Penetrative Sedimentary Intrusions in the Pennsylvanian Tensleep Formation of Wyoming: Implications for Reservoir and Baffle Compartmentalization

Abstract

The Tensleep Formation of the Bighorn Basin, Wyoming consists mainly of alternating eolian sandstones and marine dolomites and shales. The low-permeability marine intervals act as baffles to vertical flow between producing eolian reservoirs in the basin. While deformation structures have been described and interpreted in the eolian sandstones, the perceived lack of such features from marine facies leads to the erroneous conclusion that they remained unaffected. We report here the presence of discordant, tabular sedimentary intrusions in the marine intervals found at three localities within a 30 km radius. Orientations were measured, and several elements of this system were sampled for petrography. Two types of discordant, vertical, and tabular bodies are found in the marine intervals. The first and most abundant type consists of vertical, tabular bodies of dolomicrite. In most cases, their bases are connected to horizontal beds of dolomite, and drag indicators in the host beds point upward. Body widths range from 10 to 40 cm, and heights up to 150 cm. The second type also consists of vertical, more or less tabular bodies, but these are filled by fine-grained quartz sandstone containing sparse clasts of the host shale. Ptygmatically folded bodies extend downward from overlying sandstone bodies, in which soft-sediment deformation structures are common. We interpret the second type to be the result of a downward injection of fluidized sand from overlying sandstone bodies. While other examples of dolomitic dikes are extremely rare in the literature, we hypothesize that the first type of tabular bodies is caused by a remobilization of the initial carbonate mud along fracture planes. These features can be explained by hydraulic fracturing of the less permeable units, possibly associated with seismic activity. Within the study region, the presence of active faults during the Pennsylvanian have previously been proposed to account for changes in thicknesses and facies. The locations of these proposed faults would explain the spatial distribution of the dikes described here. These vertical features then provided weak points in baffle lithologies, and fractures are commonly observed in the host and sandstone bodies around these dikes. These previously undescribed features provide insights into the lateral continuity of baffles and barriers to flow in mixed eolian/marine series, and the structural context during the deposition of the Tensleep Formation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015