Biodegradation And Geochemical Evaluation Of Crude Oil In Ronier Oilfield,Bongor Basin,SW Chad

Located in the Ronier oil field in the northern slope of the Bongor Basin in Chad,the reservoir-forming combination is mainly?B+R+K?and the lower combination?M+P?.The upper combination is mainly heavy heavy oil that has undergone biodegradation,and the lower combined depth is large,and the exploration risk is high.The recovery of heavy oil varies greatly,and the development process requires high.The traditional static geology has challenged the evaluation of heavy oil reservoirs.There are no large-scale geochemical characteristics and crude oil degradation evaluation studies in the northern slope zone.Establishing biodegradation evaluation criteria applicable to the area,combined with relevant geological information,can support the prediction of biodegradable crude oil distribution throughout the northern slope zone and improve the economic benefits of crude oil production.In this paper,15 crude oil samples from the Ronier oil field were collected,and the molecular geochemical characteristics of saturated hydrocarbon biomarker compounds and aromatic hydrocarbon components were studied by chromatographic mass spectrometry.It aims to discuss the geochemical characteristics of crude oil in the study area and establish a set of crude oil evaluation and biodegradation evaluation standards applicable to the area.The tricyclic ruthenium series of crude oil in the study area is dominated by C₂₃TT,and the relative content of the ascending decane series decreases with the increase of carbon number;the average gamma wax/C₃₀ decane(Ga/C₃₀H)is 0.34;the ratio of C₂₃TT/C₃₀H is low;C₃₁ The ascending decane 22R/C₃₀H averaged 0.16;the regular decane series was dominated by C₂₉;the C₂₆ triaryl decane?20S?/C₂₈ tributary aryl?20S?averaged 0.53,which was a set of low salinity and terrestrial input.The contribution of the lacustrine source rocks.The crude oil maturity in the study area is medium,with an average of 0.57 for Ts/?Ts+Tm?;K3=2,3,6/?2,3,6+1,2,5?-TMN is 0.48,TeMNr=1,3,6,7/?1,3,6,7+1,2,5,6+1,2,3,5?-TeMN is 0.45;20S/?20S+20R?-C28 triarylnonane has an average of 0.41.The current low API°is due to biodegradation.The degradation sequence of sterane and hopane in the study area is very vague,so the biodegradation evaluation criteria of Peters and Moldowan?1993?do not apply to the study area.Through the combination of 25-Norhopane series,aromatic hydrocarbon series and demethylated tricyclic oxime series,the evaluation criteria suitable for the degradation grade 6 and above in this area have been established:6 Level is the25-norbornane series;7 Level The order of alkyl naphthalene is significantly lower than that of alkyl phenanthrene;8 Level is the appearance of demethyltricyclic guanidine series;9 Level is 2-3 ring compound disappears,and lanthanum and tribute series are relatively high.When the biodegradation grade is not more than 7?PM?7?,the series of parameters such as sterane and hopane are reliable and effective;when PM=8,tricyclic guanidine and gamma wax are degraded,and the relevant thermal maturity The parameters Ts/Tm and C₂₉Ts/(C₂₉Ts+17??H?-C₂₉-hopane)are both ineffective,only K3and TeMNr are effective;when PM?9,most of the geochemical parameters have failed.The anti-degradation ability of decane is underestimated,and the isomerization parameter 20S/20?S+R?is still effective at PM?8.At PM=6,the degradation of decane is very slight,and the degradation products can be simply superimposed for oil source comparison.The separation of a set of thick mudstones distributed in the K layer of 1450m is the main controlling factor for the significant differential distribution of crude oil in the vertical direction,and may hinder the vertical migration of the combined oil and gas.The combined oil source still needs further study,and the bottom of the thick mudstone is still a favorable layer for exploration.