A study on the hydrocarbon generation potential of the coal-bearing source rocks in the Pinghu Formation of Pingbei area, the Xihu depression
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摘要:
西湖凹陷平北地区平湖组发育海陆过渡相的煤系烃源岩，以生烃潜力为主要依据对煤岩以及暗色泥岩的有机质丰度进行了评价，并建立了相应的评价标准，评价结果显示其有机质丰度均为中等―好，煤岩表现为S1+S2＞90 mg/g，HI＞100 mg/g，氯仿沥青“A”＞1.0%；暗色泥岩表现为TOC＞1%, S1+S2＞2 mg/g，氯仿沥青“A”＞0.05%，总烃“HC”＞150×10-6。干酪根元素分析显示煤的干酪根类型主要为Ⅱ1-Ⅱ2型，富氢的显微组分含量较高，壳质组中的树脂体、孢子体及角质体含量达到10%～20%，且含有约20%的富氢镜质体，表明煤具有较高的生油潜力；暗色泥岩的富氢显微组分低于煤岩，有机质类型主要为Ⅱ2-Ⅲ型，以生气为主。根据镜质体反射率Ro以及S1+S2/TOC等指标共同确定了平北地区烃源岩的生烃门限（3 100 m）和排烃门限（3 500 m）。热模拟实验显示平湖组煤具有油气共生的特点，具有较高的产油率，可达130 mg/g TOC，产气率可达125 mg/g TOC；暗色泥岩以生气为主，产油率最高仅为50 mg/g TOC，产气率可达185 mg/g TOC，且生气周期长。基于平北地区较重的原油碳同位素和生物标志物特征对比，认为煤成油在研究区广泛存在。
Abstract:The source rocks of the Pinghu Formation in the Pingbei area of the Xihu depression consist of a set of coal-bearing deposits of marine-continental transitional facies. Organic matter abundance of the coal and dark mudstone were evaluated mainly based on potential hydrocarbon generation amount according to the evaluation criteria established. Results show that both the coal rocks and dark mudstones have high hydrocarbon generation potential and medium to high abundance of organic matter. The coal rocks are featured by S1+S2＞90 mg/g, HI＞100 mg/g,“A”＞1.0%, while the mudstones characterized by TOC＞1%, S1+S2＞2 mg/g, “A”＞0.05%, “HC”＞150×10−6. It is identified that Kerogen of the coal rocks belongs to typeⅡ1-Ⅱ2 according to the elemental data, and the content of hydrogen-rich macerals is very high. The total content of resinite, sporophyte and cuticle in exinite is up to 10%～20%, and the content of hydrogen-rich vitrinite is as high as 20%, indicating a high oil-generating potential. The content of hydrogen-rich macerals in mudstone is lower than that in coal rocks. The kerogen belongs to typeⅡ2 with higher gas-generating potential. Threshold depth of oil generation and expulsion in the Pingbei area were determined by Ro values and (S1+S2)/TOC values, and they are 3 100 m and 3 500 m respectively. The results of thermal simulation show that the coal rocks are prone to generating both oil and gas, with higher proportion of oil. Oil generation rate can reach 130 mg/g TOC and Gas generation rate about 125 mg/g TOC. The mudstone is gas-prone for a long history of gas generation. Data suggests that the oil generation rate is 50 mg/g TOC only, while the gas generation rate can reach 185 mg/g TOC. Coal-derived oil is widely existed in the Xihu depression according to the heavier crude oil carbon isotope and correlation of biomarkers between oils and source rocks.
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Key words:
- coal-bearing source rocks /
- hydrocarbon potential /
- coal-generated oil /
- Xihu depression
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表 1 研究区煤岩有机质丰度评价标准
Table 1. Evaluation criteria of organic matter abundance of coal rocks in study area
评价标准 S1+S2/（mg/g） HI/（mg/g） A/% 好 ＞150 ＞300 ＞2.0 中 90～150 150～300 1.0～2.0 差 60～90 100～150 0.5～1.0 非 ＜60 ＜100 ＜0.5 
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表 2 研究区泥岩有机质丰度评价标准
Table 2. Evaluation critera of organic matter abundance of mudstone in study area.
评价标准 TOC/% S1+S2/（mg/g） A/% HC/10-6 好 ＞2.5 ＞6.0 ＞0.15 ＞400 中 1.0～2.5 2.0～6.0 0.05～0.15 150～400 差 0.6～1.0 0.6～2.0 0.02～0.05 50～150 非 ＜0.6 ＜0.6 ＜0.02 ＜50
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表 3 平湖组煤及泥岩热模拟试验样品地化参数
Table 3. Geochemical index of the thermal simulation experiment of coal rock and mudstone in Pinghu Formation
井号 岩性 Ro/% TOC/% HI/（mg/g） S1/（mg/g） S2/（mg/g） A-X井 煤 0.4 61 320 2.34 127.44 B-X井 泥岩 0.6 2.5 236 0.54 5.86
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