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Natural Gas Geoscience ›› 2021, Vol. 32 ›› Issue (2): 205-214.doi: 10.11764/j.issn.1672-1926.2020.08.002

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Hydrocarbon characteristics and genesis in the southern depression of Bongor Basin， Chad

Zhi-gang WEN1(),Li-rong DOU1,2,Ding-sheng CHENG2,Wei LI3   

  1. 1.College of Resources and Environment，Key Laboratory of Exploration Technology for Oil and Gas Resources，Ministry of Education，Yangtze University，Wuhan 430100，China
    2.Research Institute of Petroleum Exploration and Development，PetroChina，Beijing 100083，China
    3.CNOOC Research Institute Co. Ltd. ，Beijing 100028，China
  • Received:2020-06-16 Revised:2020-07-15 Online:2021-02-10 Published:2021-03-10
  • Supported by:
    The National Natural Science Foundation of China(41972122)

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Abstract:

The discovery of a series of natural gas in the Bongor Basin， Chad has opened up a new situation of simultaneous oil and gas development. There are obvious phenomena of oil and gas co-existence in D area of the southern depression. The study on the distribution characteristics and genesis of oil and gas in D area is of great significance to the natural gas prospect of Bongor Basin， and is conducive to the further understanding of the source rock potential of Bongor Basin. Taking Well D-1 as an example， the effect of natural gas on the accumulation of crude oil is analyzed by using the parameters such as natural gas composition， isotopes， light hydrocarbons， as well as the parameters such as crude oil chromatography and chromatism. Results show that crude oil at different depths of Well D-1 is the product of the same source in the same evolutionary stage. It was mainly affected by gas washing and migration fractionation in the later period. Natural gas brings the light components from deep reservoirs to shallow reservoirs， which enriches the light components in shallow reservoirs. The chromatogram shows the double peak feature， and the main peak of the front peak has obvious low carbonization. At the same time， natural gas mainly exists in shallow reservoirs， forming the coexistence of oil and gas.

Key words: Bongor Basin, Southern depression, Natural gas, Evaporation fractionation

CLC Number: 

  • TE122.2

Fig.1

Structure outline of Bongor Basin"

Table 1

The compoment of natural gas and crude oil in Well D-1 in Bongor Basin"

编号深度/m原油族组成/%天然气组分/%
饱和烃芳 烃非 烃沥青质nC1nC2nC3nC4N2CO2
a1 531.073.9017.304.004.9073.777.946.011.347.041.56
b1 669.081.2013.102.702.9074.786.825.151.148.142.02
c1 858.058.7012.1027.202.00------
d1 950.062.708.0028.300.90------
e2 177.061.3010.3025.103.30------

Table 2

Isotopic characteristics of natural gas in Well D-1 in Bongor Basin"

深度/mδ13C/‰δD/‰
甲烷乙烷丙烷正丁烷异丁烷甲烷乙烷丙烷
1 531.0-49.5-38.5-31.8-29.5-32.8-238-248-168
1 669.0-49.535-38.3-31.8-30.6-33.1-237-248-171

Fig.2

Origin identification of natural gas in Well D-1 in Bongor Basin"

Fig.3

Distribution of steranes and trepane at different depths in Well D-1 in Bongor Basin"

Fig.4

Comparison of crude oil geochemical parameters at different depths in Well D-1 in Bongor Basin"

Table 3

The correlation diagram δ13C1-RO of gases in Well D-1 in the Bongor Basin"

深度/mSTAHL等[24]MARTIN[25]戴金星等[26]FABER等[27]
δ13C1=17LgRO-42δ13C1=14.8LgRO-41δ13C1=15.80LgRO-42.20δ13C1=15.4LgRO-41.3δ13C2=22.6LgRO-32.2δ13C3=20.9LgRO-29.7
1 531.000.360.270.350.290.530.79
1 669.000.360.270.340.290.540.79

Table 4

Comparison of light hydrocarbons parameters of gases in Well D-1 in the Bongor Basin"

深度/mHI2.4-DMP/2.3-DMPT
1 531.0021.312.910.64133.34
1 669.0020.193.040.68134.23

Fig.5

Gas chromatogram of saturated hydrocarbon of crude oil in Well D-1 well in the Bongor Basin"

Table 5

The comparison parameters of chromatographic peaks at different depths in D-1 well"

参数深度/m
1 531.01 669.01 858.01 950.02 177.0
前峰碳C13C14C15C15C15
后峰碳C25C25C25C25C25
前峰/波谷1.361.191.070.991.02
后峰/波谷1.481.321.381.151.43
前峰/后峰0.920.900.780.860.71

Fig.6

Relationship between n-alkane concentration and carbon number distribution in Well D-1"

Fig.7

Vertical characteristics of geochemical parameters of crude oil in Well D-1"

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