POROSITY PRESERVATION DUE TO AUTHIGENIC CHLORITE COATINGS IN DEEPLY BURIED UPPER TRIASSIC XUJIAHE FORMATION SANDSTONES, SICHUAN BASIN, WESTERN CHINA

Non-marine sandstones in the Upper Triassic Xujiahe Formation form an important reservoir unit in the Sichuan Basin, western China. Widespread authigenic chlorite cement is present in the second and fourth members of the formation, referred to as the Xu II and IV members. This study, based on data from 12 wells from three gasfields, focusses on chlorite development and evolution and attempts to evaluate its influence on reservoir quality. Data included results of Xray diffraction (XRD) and electron microprobe analyses (EMPA), scanning electron microscopy (SEM), and stable carbon and oxygen isotope analyses. The results showed that chlorite coatings preserve primary pore spaces in the sandstones by inhibiting the nucleation of quartz cements on grain surfaces. Iron for chlorite growth came from the dissolution of Fe-rich magmatic rocks derived from the palaeo-Longmenshan fold-belt, from dissolved and colloidal iron which had been precipitated in the Xujiahe Formation in a fluvio-deltaic environment, and from compacting mudstones in other pars of the formation. The chlorite initially precipitated from pore waters during eodiagenesis (< 70 degrees C; < 2 km burial depth). During mesodiagenesis (> 70 degrees C, > 2 km), the chlorite was modified due to exposure to Mg-rich fluids derived from the underlying carbonate-dominated Leikoupo Formation. Thus the Mg content of chlorite in the Xu II member increases with depth. However this phenomenon was not observed in the Xu IV member. The effects of authigenic chlorite on the sandstones' petrophysical properties were evaluated from correlations between chlorite content and porosity. Optimum thicknesses for chlorite coatings range from 5 to 10 microns. Thicker or thinner coatings appear to be less capable of preserving primary porosity and permeability.