Hydrocarbon Alteration as a Result of Thermochemical Sulfate Reduction in the Upper Devonian Nisku Formation, Western Canada Sedimentary Basin

B. Kim Manzano, Hans G. Machel

In the Brazeau River Area of the Western Canada Sedimentary Basin, fifteen hydrocarbon reservoirs in the Upper Devonian Nisku Formation exhibit a change from sweet oil pools updip (ca. 3100 m depth) to sweet gas condensate and then sour gas condensate pools downdip (ca. 4000 m depth). Hydrogen sulfide concentrations range from <1 mol-% to >20 mol-%. These reservoirs provide a unique possibility to investigate hydrocarbon alteration as a function of thermal maturation and thermochemical sulfate reduction (TSR) because they appear to be hydrodynamically isolated on the scale of tens of millions of years.

Thermal maturation affected mainly the C₁₅₊ fraction, whereas TSR affected mostly the lighter hydrocarbons. Within the gasoline range (C₅-C₈), normal heptane is consumed during TSR resulting in a lowering of the paraffinicity (maturity index) normal heptane/methylcyclohexane in sour gas condensate pools. TSR further resulted in a small decrease in API gravity and an increase in the sulfur content of the liquid hydrocarbons. ^dgr³⁴S values of hydrogen sulfide range from 19 to 23 ^pmil CDT, whereas organic sulfur in the corresponding liquids have higher ^dgr³⁴S values that range from 19 to 26 ^pmil. Detailed chromatography and carbon isotope analyses of various isolates of the condensates reveal further details characteristic of TSR

Petrographic, inorganic geochemical, and bulk hydrocarbon compositional data led to the recognition of oil versus gas condensate pools, and to a crude grouping of gas condensate pools into those with and without TSR. The new organic geochemical data add a further level of resolution that permits unequivocal identification pools that have been altered by TSR.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995