Geochemical Characteristics Of Oil And Oil Source Correlation In Central Hebei Depression

The Central Hebei Depression, formed after Huabei platform disintegrates, is a Mesozoic and Cenozoic era fault depression basin. Suqiao-wenan Area is a main oil and gas distribution area of Central Hebei Depression and the hydrocarbon-bearing formations are distributed from the Ordovician to Tertiary, of which Carboniferous-Permian coal seam and the third section of the Paleogene Shahejie Formation lacustrine mudstone are the the main hydrocarbon source layer. The thickest coal formation (about950m) is located in the east side of a line of Suqiao-wenan and thinning out gradually to Tyrants County Troughs. The thickest mudstone formation is located in Tyrants County Troughs with the biggest sedimentary thickness about600m at the center of it. There is a wide range of oil and gas distribution in Suqiao-wenan Area and its peripheral block, and the oil with obviously different Geochemical Characteristics, is mostly the condensate oil and light oil with a few normal density crude oil, even a small amount of heavy oil.The mixing of oil and gas in research area leads to crude oil geochemical parameters and indicators disorder. This seriously influences the genetic classification of crude oi and makes oil and gas source correlation more difficult. Detailed analysis of geochemical characteristics of oil and oil source correlation in Central Hebei Depression realizes the fine identification of monophyletic oil of study area, solves the long-standing controversy over oil and gas source in Suqiao-wenan Area, lay a foundation for the establishment of the quantitative analysis model of mixed oil and provide important basis for oil and gas research and exploration decision in this area.Based on domestic and international research achievements of mixed oil, oil and source rock of study area were analyzed systematically by geochemical analyses such as GC, GC/MS and TCP/IP MS. Geochemical characteristics of oil were studied in detail including physical properties, group composition, carbon isotopic composition, biomarker composition, aromatic hydrocarbon composition and maturity. And geochemical characteristics of typical source rock were studied in detail including group composition, carbon isotopic composition, biomarker composition and aromatic hydrocarbon composition of its extractive. On this basis, the genetic types of crude oil was discusses and Systemic oil correlation was deduced according to the geochemical parameters of biomarkers. The main conclusions are as following:(1) The characteristics of biomarker in coal samples from permo-carboniferous in studying area display:the pr/ph value is quite high, which is up to9.33, Pr/nC17>Ph/nC18, the Low carbon number (C19-C21) tricyclic terpane content is rich with C21tricyclic terpane peak; the C24tetracyclic terpane content is extremely high.The pr/ph is low in paleogene lacustrine mudstone, the value is less than1, Pr/nC17<Ph/nC18; the tricyclic terpane content is higher than that in coal samples.the low carbon number (C19-C21) tricyclic terpane content is rich, C19、C20、C21tricyclic terpane shows increasing trend with C23tricyclic terpane peak; the C24tetracyclic terpane content is obviously low.(2) According to the geochemical characteristics of coal related oil at home and abroad, combined with the biomarkers in coal sample from C-P coal formation and oil from Jizhong Depression, the oil in Jizhong Depression can be divided into coal related oil, lacustrine oil and mixed-source oils by the biomarkers which can reflect the organic types and depositional environment.(3) Most of the coal related oils in studying area is condensate and light oil, the sulfur content is low; with the characteristics of high saturated hydrocarbon content and low nonhydrocarbon and asphaltene content, saturated hydrocarbon content for absolute advantage,more than80%, the ratio of saturate to aromatics is more than8, but the nonhydrocarbon and asphaltene content is less than5%.The lacustrine oils is generally normal density oil, the sulfur content is a little higher than coal related oil; compared with the coal related oil,the saturated hydrocarbon content in lacustrine oil decrease,the value of which is71.97%, the ratio of saturate to aromatics is less than8.The mixed-source oils in Su49well and Wen102-1well have the different geochemical characteristics. The oil in Su49well is condensate, the saturated hydrocarbon is dominate, the content is up to96.29%, which shows the characteristics of coal related oil. The oil in Wen102-1well is normal density oils, the saturated hydrocarbon content only is50.57%, which shows the characteristics of lacustrine oil.(4) The carbon isotope ratio of coal related oil in studying area ranges from-27.60‰to-26.30%o, the mean value is-26.8‰,which shows the same characteristics of other coal related oil in our country. The813C value in lacustrine oil is-27.9℃, which is lower than that in coal related oil. The characteristics of carbon isotope in oils also show the difference between the two kinds of mixed-source oil, the813C value of condensate in Su49well is heavy (-26.3‰), which is nearly the same with the coal related oil; but the δ13C value of oil in Wen102-1well is-28.2%o, which is the same with the lacustrine oil.(5) The absolute concentrations of biomarkers in all kinds of oils from Jizhong Depression vary greatly. The absolute concentrations of terpane and sterane in coal related oil generally are190～1500μg/g and40～130μg/g, respectively. In lacustrine oil, take the oil in Cha33-19well for example, the two values are3709.06μg/g and2654.56μg/g, respectively. The mixed-source oils, take the oil in Su49well and Wen102-1well for example, although there are only two samples, the content of terpane and sterane vary greatly. The content of terpane and sterane in the former are423.14μg/g and185.92μg/g, respectively; in the later are up to4258.58μg/g and2759.29μg/g.The Pr/Ph value is high (Pr/Ph>2), Pr/nC17>Ph/nC18in coal related oil; In the tricyclic terpane series, the low carbon number C19-C21tricyclic terpane is dominate, C19、C20、C21display decreasing trend; The C24tetracyclic terpane content is high. In addition, low gammacerane and high C30diahopane is another obvious characteristic in this kind of oil.The Pr/Ph<1、Pr/nC17<Ph/nC18in lacustrine oil; The low carbon number tricyclic terpane content is low, C19、C20、C21display increasing trend.The gamacerane content is high, but the diahopane is low in this kind of oil.The parameters of biomarkers in mixed-source oil have double properties, the crude oil and coal related oil in Su49well is the same, the parameters of biomarkers are between lacustrine oil and coal related oil.(6) On the whole, whether the coal related oil or lacustrine oil in studying area, or mixed-source oil, the maturity parameter value shows they are mature oil, the vitrinite reflectance Ro is between0.71%and0.99%.(7) According to the geochemical characteristics of source rocks and crude oils genetic types, select the geochemical parameters which reflect the origin for the research parameters. It shows that the mudstone in paleogene and lacustrine oil from Cha33-19Wells have good affinity, and the source rocks in Carboniferous-Permian have strong similarities with coal related oil from Su402wells.