Print this page“How Is the Barnett Shale Stratified? Extracting the Answer from Cores and Logs…”
Prerna Singh1, Roger M. Slatt1, Chandra Rai2, and Chris Stamm3
1ConocoPhillips School of Geology & Geophysics, University of Oklahoma, Norman, OK
2Mewbourne School of Petroleum & Geological Engineering, University of Oklahoma, Norman, OK
3Devon Energy Corporation, Oklahoma City, OK
The Newark East field (Barnett Shale, TX) is the second largest producing field and third largest in proven reserves of natural gas in the U.S (EIA 2006 Annual Reserve Reports) and continues to be growing. However a thorough geologic knowledge, including its depositional and diagenetic history, sequence stratigraphy and characteristics as a reservoir rock, is still not well known.
The current work - which includes visual, petrographic and mineralogic analysis of several continuous long cores and the wireline log suites covering approximately 35 sq. miles - reveals several distinct shale facies. This variation in bedding, lithology and facies is related to its position within a depositional stratigraphic sequence. The systematic vertical variation in the source quality within sequences and sequence sets is also expected.
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-bedded mudstone. The Barnett stratigraphy in the study area is composed of several approx. 30ft. thick parasequences which are identified as cyclical patterns of increasing and decreasing gamma ray API values. The capping components of the gamma ray increasing upward packages are composed of calcareous lithologies represented by debris flows & intervals richer in micro- and macro-fossils in the core; these features suggest relative shoaling of the depositional environment through each paracycle. 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.
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas