煤系高频层序格架内含菱铁矿层成岩演化及封隔性能影响机理

Exploration and development of coal measure gas are the frontier research that China's coal geologists have made great efforts to make breakthroughs in recent years. However, the geological phenomenon of gas-bearing system compatibility is common in coal measure gas extraction together. Accurate evaluation of the sealing-barriering capacity of siderite-bearing strata with gas-barrier properties in the gas-bearing systems is the solution to this key scientific issue. Based on this, coal measures of the Late Permian in West Guizhou Province with typical geological conditions and certain coal measures gas exploration are selected as the research object. Based on the time-frequency analysis method, the logging high-frequency sequence identification study is combined with isotope data to divise high frequency sequence stratigraphy and fine geo-dating, and to study the diagenetic evolution of siderite-bearing strata in the high-frequency sequence stratigraphy framework of the coal measures and the mechanism of its influence on the sealing-barriering capacity. We hope to establish a set of detailed characterization theory and method of gas barrier for the gas-bearing system, and objective identification of unconventional natural gas enrichment in coal measure.

煤系气勘探、开发是我国煤地质界近年来着力突破的前缘研究方向。然而，煤系气共采中普遍存在含气系统兼容性的地质现象，对煤系含气系统具有分划性隔水阻气（封隔）性能的含菱铁矿层封隔性能准确评价是解决这一难题的核心科学问题。基于此，选择地质条件典型且取得一定煤系气勘探效果的黔西晚二叠世煤系为研究对象，基于时频分析方法的测井高频层序识别研究，结合测年资料，深化高频层序划分和精细地质定年，精细研究煤系高频层序格架内含菱铁矿层成岩演化及封隔性能的影响机理，期望建立一套基于高频层序格架内的煤系含气系统隔水阻气层封隔性能的精细刻画理论与方法，为客观识别煤系非常规天然气富集规律奠定基础。

煤系气共采中普遍存在含气系统兼容性的地质现象，对煤系含气系统具有分划性隔水阻气（封隔）性能的含菱铁矿层封隔性能准确评价是合层排采选层选段的关键依据。基于研究区钻测井数据的小波分析，在晚二叠世煤系高频层序格架内对具分划性隔水阻气（封隔）性能的含菱铁矿层样品开展了系统的薄片鉴定及激光微区原位主微量元素分析结果，揭示了各类隔水阻气的封隔性能的影响机理。结果表明,菱铁矿依微观形态差异可划分为凝胶状菱铁矿、微粉晶菱铁矿、球粒状菱铁矿3种类型,其中凝胶状菱铁矿发育于层序内部最大海泛面附近,微粉晶菱铁矿发育于海侵体系域海平面上升期或高位体系域内,球粒状菱铁矿在层序内未见明显发育规律。各类菱铁矿发育受到层序格架的制约作用,层序反映了原始沉积环境与沉积物质组成的旋回性变化,从而导致其内菱铁矿的微观形态存在差异,发育于最大海泛面附近的凝胶状菱铁矿形成于偏海相环境,沉积期溶液中Fe2+与CO32-供应充足,迅速结晶形成隐晶凝胶状菱铁矿集合体;发育于海侵体系域海平面上升期或高位体系域内的微粉晶菱铁矿形成于海陆过渡相沉积环境,陆源铁质充足但CO32-较少,菱铁矿沉淀速率缓慢最终形成亮晶菱铁矿,球粒状菱铁矿的形成与孔隙流体引发的菱铁矿多期次结晶生长有关,不受控于层序格架内部发育位置。