Distribution And Favorable Zone Prediction Of Marine Gypsum Salt Rocks In Jialingjiang Formation,Sichuan Basin

Gypsum salt rock is widely developed in the Jialingjiang formation of Sichuan Basin.As an important cap rock,it is closely related to the formation of large oil and gas fields.However,previous studies on the gypsum salt rocks of Jialingjiang formation in Sichuan Basin are limited to a certain part of the basin or a certain section of Jialingjiang formation,lacking a comprehensive understanding.In addition,the relationship between gypsum salt rock and dolomite as reservoir needs further study.Therefore,this paper intends to study the distribution of gypsum salt rock,the plane distribution relationship between gypsum salt rock and dolomite,and the sealing property of gypsum salt rock,so as to provide a more reliable geological basis for looking for favorable oil and gas accumulation blocks of Jialingjiang formation in Sichuan Basin.In this paper,gypsum salt rocks in the study area are divided into three categories and five sub categories:pure gypsum rock,dolomitic gypsum rock,dolomitic gypsum rock,gypsum dolomite and gypsum dolomite.Vertically,gypsum salt rock mainly presents three different lithologic combination types:thick dolomite with thin gypsum salt rock,rhythmic interbedding between gypsum salt rock and dolomite,thick gypsum salt rock with thin dolomite.The gypsum salt rock is characterized by wide distribution,many intercalations and great variation of single-layer thickness,mainly developed in the second,fourth and fifth section of Jialingjiang formation.On the profile,along the NW-SE direction of the basin,the gypsum salt rock gradually thickens from Jiange to Langzhong-Quxian,and then thins slightly to Shizhu;Along the NE-SW direction of the basin,the thickness gradually changes from Bazhong to Suining-Nanchong,and then to Weiyuan.On the plane,the distribution of gypsum salt rocks in the whole basin is three thick and three thin.The thickness center of gypsum salt rocks in the second section of Jialingjiang formation is located in Tongjiang-Bazhong-Nanjiang and Hechuan-Chongqing areas;The thickness center of gypsum salt rocks in the fourth section of Jialingjiang formation is located in Langzhong-Quxian-Bazhong area and Dazhou-Kaixian-Wanxian-Liangping area;The thickness center of gypsum salt rocks in the fifth section of Jialingjiang formation is located in Nanchong area and Dazhou-Wanxian area.The thickness center of gypsum salt rock starts from the second section to the end of the fifth section of Jialingjiang formation,which basically remains unchanged in Northeast Sichuan,and tends to migrate southward in Northwest Sichuan.Comparing the gypsum salt rock thickness value area with the dolomite thickness value area,it is found that dolomite often develops around the gypsum salt rock,which is explained that gypsum salt rock is favorable for the formation of dolomite.It is speculated that the reason for this phenomenon is that the precipitation of anhydrite in the evaporation environment increases the Mg²⁺/Ca²⁺value of the nearby strata,so that the adjacent strata overcome the kinetic barrier of dolomitization Mg²⁺and form dolomite.Based on the distribution law of gypsum salt rock and its influence on oil and gas sealing,combined with the source and reservoir conditions of the study area,three favorable oil and gas zones are divided in Sichuan Basin:Wulong-Fengdu area in eastern Sichuan(grade I),Chongqing-Luzhou area in southern Sichuan(grade II)and Nanjiang-Bazhong area in northern Sichuan(grade III).