Time: 11:30 am Virtual Social Icebreaker, Noon Speaker Presentation

Speakers: Dr. William Ambrose, BEG – STARR Program

Title: Deepwater and Valley-Fill Systems in the Cisco Group, Eastern Shelf of the Permian Basin

Abstract: Coastal-plain, incised-valley, shelf, and shelf-edge depositional facies in the Virgilian-Wolfcampian Cisco Group in a ~12,000 mi2 (~31,000 km2) area in the southern part of the Eastern Shelf of the Permian Basin record a significantly increased sediment supply coupled with rejuvenation of the Ouachita Fold Belt.  Extensive lower Cisco (Virgilian) fluvial-deltaic systems built out westward across the Eastern Shelf and the Canyon carbonate deposits, producing a progradational succession of regionally interstratified and cyclic delta and carbonate shelf systems deposited throughout the Virgilian and Wolfcampian Series. The Cisco section consists of a cyclic series of thirteen mudrock, limestone, and sandstone facies (top of the Home Creek to top Coleman Junction Limestone).  It forms a progradational succession from the Eastern Shelf (Bunger Limestone) to the deeper part of the Midland Basin (Coleman Junction Limestone). The top of the Home Creek Limestone coincides with a regional downlap surface for progradational lower Cisco shelf strata.

The lower and middle Cisco Group, which produces oil gas in Lake Trammel South field in Nolan County, forms a 1,000- to 1,700-ft (300- to 520-m) clinoformal succession of slope-channel deposits that lap onto and overtop aggradational carbonate-reef and mound deposits of the underlying Canyon Group.  Reservoir sandstone bodies in this slope succession occur in narrow (commonly 500 to 3,000 ft [150 to 900 m] across), sinuous and anastomosing channel trends. Distribution of successive slope-channel trends in the field are dominantly controlled by autocyclicity.  Progressive upward decrease in height of shelf-margin clinoforms indicates that accommodation decreased in the upper Cisco Group.

Lowstand valley-fill deposits of the Tannehill Sandstone in the upper Cisco Group in Nolan, Taylor, Runnels, and Coke Counties consist of narrow (commonly less than 3-mi [<4.8 km]), westward-trending belts of 20 to 60 ft (6 to 18 m) of net-sandstone genetically related to lobate, lowstand shelf-edge deltas developed along the Saddle Creek and Stockwether shelf edges. These narrow lowstand valley-fill systems record deposition of coarse-grained meanderbelts, with basal channel-fill facies composed of sandy, pebble conglomerates. Channel-fill successions are commonly capped by paleosols marked by root traces and other pedogenic structures such as soil nodules and zones of gray-green and reddish-brown, very fine grained sandstone and siltstone that record intermittent episodes of subaerial exposure in floodplain settings. These lowstand Tannehill deposits represent potential stratigraphic traps where reservoir-quality sandstone bodies are bound both vertically and laterally by low-permeability mudstones.

Bio: William A. Ambrose is a Senior Research Scientist at the Bureau of Economic Geology. He received a Master of Arts degree in geological sciences in 1983 from the University of Texas at Austin. Since joining the Bureau of Economic Geology in 1987, he has worked on a variety of projects at the Bureau, including characterization of the Woodbine Group in the East Texas Basin, Pennsylvanian reservoirs in the Eastern Shelf of the Permian Basin, Frio fluvial and deltaic reservoirs in South Texas, tight-gas reservoirs in the Cleveland Formation in the Texas Panhandle, co-production of gas and hot brine from Oligocene reservoirs in the Texas Gulf Coast, evaluation of coalbed methane reservoirs in Rocky Mountain basins, and reservoir characterization and basin analysis studies in Venezuela, Australia, and Mexico. He is currently the principal investigator of the Bureau’s STARR (State of Texas Advanced Oil and Gas Resource Recovery) program, past president of the Energy Minerals Division (EMD) of AAPG, chair of the EMD Coal Committee, and co-chair of the AAPG Astrogeology Committee. His contact information is--email: william.ambrose@beg.utexas.edu , telephone: 512-471-0258, address: Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX, 78713-8924.