Fluid Flow Histories in Permo-Triass Sediments of the Sydney Basin, SE Australia; Isotope and Fluid Inclusion Constraints

P. Joe Hamilton, G. P. Bai, P. J. Eadington, and J. B. Keene

A petrographic, isotopic and fluid inclusion investigation of Permo-Triassic sandstones in the Sydney Basin has enabled reconstruction of the history of fluid flow. Diagenetic cements in Triassic sandstones comprise, in order, grain coating clays and carbonates, pore filling carbonates, kaolin, quartz, late interstitial carbonate and illite. Diagenesis in the Permian sandstones began with pore filling siderite prior to saddle ankerite, followed by quartz and then illite and late carbonates.

Fluid rock interaction during burial resulted in increases in pore water ^dgr18O from depositional values of -17^pmil to about -12^pmil -7^pmil at the time of quartz overgrowth crystallisation and to between -7^pmil to -1^pmil at the time of illite crystallisation.

Interpretation of fluid inclusion data and maturity measurements indicates that heat flow increased during the Triassic and Jurassic, reaching a maximum of about 2.1 HFU in the Cretaceous. Late Cretaceous uplift, cooling and erosion was initiated at the same time as, and probably related to, the initiation of Tasman Sea rifting. The rocks had previously been more deeply buried by 1500-2100 m. The uplift resulted in cessation of illite diagenesis timed at 90Ma and meteoric water invasion of the sandstones in turn causing partial resetting of ^dgrD compositions of the diagenetic clays to less D-depleted values.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California