四川盆地广元-梁平古裂陷构造-沉积演化

二叠纪-三叠纪转折期,四川盆地北部广元-梁平地区发育一个北西-南东向古裂陷,称之为"开江-梁平古裂陷"。由于对广元-梁平古裂陷的认识直接影响到川北和川东北地区的油气勘探部署,因此有必要对该古裂陷的构造-沉积演化进行深入研究。通过系统收集广元-梁平古裂陷槽内外的露头和钻井资料,并根据地层不整合面特征、沉积充填特征、测井曲线突变特征、沉积物地球化学特征等沉积记录以及地震剖面约束,发现广元-梁平古裂陷槽内仍存在茅口组三段(茅三段)和四段沉积物,槽内茅口组顶部缺乏广泛岩溶,表明茅口组减薄并非岩溶侵蚀造成,而是由相变导致的。广元-梁平古裂陷的雏形最早形成于茅三段和茅四段沉积期,吴家坪期在此基础上继承性发展,长兴期拉张裂陷活动加剧,热液活动加强,构造沉积分异明显,为古裂陷发育的高峰期,直至飞仙关组四段沉积期,古裂陷才完全被填平补齐。广元-梁平古裂陷对环裂陷槽中-上二叠统多层系储层具有明显控制作用,不仅控制着茅口组生屑滩储层的发育,还控制着吴家坪组颗粒滩储层的发育,且吴家坪组储层中存在油气运聚过程,发育残余孔隙,为下一步油气勘探的有利目标。

Tectonic-sedimentary evolution of Guangyuan-Liangping paleo-rift in Sichuan Basin

During the Permian-to-Triassic transition, a NW-SE-trending paleo-rift, also known as Guangyuan-Liangping paleo-rift, was developed in the Guangyuan-Liangping area of northern Sichuan Basin.The paleo-rift is of great significance to the oil and gas exploration and development in the northern and northeastern parts of the Basin and therefore calls for an in-depth study on its tectonic-sedimentary evolution.A systematic collection and study of outcrops and drilling data from both inside and outside of the paleo-rift, combined with analyses of corresponding sedimentary records, including unconformities, sedimentary filling, abrupt log response changes, and sediment geochemistry, as well as seismic profiles, shows that the sediments of the 3^rd and 4^th members of the Maokou Formation extend into the paleo-rift.The absence of an extensive karstification on top of the Maokou Formation in the paleo-rift suggests that, it was sedimentary facies changes, rather than karst erosion, that caused the thinning of the Maokou Formation in the paleo-uplift.The paleo-rift was initially taking shape during the deposition of the 3^rd and 4^th members of the Maokou Formation and continued to grow during the Wujiapingian stage.It entered the peak development phase during the Changxingian stage when the extensional rifting and hydrothermal activities intensified and tectonic-sedimentary differentiation escalated.It wasn't until the deposition of the 4^th member of the Feixianguan Formation that the paleo-rift was completely filled up.The paleo-rift had strong control over the development of the Middle-to-Upper Permian reservoir rocks around it, including the biodetritus beach facies of the Maokou Formation and the grain beach facies in the Wujiaping Formation.Moreover, signs of hydrocarbon migration and accumulation and residual pores in the Wujiaping Formation indicate that the Formation is promising in terms of further hydrocarbon exploration.