Structural Complexity in Structurally Simple Fractured Reservoir Analogs

Laubach, Stephen E.¹, Meghan E. Ward¹ (1) The University of Texas at Austin, Austin, TX

Middle Triassic to Lower Jurassic La Boca Formation sandstones, northeastern Mexico have fracture size distributions and patterns including internal textures—that those found in cores from many dbasins. We show that many attributes that are challenging to discover in buried rocks are preserved here. This fractured reservoir analog has abrupt differences fracture sizes, clustering, and porosity preservation over vertical distances of a few meters and lateral distances of 10s of meters. Heterogeneity results from interplay of stratigraphy, structure, and diagenesis during fracture growth and aging. These rocks provide a rare opportunity to compare what can be sampled using core with attributes that cannot be measured using core-based methods.

Using diagenetic and mechanical modeling we show how this heterogeneity likely arises. Results imply that mechanical and chemical feedbacks resulting from cementation in fractures and surrounding rock mass during and after fractures form influence size and shape of fracture pore space. We show that diagenetic history is a prime control on persistent porosity in fractures in basins and thus their capacity to conduct fluid or to create a perceptible seismic response. Even structurally simple reservoirs can have great fracture, and thus fluid flow, heterogeneity.