SPRINGHILL FORMATION, MAGELLAN BASIN, CHILE - FORMATION WATER CHARACTERISTICS AND MINERALOGY

The Early Cretaceous Springhill Formation is a transgressive sequence of quartz sandstones with intercalations of shale and represents the p rincipal hydrocarbon producer of the Magellan Basin. Analyses of its f ormation waters from the Chilean sector (surface area approximately 20 ,000 km(2)) are classified and linked to the diagenetic alteration of the formation. Local delta(18)O determinations and core leaching have been carried out to evaluate homogeneity and whole rock contribution t o Springhill Formation fluid phases. Formation waters are of the chlor ine-calcium type with Na+/Cl- < 1, (Na+ -Cl-)/SO42- < 0 and (Cl- -Na+) /Mg2+ > 1. Together with an increase in organic maturity (Ro) between 2000 and 4000 m, a change is observed from authigenic kaolinite to ill ite and corrensite, accompanied by a loss of porosity and permeability . With respect to the distribution of the authigenic pore phyllosilica tes, the shallowest kaolinite dominated area is limited by faults agai nst deeper sectors where corrensite and illite/smectite prevail. A lat er influx of high salinity fluids into the kaolinite area from the bas in centre or continental graben sediments has let to the present day d istribution of formation waters. Low scatter delta(18)O ratios of kaol inite and corrensite reflect lithological consistency on a m-scale. Re sidual salt analyses shows a direct relationship between the dissoluti on of Ca and a high CaO content in the whole rock sample. (C) 1998 Els evier Science Ltd. All rights reserved.