湘西北牛蹄塘组探井页岩气富集要素的对比和启示

Comparison of factors for shale gas accumulation in Niutitang formation wells in northwestern Hunan and its implications

基于湘西北地区牛碲塘组是近年来南方页岩气勘探的重点目标,以该区井花页1井、慈页1井及常页1井为研究对象,根据目的层实钻地层及气显示特征,系统对比页岩气生烃、储集及保存等方面的富集条件。研究结果表明:慈页1井和常页1井的原牛牛蹄塘组可进一步划分为杷榔组和牛碲塘组,与花页1井一致;牛蹄塘组烃源岩具有良好的生烃条件,已经历大规模生烃的过程;各探井以吸附状态页岩气为主,且均含有一定的氮气,历史上牛蹄塘组含气层可能受到过破坏;牛蹄塘组基本表现出极低孔隙度及渗透率的物性特征,有较强的压裂改造能力;研究区具有生烃时期早、抬升时期早及幅度大的特征,大规模构造改造及断裂发育,总体上对页岩气保存不利;有机质丰度、成熟度、埋深、地层压力、储层物性、脆性矿物质量分数等因素与地层含气量的相关性均不明显,高吸附能力仅是牛蹄塘组页岩气富集的1个必要非充分条件;慈页1井及常页1井在局部较好的保存条件下有较高的含气性,常页1井高含气性反映出最后1次抬升的时间晚,对页岩气的保存非常有利,因此,保存条件是页岩气富集的关键指标,在勘探过程中需从构造样式、断裂空间分布特征、目的层产状和完整程度、埋藏深度及演化历史等方面进行综合分析和重点评价。

Considering that the Niutitang formation in northwestern Hunan is a key target for shale gas exploration in south China in recent years,the key parameter wells including Huaye 1,Ciye 1 and Changye 1 in the study area were studied.According to the characteristics of target stratum and gas showings,the conditions of shale gas enrichment such as hydrocarbon generation,accumulation and conservation were systematically compared.The results show that the original Niutitang formation of Ciye 1 and Changye 1 well can be further divided into Palang and Niutitang formation,which is the same as those with Huaye 1 well.The source rocks in these wells have good conditions of hydrocarbon generation and experience a large-scale hydrocarbon generation.The shale gas in Niutitang formation of the wells is in absorbed state,and all of wells have some nitrogens,which shows that the enriched shale gas may be destroyed in history.The Niutitang formation has the characteristics of very low porosity and permeability,but has good abilities of fracture reconstruction.In northwestern Hunan,the Niutitang formation source rocks have the characteristics of hydrocarbon generation in the early stage,and uplift with early time and large extent through the buried evolutionary history.The large scale of tectonic reconstruction and fracture is developed,which is not good for shale gas enrichment.Many factors such as the abundance of organic matter,maturity,buried depth,formation pressure,reservoir features,brittle minerals content and so on,have no significant correlation with shale gas content.High adsorption ability of shale is only a necessary but not sufficient condition for shale gas enrichment.In relatively well preservation local condition,Ciye 1 and Changye 1 show the relatively high shale gas content,which shows that the late time of last tectonic uplift is very beneficial for shale gas preservation.Therefore,the preservation conditions are the key factors for shale gas enrichment,and must be synthetically analyzed and evaluated from many aspects in the shale gas exploration process,such as tectonic styles,the distribution characteristics of faults,the attitudes and completeness of target stratum,the buried depth and history and so on.