Geochemical Characterization And Biodegradation In Caoqiao Oilfield From Dongying Depression

The Caoqiao oilfield in the Dongying Depression is a heavy oil reservoir with most heavy oils formed by biodegradation in order to accurately predict the degree of biodegradation and effectively reduce the risk of oil and gas exploration,30 oil sand samples from 6 wells in the Caoqiao area were selected for pyrolysis and gas chromatography-mass spectrometry?GC-MS?analysis.The experimental results show that most of the studied samples in the Caoqiao area is extra-heavy oil and a small proportion of samples is bitumen,while some samples still exhibit high residual hydrocarbon index and high saturated hydrocarbon content.GC-MS analysis showed that partial of n-alkanes and isoprenoids was severely biodegraded which show incomplete distribution characteristics.The absolute concentration of n-alkanes in each well diiffers significantly ranging from312 to 4650ppm.The pristane is greater than n-C₁₇,and phytane is larger than n-C₁₈.The concentration of pregnanes and diasteranes do no change considerably,however,regular steranes are biodegraded to varying in different locations,particularly in wells C105,C108,C124 with concentration in the range of1323 and 3476ppm.The concentrations of tricyclic terpenes and tetracyclic terpenes age generally low,which is dominated by C₂₃ component.The series of hopane is dominated by C₃₀??-hopane,and its concentration varies between 3540 and 9960ppm.Most samples have complete pentacyclic terpanes distribution with hopane series being enriched in the Caoqiao area.The series of 25-norhopanes have been detected in all samples.Aromatic hydrocarbons such as the series of naphthalene and phenanthrene are mostly consumed by bacterial,and the series of mono-aromatics and triaromatic steroids are relatively intact.The biodegradation degree in Caoqiao oilfield is about 6-9 on the Peters and Moldowan?1993?biodegradation scale.The coexistence of n-alkanes and 25-norhopanes indicates that crude oil in Caoqiao area is affected by late charged.Different degrees of enrichment of n-alkanes,steranes,and hopanes content indicate that late charged crude oil is also subjected to varying degrees of biodegradation.There is a good correlation between the trend of maturity of each well and the gradient of biodegradation.The late oil charge is probably affected by the oil-water contact and bedded shaly barrier.The overall oil maturity is low with most samples containning high levels of beta-carrotane and gammacerane.Biomarker parameters indicate that organic matter is mainly derived from the input of higher plants,algae,and bacteria.The depositonal environment is anoxic,high-saline,lacustrine.