Abstract:

The exploration and development potential of shale oil in the Jurassic Lianggaoshan Formation of the Sichuan Basin is enormous. Understanding the depositional and evolutionary patterns of mudstone has become increasingly urgent,and astronomical forcing is an important mechanism driving mudstone deposition and evolution. In this study,the natural Gamma Ray logging curve was selected as a proxy for paleoclimate,and a cyclostratigraphic analysis of the Lower Liang 2 Submember in the Fuxing area of the Sichuan Basin was conducted to establish a high-frequency sequence stratigraphic framework and explore the spatial and temporal distribution patterns of mudstone lithofacies within the sequence pattern. The following results were obtained: (1)Multiple astronomical cycles of 405ka,128ka,43ka,and 21ka were identified in the Lower Liang 2 Submember,corresponding to the fourth and fifth-order sequences. High and low-eccentricity cycles were used to achieve high-precision stratigraphic subdivision and correlation. (2)Combining the palaeoenvironmental parameters of elemental geochemistry with the trend analysis of Total Organic Carbon(TOC),we have demonstrated the close relationship between palaeoclimate,terrestrial input conditions,TOC,and astronomical cycles. This study has elucidated the significant control exerted by long-and short-eccentricity cycles on lithofacies development and their combinations. Eccentricity emerges as a crucial factor governing climate evolution and lithofacies spatial distribution. The Long Eccentricity is a key factor in controlling climate evolution,organic matter enrichment,and the spatial distribution of lithofacies,while the Short Eccentricity plays a role in local regulation. (3)Based on the astronomical response of lithofacies and lithofacies combinations,we have summarized the climate and sedimentary cyclicity patterns primarily controlled by Eccentricity. During periods of high Eccentricity,intensified insolation,precipitation,and river runoff lead to lake expansion,with deepwater laminated mudstone rich in organic matter forming in the basin center. Conversely,during periods of Low Eccentricity,the climate becomes arid,monsoons weaken,and weathering intensifies,resulting in severe river incision and the development of large-scale deltaic sand bodies on basin slopes. Turbidite sandstones mainly form in the downslope and deepwater areas. This research is of great significance for understanding the origin of organic-rich mudstones and predicting high-quality mudstone reservoirs.

Key words: continental shale oil, lithofacies, lithofacies association, astronomical cycles, Lianggaoshan Formation, Jurassic, Sichuan Basin

摘要：

四川盆地侏罗系凉高山组页岩油的勘探开发潜力巨大,对泥岩沉积演化模式的认识越来越迫切,而天文旋回是驱动泥岩沉积演化的重要机制。本研究选取自然伽马测井曲线为古气候替代指标,对四川盆地复兴地区凉高山组凉二下亚段展开旋回地层学分析,建立高频层序地层格架,探索在层序格架内部泥页岩岩相的时空分布规律。得出以下结果: (1)在研究区凉二下亚段中识别出405ka、128ka、43ka和21ka等多级天文周期,长、短偏心率旋回分别对应四级、五级层序,由此进行高精度地层划分与对比。(2)结合元素地球化学的古环境参数和TOC趋势分析,证明了古气候、陆源输入条件和TOC与天文周期之间关系密切,明确了长、短偏心率对岩相及其组合发育的控制作用。长偏心率是控制气候演化、有机质富集以及岩相空间展布的关键因素,短偏心率进行局部调控。(3)根据岩相及岩相组合的天文响应,总结了偏心率主控的气候和沉积旋回模式。偏心率极大值附近,日晒、降水和河流径流增强导致湖泊扩张,盆地中心多发育富有机质深水纹层状泥岩; 偏心率极小值附近,气候干旱,季风、降水变弱,风化程度较高,河流下切侵蚀严重,盆地上斜坡发育大规模三角洲砂体,下斜坡区和深水区主要发育异重流砂岩。对于认识富有机质泥岩成因、预测优质泥岩储集层具有重要意义。

关键词: 陆相页岩油, 岩相, 岩相组合, 天文旋回, 凉高山组, 侏罗系, 四川盆地

CLC Number: 

• P512.2