Heterogeneity Of Hydrocarbon Molecular Composition And Its Significance In The North District Of Bozhong Sag

The Bozhong Sag and its surrounding area have great potential for oil and gas resources,after years of oil and gas exploration and development in the north district of Bozhong Sag,it has entered the middle and late stages of exploration,and the difficulty of oil and gas exploration in this area is increasing,so figure out the distribution characteristics of oil and gas in this area and its enrichment rules will greatly promote the subsequent exploration work.Oil and gas geochemical methods and theories can be a good measure for the heterogeneity of hydrocarbon molecular composition,which provides a possibility to further reveal the spatial distribution characteristics and enrichment rules of oil and gas in this district.In this article,the samples of crude oil and reservoir rocks taken from different wells in the north district of Bozhong Sag were analyzed by gas chromatography of light hydrocarbons and GC-MS of saturated hydrocarbons,discussed the sources and types of organic matter,sedimentary environment and medium conditions,thermal evolution maturity and microbial degradation,etc.Based on the combination characteristics of biomarkers,the source and type of crude oil are divided,and the spatial distribution and variation characteristics of oil and gas are combined to reveal the enrichment of oil and gas.The mainly results from this district are as follows:?1?Based on the analysis of molecular composition characteristics of oil and gas in the north district of Bozhong Sag,it is found that microbial degradation is one of the most common secondary alteration phenomena in this district.Those samples from the tectonic high site and shallow position suffered from severe microbial degradation,the light hydrocarbon components basically disappeared,and the distribution of normal paraffins is incomplete,due to serious microbial degradation,the hopane and sterane were also affected,and has the ability to detect 25-norbornane,severe microbial degradation also increases the viscosity and density of oil and gas.Some samples from the tectonic low site and deeper position suffered from light-moderate microbial degradation,the light hydrocarbon components have a certain degree of loss,and the distribution of normal paraffins is still preserved,but for those which have not suffered microbial degradation have a relatively complete distribution of light hydrocarbons and normal paraffins.?2?Utilize the gas chromatography analysis of light hydrocarbons for those samples that have not suffered from microbial degradation or with extremely low levels of microbial biodegradation,and according to the carbon ring dominance theory of C₇light hydrocarbon molecules,it shows that those samples from the western section and its southwestern slope zone of Shijiutuo uplift,and the southern margin adjacent to the secondary sag and the middle section of Shijiutuo uplift presents the six-ring advantage,the abundance of TOL+MCC₆ is in 45%⁵5%,but does not have obvious aromatic ring advantage,the six-ring advantage often indicates that the source rock has the contribution of terrestrial organic input.Some samples from the eastern section of Shijiutuo uplift for the QHD29-2E well-zone not only have a six-ring advantage but also have an aromatic ring advantage,and very few samples from this well-zone with an abundance of P₂+P₃ in 38%⁵0%which represent the three-ring advantage.According to the Thompson maturity parameter,it shows that the southwestern slope zone of Shijiutuo uplift for the CFD6-4-2 well-zone has a H value of less than 18 and an I value of less than 0.8,which shows the characteristic of microbial degradation oil.Most of the samples from western section of Shijiutuo uplift,part of its southwestern slope zone,the southern margin adjacent to the secondary sag,the middle section,and part of the eastern section of Shijiutuo uplift has a H value is mainly concentrated between 20 to 44and the I value is between 0.6 to 4,it shows that crude oil has entered the stage of thermal evolution and maturity,and most of the samples with the characteristics of mature crude oil.?3?The GC-MS analysis of saturated hydrocarbons shows that for those samples have not suffered from microbial degradation or with extremely low levels of microbial biodegradation have a certain normal distribution of the normal paraffin,the carbon number range is mainly between nC₁₂ⁿC₃₈,odd and even carbon advantage is not obvious.The Pr/Ph is in 0.41¹.49,Pr/nC₁₇7 is in 0.35².61,Ph/nC₁₈ is in 0.32¹.94,the Pr/nC₁₇ and Ph/nC₁₈ relationship diagrams indicates the source rocks are mostly formed in a lake environment with weak oxidation-bias reduction conditions.The C₂₇/C₂₉???20R steranes ratio is concentrated between 0.5 and 1.5,and combined with the relative abundance of C₂₇-C₂₈-C₂₉???R steranes in the north district of Bozhong Sag shows that the source of organic matter includes both aquatic species and terrestrial higher plants.According to the values of Ts/Tm,C₂₉-???20S/????20S+???20R?and C₂₉-???/????+????shows that most of the samples are in the stage of thermal evolution and maturity,which represent the characteristics of mature crude oil.?4?By focusing on the characteristics of biomarkers in the north district of Bozhong Sag,the crude oil in this area can be divided into:main crude oil,higher G/H30R values crude oil and higher Tet24/TT26S values crude oil.The main crude oil with a higher abundance of 4-methyl steroid also have a low values of G/H30R and Tet24/TT26S,which indicate the mainly contribution of the S3 Formation source rocks,it also can be the mix in of S1 Formation hydrocarbon source.The higher G/H30R values crude oil with a higher abundance of gammacerane also have a low values of Tet24/TT26S and lower abundance of 4-methyl steroid,which indicate the contribution of the S1 Formation source rocks,but the Dongying Formation hydrocarbon source is limited.The higher Tet24/TT26S values crude oil with a higher abundance of C₂₄-Tet also have a very low values of G/H30R and lower abundance of 4-methyl steroid,which symbolizes the mainly contribution of the Dongying Formation source rocks.The main crude oil is the most widely distributed in the north district of Bozhong Sag,mainly concentrated in the western section and its southwestern slope zone of Shijiutuo uplift,and the middle section of Shijiutuo uplift.The higher G/H30R values crude oil mainly distributed in the eastern section of Shijiutuo uplift for the QHD29-2E well-zone.The higher Tet24/TT26S values crude oil mainly distributed in the southern margin of Shijiutuo uplift where near the secondary Bozhong Sag for the BZ1-1 and BZ2-1well-zone with the injection characteristics of the Dongying Formation source rock,which represent the dominant migration path of hydrocarbon molecules in the Dongying Formation source rocks,it also can be the focus of exploring the late injection of oil and gas and the breakthrough of tracking the oil source.