Hydrous Pyrolysis in the Field: Closed-System Diagenesis at High Fluid Flow

Ian Hutcheon, Hugh Abercrombie, Maurice Shevalier, Cynthia Nahnybida

Diagenetic processes are studied by observing natural systems or by experimental hydrous pyrolysis of water-organic-rock mixtures. Steam-enhanced recovery is similar to hydrous pyrolysis but is done in a previously undisturbed geological setting with mass, time, and temperature closer to natural diagenetic systems.

Chemical and isotopic compositions of produced water and gas were determined for wellhead samples obtained from quartz-rich and lithic reservoirs. Estimates of reservoir temperature were made using the silica and Na-K geothermometers and agree with temperatures estimated from ¹²C/¹³C partitioning between bicarbonate and CO₂. Temperature and fluid composition data are portrayed on activity diagrams and show that minerals (illite, chalcedony, chlorite, analcime, and smectite) rapidly reach equilibrium with waters. Mineral reactions inferred from produced waters are different in quartz-rich and lithic reservoirs and agree with mineral reactions observed in post-steam cores. Carbon isotopic data indicate that carbonate minerals are the source of produced C ₂.

Comparison of the buffering potential of aqueous carbonate species, carbonate minerals, organic acids, and silicate hydrolysis shows that silicates have the greatest potential to buffer pH. Our data are consistent with pH control by silicate hydrolysis and indicate that silicate-carbonate reactions may be a major source of CO₂ during diagenesis. More generally, our results show that a diagenetic system of high fluid flow can be approximated by closed-system behavior.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.