Abstract

The Early Cretaceous Springhill Formation is a transgressive sequence of quartz sandstones with intercalations of shale and represents the principal hydrocarbon producer of the Magellan Basin. Analyses of its formation waters from the Chilean sector (surface area approximately 20,000 km 2) are classified and linked to the diagenetic alteration of the formation. Local ? 18O determinations and core leaching have been carried out to evaluate homogeneity and whole rock contribution to Springhill Formation fluid phases. Formation waters are of the chlorine-calcium type with Na + Cl - < 1, (Na +-Cl -)/SO 4 2- < 0 and (Cl --Na +)/Mg 2+ > 1. Together with an increase in organic maturity (Ro) between 2000 and 4000 m, a change is observed from authigenic kaolinite to illite and corrensite, accompanied by a loss of porosity and permeability. With respect to the distribution of the authigenic pore phyllosicates, the shallowest kaolinite dominated area is limited by faults against deeper sectors where corrensite and illite/smectite prevail. A later influx of high salinity fluids into the kaolinite area from the basin centre or continental graben sediments has let to the present day distribution of formation waters. Low scatter ? 18O ratios of kaolinite and corrensite reflect lithological consistency on a m-scale. Residual salt analyses shows a direct relationship between the dissolution of Ca and a high CaO content in the whole rock sample. The Early Cretaceous Springhill Formation is a transgressive sequence of quartz sandstones with intercalations of shale and represents the principal hydrocarbon producer of the Magellan Basin. Analyses of its formation waters from the Chilean sector (surface area approximately 20,000 km 2) are classified and linked to the diagenetic alteration of the formation. Local ? 18O determinations and core leaching have been carried out to evaluate homogeneity and whole rock contribution to Springhill Formation fluid phases. Formation waters are of the chlorine-calcium type with Na +/Cl - < 1, (Na +-Cl -)/SO 4 2- < 0 and (Cl --Na +)/Mg 2+ > 1. Together with an increase in organic maturity (Ro) between 2000 and 4000 m, a change is observed from authigenic kaolinite to illite and corrensite, accompanied by a loss of porosity and permeability. With respect to the distribution of the authigenic pore phyllosilicates, the shallowest kaolinite dominated area is limited by faults against deeper sectors where corrensite and illite/smectite prevail. A later influx of high salinity fluids into the kaolinite area from the basin centre or continental graben sediments has let to the present day distribution of formation waters. Low scatter ? 18O ratios of kaolinite and corrensite reflect lithological consistency on a m-scale. Residual salt analyses shows a direct relationship between the dissolution of Ca and a high CaO content in the whole rock sample.