A FLUID INCLUSION AND CATHODOLUMINESCENCE APPROACH TO MODEL FRACTURE GROWTH IN THE TRIASSIC-JURASSIC LA BOCA FORMATION, NORTHEASTERN MEXICO

Successful prediction of fracture hydraulic aperture, spacing, and connectivity is essential for the development and production of low-permeability tight-gas sandstone reservoirs. In these deep (>3 km) reservoirs, the hydraulic apertures of natural fractures are significantly affected by fracture cements requiring an integrated structural and diagenetic approach to fracture prediction. We report initial results from an ongoing study of fracture growth histories in outcrop analogs of tight-gas sandstones. Outcrop samples from the Triassic-Jurassic La Boca Fm., northeastern Mexico, show localized quartz cement deposits with crack-seal texture and multiple fluid inclusion assemblages. Following methods developed for tight-gas reservoir sandstones in the East Texas basin, we combine fluid inclusion microthermometry and SEM-based cathodoluminescence imaging of quartz fracture cement to determine timing and rate of fracture opening. Cathodoluminescence imagery reveals multiple stages of crack-seal texture indicative of fracture cement growth coeval with fracture opening. Early crack-seal textures grow symmetrically toward the center of the fracture. Later stages of crack-seal opening and cementation appear to initiate asymmetrically away from the center of the fracture. Euhedral quartz cement precipitates simultaneously with crack-seal cement into open fracture space. Typically, late-stage carbonate cements precipitate in remaining pore-space. Fluid inclusion homogenization temperatures of crack-seal cement range from 164°C to 185°C. Temperatures are higher towards the center of the fracture suggesting fracture opening and cementation during prograde burial. Low-temperature (150°C to 160°C) homogenization temperatures of late crosscutting cements indicate fracture opening continued exhumation.