川西北中二叠统栖霞组沉积特征及古地貌响应

Sedimentary Characteristics and Paleogeomorphic Responses of the Middle Permian Qixia Formation in the Northwestern Sichuan Basin

川西北中二叠统栖霞组发育厚层白云岩储层,成为近年来四川盆地油气勘探开发的重点领域,但栖霞组有利相带分布和储层发育的主控因素并不明朗,制约了该地区油气勘探进展。根据野外露头、岩心、薄片、测井和地震资料,结合区域地质演化,对川西北栖霞组沉积特征、沉积格局以及古地貌进行研究发现:川西北栖霞组主要为碳酸盐岩台地沉积,自西北向东南方向依次发育盆地—斜坡相、台地边缘相和开阔台地相,并发育台缘滩或台内滩等若干亚相类型。其中栖一期沉积水体较深,为弱镶边碳酸盐岩台地形成期,台缘滩厚度薄,展布范围有限;栖二期为栖霞组主要成滩期,滩体厚度大,展布范围广,具典型镶边碳酸盐岩台地特征。栖霞组白云岩主要为台地边缘高能滩相沉积背景,栖霞组沉积期,研究区具西北高、东南低,西南高、东北低的古地貌格局,其不仅控制着台地边缘高能滩相的沉积分布,并进一步为该区白云岩储层发育奠定了基础,白云岩具有西北向东南、西南向东北逐渐减薄的特点。结合沉积演化和古地貌分析结果,认为古地貌为栖霞组白云岩发育的主要控制因素,西北及西南方向等古地貌高地为研究区栖霞组白云岩储层发育的有利沉积相带。

The Middle Permian Qixia Formation in northwestern Sichuan Basin has developed thick dolomite reservoirs,which have become a key area for oil and gas exploration and development in recent years;however,the main controlling factors of favorable sedimentary facies distribution and reservoir development in the Qixia Formation are not clear,which restricts the oil and gas exploration progress in this area.Using field outcrops,cores,thin section,well logs,and seismic data,combined with regional geological evolution,the sedimentary characteristics and framework of the Qixia Formation in the northwestern Sichuan were examined.The Qixia Formation mainly consists of carbonate platform deposits;from northwest to southeast,basin-slope facies,platform margin facies,and open platform facies are developed in turn,and several subfacies,such as platform margin banks or inner platform banks,are developed.Among them,the sedimentary water body in the Early Qixia stage is relatively deep,and it is the formation period of a weakly rimmed carbonate platform;the Late Qixia stage is the main bank formation period of the Qixia Formation.The banks are thick and have a wide range of distribution.At this time,the study area has the characteristics of a typical rimmed carbonate platform.The dolomite reservoir of the Qixia Formation is mainly a high-energy bank facies sedimentary background of a platform margin.In the sedimentary period of the Qixia Formation,the study area has a paleogeomorphic framework that is high in the northwest,low in the southeast,high in the southwest,and low in the northeast,it not only controls the deposition and distribution of high-energy beach facies on the platform margin,but also further controls the development of dolomite reservoirs in the area,and the thickness of dolomite reservoirs gradually decreases from southwest to northeast and northwest to southeast.Combining sedimentary evolution and framework,we determine that paleogeomorphology is the main controlling factor for the development of dolomite in the Qixia Formation.Paleogeomorphic highlands such as in the northwest and southwest,are favorable sedimentary facies zones for the development of dolomite reservoirs in the Qixia Formation.