DIAGENESIS OF THE TALISMAN‐1 RESERVOIR SEQUENCE, DAMPIER SUB‐BASIN, WESTERN AUSTRALIA

The Talisman‐1 well was drilled in 1984 at the NE end of the Dampier Sub‐basin, Western Australia, in permit WA‐191‐P. Testing of the Lower Cretaceous reservoir sequence produced an aggregated flow of 11,239 barrels of oil per day (b/d) from three drill‐stem tests. The reservior sequence consists of Berriasian to Tithonian mass‐flow sandstones (‘B’ and ‘C’ and Sands), and Hauterivian to Valanginian glauconitic shelf snadstones (‘A’ Sand), separated by claystone. An understanding of the diagenetic history of the Talisman‐1 reservoir sequence has been developed from thue integrated evaluation of petrography and petrophysics with hydrocarbon show, sulphur isotope, fromation water and oil composition data. This has, in turn, provided answers concerning controls on reservoir quality and the histroy of oil entrapment in the Talisman structure. Two pyrite/ferroan dolomite‐cemented sandstone intervals, below the present oil‐water contact in the Berriasian to Tithonian reservoir snadstones, are interpreted to have developed at palaeo‐oil‐water contacts in stacked oil accumulations formed during the Late Cretaceous to Early Tertiary. The pyrite and ferroan dolomite are considered to have developed as a result of anaerobic bacterial reduction of formation water sulphate. H 2 S and CO 2 were generated, and reacted with iron derived from dissolution of glauconite to form pyrite and ferron dolomite. Glauconite dissolution has enhanced the porosity and permeability of the reservoir sandstone. Miocene tectonism produced minor faulting in the reservoir sequence, allowing cross‐feed of oil between the stacked “pays”, redistributing the oil accumulation to its present configuration. Lighter hydrocarbons preferentially leaked up the fault during this phase of tectonism, with some accumuluting in thc Hauterivian to Valanginian reservoir sandstone