川中地区茅口组碳酸盐岩层序地层及沉积相特征

Sequence stratigraphy and sedimentary facies characteristics of the Maokou Formation carbonate rocks in central Sichuan Basin

四川盆地二叠系茅口组碳酸盐岩是油气勘探的重要层位,川中八角场—磨溪—龙女寺地区茅二段目前已获得勘探突破,但因其层序地层结构尚不明确,制约了茅口组沉积演化研究与储集层预测。本研究基于川中广安华蓥山二崖剖面茅口组高精度岩相特征,综合实际钻井资料,开展茅口组沉积相和层序地层格架分析,并解释层序格架对储集层发育特征和分布规律的控制作用。研究结果表明:(1)川中地区茅口组碳酸盐岩可分为8类岩石微相,在茅口组沉积中晚期,构造作用驱动茅口组由缓坡沉积体系向“台地—台内海槽”沉积体系转换。(2)栖霞组上部层序界面SB1和茅二下亚段上部层序界面SB2均为Ⅱ型岩相转换界面,茅二上亚段顶界面SB3为Ⅰ型淹没不整合界面,茅三段顶界面SB4为Ⅰ型剥蚀不整合面。(3)二崖剖面茅口组可划分为3个三级层序、5个四级层序:茅一段构成三级层序SQ1的海侵体系域;茅二下亚段中—下部对应三级层序SQ1的高位体系域,而上部发育三级层序SQ2陆架边缘体系域;茅二上亚段和茅三段分别对应三级层序SQ2和SQ3。(4)磨溪—龙女寺地区茅二下亚段云化储集层分布在三级层序SQ1高位体系域和三级层序SQ2陆架边缘体系域,分别以准同生晚期和准同生早期云化作用为主,受控于Ⅱ型层序界面SB2;广安—八角场地区茅二上亚段台缘滩相白云岩储集层分布在三级层序SQ2高位体系域并且受控于Ⅰ型层序界面SB3,经历了准同生早期基质云化—准同生晚期颗粒溶蚀—埋藏期云化作用阶段。该研究为四川盆地茅口组白云岩储集层分布规律的预测和成因研究提供了层序和沉积约束,对茅口组的油气勘探具有一定的参考意义。

The carbonate rocks of the Permian Maokou Formation in the Sichuan Basin are crucial for oil and gas exploration.Significant breakthroughs have been achieved in the exploration of the Maokou Formation in the Bajiaochang-Moxi-Longnüsi area of central Sichuan Basin.However,the unclear stratigraphic structure impedes the study of sedimentary evolution and reservoir prediction of the Maokou Formation.This research is based on high-precision lithofacies characteristics of the Maokou Formation at the Er’ya section in the Huaying Mountain of Guang’an in central Sichuan Basin.It integrates actual drilling data to analyze the sedimentary facies and sequence stratigraphic framework of the Maokou Formation,explaining the influence of the sequence framework on the development characteristics and distribution patterns of the reservoirs.The research findings indicate that:(1)The carbonate rocks of the Maokou Formation in the central Sichuan Basin can be divided into eight lithofacies.During the late stage of deposition of the Maokou Formation,tectonic activity facilitated the transition from a gentle slope sedimentary system to a“platform-platform interior sea trough”sedimentary system.(2)The upper sequence interface SB1 of the Qixia Formation and the upper sequence interface SB2 of the lower submember of Member 2 of the Maokou Formation are both classified as type II lithofacies transition interfaces.The top interface SB3 of the upper submember of Member 2 of the Maokou Formation is identified as a type I drowning unconformity interface,while the top interface SB4 of Member 3 of the Maokou Formation is classified as a type I erosion unconformity interface.(3)The Maokou Formation in the Er’ya section can be divided into three third-order sequences and five fourth-order sequences.Member 1 of the Maokou Formation constitutes the transgressive system tract of third-order sequence SQ1.The middle to lower parts of the lower submember of Member 2 correspond to the high-position system tract of third-order sequence SQ1,while the upper part develops the transgressive system tract of third-order sequence SQ2 at the margin of the continental shelf.The upper submember of Member 2 and Member 3 of the Maokou Formation correspond to third-order sequences SQ2 and SQ3,respectively.(4)In the Moxi-Longnüsi area,the dolomitized reservoirs of the lower submember of Member 2 of the Maokou Formation are distributed in the high-position system tract of third-order sequence SQ1 and the continental shelf margin system tract of third-order sequence SQ2.These reservoirs are primarily dominated by late diagenetic dolomitization and early diagenetic dolomitization,controlled by type II sequence boundary SB2.In the Guang’an-Bajiaochang area,the dolomite reservoirs of the upper submember of Member 2 of the Maokou Formation in the platform margin shoal facies are found in the high-position system tract of third-order sequence SQ2 and are controlled by type I sequence boundary SB3.These reservoirs undergo stages of early diagenetic matrix dolomitization,late diagenetic grain dissolution,and burial dolomitization.This study provides constraints on sequence and sedimentation for predicting the distribution patterns and investigating the genesis of dolomite reservoirs in the Maokou Formation in the Sichuan Basin,which is significant for oil and gas exploration in the Maokou Formation.