上斜坡区筇竹寺组沉积环境及其页岩气勘探潜力:以四川盆地威远地区威207井为例

Sedimentary environment and shale gas exploration potential of Qiongzhusi Formation in the upslope area:A case study on Well W-207,Weiyuan area,Sichuan Basin

为深入分析早寒武世(541~509 Ma)海洋化学状态、有机质富集机制,积极评价四川盆地西南部下寒武统筇竹寺组页岩气资源潜力,基于四川盆地威远地区威207井筇竹寺组岩石学、地球化学、主微量元素、孔隙结构与吸附特征,对寒武纪早期上斜坡区古海洋环境、有机质富集控制因素及页岩含气潜力进行了探讨。沉积旋回显示筇竹寺组细粒沉积期发育多个海平面升降控制下深水-浅水陆棚交互转换旋回,但以浅水陆棚相为主,发育典型斜坡浊积体(扇)、重力流沉积。以威远地区为代表的上斜坡区未长期处于深水区,富有机质黑色泥页岩沉积厚度受限。有机地球化学测试结果显示,威207井筇竹寺组有机质以Ⅰ型干酪根为主,热演化程度高,残余烃较少,生烃能力偏低。氧化还原参数指示地处上斜坡的威远地区寒武纪初期海洋环境处于中等限制程度,仅存在一定程度的上升流,海水经历了“缺氧-氧化-缺氧-次氧化-氧化”的转变过程,致使上斜坡区古海洋生产力水平整体较低,由下至上呈明显下降趋势。微观孔隙结构与氮气吸附曲线指示筇竹寺组储层以复杂的、无规则狭缝型孔为主,甲烷吸附量和w(TOC)呈正相关性,但与温度呈负相关性,表明筇竹寺组现今普遍面临的高压高温条件不适于甲烷吸附。上述证据指示上斜坡区筇竹寺组页岩气地质条件较复杂,勘探风险较高,建议资源评价方向应往以深水陆棚相沉积为主的拉张槽区转变。

To deeply analyze the thalassochemical conditions and organic matter enrichment mechanism during the Early Cambrian(541-509 Ma)and actively evaluate the potential of shale gas resources of the Lower Cambrian Qiongzhusi Formation(Fm)in the southwestern Sichuan Basin,based on the analysis of the petrology,organic geochemistry,element characteristics,pore structure and adsorption capacity of Qiongzhusi Formation,Well W-207,Weiyuan area,this study has discussed the Early Cambrian paleo-ocean environment,organic matter enrichment control factors and gas-bearing properties of shale gas in the upslope area of the Upper Yangtze Platform.Sedimentary cycle shows that multiple interactive conversioncycles of deep-water continental shelf and shallow-water continental shelf are developed during the fine-grained deposition period of Qiongzhusi Formation under the control of eustasy.In particular,the slope turbidite(fan)and gravity flow sediments indicate that shallow-water continental shelf facies are dominant,and the wells in the upslope of the Weiyuan area are not in deep-water for a long time,with the sedimentary thickness of organic-rich black shale limited.Organic geochemistry evidence indicates that the organic matter of Qiongzhusi Formation in Well W-207 is mainly Type-I kerogen,with a high degree of thermal evolution,fewer residual hydrocarbons and a low hydrocarbon generation capacity.The redox parameters indicate that the marine environment on the upslope has a medium restrictive degree,and there is a certain degree of upwelling.The seawater has experienced the transformation process of"anoxic-oxidation-anoxic-secondary oxidation-oxidation".Therefore,the paleo-ocean productivity level in the upslope area is generally low,with an obvious downward trend from bottom to top.The pore structure and nitrogen adsorption curve show that the reservoirs of the Qiongzhusi Formation are mainly complex and irregular slit pores.The methane adsorption capacity is positively correlated with TOC but negatively correlated with temperature,indicating that the high-pressure and high-temperature conditions generally faced by the Qiongzhusi Fm are not suitable for methane adsorption.As a result,the geological conditions of shale gas for Qiongzhusi Fm in the upslope area are complex.With high exploration risk,this study suggests that the resource evaluation direction should change to the intracratonic sag(downslope area),which is characterized by deep-water continental shelf facies.