Soft-Sediment Deformation Features and Their Influence on Fluid Flow and Reservoir Characteristics: Jurassic Navajo Sandstone, Southeast Utah

Abstract

Intervals of soft-sediment deformation, especially clastic injectites, are recognized as preferential fluid pathways and reservoir targets in petroleum systems. Erg margin deposits of the Jurassic Navajo Sandstone, southeastern Utah, contain a wide variety of soft-sediment deformation features: 1) centimeter-scale disruptions of laminae by synsedimentary faults and slumps; 2) meter-scale contorted bedding and fluid escape or dish structures within one or more beds; and 3) meter- to decameter-scale disturbances of clastic pipes and plugs (injectites), as well as zones of massive sandstone that cut across multiple beds. Eolian bounding surfaces typically have low transmissibility and act as fluid baffles. However, soft-sediment deformation features may disrupt original depositional textures to create new preferential flow paths across bounding surfaces.

Small soft-sediment deformation structures are commonly removed during upscaling in reservoir simulation models. However, the structures that cut across multiple beds could be significant vertical, preferential flow paths. Complex diagenetic histories in Jurassic pipe examples from Navajo, Carmel, and Entrada formations suggest multiple episodes of fluid migration and cement precipitates, including evidence of hydrocarbon migration from diagenetic bleaching and trapped hydrocarbons in calcite cement fluid inclusions. Massive units within the Navajo Sandstone commonly weather as smooth outcrop surfaces due to the homogenized texture and lack of cement resulting from removal by bleaching diagenetic fluids.

Field relationships with allied textural and diagenetic studies suggest that massive sandstone zones can allow fluid migration both laterally and vertically within the disrupted zone for tens of meters, bypassing bed set boundaries that might normally be permeability baffles in eolian reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018