Difference Of Gas Generation Potential Between Reservoired Oil And Source Rock Residual Bitumen During Thermal Cracking And Gas Origin Assessment In Anyue Gas Field

The Anyue gas field,located in Gaoshiti-Moxi paleo-uplift structure of cen tral Sichuan basin,is the largest Chinese gas field ever discovered in carbonate reservoirs of the Longwangmiao Formation and the 4th member of the Dengy ing Formation.Natural gas is mainly derived from thermal cracking origin but the relative contribution from reservoired oil,residual bitumen within source ro cks and residual bitumen within second migration pathway remains controversia l.In order to understand relative contributions from residual bitumen and res ervoired oil to gas accumulation in Anyue gas field,gold tube pyrolysis experi mental results of different types of oil and source rocks have been systematical ly evaluated in the present study.The difference of gas generation potential fro m different types of oil,different bulk fractions of oil and dispersed bitumens in source rocks has been systematically evaluated.The initial and peak gas gen eration maturity and their corresponding geological time have also been investi gated.Geochemical compositions of oil and solid bitumen have been incorporat ed into mass balance calculation model to estimate gas generation potential fro m various source types.Gas accumulation time was assessed based on kinetic reaction of different source types and basin tectonic evolution history.Significant difference of gas generation potentials were noticed from differ ent source types.Heavy oil and dispersed bitumen in source rocks have lower initial gas generation temperature than those from light oil and normal oil.In t he early stage of oil cracking,methane yields from heavy oil and dispersed bit umen are higher than those from normal oil and light oil.With the increase of temperature at the main thermal cracking stage,methane yields from light and normal oil are much higher than those from heavy oil and dispersed bitumen.The main gas generation period of heavy oil is at EasyRo of 1.0―2.1%,whe reas the main gas generation period of normal oil and light oil are at EasyRo of 1.2―2.3% and 1.5―2.3%,respectively.Heavy oil is rich in resins and asph altenes which have low activation energies and easy to crack,while its lower methane yield is controlled by low H/C ratios.Light oil is rich in saturated hy drocarbons which have high activation energies and hard to crack,whereas its higher methane yield is governed by high H/C ratios.The cracking of residual bitumen within source rocks share some similarit y as heavy oils because the chemical compositions of residual bitumen after ex pulsion and primary migration are similar to heavy oil.Meanwhile,catalytic ef fect from clay minerals in source rocks can further facilitate thermal cracking process.Simulation results suggest that thermal cracking of residual bitumen sta rts at EasyRo 1.0% and the main gas generation period is about EasyRo 1.0―2.1%.Residual bitumen within source rocks has limited contribution to gas acc umulation due to low hydrogen content before thermal cracking.If expulsion ef ficiency is 60%,the calculated contribution of residual bitumen within source r ocks is about 15%.If expulsion efficiency rise to 70%,the calculated contribut ion of residual bitumen within source rocks is only about 6%.The calculated contribution of residual bitumen within secondary migration pathway is about 7.9―12.9%.Gas in Anyue gas field is dominated by oil thermal cracking origin.