页岩层层内烃类微运移地球化学效应——以南襄盆地泌阳凹陷古近系核桃园组三段Ⅲ号砂层组5号页岩层为例

Geochemical effects of intra-layer hydrocarbon micro-migration in shale layers: a case study of the No. 5 shale layer of the No. Ⅲ sand group in the upper submember of the third member of the Paleogene Hetaoyuan Formation in the deep sag area of Biyang Sag, Nanxiang Basin

张冬梅 李水福 张延延 苏鹏 周畅然

ZHANG Dongmei LI Shuifu ZHANG Yanyan SU Peng ZHOU Changran

中国地质大学(武汉) 资源学院, 武汉 430074

School of Earth Resources, China University of Geosciences (Wuhan), Wuhan, Hubei 430074, China

烃源岩层内烃类微运移是页岩油富集的重要途径，所产生的地球化学效应是其发生的有力证据。为了证实南襄盆地泌阳凹陷深凹区烃类微运移存在，研究页岩油富集机理，以泌页1井和程2井核桃园组三段上亚段Ⅲ号砂层组5号页岩层为研究对象，运用岩石热解、抽提色层、色谱—质谱等地球化学分析手段，以及岩石薄片观察等技术，揭示了研究区页岩层内烃类微运移的存在及其所产生的地球化学效应。结果表明：根据热释烃(S1)含量与总有机碳(TOC)含量关系，将研究区页岩层内烃类微运移划分为弱排烃(Ⅰ类)、强排烃(Ⅱ类)和外来烃(Ⅲ类)三种情况，并定量计算出相应的排烃强度；三类烃类微运移的地球化学特征：Ⅲ类样品饱芳比最高，Ⅱ类样品饱芳比最小，Ⅰ类样品介于两者之间；正构烷烃nC19与甲基菲比值出现相似的规律。另外，岩石薄片观察进一步证实：Ⅲ类样品具有较多的储集空间，外来烃可进入赋存；Ⅱ类样品裂缝发育，油气可顺畅排出；Ⅰ类样品的孔喉半径小、渗透率低、毛细管阻力大且裂缝不发育，其排烃不畅，这与其产生的地球化学效应相吻合。

Hydrocarbon micro-migration within source rock layers is an important pathway for shale oil enrichment, and the resulting geochemical effects provide strong evidence for its occurrence. To verify the existence of hydrocarbon micro-migration in the deep sag area of the Biyang Sag of Nanxiang Basin and to investigate the enrichment mechanism of shale oil, the No. 5 shale layer of the No. Ⅲ sand group in the upper submember of the third member of the Paleogene Hetaoyuan Formation in wells BY1 and Cheng 2 was selected as the research object. By using geochemical analysis methods such as rock pyrolysis, extraction and chromatographic techniques, chromatography-mass spectrometry, and rock thin-section observation, the existence of hydrocarbon micro-migration within the shale layer in the study area and its resulting geochemical effects were revealed. The results indicated that, based on the relationship between pyrolytically desorbed hydrocarbon (S1) and total organic carbon (TOC), hydrocarbon micro-migration within the shale layer in the study area was divided into three types: weak hydrocarbon expulsion (type Ⅰ), strong hydrocarbon expulsion (type Ⅱ), and indigenous hydrocarbons (type Ⅲ), and their corresponding hydrocarbon expulsion intensities were quantitatively calculated. The geochemical characteristics of the three types of hydrocarbon micro-migration were as follows: type Ⅲ samples had the highest saturate/aromatic ratio, type Ⅱ samples had the lowest saturate/aromatic ratio, and type Ⅰ samples were in between. The ratio of n-alkane nC19 to methylphenanthrene (MP) showed a similar pattern. Additionally, rock thin-section observation further confirmed that type Ⅲ samples contained more reservoir space, allowing for indigenous hydrocarbon influx and storage. Type Ⅱ samples developed fractures, enabling smooth expulsion of oil and gas. Type Ⅰ samples had small pore and throat radii, low permeability, high capillary resistance, and undeveloped fractures, leading to poor expulsion capacity, which aligned with their resulting geochemical effects.

国家自然科学基金“烃源岩抬升过程中生排烃转换、压力保存与页岩油气富集” 41672136;“干酪根有效碳含量对干酪根吸附液态烃能力的影响及其页岩油地质意义” 42073067

https://doi.org/10.11781/sysydz2025040847