Barnett Shale–Unfolded: Sedimentology, Sequence Stratigraphy and Regional Mapping

Extending sequence stratigraphic concepts to distal marine, clastic fine-grained rocks requires an understanding of spatial and temporal transport, depositional, and post-depositional processes which differ from those of their coarser-grained counterparts. In this study, we have developed a preliminary sequence stratigraphic framework for fine-grained rocks comprising the Barnett Shale, North Texas. This framework is regionally mappable over the study area comprising Wise, Denton, Parker and Tarrant counties of the Fort Worth basin.

Integrated study of several long continuous cores and wireline logs led to defining nine distinctive lithofacies on the basis of their physical, chemical and biological characters: 1) Siliceous, non calcareous mudstone 2) Siliceous, calcareous mudstone 3) Dolomitic mudstone 4) Concretionary zones 5) Calcite-rich laminated deposit (bottom current deposit) 6) Fossiliferous deposit 7) Phosphatic deposit 8) Silty-shaly (wavy) interbedded deposit and 9) Micrite/Lime mudstone. These nine lithofacies record considerable variation in depositional environments and processes through time, from quiet water deposition of low energy, muddy facies to high energy, phosphatic shelly lags and wavy-bedded mudstones.

The cyclical stratal stacking style of lithofacies and corresponding API gamma ray log patterns has led to identification of high resolution depositional parasequences within the Barnett Shale. Each parasequence is approximately 30ft. (9.1 m) thick, laterally continuous and mappable. Isopach maps of the Gamma Ray parasequences in the Lower Barnett and the Upper Barnett reveal regionally variable thickness patterns and trends. Sites of maximum parasequence thickness represent either the sites of greatest accommodation or variations in sediment source area over the time interval of deposition of the Barnett Shale. In this work, we attempt to determine which of these two possibilities are the most probable.