Influence of mechanical stratigraphy on clastic injectite growth at Sheep Mountain anticline, Wyoming: A case study of natural hydraulic fracture containment

Jennifer Beyer,W Ashley Griffith

doi:10.1130/ges01349.1

Abstract

Sandstone injectites ranging from 1 km in outcrop length intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain anticline (Bighorn Basin, Wyoming, USA). These injectites were sourced from the Peay Sandstone Member of the overlying Cretaceous Frontier Formation and represent a significant possible fluid pathway through impermeable shales. Sand injection occurred along dikes and sills, interacting with bedding discontinuities and preexisting joints in the Mowry Formation during the early folding of Sheep Mountain anticline. We argue that, in contrast to the passive sweeping of sediments into fissures characteristic of Neptunian dike formation, downward intrusion of the Peay sand was forceful and made possible by a highly stratified horizontal stress field resulting from the deposition, burial, and lithification history of the rock units in the area. The internal structure of the injectites is dominated by two sets of mutually offsetting deformation bands. The deformation bands have shear and compaction components, exhibiting significant porosity loss, as well as cataclasis and minor pressure solution. After formation of the deformation bands, subsequent faulting was localized along the margins of deformation bands, evidenced in the field by slickensided surfaces. A detailed kinematic analysis of slickenline lineations yields shortening and extension axes consistent with deformation band formation during early Laramide–oriented shortening, and continuing through the folding of Sheep Mountain anticline. Beyond the formation and deformation of these sandstone injectites, this study highlights the importance of mechanical stratigraphy in the containment of hydraulic fractures.

Highlights

Clastic injectites are intrusive bodies of sediment, commonly sand, that cut across lower permeability rocks in a manner analogous to igneous intrusions (Jolly and Lonergan, 2002)
More recently geologists have recognized that clastic injectites occur in most sedimentary environments, known locations of injectites are perhaps biased toward deep-water marine settings (Jolly and Lonergan, 2002)
In cases where clastic injectites intrude impermeable units, they act as both seal risks and migration pathways that can potentially increase connectivity between reservoirs isolated by units of low permeability (Jolly and Lonergan, 2002)

Summary

Introduction

Clastic injectites are intrusive bodies of sediment, commonly sand, that cut across lower permeability rocks in a manner analogous to igneous intrusions (Jolly and Lonergan, 2002). Clastic injectites have been found to intrude granitic rock, mafic sills, and pillow basalts (Jolly and Lonergan, 2002; Hurst and Cartwright, 2007). The importance of clastic injectites is being recognized in areas where these soft sediment deformation and intrusion processes directly affect hydrocarbon reservoirs (Braccini et al, 2006). The study area is located at SMA in the eastern Bighorn Basin, located in north-central Wyoming (Fig. 1). SMA trends northwest-southeast and formed subperpendicular to the inferred direction of maximum Laramide orogenic contraction (northeast-southwest), similar to many Laramide-age (latest Cretaceous–early Eocene) folds (Erslev, 1993). The fold plunges as steeply as 20° toward the northwest at the northern extent and ~10° southeast at the southern extent (Bellahsen et al, 2006; Amrouch et al, 2010)

Methods

Discussion

Conclusion