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Stratigraphic Architecture and Reservoir Distribution in the Tensleep and Amsden Formation Influenced by Gondwanan Glaciations (Mississippian-Pennsylvanian, Northern Wyoming)

Abstract

During the Pennsylvanian, Gondwanan ice sheets waxed and waned dynamically. We investigate how changes in southern hemisphere glacial conditions influenced the stratigraphic architecture of the Amsden and Tensleep Formations in northern Wyoming, USA, and its impact on the distribution and heterogeneity of potential petroleum reservoirs. To evaluate changes in facies distribution and stratigraphic style in these late Visean to late Moscovian/early Kasimovian formations, we measured 5 detailed sections from surface outcrops in the eastern Bighorn Basin and compared them to available subsurface data. The base of the Amsden Formation, which unconformably overlies platform carbonates of the Madison Formation, is a fine sandstone member overlain by up to 27 m of red siltstones and calcretes interpreted as floodplain deposits. The limited range of facies indicates only modest variations in depositional environment. In every section, the upper Amsden is characterized by dolomite beds containing sparse marine fossils of inferred Serpukhovian-Bashkirian age and thus spanning the Mississippian-Pennsylvanian boundary. The overlying Tensleep Formation is characterized by alternations between eolian sandstone, sabkha deposits and shallow marine siltstone. The latter are major bounding surfaces that indicate repeated flooding of a coastal erg. The wider range of facies in this part of the formation indicates an increase in the amplitude of sea-level fluctuations. The presence of Moscovian fusulinids in these intervals suggests that these facies variations correspond to a major phase of Pennsylvanian glaciation (i.e. latest Bashkirian-mid Moscovian C4 glacial of eastern Australia). The low permeability marine siltstones can be traced across tens of kilometers and may now serve as barriers to vertical hydrocarbon flow between eolian reservoir bodies. Additional potential barriers include lateral facies changes from eolian dunes to sabkha and interdune deposits. Preliminary results suggest that the Amsden and Tensleep Formations record a change from low to high-amplitude sea-level change, with the highest amplitudes recorded by spatially extensive marine siltstones barriers between eolian storeys. This study provides new insight into the paleotropical response to the waxing and waning of Gondwanan glaciers and its impact on reservoir architecture.