川东地区新钻区块礁滩沉积模式及有利相带——以板东南缘上二叠统长兴组为例

Reef-beach sedimentary model and favorable facies zones of new block drilled in eastern Sichuan Basin:A case study of Upper Permian Changxing Formation in south margin of Bandong area

近年来,四川盆地东部武胜-石柱台内洼地两侧上二叠统长兴组礁滩气藏勘探开发不断突破,卧龙河南缘地区3口生物礁钻井已获天然气近5×10^(8)m^(3),与其相邻的板东南缘长兴组也发现大面积相同的礁滩地震异常体,但其“亚平行层状反射”特征与典型的生物礁地震响应特征具有明显差异,由于该区块钻井少(仅板东18井1口井),研究程度较低,对于该异常体是礁还是滩的地质认识仍然模糊不清。因此,亟待建立本区的沉积模式,为下一步的勘探部署提供理论依据。在对比分析卧龙河南缘及广安高带地区的薄片、测井曲线及地震资料的基础上,结合正演及古地貌分析,开展了本区沉积储层模式及有利相带预测的研究。结果表明:①板东南缘地区长二段岩性以生屑灰岩为主,长三段见生物礁灰岩,认为板东南缘地区具有生物礁和生屑滩纵向叠置发育的条件;②板东南缘台洼边缘前端长顶上隆、振幅减弱、内部断续及空白反射为生物礁的响应特征,台洼边缘后端长顶上隆、振幅减弱、内部亚平行层状反射为礁滩叠置体的响应特征;③恢复了长兴组古地貌,预测了礁滩体有利相带共8.8km^(2);④通过与邻区的井震对比及正演分析,建立了本区台洼边缘前端生物礁、台洼边缘后端礁滩叠置体的沉积模式。板东南缘地区长兴组礁滩沉积模式的建立对台洼礁滩气藏的勘探开发具有重要的指导意义。

Successive breakthroughs have been made in the exploration and development for reef-beach gas reservoirs of the Upper Permian Changxing Formation located in both sides of the Wusheng-Shizhu intra-platform depression of eastern Sichuan Basin in recent years.Three wells targeting organic reefs in the south margin of Wolonghe area have produced natural gas of 5×10^(8)m^(3)approximately.In the adjacent south margin of Bandong area,a same reef-beach seismic anomalous body in large scale was found in the Changxing Formation,but its subparallel layered reflection characteristics are significantly different from the typical seismic response characteristics of organic reefs.It is still unclear whether the anomalous body belongs to reef or beach,since this area has been scarcely studied due to less drilling(only one well drilled,Well Bandong18).Therefore,it is urgent to establish the sedimentary model of the area,so as to provide a theoretical basis for subsequent exploration deployment.The sedimentary model and favorable facies zones of this area were investigated,based on the comparative analysis of thin sections,logging curves and seismic data from the south margin of Wolonghe area and the high zones of Guang’an,together with forward and paleogeomorphic analyses.The results show that,(i)bioclastic limestones are dominant in the second member of Changxing Formation,and organic-reef limestones can be seen in the third member,indicating that the south margin of Bandong area has the condition for developing vertically superimposed organic reefs and bioclastic beaches;(ii)in the front edge of the platform depression of the study area,the top uplift,weakened amplitude,and internal discontinuous and blank reflections are the seismic response characteristics of organic reefs,while in the rear edge,the top uplift,weakened amplitude,and internal subparallel layered reflections are the response characteristics of superimposed reef-beach bodies;(iii)the paleogeomorphology of Changxing Formation was restored,and the favorable facies zones of reef-beach bodies predicted are 8.8km^(2)in total;and(iv)the sedimentary model that organic reefs in the front edge and superimposed reefbeach bodies in the rear edge of the platform depression was established through the logging-seismic correlation with adjacent area and the forward analysis.The establishment of the reef-beach sedimentary model of Changxing Formation in the south margin of Bandong area is of great guiding significance for the exploration and development of reef-beach gas reservoirs in platform depression.