Eichhubl, Peter¹, W. Lansing Taylor², David Pollard¹, Atilla Aydin¹
(1) Stanford University, Stanford, CA
(2) Anadarko Petroleum Corporation, The Woodlands, TX

ABSTRACT: Structural and Stratigraphic Control on Migration Pathways in Jurassic Aztec Sandstone at the Valley of Fire, Nevada

The distribution of oxidation and reduction reactions in permeable Jurassic Aztec Sandstone was mapped to assess the impact of structural and depositional heterogeneities on outcrop to reservoir scale fluid flow. The Aztec Sandstone is characterized by early diagenetic hematite grain coats resulting in a uniform red color. Two subsequent stages of fluid flow and diagenetic alteration caused bleaching by hematite dissolution and partial iron oxide reprecipitation. Based on structural crosscutting relations, the first stage of bleaching and iron remobilization is correlated with Late Cretaceous Sevier thrusting and foreland deposition of clastic sediments. The second stage occurred during and after Miocene strike-slip faulting associated with Basin and Range tectonics.
The distribution of diagenetic alteration patterns indicates that regional fluid migration pathways were controlled by stratigraphic contacts and by thrust faults. Outcrop-scale focusing of flow was controlled by structural heterogeneities such as joints, joint-based faults, and deformation bands as well as the sedimentary architecture. Slight shearing increases the efficiency of flow along joint zones resulting in haloes of enhanced wallrock alteration. Diagenetic alteration zones are deflected adjacent to joint-based faults with large offset, consistent with their dual role as aquitards for cross-fault flow and preferred pathways for fault-parallel flow. Fluid flow is similarly impeded across deformation bands resulting in alteration gradients adjacent to these structures. The complex interaction of structural heterogeneities with alteration is consistent with their measured hydraulic properties demonstrating the significance of structural heterogeneities for focused fluid flow in a porous sandstone aquifer.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.