Organic-Inorganic Interaction and Alteration of Petroleum Reservoirs: A Case Studies from Puguang Gas Field, Sichuan Basin, China

Fang Hao
China University of Petroleum, Changping, Beijing, China

The carbonate reservoirs in the Puguang gas field had been buried to a depth of about 7000m and experienced maximum temperature up to 220C before they were elevated to the present-day depth of 5000 to 5500m, with present-day thermal maturity between 2.0 and 3.0%Ro. The Puguang gas field evolved from a paleo-oil field. The combination of thermal cracking of crude oils and thermochemical sulfate reduction (TSR) had not only led to the generation of wide-spread pyrobitumens with high reflectance and high sulfur content and the generation of CO₂ and H₂S gases, but also caused the conversion from reversed to normal isotope distributions between methane and ethane. The salinity of primary aqueous fluid inclusions decreased from about 20 wt% NaCl to less than 5 wt% NaCl as homogenization temperature increased from less than 100C to higher than 200C, suggesting that substantial amount of “freshwater” had been generated and added to the petroleum pools. Complicated precipitation and solution occurred in the hydrocarbon-water-rock system. Intensive solution of carbonate minerals resulted in the local occurrence of abnormally high porosity (up to 30% at depth greater than 5100m) and permeability (up to 9800 md), whereas intensive precipitation of 13C-depleted calcite had locally reduced the porosity and permeability of the reservoir rocks. It appears that post-accumulation processes especially thermochemical sulfate reduction during deep burial had profoundly altered the composition and characteristics of the petroleum fluids, the physicochemical conditions in the petroleum pools and the properties of the reservoir rocks. Understanding the complicated post-accumulation processes is of great significance for petroleum exploration in superimposed basins in China.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas