Print this pageGeological Anatomy of Two "HOT" Exploration Plays in Carbonates: Cotton-Valley-type (Jurassic) and Lodgepole-type (Mississippian) Buildups
EBY, DAVID E
Two carbonate plays that have recently helped revitalize U.S. onshore
exploration are in the Upper Jurassic Cotton Valley Lime interval along the
western portion of the East Texas Basin and the Lower Mississippian Lodgepole
Formation along the southwestern portion of the Williston Basin. Intriguing
similarities are that: (1) Both buildup types are almost entirely limestones;
(2) Major contributors to the "framework" and mud components appear to be from
microbial organisms responding to depositional energy; (3) Significant matrix
porosity within these limestone buildups can be maintained at present depths of
10-15,000 ft., and probably deeper; (4) Matrix porosity within these buildups is
typically a combination of preserved primary porosity and selective dissolution
of mud matrix or some of the skeletal material in the subsurface; (5) Early
cementation, 'stromatactoid' structures and bioerosion are important for the
development of buildup core and flank relationships; (6) Fracturing can be
important in the burial diagenesis and reservoir performance; (7) Several growth
stages in each buildup occur under different relative sea-level positions; (8)
Significant worldwide source rock intervals are present Late Jurassic and Early
Carboniferous times; (9) Both buildup types can be envisioned in "deeper water"
depositional patterns in offshore to toe-of-slope positions as suggested by
Wilson's (1969) model of bathymetries in; oxygen-stressed basins; (10)
Paleostructural control on the position of nucleation sites for lines or
clusters of these types of buildups may be important.
Interpretation of these similarities is significant for understanding the anatomy and location of these types of problematical microbial buildups.