Clastic Injectites: Fracture Mechanics, Subsurface Architecture, and Implications for Basinal Fluid Flow

Abstract

Injectites are examples of natural hydraulic fracturing, which require a pressure differential between the source of the injecting sediment and the tip of the developing fracture to sustain propagation. Understanding how injectites form is important, as they are increasingly recognized as significant components of sedimentary basin-fills, but are not predictable using traditional facies models. We present a multi-scaled analysis of injectites to understand their propagation process(es), their 3D architecture, and their impact on fluid migration pathways. First, the fracture patterns on the margins of dykes are used to interpret propagation direction and a laminar flow regime. Centimeter-scale ptygmatic folding demonstrates complex 3D architecture of small-scale dykes, which is governed by host mudrock resulting in highly variable fracture apertures. Second, these insights are applied to larger scale (100s m to km) outcrop studies in the Karoo Basin, where extensive outcrop and well constrained paleogeography permits the large-scale injectite geometry to be related to parent sandstone facies and architecture. The outcrop data permits construction of forward seismic models that validate seismic mapping, demonstrating that injectite architecture is scale invariant, which supports the use of outcrop-scale data in seismic-scale interpretations. Third, from this multi-scaled analysis we assess the influence of fluid flow pre-, syn- and post-injection across several scales. Injectites are widely identified in subsurface data with specific targeting of clastic injectite complexes as hydrocarbon reservoirs. The initial hydraulic fracturing process leads to particular architecture of injectites. Increasing predictability in the location and character of injectites allows subsurface uncertainty in the impact clastic injectites to be reduced.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016