THERMO-CHEMICAL CONTROLS ON DIAGENETIC PROCESSES: IMPACT ON GEOLOGIC MODELS FOR GEOPRESSURE, FLUID MIGRATION, BIODEGRADATION, AND OPERATIONAL SAFETY

摘要

Proposed models for the effect of clay diagenesis on shale/claystone permeability based on precipitation of clay minerals in pore networks and exponential decreases in permeability (Bjorkum and Nadeau, 1998) have been confirmed by subsurface studies (Nadeau et al., 2002). These results have important implication for modelling fluid flow at the basin and field scale, including: 1. overpressure development; 2. hydrocarbon migration; 3. fluid flow through oil columns, shale top seals, and possible controls on biodegradation. This communication further develops the proposed model, and evaluates the implications for petroleum systems analysis, including models for biodegradation, as well as drilling/operational safety. Conventional models for fluid flow in shales/claystones are mainly a function of porosity reduction resulting from mechanical compaction and increasing effective stress. The resulting models predict that shale permeability is mainly a function of porosity reduction with increasing depth and effective stress. Clay diagenesis models often consider that diagenetic clay minerals, typically illite, form by transformation of pre-existing smectite layers by K-fixation and layer collapse. Neoformation models for illite diagenesis, however, indicate that illitic clays precipitate within pore networks from the dissolution of mineral reactants, inlcuding smectite or kaolinite, and K-feldspar. This reaction is mainly a function of temperature, and typically begins at 60° to 80°C, and completed at circa 100°C (Figure 1).