STRATIGRAPHY OF THE BARNETT SHALE, NEWARK EAST FIELD, TEXAS

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 nine facies have been identified: 1) Non calcareous mudstone 2) Calcareous mudstone 3) Dolomitic mudstone 4) Concretionary zones 5) Calcite-rich laminae deposit 6) Shelly deposit – in situ fossil and transported shelly debris 7) Phosphatic deposit 8) Wavy to flaser-bedded deposit and 9) Micrite/Lime mudstone. A careful analysis reveals that 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 wavy to flaser-bedded deposits. 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.