Source Rocks Evaluation And Oil Source Analysis Of Baobab Structural Belt In Bongor Basin

Bongor basin is a Meso-Cenozoic rift basin which located in the interaction of western and central African rift systems and impacted by both of them. Because of developmental fracture structure、complexity structural evolution history and obvious strike-slip and inverse structural deformation, the oil-gas source problem is of complexity. Determination of chief source rocks has direct relation to the forecast of oil-gas resources and select of exploration planning. Baobab structural belt is oil-gas enrichment area in Bongor basin, which has important significance for the systematic study on hydrocarbon source rocks and oil-gas sources.On the basis of collection and collation of the geological data about the Lower Cretaceoin Baobab structural belt, combining with the previous research results and geologging plot, the cuttings, core and oil samples have been collected and screened firstly. Then hydrocarbon source rock samples of K, M and P formation in Baobab structural belt are measured for organic carbon, rock-eval, Kerogen carbon isotopes, Kerogen microscopic andvitrinite reflectance, etc. According to the evaluating standard of lacustrine source rocks, the source rock has been evaluated synthetically. On this basis, some typical source rock sampes are selected to have the analysis of chloroform bitumen "A" and group composition. Then the experimental analyses, which include the gas-facies chromatogram of saturated hydrocarbon and chromatograph-mass spectr-ometer of, have been proceeded in these source rock and oil samples. The oil source analysis has been discussed by the compositional characteristics of the biomarkers. The main results are as follows:1. Statistical results show that most of the dark mudstones in Baodab structural belt are well developed. For example,the total thickness of dark mudstone in Baobab SE-1well is to1164.00m,accounting for56.34%in the overall thickness of the well.In study area,the dark mudstone is thick and developed well in K、M and P formation.The percentage range of thickness are respectively49.09%～67.16%,58.34%～84.57%,41.28%～77.82%in K、M and P formation, the average percentage are57.07%,75.65%and63.88%.These source rocks are characterized by high abundance of organic matter.The range of the organic carbon content is0.28%-25.50%, while the mean is up to2.54%; The hydrocarbon potential ranges from0.32to256.65mg/g with the mean of14.43mg/g.It belongs to the good-better source rocks as a whole.The hydrocarbon source rocks are with the average value of TOC of3.93%and the average value of S1+S2of28.96mg/g in K formation, so the source rocks of K formation are good-better.The average organic matter abundance of source rocks are respectively2.13%and2.41%in M and P formation.The average hydrocarbon potential are respectively12.38mg/g and12.97mg/g.Thus the source rocks are moderate-good in M and P formation. 2. The kerogen carbon isotope ranges from-30.2%o to-28.8%o in an important exploration well of Baobab S-1well. It can be seen that the organic matter type of source rocks is I in study area. The result of kerogen microscopic examination shows that the organic macerals components in every formation are mainly sapropel and exinite with high hydrocarbon-generating potential. Both content are more than71%of the macerals. The value of type index Ti is between49.3and83, while the average value is68.2. The organic matter type is Ⅱ1type overall. Some individual samples have high Ti value, which exceeding I type organic matter standard (Ti=80). Its organic matter belongs to Ⅰ type. The pyrolysis datas show that most of the source rock samples are better with Ⅰ-Ⅱ1type in Baobab structural belt. A small amount of samples are Ⅱ2type. There are a little samples of III type in K formation, but there is no III type in in M and P formation.3. The degree of thermal evolution is rather low with Ro ranged from0.43%to0.74%in study area. Most of them were still in immaturity or lower maturity stage. With the increase of the depth, the maturity increases, but parts of source rocks samples are maturity in P formation. The pyrolysis parameter (Tmax) are between412℃and555℃, which show most of source rocks samples was still immaturity or lower maturity.Few samples are maturity. This result agrees with Ro.4. Based on the molecular geochemical characteristics of hydrocarbon source rocks in K、M and P formation, it is found that source rocks forms in lacustrine sedimentary environment in Baobab Structural Belt. It has obvious even-odd predominance in high carbon number (nC22-nC32) of nalkanes. The value of Pr/Ph ranged from0.99to2.38, which reveals the certain advantage of pristan. The tricyclic terpanes show normal distribution.17a (H)-C3o-hopanoid is the main peak. It has low abundance of hopane series with the hopane index ranged from0.03to0.07. The gammacerane index is low as between0.19and0.50, with an average of0.32. The distribution pattern of regular sterane (C27, C28, and C29) presents the V curve. The level of C29sterane is the highest. The ratio of C27and C29is0.77. The maturation of source rock samples is characteristic of low maturity to maturity by C29S/(S+R) ranged from0.28to0.48and C29ββ/(ββ+αα) ranged from0.11to0.36in regular sterane.5. According to the molecular geochemical characteristics of saturated hydrocarbon and aromatic hydrocarbon in crude oil samples, the results show that oil samples are typical lacustrine oils and similar. It is suggested that the oils belong to the same oil family by the research of the genetic type and the maturity of oils. It has a certain even-odd predominance in high carbon number (nC22-nC28) of nalkanes. The CPI and OEP are more than1.1and Pr/Ph ranges from1.93to2.67. The threefluorene series are mainly in fluorine series. The second is dibenzothiophene series, the dibenzofuran are lowest. 6. In view of the geochemical characteristics of hydrocarbon source rocks and crude oils in study area, combined with the divide of the genetic type and the maturity of oils and the optimal selection parameters for oil sources correlation including Pr/Ph, C21TT/C23TT and the gammacerane index, the oil sources correlation has been researched subtly. The result shows that the crude oil has the best genetic relationship with source rock of well Prosopis-3(1810-1875m at the bottom of K formation), Baobab S-1well (1695-1775m P formation) and Ronier4-1well (1800-1850m at the bottom of K formation).That is to say, the crude oils are most likely from high abundance of immature source rock from the bottom of K formation and P formation in Baobab structural belt.