Dhm. Alderton et Re. Bevins, P-T CONDITIONS IN THE SOUTH-WALES COALFIELD - EVIDENCE FROM COEXISTING HYDROCARBON AND AQUEOUS FLUID INCLUSIONS, Journal of the Geological Society, 153, 1996, pp. 265-275
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.