An Outcrop Study of Clastic Injection Structures in the Carboniferous Tesnus Formation, Marathon Basin, Trans-Pecos Texas
Published:January 01, 2007
Timothy N. Diggs, 2007. "An Outcrop Study of Clastic Injection Structures in the Carboniferous Tesnus Formation, Marathon Basin, Trans-Pecos Texas", Sand Injectites: Implications for Hydrocarbon Exploration and Production, Andrew Hurst, Joseph Cartwright
Download citation file:
The Carboniferous Tesnus Formation in the Marathon basin of west Texas was deposited as a large submarine-fan complex in a tectonically active, migrating foredeep. Primary depositional fabrics in siliciclastic mass-flow deposits of the Tesnus Formation were extensively modified during intense soft-sediment deformation. Fluidization and clastic intrusion were common processes and produced clastic injection structures possessing a remarkable array of shapes and orientations with respect to bedding. The most commonly recognized intrusive bodies are dikes, which were nonsystematically injected into overlying host sediments at angles ranging from just greater than 0–90°. Dike orientations, corrected for tectonic deformation, show no correlation with paleoslope or later structural trends and, based on traditional decompaction methods, are interpreted to have been injected at relatively shallow burial depths (tens to hundreds of meters). Clastic sills are nearly as common as dikes and, based on their generally greater thickness, are interpreted to have been responsible for accommodating a greater proportion of postdepositional sediment remobilization than any other type of intrusion. Other, more unusual clastic injection structures in the Tesnus include concordant and discordant cylindrical clastic pipes; irregularly shaped, wavelike intrusions in which one surface is concordant and the other is discordant; and intrusions that are both concordant and discordant along different parts of the injection body. With clastic dikes and sills as end members, previously undescribed clastic injection structures in the Tesnus Formation define a spectrum of features with geometries transitional between concordance and discordance.
Figures & Tables
Sand Injectites: Implications for Hydrocarbon Exploration and Production
Sand injectites are described in scientific literature as an increasingly common occurrence in hydrocarbon reservoirs, in particular in deep-water clastic systems, where they are known to influence reserves distribution and recovery. Seismically-detectable injected sand bodies constitute targets for exploration and development wells and, subseismic sand bodies provide excellent intra-reservoir flow units that create field-wide vertical communication through depositionally extensive, low-permeability units. As sand injectites form permeable conduits in otherwise low-permeability units they facilitate the expulsion of basinal fluids; hence they act both as a seal risk and mitigate timing and rate of hydrocarbon migration. Injected sand bodies form intrusive traps, which are distinct from structural or stratigraphic traps. Included in this publication are 10 chapters on subsurface examination of sand injectites, 1 chapter on theoretical considerations, and 13 outcrop analogs in reservoirs across the world. Captured in this volume is at least a taste of the global and stratigraphic distribution of sand injectites, and an attempt to introduce readers to sand injectites and their significance in the context of hydrocarbon exploration and production. The book is not intended as a complete review of the field-based literature, but emphasizes high quality case studies from the surface and subsurface. The geographic scope of the book is large, and illustrates the diversity of geological settings in which these fascinating and economically significant features are found.