Stratigraphy of the Barnett Shale, Newark East Field, Texas.

Prerna Singh¹, Roger Slatt¹, Chandra Rai², Carl Sondergeld², and Chris Stamm^3
1 School of Geology and Geophysics, University of Oklahoma, Norman, OK;
² Mewbourne School of Petroleum and Geological Engineering, University of Oklahoma, Norman, OK.;
³ Devon Energy Corporation, Oklahoma City, OK

The Barnett Shale is currently the most prolific shale gas play in the U.S. Although ample work has been done to characterize Barnett outcrops in the Llano area of central Texas, much is still unknown about its depositional and diagenetic history, stratigraphy and characteristics as a reservoir rock in the Newark East Field of the Fort Worth Basin. Visual, petrographic and mineralogic analysis of a continuous long core from the field reveals several distinct shale facies arranged in a complex stratigraphic stacking pattern. The following ten facies have been identified: 1) Mudstone with 3-5% calcite cement and app. 50% agglutinated forams 2) Mudstone with app. 50% agglutinated forams but without calcite cement 3) Flaser-bedded mudstone 4) Concretionary zones 5) Calcite-rich laminae, 6) Shelly deposit 7) in situ Phosphatic laminae 8) Phosphatic lags, 9) Dolomitic mudstone and 10) Lime mudstone. A careful analysis reveals the significance of depositional processes which have constrained the facies successions, geometries and cyclicity in the study area. There has been considerable variation in depositional environments through time from quiet water conditions which are manifested by low energy, muddy facies to high energy conditions manifested by phosphatic and shelly lags and flaser-bedded mudstone. Thus, the Barnett Shale in this area, rather than representing a continuous depositional succession, consists of stacked low-energy shale packages bounded by surfaces indicative of higher-energy environments. Subtle variation in the organic content, mineral composition and petrophysical properties of the facies parallel this stratigraphic stacking, and facies exhibit distinctive well log patterns. Current work is addressing the timing of high-energy events and paleowater depths in order to develop a high resolution sequence stratigraphic framework. Diagenetic history is also being analyzed as it plays a significant role in alteration and formation of certain intervals which are rich in secondary mineralization e.g. Dolomitic mudstone and concretionary zones.

AAPG Search and Discover Article #90065©2007 AAPG Southwest Section Meeting, Wichita Falls, Texas