四川盆地北部栖霞组-茅口组热液白云岩特征与成因

以四川盆地北部广元峡沟煤矿剖面白云岩为研究对象,剖面由下部地层向上,白云岩化程度逐渐增强,颜色由深到浅,岩性由生屑灰岩逐渐过渡到斑状白云岩,然后再到中-粗晶白云岩,上部覆盖茅口组泥灰(云)岩。中-粗晶白云岩发育于栖霞组顶部和茅口组底部,由中晶到粗晶的半自形-自形白云石组成,其中可见鞍状白云石。岩石中溶蚀孔洞发育,孔隙内有沥青充填。微晶灰岩的δ¹³C(PDB)值为3.23‰,δ¹⁸O(PDB)值为-5.03‰;白云石样品的碳、氧同位素具有一定的差异,δ¹³C(PDB)值为3.29‰~4.14‰,δ¹⁸O(PDB)值为-6.07‰~-6.75‰。流体来源可能为岩浆热液,或者是经历了深循环的大气降水。在断裂活动时,深部热液流体沿断裂向上运移,当运移到栖霞组和茅口组时,由于吴家坪组泥灰岩和泥云岩的封堵,流体的运移被减缓甚至停滞,促使围岩发生白云岩化,溶蚀作用也同时发生,产生溶蚀孔隙。由此,在栖霞组顶部和茅口组底部可能形成热液白云岩储层,该储层具有一定的储集意义。

Characteristics and genesis of hydrothermal dolomites of Qixia and Maokou Formations in northern Sichuan Basin

Taking outcrop dolomites in Guangyuan Xiagou coal mine in northern Sichuan Basin as an example,this paper studies the characteristics and genesis of hydrothermal dolomite reservoir.From the bottom to the top of the outcrop cross section,the dolomitization gradually enhances;the color of rocks changes from dark to light;the lithologies change from bioclastic limestones to patchy dolostones,then to medium-and coarse-grained dolomites,and end up with the Maokou Formation marlstones.The medium-to coarse-grained dolomites occur at the top of the Qixia Formation and the bottom of the Maokou Formation,and are composed of medium-to coarsegrained subhedral or euhedral dolomites and some saddle-like dolomites.Dissolved caverns and pores are welldeveloped and filled with asphaltares.The δ13C value(PDB) and δ18O value(PDB) of micritic limestones are 3.23‰ and-5.03‰,respectively.The δ13C value(PDB) and δ18O value(PDB) of the dolomites are 3.29 ～ 4.14‰ and-6.07 ～-6.75‰ respectively.Fluids may be sourced from magmatic hydrothermal fluid,or meteoric water experiencing deep circulation.The deep hydrothermal fluids migrated up along open faults and slowed down or even stopped when arriving at the Qixia and Maokou Formations due to the sealing of marl and politic dolostones in the Wu Jiaping Formation.The stagnant thermal fluids caused dolomitization dissolution of the surrounding rocks.Therefore,it is possible that hydrothermal dolomite reservoirs occur at the top of Qixia Formation and the bottom of Maokou Formation.