基于机器学习测井反演的煤系岩性组合识别与评价——以鄂尔多斯盆地米脂北地区为例

    Identification and evaluation of lithological assemblages in the coal measure based on machine learning log inversion: A case study of the northern Mizhi area of Ordos Basin

    • 摘要: 煤系岩性组合影响了煤储层的气体封闭样式,对其进行准确识别与预测对评价生储盖配置和预测含气性具有重要意义。传统的岩性组合识别依赖人工解释,具有高成本、低效率、主观性过强等缺点。使用轻量级梯度提升机(light gradient boosting machine,LightGBM)、随机森林(random forest,RF)和极限梯度提升(extreme gradient boosting,XGBoost)3种机器学习模型进行对比,以鄂尔多斯盆地米脂北地区本溪组8号煤层及其顶底板为研究对象,基于地球物理测井数据与岩心观察资料,采用最优算法对区内的煤系岩性组合进行测井参数反演,并结合煤层气含量进行有利组合评价。结果表明: ①LightGBM模型预测平均准确率最高,达到0.93,RF与XGBoost模型预测平均准确率分别为0.92和0.91; ②使用LightGBM模型对全区进行反演,结果显示区内泥岩-煤-泥岩组合分布最广,砂岩-煤-泥岩和泥岩-煤-砂岩次之; ③结合研究区煤层厚度和实测含气量综合判断,灰岩-煤-泥岩组合含气丰度最高,平均达到168.6 m3/m2,在研究区现有样本和含气丰度指标下,判断其为区内最优岩性组合,主要分布于东南部。研究成果有效解决了传统岩性组合识别的局限,为煤系非常规天然气综合勘探开发提供了新的思路与技术参考。

       

      Abstract: The gas sealing behavior of coal reservoirs is affected by lithological assemblages in coal measures, and the accurate identification and prediction of such coal assemblages is critical for evaluating source reservoir cap configurations and predicting gas bearing potential. Traditional lithological assemblages rely on labor interpretation, with disadvantages of high cost, low inefficiency and strong subjectivity. Three machine learning algorithms were compared in this research, including light gradient boosting machine (LightGBM), random forest (RF) and extreme gradient boosting (XGBoost), and the roof/floor strata of No.8 coal seam of Benxi Formation in the northern Mizhi area of Ordos Basin was chosen as the study area. And the favorable lithological assemblages in the coal measure were assessed using logging parameters inversion by optimal algorithm in conjunction with coalbed methane content, based on well logging and core observation data. Results indicated that LightGBM model yielded the highest prediction accuracy at 0.93, compared with 0.92 for Random Forest and 0.91 for XGBoost. Regional inversion by LightGBM showed that mudstone-coal-mudstone assemblage was the most widely distributed, followed by sandstone-coal-mudstone assemblage and the mudstone-coal-sandstone assemblage. Integrated analysis of coal thickness and measured gas content revealed that the limestone-coal-mudstone assemblage exhibited the highest gas abundance, with an average of 168.6 m3/m2. And it was the most favorable lithological assemblage in the study area under present samples and gas bearing content indices, and it was predominantly distributed in the southeastern part of the study area. This study effectively overcomes the limitations of traditional lithological assemblage identification, providing new insights and technical references for the comprehensive exploration and development of coal-measure unconventional natural gas.

       

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