Paleogeographic evolution during the Eocene Upper Wilcox in the Houston embayment with consideration of the Yoakum Canyon fill




Conwell, David Andrew

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The Eocene Upper Wilcox clastic wedge represents the second major pulse of terrigenous material into the Gulf of Mexico basin. Recent interest in the Wilcox has been reinvigorated by the drilling of the Baha prospect in 2001, and the associated discovery of 2.5 billion barrels of producible oil in deepwater Wilcox aged turbidite deposits. To better characterize and understand the deepwater deposits, research in the delivery systems that transported sediment from the Laramide uplift to the deep Gulf of Mexico is required, with a special focus on the Wilcox shelf margin. This study incorporates over 300 well logs, and outcrop to analyze the Upper Wilcox shelf deposits in the Houston Embayment. The area of this study extends from the outcrop belt in the north-northwest of the embayment down into the subsurface to the limit of down dip well control, around 150 km to the south-southeast. From west to east it extends from Gonzales County to Grimes County, around 200 km. In the Houston embayment the Upper Wilcox was previously interpreted as mainly being fluvial deposits that prograded across the relatively stable substrate provided by the underlying delta complexes of the Lower Wilcox. Previous authors have asserted that the Yoakum Canyon (Middle Wilcox) in the southwest of the field area had its entire 3,000’ filled with prodelta muds prior to the progradation of the Upper Wilcox. However, the present work shows that upper reaches of the Yoakum Canyon were filled by the sandstone units of the Upper Wilcox. Paleogeography maps, generated by differentiating between marine and terrestrial log signatures identify five distinct sequences in the Upper Wilcox. Each sequence shows linear sand trends across the shelf, generally in a north to south direction. A majority of sand was deposited in sequences three and four, through aggradation in the east and progradation in the west over the Yoakum Canyon region. The corresponding shorelines for sequences one through four remain largely pinned along the inherited shelf edge of the Lower Wilcox in the east, whereas the shorelines strongly prograde in the southwest over the Yoakum Canyon. With this new interpretation, that the Yoakum Canyon was not completely filled at the time of Upper Wilcox deposition, it is possible that the canyon delivered 2.86-7.15*106t/yr of sediment to the deepwater reservoirs. Outcrop measurements of cross strata, taken in Bastrop County, confirm the fluvial well log interpretation and provided a base for the volume calculations.


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