Geochemical Fingerprints: A Clue to Fluid Migration and Diagenetic History

Sharon A. Stonecipher

The Talisman #2 well, northwest offshore Australia, was drilled and abandoned in 1985. Cementation patterns and geochemical indicators have been used to delineate an oil reservoir which no longer exists.

The main unit of interest in the Talisman core consists of a Hautervian to Valanginian, burrowed, clayey, quartz/glauconite sandstone cemented by siderite, ferroan dolomite, and pyrite. The unit represents a generally coarsening- and cleaning-upward sequence typical of shelf sand buildups.

It appears that sideritized intervals all represent originally porous and permeable zones situated beneath impermeable layers. This distribution and carbon isotope data (^dgr¹³C ranging from -8 to -20 ^pmil both suggest formation in a gas cap. The gas cap served, in part, as the source of carbon for the siderite which began precipitating preferentially along iron oxide-rich clay laminae. Oxygen isotope values are around zero, suggesting low temperature formation.

Ferroan dolomite cements are also most common near the top of the reservoir. Carbon isotope values are again light (avg. ^dgr¹³C of -16 ^pmil), again suggesting a hydrocarbon source, but oxygen isotope values are also light (avg. ^dgr¹⁸O of -8.8 ^pmil), suggesting higher temperatures of formation. This is consistent with petrographic evidence that the ferroan dolomite formed after the siderite.

Pyrite cements are concentrated in the lower portions of the reservoir and in fractures in the underlying claystone. Pyrites from the Talisman well exhibit ^dgr³⁴S values ranging from -6.5 to +22.9% ^pmil. Bacterial reduction commonly produces pyrites with ^dgr³⁴S values ranging from -30 to -10 ^pmil. Sulfate reduction in a closed system could produce pyrite as heavy as the seawater from which it formed; however, the secular variation curve for seawater sulfate suggests that Jurassic/Cretaceous seawater had a ^dgr³⁴S of only +16 ^pmil. Another mechanism that would explain the observed ^dgr³⁴S values involves the interaction between Talisman oil, H₂S, and pre-Jurassic sulfate. Orr discussed a mechanism for the reduction of hydrothe mally introduced sulfate through contact with reservoired oil. The reaction requires temperatures of 80° to 120°C (similar to Talisman temperatures) and a source of dissolved sulfate. Formation fluids contain up to 1,000 ppm SO = ₄ with a ^dgr³⁴S of +31.3 ^pmil which suggests a source of Devonian age or older.

Another important piece of information provided by the pyrite isotope data comes to light when they are plotted as a function of depth. Pyrites from the reservoir interval and the top of the claystone exhibit ^dgr³⁴S values of +19.7 to +22.9 ^pmil whereas pyrites from below the most heavily pyritized zone in the claystone exhibit ^dgr³⁴S values ranging from -6 to +5 ^pmil. The one sample from within the pyritized zone shows a value of +11.1 ^pmil. This heavily pyritized zone is interpreted to represent a paleo-oil/water contact. Subsequent tectonic events appear to have allowed leakage of oil out of the reservoir leaving only this geochemical fingerprint as a record of its passing.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.