Petrofacies of Eocene sand injectites of the Tumey Giant Injection Complex, California (USA)

The forearc succession of Great Valley Group in Central California provides some of the best examples of giant sand injection complexes in the world and is therefore considered valuable analogues for injectite systems in the subsurface. Several sand injection complexes are well described in the outcrop and subsurface, however the petrographic characteristics of injectites are still poorly documented. In this paper, we present the results of an integrated study of field observations, quantitative and qualitative sandstone petrography, provenance, and petrofacies analysis of the Tumey Giant Injection Complex (TGIC) in order to understand its lithostratigraphy and petrological evolution, and its impacts on reservoir petrofacies characteristics and fluid migration. The TGIC intrudes into a 450 m thick deep-water succession of slope mudrocks and sandy channel-fills of the Kreyenhagen Shale (Eocene), forming an interconnected network of sandstone sills, dykes and injection breccias. The complex generated a horizontal and vertical plumbing system for fluid migration, connecting isolated sandy channel-fills among low-permeability mudrocks. The primary detrital composition, diagenetic products, microtextures, and provenance signatures allowed for the definition and discrimination of depositional and intrusive petrofacies and their genetic relations. Petrofacies associations confirm that the gypsum-cemented feldspathic litharenites from the Kreyenhagen Shale channel-fills are the only source for the injection complex. Eodiagenetic compaction and extensive gypsum cementation reduced the primary porosity of the complex, while telodiagenetic dissolution of autigenic constituents formed pervasive secondary porosity. The underlying calcite-cemented arkosic sandstones of the Lodo and Domengine formations acted as barriers for fluid flow, aiding lateral fluid migration and overpressure buildup within the overlying Kreyenhagen channel-fills. Intense grain microfracturing occurred during sand remobilization and injection along with erosion of the host mudrocks. This study has significance for the generation of reliable stratigraphic and petrological models for sand injection complex genesis and evolution that consequently can help the understanding and exploration of injectite complexes elsewhere.