Origin and exploration prospect of Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin
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摘要: 鄂尔多斯盆地杭锦旗地区上古生界海陆过渡相有一套良好的生储盖组合,目前已发现了东胜大气田。该区二叠系下石盒子组产出少量中质原油,但原油的成因及其勘探前景尚不明确。基于对该区原油的物性、饱和烃色谱、生物标志物及碳同位素等地化特征的研究,试图揭示原油的成因和勘探前景。杭锦旗地区上古生界原油具有低硫、高蜡、中等成熟度特征,其甾烷分布显示以陆生植物为主,少量水生生物;Pr/Ph值为2.16~2.26,重排甾烷和重排藿烷含量高,伽马蜡烷含量低,原油碳同位素值为-27.7‰~-26.8‰,正构烷烃单体烃碳同位素前重后轻,这些特征均指示该原油是由沉积环境为弱氧化的陆相煤系地层中的暗色泥岩生成。二叠系山西组烃源岩有机岩石学、生烃热模拟实验及岩石热解等综合分析表明,该区山西组暗色泥岩具有一定生油潜力。结合原油产出井的位置和烃源岩的空间展布,山西组暗色泥岩可能是潜在的烃源岩。杭锦旗地区上古生界良好的生储盖组合条件暗示着该区的原油勘探值得重视,泊尔江海子断裂两侧邻区是原油聚集的潜在有利区。Abstract: A set of favorable source-reservoir-caprock assemblage was found in the Upper Paleozoic transitional facies in the Hangjinqi area of the Ordos Basin, and the large-scale Dongsheng gas field has been discovered. A small amount of crude oil has been produced from the Lower Shihezi Formation reservoir in the area; however, the origin and exploration prospect of the oil are unclear. Based on the studies including physical property, n-alkanes distributions, biomarker compositions, and carbon isotopic ratios, the origin and exploration prospect of the crude oil has been discussed. Results indicate that the Upper Paleozoic crude oil from the Hangjinqi area is mature oil with a low sulfur content and a high wax content, and the steranes are dominated by terrestrial plants and assisted by a small amount of aquatic organism. The Pr/Ph ratio is 2.16-2.26, and the diasteranes and rearranged hopanes are in high contents with a relative low content of gammacerane. The carbon isotopic ratios of the oil range from -27.7‰ to -26.8‰, [JP+1]and the carbon isotopic[JP]compositions of individual n-alkanes are heavy at first and then become light with the increasing of carbon numbers. These characteristics suggest that the oil was generated by the dark mudstone in terrigenous coal measures under weak oxidation environment. The comprehensive analysis combining organic petrography, hydrocarbon generation simulation, and pyrolysis of source rocks in the Shanxi Formation indicates that, the dark mudstone in the Permian Shanxi Formation in the Hangjinqi area displays a certain amount of oil generation potential. In combination with the location of oil producing wells and the spatial distribution of source rocks, the dark mudstone in the Shanxi Formation might be the potential source rocks for the produced oil. The oil exploration of the Upper Paleozoic strata in the Hangjinqi area deserves attention as suggested by the favorable conditions of source-reservoir-caprock assemblage, and the adjacent areas of the Borjianghaizi fault are potential favorable areas for oil accumulation.
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图 5 鄂尔多斯盆地杭锦旗地区上古生界原油正构烷烃单体碳同位素分布曲线
川科1井和雷北1井数据引自文献[27]。
Figure 5. Distribution curves of carbon isotopic compositions of individual n-alkanes in Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin
表 1 鄂尔多斯盆地杭锦旗地区上古生界原油物理性质和族组成
Table 1. Physical properties and group compositions of Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin
样品号 层位 密度(20 ℃)/(g·cm-3) 黏度(50 ℃)/(mPa·s) 硫/% 蜡/% 饱和烃/% 芳香烃/% 非烃+沥青质/% 饱芳比 (饱+芳)/(非+沥) JPH-353 P1x 0.86 6.96 0.08 15.60 78.36 17.28 4.36 4.53 21.94 JPH-2 P1x 0.89 10.42 0.27 12.15 67.53 26.26 6.21 2.57 15.10 表 2 鄂尔多斯盆地杭锦旗地区上古生界原油地球化学参数
Table 2. Geochemical parameters of Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin
样品号 地球化学参数 Ⅰ1 Ⅰ2 Ⅰ3 Ⅰ4 Ⅰ5 Ⅰ6 Ⅰ7 Ⅰ8 Ⅰ9 Ⅰ10 Ⅰ11 Ⅰ12 JPH-353 2.26 0.45 0.69 0.71 0.34 1.62 0.69 1.87 0.64 0.34 0.09 -27.7 JPH-2 2.16 0.48 0.48 1.35 0.28 2.44 0.66 1.27 0.46 0.38 0.09 -26.8 注:Ⅰ1.姥鲛烷/植烷;Ⅰ2.C29规则甾烷/C27-C29规则甾烷;Ⅰ3.C29重排甾烷/C27-C29重排甾烷;Ⅰ4.C27-C29重排甾烷/C27-C29规则甾烷;Ⅰ5.规则甾烷/17α藿烷;Ⅰ6.C24四环萜烷/C26三环萜烷;Ⅰ7.C35藿烷(22 S)/ C34藿烷(22 S);Ⅰ8.18 α(H)-22, 29, 30-三降藿烷(Ts)/17α(H)-22, 29, 30-三降藿烷(Tm);Ⅰ9.18 α(H)-30-降新藿烷(C29Ts)/17 α(H), 21 β(H)-30-降藿烷(C29H);Ⅰ10.C30重排藿烷/17 α(H), 21 β(H)-C30藿烷;Ⅰ11.伽马蜡烷/21 β(H)-C30藿烷;Ⅰ12. δ13C/‰。 表 3 鄂尔多斯盆地杭锦旗地区上古生界原油成熟度参数
Table 3. Maturity parameters of Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin
成熟度参数 JPH-353 JPH-2 OEP 1.02 0.95 CPI 1.08 0.97 C21-/C22+ 1.36 2.26 庚烷值 30.57 异庚烷值 2.83 C29甾烷20 S/(20 S+20 R) 0.53 0.50 C29甾烷ββ/(αα+ ββ) 0.48 0.50 Ts/(Ts+Tm) 0.65 0.56 C31藿烷22 S/(22 S+22 R) 0.54 0.58 MPI1 0.70 0.59 MPI2 0.74 0.64 MPR 0.52 0.92 单金刚烷指数/% 63 64 双金刚烷指数/% 47.1 33.3 注:MPI1=1.5(2-MP+3-MP)/(P+1-MP+9-MP);MPI2=3×2-MP/(P+9-MP+1-MP);MPR=(3-MP+2-MP)/(9-MP+1-MP)。样品JPH-2的OEP、CPI、C21-/C22+数据是根据色质m/z 85的峰积分得来的。 -
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