Springhill Formation, Magellan Basin, Chile: formation water characteristics and mineralogy (vol 15, pg 651, 1999)

The Early Cretaceous Springhill Formation is a transgressive sequence of qu artz sandstones with intercalations of shale and represents the principal h ydrocarbon producer of the Magellan Basin. Analyses of its formation waters from the Chilean sector (surface area approximately 20,000 km(2)) are clas sified and linked to the diagenetic alteration of the formation. Local delt a(18)O determinations and core leaching have been carried out to evaluate h omogeneity and whole rock contribution to Springhill Formation fluid phases . Formation waters are of the chlorine-calcium type with Na+/Cl- < 1, (Na- Cl-)/SO42- < 0 and (Cl- -Na+)/Mg2+ > 1. Together with an increase in orga nic maturity (Ro) between 2000 and 4000 m, a change is observed from authig enic kaolinite to illite and corrensite, accompanied by a loss of porosity and permeability. With respect to the distribution of the authigenic pore p hyllosilicates, the shallowest kaolinite dominated area is limited by fault s against deeper sectors where corrensite and illite/smectite prevail. A la ter influx of high salinity fluids into the kaolinite area from the basin c entre or continental graben sediments has let to the present day distributi on of formation waters. Low scatter delta(18)O ratios of kaolinite and corr ensite reflect lithological consistency on a m-scale. Residual salt analyse s shows a direct relationship between the dissolution of Ca and a high CaO content in the whole rock sample. (C) 1998 Elsevier Science Ltd. All rights reserved.