页岩气成藏的四性特征及对下扬子地区页岩气勘探的启示

Four Characteristics of Shale Gas Play and Enlightenment to Shale Gas Exploration in Lower Yangtze Area

页岩气藏是连续型天然气藏,是生储盖为一体的封闭天然气系统,其成藏具有生气的唯一性、储气的多样性、富气的变化性和供气的改造性。生气的唯一性是指页岩气藏的烃源岩来源为富有机质页岩,且唯一来源于成熟的有机质生烃作用;而储气的多样性则揭示了页岩气藏中天然气的主要赋存形态:游离态和吸附态,两种状态的天然气含量变化较大,天然气分子主要富集在有机质的微孔隙、微裂缝及页岩的孔隙、裂缝内;富气的变化性是指多种地质指标,即"五度"指标影响页岩气藏的形成,"五度"包括深度、厚度、有机质丰度、热演化程度和脆性度,合适的"五度"指标才能形成商业开采的页岩气藏;供气的改造性凸显了页岩储层的最大特点,纳米级孔隙的非达西流渗流定律,该特点影响单井产量,造成产量低、递减慢、生产周期长,只有经过后期压裂改造才能获得工业气流。以上页岩气藏的四性变化为指导下扬子地区下寒武统黄柏岭组页岩气勘探提供主要技术依据,结合区域地质、地化特征,认为下扬子东南缘、东北部地区下寒武统黄柏岭组最有利于页岩气藏形成,并可进行必要的商业开采。

Shale gas play is a continuous gas play, which is a closed natural gas system with combination into one body of source rock, reservoir and cap rock. And there are four characteristics of shale gas accumulation and distribution conditions, i.e. uniqueness of gas generating, diversity of gas storage, variation of gas enrichment, and transformation of gas supply. The uniqueness of gas generating means that the source rock of shale gas play is organic-rich shale, and only roots in mature organic matter hydrocarbon generation; the diversity of gas storage shows that free gas and adsorbed gas are main modes of occurrence as to shale reservoir. The content of natural gas of the two modes of occurrence varies a lot. Gas accumulates in microfracture and micropore of organic matter and pore and fracture of shale. The variation of gas enrichment refers to multiple geological indexes, i.e. òfive-parameteróindexes affect the formation of shale gas play. The òfive parametersóinclude depth, thickness, TOC, Ro and brittleness, and appropriate parameters can make the formation of shale gas play that could be exploited commercially. The transformation of gas supply highlights the biggest characteristic of shale reservoir nano-darcy permeability, affecting single well production. So production is low, declines slowly and the production period is long. Based on above reasons, only systematic fracturing technology is key to get industrial airflow. The analysis of four characteristics provides major technical basis for shale gas exploration in Huangboling Formation of Lower Cambrian in Lower Yangtze area. Combined with regional geology and geochemical characteristics, it is regarded that the southeastern edge of Lower Yangtze and the Huangboling Formation are most favorable for the formation of shale gas play, and they could be exploited commercially.