Acta Petrolei Sinica ›› 2023, Vol. 44 ›› Issue (8): 1274-1298.DOI: 10.7623/syxb202308006

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A new method for isochronous stratigraphic division of Longmaxi Formation in Sichuan Basin, China

Liang Feng1,2, Zhao Qun1,2, Shi Xuewen3, Wang Yuman1,2, Zhang Qin1,2, Zhou Shangwen1,2, Qi Lin4, Cui Huiying1   

  1. 1. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China;
    2. National Energy Shale Gas R & D (Experimental) Center, Hebei Langfang 065007, China;
    3. Shale Gas Institute, PetroChina Southwest Oil & Gasfield Company, Sichuan Chengdu 610051, China;
    4. Geological Exploration and Development Research Institute, CNPC Chuanqing Drilling Engineering Company Ltd., Sichuan Chengdu 610051, China
  • Received:2023-02-05 Revised:2023-06-07 Online:2023-08-25 Published:2023-09-06

四川盆地龙马溪组地层等时划分新方法

梁峰1,2, 赵群1,2, 石学文3, 王玉满1,2, 张琴1,2, 周尚文1,2, 漆麟4, 崔会英1   

  1. 1. 中国石油勘探开发研究院 北京 100083;
    2. 国家能源页岩气研发(实验)中心 河北廊坊 065007;
    3. 中国石油西南油气田公司页岩气研究院 四川成都 610051;
    4. 中国石油川庆钻探工程有限公司地质勘探开发研究院 四川成都 610051
  • 通讯作者: 张琴,女,1985年11月生,2019年获中国地质大学(北京)矿产普查与勘探专业博士学位,现为中国石油勘探开发研究院高级工程师,主要从事页岩气储层与地质评价相关工作。Email:zhangqin2169@petrochina.com.cn
  • 作者简介:梁峰,男,1982年11月生,2018年获中国矿业大学地质资源与地质工程专业博士学位,现为中国石油勘探开发研究院高级工程师,主要从事页岩气地质与开发评价相关工作。Email:liangfeng05@petrochina.com.cn
  • 基金资助:
    中国石油天然气股份有限公司前瞻性基础性科技项目"海相页岩气勘探开发技术研究"(2021DJ1904)资助。

Abstract: Based on the study of graptolite biostratigraphic classification, electrical characteristics and distribution of bentonite condensed sections of 53 shale gas wells in Sichuan Basin, the paper clarifies the origin and isochronous attribute of natural gamma (GR) peak, and specifically, its formation is closely related to the tectonic activities of bentonite condensed sections. Moreover, a tectonic activity response interface with industrial significance was found in the middle Rhuddanian period (LM4); an isochronous division method was built for the GR logging curves, i.e., thorium (Th) and compensated neutron (CNL) logging curves, of Longmaxi Formation, and the distribution law of shale in different periods was re-depicted. The results show as follows:(1) There are a total of 8 GR peak intervals (No. 1-8) developed in Longmaxi Formation. The GR peak has a good correlation with biostratigraphy and bentonite condensed sections, and its formation is closely related to the episodic tectonic activities of Kwangsian Orogeny. Deflection leads to the formation of high GR shale in the sedimentation area (depocenter), and the source area uplifted by flexure provides a large amount of material sources, resulting in an increase in Th content in the high GR interval. (2) It was proposed that the GR peak (high Th) of No.3 GR peak interval (LM4) is the response of first episode of strong tectonic activity of Kwangsian Orogeny (dense and thick bentonite sections) in Early Silurian in Sichuan Basin. This stage of tectonic activities significantly accelerated the deposition rate in the basin, increased the clay mineral and other source minerals from Th reaction, decreased the reducibility of water body, and deteriorated the quality of shale reservoirs. (3) Based on the identification of three GR peaks with obvious characteristics (No. 1-3), in combination with Th or CNL logging curves, the paper summarizes 5 types of GR logging curve in the early depositional period of Longmaxi Formation in Sichuan Basin, determines the distribution range of different curve types, and establishes the isochronous division method for GR logging curve strata. (4) Based on the new method, the regional distribution map of key intervals in Wufeng Formation and Longmaxi Formation was built. In Sichuan Basin, the WF2-WF4, LM1-LM3, and LM4-LM5 shales show an NE-SW trending sedimentary pattern of "multi-row and multi-center" and "alternating uplifts and depressions" with a significantly undulated seabed, and the LM6-LM8 shale shows a trend of being thick in the southeast and thin in the northwest, and the great variation in thickness indicates a drastic change in sedimentary pattern. This finding has guiding significance for isochronous correlation, paleogeographic analysis and reconstruction, as well as the prediction of high-quality reservoirs of Longmaxi Formation in Sichuan Basin.

Key words: Wufeng Formation-Longmaxi Formation, graptolite biostratigraphy, GR peak, key tectonic interfaces, isochronous stratigraphic correlation, shale distribution

摘要: 通过对四川盆地53口页岩气井进行笔石生物地层划分、电性特征和斑脱岩密集段分布研究,明确了自然伽马(GR)峰具有等时属性,其形成与斑脱岩密集发育段(构造活动)密切相关,并在鲁丹阶沉积中期(LM4)发现一个具有重要工业意义的构造活动响应界面,建立了龙马溪组GR测井曲线[钍(Th)、补偿中子(CNL)测井曲线]等时划分方法,重新刻画了不同时期页岩展布规律。研究结果表明:①龙马溪组共发育8个GR峰值层段(编号①—编号⑧),GR峰值与生物地层、斑脱岩密集段具有良好的对比关系,其形成与广西运动的"幕式构造活动"密切相关,挠曲导致沉降区(沉积中心)形成高GR页岩,而挠曲抬升的源区提供了大量物质来源,致使高GR层段Th含量增高;②提出编号③(LM4)的GR峰(高Th)是早志留世广西运动第1幕强构造活动(斑脱岩层段密集,厚度大)在四川盆地内的响应,该期构造活动致使盆地内沉积速率明显加快,黏土矿物和Th等反应物源的矿物明显增多,水体的还原性明显下降,页岩储层品质变差;③以识别编号①、编号③和编号⑥3个特征明显的GR峰为基础,Th或CNL测井曲线为辅助,总结了四川盆地龙马溪组沉积早期5种GR测井曲线类型,圈定了不同曲线类型的分布范围,建立了GR测井曲线地层等时划分方法;④基于新方法开展了五峰组—龙马溪组关键层段区域分布编图,四川盆地WF2—WF4、LM1—LM3、LM4—LM5页岩呈NE—SW向"多排多中心"、"隆坳相间"的沉积格局,海底起伏明显,LM6—LM8页岩厚度呈现东南部厚西北部薄趋势,厚度趋势显著变化预示沉积格局剧变。这一研究成果对四川盆地龙马溪组地层等时对比、古地理分析与重建及优质储层的预测具有重要的指导意义。

关键词: 五峰组—龙马溪组, 笔石生物地层, GR峰, 关键构造界面, 地层等时对比, 页岩展布

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