海相页岩定量古地理编图及深水陆棚沉积微相划分——以川南泸州地区五峰组—龙马溪组龙一11⁃4小层为例

Quantitative Paleogeographic Mapping and Sedimentary Microfacies Division in a Deep-water Marine Shale Shelf:Case study of Wufeng Formation-Longmaxi Formation shale,southern Sichuan Basin,China

海相页岩古地理编图和深水陆棚亚相微相细分一直是细粒沉积学研究的热点和难点。以川南泸州地区五峰组—龙一_(1)^(4)小层为编图单元,通过页岩编图关键单因素的选取和典型钻井矿物组分测井数据的统计分析,编制了关键单因素图件。在此基础上,通过深水陆棚亚相各微相划分标准的确定及多因素图件综合分析,编制了海相页岩定量古地理图。结果表明:1)海相页岩定量古地理编图的关键单因素是地层厚度(m)、碳酸盐矿物含量(%)、石英含量(%)和黏土矿物含量(%)。其中,地层厚度(m)反映了该地层单元在区域上的分布范围、厚度变化及沉积时期的大地构造背景;碳酸盐矿物含量(%)及分布能够反映古水深,水深越浅,碳酸盐矿物含量(%)越高;石英含量(%)及分布主要受古水深控制,水深越大,石英含量越高;黏土矿物含量(%)和分布可以反映古物源方向,黏土矿物含量越高,受陆源碎屑影响越大。2)依据海相页岩的碳酸盐矿物含量(%)、石英含量(%)和黏土矿物含量(%),深水陆棚亚相可划分出深水斜坡、深水洼地、重力流沉积和深水平原4种沉积微相。其中,深水斜坡碳酸盐含量最高(15%~20%),深水洼地石英含量最高(>55%),重力流沉积黏土矿物含量最高(>40%),深水平原各矿物含量介于其他微相之间。3)五峰组—龙一_(1)^(4)小层发育时期,川南泸州地区发育深水斜坡、重力流沉积、深水洼地和深水平原4种沉积微相,整体呈“3高1洼1复合体”的分布格局,其中,深水斜坡分布于研究区西北部和东北部,重力流沉积发育于研究区东南部,深水洼地发育于研究区中部,其他地区发育深水平原。沉积微相分布是该时期古地形及古构造活动的产物。

Paleogeographic mapping of marine shale and sedimentary microfacies division in a deep-water shelf are key points in the search for the“sweet spot”in shale gas exploration.This analysis of logging curves and the prepara⁃tion of single-factor maps optimizes important single factors in layer of Wufeng Formation-Longmaxi Formation in the Luzhou area,southern Sichuan Basin.A quantitative paleogeographic map of the marine shale was systematically compiled by classifying the sedimentary microfacies in the deep-water shelf using comprehensive multi-factorial anal⁃ysis.The results show that(1)the single factors for consideration are:strata thickness(m),carbonate mineral con⁃tent(%),quartz content(%),and clay mineral content(%).The strata thickness reflects the regional distribution tendency,since variation in thickness reflects the paleotectonic background of the stratigraphic unit during sedimen⁃tation.The mineral content and distribution directly indicate the topographic relief of the deep-water shelf.As the ter⁃rain level rose,carbonate mineral content increased.Quartz was more likely to develop in the underwater sub-sag.The clay mineral content reflects the locations of the ancient provenances;high content clay mineral indicates that the deposition pathway was significantly influenced by the clastic supplies.(2)The contents of carbonates,quartz and clay minerals are evidences of four types of sedimentary microfacies dispersed in the deep-water shelf,distin⁃guished by their content:subaqueous slpoe,subaqueous sub-sag,gravity flow deposit and subaqueous plain.Of these,carbonate mineral content is highest in the subaqueous slope(16%⁃20%),quartz is highest in the subaqueous sub-sag(>55%),and clay mineral content is highest in the gravity flow deposit(>40%).In the subaqueous plain,the content of these three minerals is mixed with other microfacies.(3)In the Luzhou area,the Wufeng⁃L11⁃4 layer con⁃tains the four microfacies types in three slopes in the northwest and northeast of the study area,one sub-sag and one gravity flow deposit in the southeast.An sub-sag occurs in the center of the study area,and others in the subaqueous plain.The microfacies distribution is the result of paleotopographical changes and paleotectonic activity during depo⁃sition of the Wufeng Formation-Longmaxi Formation.