黔北地区海相页岩气保存条件——以贵州岑巩区块下寒武统牛蹄塘组为例

中国南方地区发育多套海相富有机质页岩，页岩气资源丰富，但与北美地区相比，具有形成时代老、热演化程度高、构造期次多、构造变形复杂、地应力状态与地表条件复杂及保存条件差异大等特点。与已获得突破的龙马溪组相比，下寒武统牛蹄塘组形成时代更老，储层特殊性与复杂性更大，对保存条件要求更高。为此，针对南方复杂构造区特殊地质条件，以贵州岑巩区块牛蹄塘组为例，通过地震资料精细解释与构造演化分析，结合物质基础、顶底板与区域盖层、天然气组分、地层水条件、地层压力等因素对页岩气保存条件进行了研究。结果表明，以断裂和剥蚀为主的后期改造是页岩气散失的根本原因。研究区构造演化具有“早期小幅抬升，长期相对稳定，后期强烈改造”的特点。区内北东-北北东向隔槽式褶皱、高角度逆冲与走滑断裂发育，中部宽缓构造部位断裂较少，埋深适中，保存条件较好。天然气组分与地层水特征表明，优良的顶、底板与区域盖层有效减缓了页岩气的散失与地层水的侵入。最后，提出了页岩气构造保存条件评价指标体系，并对研究区构造保存有利区进行了优选。

Gas preservation conditions of marine shale in northern Guizhou area: A case study of the Lower Cambrian Niutitang Formation in the Cen'gong block,Guizhou Province

South China,where develops several sets of marine organic-rich shales,is an important area for shale gas E & D.Compared with North America,the marine organic-rich shales in southern China have the characteristics of old geologic age,high maturity,multi-episodic tectonic movements,complex surface conditions,complicated earth stress state and large differences of preservation conditions.In comparison with the Longmaxi shale that has achieved a breakthrough in shale gas exploration and development in Sichuan Basin and its periphery,the Lower Cambrian Niutitang shale has an older age,greater particularity and complexity and higher requirement to preservation conditions.On the basis of the geological conditions of the complex structure areas in southern China,the preservation conditions of the Niutitang shale in the Cen'gong block were analyzed through precise seismic interpretation,tectonic evolution,material foundation,cap rocks,reservoir pressure,and component of natural gas and aquifers.The results indicate that late tectonic reconstruction dominated by denudation and faulting is the major factor leading to the dissipation of shale gas.The tectonic evolution of the study area has the characteristic of "moderate early uplifting,long-term stability and intensive late reformation".Besides,NE-NNE-trending trough-like folds,high dip-angle thrust faults and strike-slip faults are well-developed in the study area,and the central wide and gentle structure zone has less faults,moderate buried depth and good preservation conditions.In addition,the component of the natural gas and strata water shows that the superior roof,floor and regional cap rocks can effectively retard the gas dissipation and strata water invasion.Finally,an evaluation index system of structural preservation conditions is put forward and applied to identify the favorable preservation zones in the Cen'gong block.