川北地区中二叠统茅口组溶蚀作用发育特征及控制因素

DEVELOPMENT CHA RACTERISTICS AND CONTROLLING FACTORS OF DISSOLUTION IN THE MIDDLE PERMIAN MAOKOU FORMATION IN NORTHERN SICHUAN REGION

溶蚀作用是四川盆地茅口组储层形成的重要成岩作用之一,为确定川北地区中二叠统茅口组碳酸盐岩岩溶储层发育情况与成因,本次研究通过野外露头、钻井岩心以及薄片观察、同位素地球化学特征分析,对溶蚀作用的发育特征与控制因素展开研究。川北地区茅口组溶蚀作用的发育特征主要表现为晶间溶孔、粒间溶孔(铸模孔)、溶蚀孔、溶洞以及溶蚀缝等不同程度发育,溶蚀孔洞缝方解石充填物可划分为同生-准同生阶段的CC_(1)方解石与表生阶段的CC_(2)方解石,CC_(1)方解石具有高的δ^(13)C值(2.80‰~4.72‰,均值为3.75‰)与δ^(18)O值(-5.30‰~-8.23‰,均值为-6.93‰),且具有与围岩接近的^(87)Sr/^(86)Sr值(0.706 210~0.708 178),形成于同沉积的海水环境中,受到部分大气淡水的影响;相比较下,CC_(2)方解石具有较低的δ^(13)C值(-2.13‰~-0.34‰,均值为-1.12‰)与δ^(18)O值(-9.59‰~-7.00‰,均值为-6.56‰),以及较高的^(87)Sr/^(86)Sr值(0.708 316~0.711 365),在大气淡水主控下形成,伴有地壳流体的影响。综合溶蚀作用发育与充填物特征将川北地区茅口组溶蚀作用划分为3期:(1)同生期溶蚀作用,流体主要为海水与大气淡水的混合水,受沉积相带控制易发育晶间溶孔、粒内溶孔(铸模孔),充填有CC_(1)方解石;(2)表生期溶蚀,流体为大气淡水与地下水,受古地貌控制主要发育在茅口组顶部,溶蚀孔、溶洞与早期溶蚀缝的大规模发育且伴有CC2方解石与钙泥质的充填;(3)埋藏期溶蚀,主要表现深部热液与酸性流体对构造裂缝与CC2方解石的再溶蚀,导致晚期溶蚀缝的发育,以沥青、黄铁矿、石膏等热液矿物的充填为主。综合各期溶蚀作用的发育规模与充填情况,研究认为川北地区茅口组溶蚀空间的发育主要来源于表生期溶蚀作用,有效溶蚀空间多发育于峰丛洼地微古地貌单元处。

Dissolution is one of the important diagenetic effects in the formation of reservoirs in the Maokou Formation in the Sichuan Basin.In order to determine the development and genesis of the carbonate karst reservoirs of the Maokou Formation of the Middle Permian in the northern Sichuan area,this paper investigates the developmental characteristics and controlling factors of dissolution by means of field outcrops,drilling cores,and thin-section observations,as well as isotope geochemical characterization.The calcite filled in the solution cavity and fracture can be divided into CC_(1) calcite in syngenetic and quasisyngenetic stage and CC_(2) calcite in supergenetic stage.Calcite in CC_(1) stage has high δ^(13)C values(2.80‰ to 4.72‰,with an average value of 3.75‰)and δ^(18)O values(-5.30‰ to-8.23‰,with an average value of-6.93‰),and^(87)Sr/^(86)Sr values(0.706210 to 0.708178),which are close to the surrounding rock and formed in the synsedimentary seawater environment and affected by some atmospheric fresh water.CC_(2)calcite has low δ^(13)C values(-2.13‰ to-0.34‰,mean-1.12‰)and δ^(18)O values(-9.59‰ to-7.00‰,mean-6.56‰),and high^(87)Sr/^(86)Sr values(0.708316 to 0.711365),which are thought to be controlled by atmospheric fresh water and influenced by crustal fluids.The dissolution of Maokou Formation in northern Sichuan can be divided into three stages based on the development of dissolution and the characteristics of infilling materials.(1)Syngenetic dissolution is mainly controlled by the mixed water of seawater and atmospheric fresh water and is related to the sedimentary facies zones.It is easy to develop intergranular dissolution pores(mold holes)and filled with CC_(1) calcite;(2)In the supergenetic stage of dissolution,the fluid was atmospheric fresh water and groundwater,mainly developed at the top of Maokou Formation under the control of paleogeomorphology.Dissolution holes,caves and early dissolution fractures developed on a large scale and were accompanied by CC_(2)calcite and calcareous infilling;(3)Burial dissolution mainly reflects the re-dissolution of structural fractures and CC_(1) calcite by deep hydrothermal and acidic fluids,leading to the development of late dissolution fractures,which are filled by hydrothermal minerals such as asphalt,pyrite and gypsum.Based on the development scale and filling conditions of each stage of dissolution,it is considered that the dissolution space of Maokou Formation in northern Sichuan was mainly derived from the supergenetic dissolution,and the effective dissolution space was mostly developed in the micro-paleogeomorphic units of the peak cluster and low-lying land.