluctuations and more oxygenated conditions under a TST.

Understanding the sequence stratigraphy of these organic-rich intervals provides a powerful tool not only for regional-to-local stratigraphic correlations, but also for high-grading stratigraphic intervals most favorable for preservation of organic matter and hydrocarbon generation and storage


AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013