Elizabeth S Petrie

Fracture-related fluid flow in sandstone reservoirs: insights from outcrop analogues of south-eastern Utah

Fault- and fold-related fractures influence the fluid circulation in the subsurface, thus being of high importance for CO2 storage site assessment, especially in terms of reservoir connectivity and leakage. In this context, discrete regions of concentrated sub-parallel fracturing known as fracture corridors are inferred to be preferential conduits for fluid migration. We investigate fracture corridors of the middle-late Jurassic Entrada and Curtis formations of the northern Paradox Basin (Utah), which are characterized by discoloration (bleaching) due to oxide removal by circulating CO2- and/or hydrocarbon-charged fluids. The analyzed structures are located in the footwall of a km-scale, steep normal fault with displacement values on the order of hundreds of meters. They trend roughly perpendicular and subordinately parallel to the main fault direction, and define a systematic network on the hundreds of meters scale. The fracture corridors pinch- and fringe-out laterally and vertically into single, continuous fractures, following the axial zones of open fold systems related to the evolution of the main fault. Based on the presented data we hypothesize that such fracture corridors, connecting localized reservoirs at different stratigraphic levels up towards the surface, represent preferred fluid migration pathways rather than the main faults.

Comparison of Mechanical and Fracture Stratigraphy between Failed Seal Analogues

The presence of discontinuities in a seal affects both their mechanical and hydrogeologic properties; migration of fluids or gas through mm- to cm-scale discontinuity networks can lead to the failure of gas or fluid traps. The presence of discontinuity networks increases the volume of rock matrix in contact with subsurface fluids creating preferential pathways for fluid flow and changing the mechanical properties of the seal. We examine the mechanical and fracture stratigraphy of failed seals analogues exposed in central and south-east Utah. We use outcrop surveys to identify relationships between occurrence of discontinuities and sedimentologic variability, and to understand the nature of alteration associated with fluid flow through these fractures. We use data from each locality to quantitatively define mechano-stratigraphic units, based on consistency in fracture distribution, bed thickness, lithologic stacking pattern, field-derived compressive strength and permeability. These data allow us to define mechano-stratigraphic units at each locality and to compare the changes in deformation behaviour between localities. Using this systematic methodology to quantify differences and similarities between field analogues we improve our understanding of the important role of micro- to meso-scale fracture networks play within sealing lithologies and how various seal lithologies respond the changes in stress.