P-T CONDITIONS IN THE SOUTH-WALES COALFIELD - EVIDENCE FROM COEXISTING HYDROCARBON AND AQUEOUS FLUID INCLUSIONS

Siderite nodules in the Carboniferous Coal Measures of South Wales con tain cavities which are often infilled with quartz, carbonates, sulphi des, and hydrocarbons. The quartz contains a mixture of hydrocarbon an d aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperat ures in the range 97-212 degrees C (mean 143 degrees C). The hydrocarb on fluid inclusions are dominated by methane with a small component of higher order hydrocarbons; their homogenization temperatures are in t he range 35-78 degrees C (mean 54 degrees C). It is assumed that the t wo fluids were trapped simultaneously during growth of the quartz and thus a P-T estimate of entrapment can be obtained by graphical interse ction of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This me thod gives temperatures between 130 and 160 degrees C, and pressures b etween 40 and 55 MPa. The timing of mineralization is uncertain, but i t is suggested that it took place during burial and low grade metamorp hism of the subsiding sedimentary basin (i.e. in the Upper Carbonifero us). The hydrothermal fluids were probably derived from evolved meteor ic or connate waters expelled during subsidence and sediment compactio n.