上扬子区志留系页岩气成藏条件

Accumulation conditions of shale gas reservoirs in Silurian of the Upper Yangtze region

广泛分布于中国南方上扬子地区的志留系为一套静水、缺氧环境下沉积的良好烃源岩,尤以四川盆地的下志留统龙马溪组黑色笔石页岩最为发育。从该套地层的有机地化参数、四川盆地南部长宁构造新完钻的页岩气浅井的分析测试数据看,龙马溪组有机质丰度高、成熟度较高、微裂缝发育、埋深适中,具有形成页岩气藏的优越条件。川东石柱—鄂西利川、川南泸州和川北镇巴地区的页岩有机碳含量大于2%,平均厚40～80m;泸州—宜宾—自贡地区,黔中麻江—瓮安、川东南的秀山、松桃及湘西吉首地区,页岩的成熟度多小于3%;阳深2、阳63、阳9、付深1、隆32、太15等探井,在钻遇志留系页岩层段时均见到气测异常、气侵、井涌现象,长芯1井也证实了浅层志留系页岩气的存在,页岩气含量平均为0.15m3/t;野外和岩心观察该套页岩微裂缝发育,岩矿组成显示石英和方解石含量较高,表明该区页岩脆性的特征;采用体积法初步估算的四川盆地及邻区龙马溪组页岩气资源量为4.0×1012～12.4×1012m3,显示了该区页岩气巨大的勘探潜力。

Silurian which is widely distributed in Upper Yangtze area in South China develops a set of excellent hydrocarbon source rocks deposited in the anoxic environment with still water,especially the Lower Silurian Longmaxi formation in the Sichuan basin in which black graptolite shale is most developed.Judged from the organic geochemical parameters of this formation and testing data of new drilled shallow shale gas wells in Changning structure in southern Sichuan basin,Silurian Longmaxi formation is characterized by high abundance of organic matter,higher maturity,rich microfracture and moderate buried depth,thus possessing favorable conditions to form shale gas reservoirs.The organic carbon content of shale with average thickness ranging from 40-80 m is more than 2% in such areas as Shizhu in East Sichuan,Lichuan in West Hubei,Luzhou in South Sichuan and Zhenba in North Sichuan while the maturity criteria of shale is mostly less than 3% in regions such as Luzhou-Yibing-Zigong,Majiang-Weng'an in middle Guizhou,Xiushan-Songtao in South Sichuan and Jishou in West Hunan.There encounters gas logging abnormality,gas cut and well kicks when drilled Silurian shale formation in such exploratory wells as Yangshen-2,Yang-63,Yang-9,Fushen-1,Long-32 and Tai-15 etc.The existence of Silurian shallow shale gas is also proved by Changxin-1 well in which the average volume of shale gas is 0.15 m3/t.There is good microfracture growth and high content of quartz and calcite in this set of shale characterized by brittleness.It is estimated preliminarily by the volume method that the Silurian Longmaxi formation has(4.0-12.4)×1012 m3 of shale gas in the Sichuan basin and its neighboring area,showing an enormous potential in exploring shale gas in this area.