Stratigraphic and sedimentary characteristics of Middle Permian Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain
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摘要: 川西龙门山前带栖霞组是近期油气勘探的热点，龙门山中段栖霞组勘探程度低，明确其地层沉积特征，对于天然气勘探具有重要的指导意义。基于对龙门山中段北川通口剖面实测，结合薄片鉴定和主量、微量元素分析，对栖霞组层序地层及沉积特征进行了研究，明确了沉积环境。北川通口剖面梁山组—栖霞组识别出2个三级和1个四级层序界面，划分为1个三级层序和2个四级层序。梁山组—栖霞组发育滨岸—沼泽、开阔台地及台地边缘3种沉积相，开阔海、滩间海、台内滩和台缘滩4种亚相，台内生屑滩、台缘生屑滩和台缘砂屑、生屑滩3种微相。栖一段至栖二段下部整体处于开阔台地，栖二段中上部则处于台地边缘，建立了龙门山中段栖霞组镶边碳酸盐岩台地沉积模式。栖霞组沉积期整体处于温暖、湿润、富氧、古生物大量繁殖的海水—陆表海沉积环境，栖二段中上部高位体系域Sr/Ba比值整体最低，MgO/CaO比值最高，平均古水温高，V/(V+Ni)、V/Cr低值，古盐度m值明显较高，代表远离陆地的浅水、高能台地边缘沉积环境，厚大高能台缘砂屑、生屑滩发育，是规模优质滩相孔隙型白云岩储层发育有利部位。龙门山中段栖二段与北段、南段具有相似的沉积相带和储集条件，是栖霞组油气勘探的有利区带，有望成为实现规模增储的重要战略接替阵地。Abstract: The Qixia Formation in the foothills of Longmen Mountain in the western Sichuan Basin is gaining attention for oil and gas exploration. However, exploration of the Qixia Formation in the middle Longmen Mountain is insufficient. Consequently, it is essential to understand its stratigraphic sedimentary characteristics for guiding natural gas exploration. This study focuses on the Beichuan Tongkou section, combines field measurements, thin-section observation and trace element analyses to investigate the sequence stratigraphy and sedimentary features of the Qixia Formation, thereby defining its sedimentary environment. Within the Beichuan Tongkou section, which spans from the Liangshan Formation to the Qixia Formation, researchers have identified two third-order and one fourth-order sequence boundaries, delineating one third-order sequence and two fourth-order sequences. The sedimentary facies found in this interval include shore-swamp, open platform, and platform edge environments, which are further classified into four subfacies including open sea, intertidal sea, intra-platform beach, and platform edge beach. Additionally, three microfacies are identified including intra-platform bioclastic beach, platform edge bioclastic beach, and platform edge sand debris and bioclastic beach. The Qi1 member and lower part of the Qi2 member is located on an open platform, while the upper-middle part of the Qi2 member is found at the platform edge. This setup defines a sedimentary model for a carbonate-rimmed platform in the Qixia Formation of the middle Longmen Mountain. During the sedimentary period of the Qixia Formation, a warm, humid, and oxygen-rich marine environment supported abundant ancient organisms. In the highstand system tract of the upper-middle part of the Qi2 member, the Sr/Ba ratio is lowest, while the MgO/CaO ratio is highest, indicating elevated ancient water temperatures and specific chemical compositions. The m value, reflective of paleosalinity, is notably high, suggesting a shallow, high-energy platform edge environment away from the mainland. Thick, high-energy platform edge sand debris and bioclastic beaches in this area promote the development of high-quality porous dolomite reservoirs of beach facies. The sedimentary facies and reservoir conditions of the Qi2 member in the middle Longmen Mountain are comparable to those in other sections, making it a promising target for oil and gas exploration within the Qixia Formation and a potential strategic replacement area for reservoir expansion.
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图 1 川西龙门山前带地质背景及北川通口剖面位置
Figure 1. Geological background of foothills of Longmen Mountain in western Sichuan Basin and location of Beichuan Tongkou section
图 2 龙门山中段北川通口剖面梁山组—栖霞组综合柱状图
Figure 2. Comprehensive column of Liangshan-Qixia formations in Beichuan Tongkou section, middle Longmen Mountain
图 3 龙门山中段北川通口剖面梁山组—栖霞组层序界面特征
a.石炭系(左)与梁山组(右)角度不整合接触界面(红色虚线)；b.栖一段(左)与栖二段(右)岩性转换面(蓝色虚线)，同时存在一切穿栖一段和栖二段的逆断层(红色实线)；c.栖霞组(下)与茅口组(上)平行不整合接触界面(红色虚线)。
Figure 3. Sequence boundary characteristics of Liangshan-Qixia formations in Beichuan Tongkou section, middle Longmen Mountain
图 4 龙门山中段北川通口剖面栖霞组主要岩石类型
a.泥晶灰岩，栖一段，单偏光；b.生屑藻泥晶灰岩，栖一段，单偏光；c.泥晶生屑藻灰岩，栖二段，单偏光；d.泥亮晶生屑藻团块灰岩，栖二段，单偏光；e.细—中晶残余砂屑、生屑白云岩，蓝色为铸体(孔隙)，栖二段，单偏光；f.细—中晶残余砂屑、生屑白云岩，蓝色为铸体(孔隙)，栖二段，单偏光。
Figure 4. Main rock types of Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain
图 5 龙门山中段北川通口剖面栖霞组典型相序
Figure 5. Typical facies sequence of Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain
图 6 龙门山中段北川通口剖面栖霞组主要古生物类型
a.有孔虫，栖一段，单偏光；b.腕足类生物化石，箭头所示，栖二段；c.藻类(蠕孔藻)，栖二段，单偏光；d.假纺锤虫筳，栖二段，单偏光；e.双壳类生物化石，栖二段；f.棘屑(海百合茎)，箭头所示，栖二段。
Figure 6. Main palaeontological types of Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain
图 7 龙门山中段北川通口剖面栖霞组镶边碳酸盐岩台地沉积模式
Figure 7. Sedimentary model of carbonate platform rimmed by Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain
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