--> Abstract: Fracture-Fill, Albian/Cenomanian Planktic Foraminifers in the Stuart City/Edwards Shelf Margin, Pawnee Field, South Texas, by Lowell Waite, Robert W. Scott, and Travis Loseke; #90078 (2008)

Datapages, Inc.Print this page

Fracture-Fill, Albian/Cenomanian Planktic Foraminifers in the Stuart City/Edwards Shelf Margin, Pawnee Field, South Texas

Lowell Waite1, Robert W. Scott2, and Travis Loseke1
1Pioneer Natural Resources, Irving, TX
2Precision Stratigraphy, Cleveland, OK

Core in the Lower Cretaceous Stuart City Formation at Pawnee Field, Bee County, Texas, verifies the occurrence of Upper Albian - Lower Cenomanian planktic foraminifera deep within the massive Fredericksburg (Middle - Upper Albian) shelf margin. The planktic fauna in the Pioneer Natural Resources #1 Schroeder well consists of the foraminifers Favusella washitensis and Rotalipora appeninica in several carbonate mud- and wackestone intervals. The dark-gray carbonate mudstone contrasts markedly with the surrounding rudist-bearing reefal rock. Six planktic-bearing lithologies occur from 235 - 617 ft (72 - 188 m) below the top of the reef. The carbonate mud-filled zones occur within small (0.5 in / 1.3 cm diameter) to large (9 ft / 2.8m), vertical and horizontal conduits that cross-cut normal reefal depositional fabrics. This morphology suggests the material percolated down large fractures or crevasses that extended deep into the reef at Pawnee Field. Similar features and lithologies are noted in the Shell #1 Chapman core in Waller County, Texas.

The interpretation of crevasse-fill lithology deep in the Stuart City/Edwards Formation has important geologic implications for the Comanche margin. The age of the foraminifers constrains the timing of fill to a narrow interval after deposition of the Stuart City Formation and before the overlying Georgetown Formation of the Washita Group. The most likely scenario, supported by 3D seismic, involves the propagation of large cracks at the top of the reef during the foundering and early burial of the over-steepened Stuart City margin. The presence of large fractures in the reef, particularly if open or partially-filled, may provide significant enhancement of reservoir properties along trend.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas