Carbon dioxide genesis and prediction in the offshore Gippsland BasinElizabeth J Walker
Honours - Bachelor of Science (Petroleum Geology)- 2007
Australian School of Petroleum
The University of Adelaide
Elevated levels of carbon dioxide (>10%) are known to have detrimental effects on the commercial value of oil and gas discoveries. CO2 reduces the value of the hydrocarbon accumulation and requires extra infrastructure to combat corrosion, scale formation and the disposal of gases. Understanding the genesis, abundance, distribution and controls of CO2 can assist in exploration risk management, specifically targeting regions and depths known for their lower CO2 concentrations.
A multifaceted approach combining 181 PVT analyses from 81 wells, in conjunction with δ13Cco2 isotopes, regional geology, depth, temperature and Pco2 was used in the offshore Gippsland Basin to evaluate carbon dioxide. Many wells in the region did not have high CO2, with almost half having <5%. Low concentrations were commonly found in production wells and were assumed to have been influenced by development decisions. Yet the amount of CO2 was not exclusively considered a good discriminator for CO2 origin in the Gippsland Basin. Ranges in δ13Cco2 instead provided a simple classification scheme for CO2 in the basin. Three different groups were recognized using δ13Cco2: Group-1 <-10‰, Group-2 >-10‰ to 0‰ and Group-3 >0‰.
• Group-1 CO2 was organic and mostly likely derived from the thermal maturation of organic matter (predominately coaly source rocks). Samples were isotopically light with a δ13Cco2 <-10‰ and concentrations ranged from low (1-2%) to very high (>50%). Wells with Group-1 CO2 were located throughout the Central Deep, typically away from the Rosedale Fault System. High levels of nitrogen concentration (>1%) were related to Group-1 and methane associated with the CO2 was geochemically ‘wet'. Group-1 CO2 did not appear restricted by depth, temperature or palynological zones; however CO2 concentration did increase with depth and temperature.
• Group-2 CO2 was considered to be inorganic and came from regions where deep mantle access could occur, typically close to the Rosedale Fault System. The CO2 ranged from 1-2% to >10% and the δ13Cco2 was isotopically heavy ranging from -10‰ to 0‰. Nitrogen concentration was low (<0.1%) in relation to Group-2 and the methane associated with the CO2 was again geochemically ‘wet'. Group-2 CO2 did not appear restricted by depth, temperature or palynological zones; however CO2 concentration did increase with depth and temperature.
• Group-3 CO2 was organic and derived from biogenic processes (biodegradation). The δ13Cco2 was isotopically heavy and positive (>0‰) and the CO2 concentration was <2%. Wells were located to the west of the basin where a freshwater lens associated with hydrocarbon biodegradation was present. Methane associated with the CO2 was marginally drier than both Group-1 and Group-2 wells. Group-3 samples were restricted by depth and temperature (<1500m and <80°C), but were not restricted by palynological zones.
The Pco2/depth relationship within the basin implied Group-1, Group-2 and Group-3 were not controlled solely by the inorganic chemical equilibrium between the mineral system and the pore water chemistry.
Recommendations from the study indicate the highest risk for elevated CO2 levels in hydrocarbon accumulations occurs for prospects close to the Rosedale Fault System. In addition the trend of increasing CO2 with depth suggests targeting deeper plays will be associated with higher concentrations of CO2. In future studies the use of helium (He) isotopes in combination with δ13Cco2 is suggested. The technique could help differentiate types of inorganic sources, such as magmatic/mantle-derived from carbonates.