湘中坳陷涟源凹陷湘页1井大隆组页岩气地质特征

Geological characteristics of shale gas in Dalong Formation of Well Xiangye 1 in Lianyuan sag,Xiangzhong depression

为评价湘中坳陷涟源凹陷二叠系大隆组页岩气成藏地质条件,部署实施了该区首口以大隆组为目的层的页岩气参数井——湘页1井。充分利用湘页1井钻井、岩心、测井及分析测试资料,开展大隆组岩石矿物组成特征、沉积特征、有机地球化学特征、储集物性特征、含气性等地质条件分析。研究结果表明:湘页1井大隆组页岩形成于缺氧的较深水台盆相沉积环境,有利于页岩发育和有机质保存,优质页岩厚度大(TOC大于2%的页岩厚92.8m)、有机质丰度高(TOC为0.41%~10.47%,平均为3.91%)、热演化程度适中(R_(o)为1.50%~1.72%,平均为1.58%),微观孔隙结构以中孔和大孔为主,微裂缝发育,页岩脆性指数高,两向水平主应力差值小,有利于压裂形成复杂网络裂缝,具备形成页岩气藏的基本地质条件;保存条件是影响页岩含气性的关键因素,湘页1井大隆组页岩进入生烃期以后,经历了3期明显的构造抬升剥蚀,以致现今埋藏浅,距离剥蚀区近,向斜构造页岩气易横向逸散,同时腐殖型干酪根产烃率相对较低也影响其含气性。构造形态宽缓、埋深较大、远离剥蚀区、翼部发育反向逆断层有利于残留向斜型页岩气富集。

The first shale gas parameter well targeting the Permian Dalong Formation,Well Xiangye 1,was deployed and drilled to evaluate the geological conditions for shale gas accumulation in the Permian Dalong Formation in Lianyuan sag,Xiangzhong depression.Based on data from drilling,core,well logging and lab test analysis of Well Xiangye 1,geological conditions of shale target in Dalong Formation are analyzed,including sedimentary characteristics,rock minerals,organic geochemical characteristics,reservoir physical property and gas bearing property.The results show that the shale of Dalong Formation in the study area is deposited in relatively deeper water platform basin environment under anoxic condition,which is favorable for shale development and organic matter preservation.The organic-rich shale has a great thickness(92.8m with TOC more than 2%),high organic matter abundance(TOC ranges 0.41%-10.47%,with an average of 3.91%),moderate thermal evolution degree(R_(o) ranges 1.50%-1.72%,with an average of 1.58%).The micro pore structure of the shale reservoir is dominated by mesopores and macropores,with micro fractures developed.The brittleness index of shale is high.The difference between two-way horizontal principal stresses is small,which is conducive to the formation of complex network fractures after simulation.All these parameters indicate that the basic geological conditions are favorable for shale gas accumulation.However,the preservation condition is the key factor affecting shale gas bearing property.After entering the period of hydrocarbon generation,the shale of Dalong Formation in Well Xiangye 1 experienced three phases of obvious tectonic uplifting and denudation,which resulted in shallow burial and close to denudation area at present.Shale gas was easy to escape laterally in synclinal structures,and the hydrocarbon generation rate of humic kerogen was relatively low,which both affected the gas bearing property of shale in Dalong Formation.Favorable areas for residual shale gas enrichment are where the structures are wide and gentle,the burial depth is relatively large,shales are far away from the denudation area,and reverse faults are developed at both wings of the syncline.