Abstract: Fluid Flow History of Permo-Triassic Sediments of the Sydney Basin, Australia

P. Joe Hamilton, Peter E. Eadington, G. P. Bai

Petrographic, isotopic and fluid-inclusion investigations of diagenetic cements from Permian-Triassic sandstones in the Sydney basin has enabled

reconstruction of the fluid flow history of pore waters and hydrocarbons.

Diagenetic cements comprise, in order of formation, grain-coating clays and carbonates, pore-filling carbonates, kaolin, quartz, late interstitial carbonate and illite.

Evolution of pore-water oxygen isotope composition (^dgr¹⁸O) during burial can be modelled from the observed paragenetic sequence and measured data for mineral ^dgr¹⁸O and fluid-inclusion temperatures. Pore water ^dgr¹⁸O remained similar throughout the burial interval characterized by early clay and pore-fill carbonate diagenesis. The value of pore water ^dgr¹⁸O is estimated to have been about -15 per mil, typical for Triassic meteoric waters. This period of diagenesis was dominated by an open fluid-flow regime with sustained meteoric-water recharge. Thereafter, diagenesis proceeded under closed system conditions, and consequently pore waters became enriched in ¹⁸O by about 10 per mil by the time of quartz and illite diagenesis at temperatures of about 76-100°C.

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 compared with 1.7-1.9 HFU currently. During the Middle Jurassic, the rocks were more deeply buried by 1500-2100 m.

Calculations suggest that significant volumes of oil were generated in and expelled from associated coal measures, and this is supported by the observation of fluorescing liquid hydrocarbon inclusions in quartz cement.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994