四川盆地北部晚二叠世—早三叠世碳酸盐岩斜坡相带沉积特征

Carbonate slope facies sedimentary characteristics of the Late Permian to Early Triassic in northern Sichuan Basin

四川盆地东北部宣汉县樊哙以东地区晚二叠世—早三叠世飞仙关期碳酸盐岩台地相—海槽相剖面均出露地表。分隔台地相和海槽相的碳酸盐岩斜坡相的鸡唱剖面距台地边缘相剖面不足1km。根据地面露头碳酸盐岩斜坡相带的特征,可在四川盆地北部环开江—梁平海槽的地震反射剖面上有效地识别出碳酸盐岩斜坡相,并划分出浅水、深水碳酸盐沉积区。斜坡相具有明显的倾斜反射,海槽相区大隆组及飞仙关组底部泥岩的强振幅反射在斜坡下部向台地上超并消失。深水相区上二叠统地震波双程反射时间都小于100ms且明显短于飞仙关组,而台地相区上二叠统双程反射时间多大于150ms,且与上覆飞仙关组地震波反射时间相近或略长。碳酸盐岩斜坡相倾斜反射的特征表明环海槽的碳酸盐岩斜坡开始发育于晚二叠世中期,并随海侵向碳酸盐岩台地方向迁移,在晚二叠世末期海侵达到顶峰时形成最陡峭的斜坡。飞仙关期斜坡随海平面缓慢下降而逐渐向海槽方向迁移并变缓。晚二叠世末期海槽南端梁平地区斜坡坡度在3°左右,向北东、北西方向变陡,宣汉地区约20°,苍溪、仪陇地区最高达40°以上,海槽呈向北逐渐加深的箕形。根据晚二叠世末期斜坡倾斜反射高度计算,开江—梁平海槽的水深在350～450m左右。苍溪、仪陇及梁平地区飞仙关组斜坡进积明显,宣汉地区的斜坡以加积为主。目前钻遇的最厚鲕滩储集层位于宣汉加积斜坡附近的台地边缘,单井的鲕粒白云岩储集层厚度超过300m。

The carbonate platform and trough facies of the Late Permian to the Early Triassic Feixianguan Formation are outcropped in the east Fankuai in Xuanhan County, northeastern Sichuan Basin. The distance between the Jichang carbonate slope facies section which separates the platform facies and trough facies, and the platform margin facies section is less than one kilometer. According to the characteristics of the outcrops of carbonate slope facies, the carbonate slope facies can be identified on the seismic sections of Kaijiang-Liangping trough in northern Sichuan Basin and the shallow water carbonate sedimentary area and deep water carbonate sedimentary area was subdivided. Slope facies is characterized by the obvious inclined reflection. High amplitude reflection caused by the mudstone of Dalong Formation and the bottom of the Feixianguan Formation of the trough facies onlapped towards the platform and disappeared. The twoway travel time of trough facies in the Upper Permian is less than 100 ms and is far less than that of the Feixianguan Formation, while the two-way travel time of platform facies is more than 150 ms which is similar to or slightly longer than that of the Feixianguan Formation. The incline reflection characteristics of slope facies, indicate that the carbonate slope of Kaijiang-Liangping trough was developed in the middle Late Permian and migrated to the carbonate platform with transgression occurred. It became the steepest slope when the transgression reached the maxium at the end of the Late Permian. In the Feixianguan Period, slope gradient decreased with the sea level fall and the slope migrated to the trough. At the end of the Late Permian, the slope gradient of Liangping area which is the south of the trough was around 3°and slope became steep towards the northeast and northwest. In the Xuanhan area, the slope gradient was about 20°, and it achieved the highest gradient of 40° in Yilong area. The trough was a basin which became deeper to wards the north. According to the height of the inclined reflection of slope in the Late Permian, it is calculated that the depth of the Kaijiang-Liangping trough was around 350 to 450 meters. The slopes are characterized by obvious progradation of the Feixianguan Formation in Cangxi, Yilong and Liangping areas and the slope is dominated by aggradation in Xuanhan area. The currently drilled thickest oolitic beach reservoir is located in the platform margin adjacent to the aggradational slope in Xuanhan area. The thickness oolitic dolomite reservoir in individual wells is larger than 300 meters.