Salt Accumulation, Potassium Formation Mechanism and Enrichment Model of Triassic in Northeast Sichuan Basin
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摘要:
川东北地区三叠系发育巨厚的蒸发岩层，并赋存优质的深层富钾卤水. 因构造沉积条件的整体差异，川东北富钾卤水的成因机制与富集规律未有详尽研究. 以“气钾兼探”为指导思想，综合利用野外露头-岩心-测井等资料，对川东北地区三叠系蒸发岩分布规律以及岩相古地理演化进行分析，明确了膏盐盆在T1j4-5-T2l1期间规模逐渐缩小、向SW逐渐迁移的分布规律. 结合岩心与卤水地球化学分析，判定川东北地区富钾卤水成因机制以海水蒸发浓缩与表层淡水淋滤为主，深部水岩反应较弱. 结合井震分析，提出富钾卤水具备“膏盆盐盆控卤、孔隙裂缝储卤、背斜核部富卤”的富集规律. 综合认为富钾卤水为“构造-沉积”双重因素控制的“次级深凹”富集模式.
Abstract:During the Triassic period, the Northeast Sichuan developed a huge evaporative salt layer and accumulated high quality deep potassium-rich brine. Due to the overall differences in tectonic depositional conditions, the genetic mechanism and enrichment rules of potassium-rich brine in Northeast Sichuan Basin have not been studied in detail. In this study it takes "gas-potassium exploration" as the guiding ideology, and comprehensively utilizes the field outcrop-core-logging data to analyze the distribution and lithofacies paleogeographic evolution of the Triassic evaporite in the Northeast Sichuan basin, and finally it shows the size of salt basin in Northeast Sichuan decreased gradually and migrated to southwest during T1j4-5-T2l1. Combined with core and brine geochemical analysis, it is concluded that the genesis mechanism of potassium-rich brine in Northeast Sichuan is mainly seawater evaporation-concentration mechanism and surface fresh water leaching mechanism, and the deep water-rock reaction is weak. Combined with well logging and seismic interpretation, it is proposed that the potassium-rich brine has the enrichment rule of "controlling brine by the gypsum basin and salt basin; storing brine in the pores and fractures; enriching brine in the core of anticline". It is considered that the potassium-rich brine is a "secondary deep depression" enrichment model controlled by "structure-deposition" dual factors.
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图 1 研究区构造位置 (a, b) 和综合柱状图 (c)
图a据龚大兴等（2014）修改；图b据李杰（2018）修改
Fig. 1. The structural location (a, b) and comprehensive histogram (c) of the study area
图 2 川东北地区三叠纪典型岩心、野外露头及镜下薄片特征
a. A井，泥微晶灰岩，T2l1；b. 营24S井，白云质灰岩，T1j4-5；c. A井，微晶白云岩，T2l1；d. A井，灰质白云岩，T2l1；e. A井，膏质白云岩，T2l1；f. 龙岗001-12井，颗粒白云岩，T1j4-5；g. 营24S井，石膏，T1j4-5；h. 四川广安，谢家槽剖面膏溶角砾岩，T1j4-5；i. 营24S井，盐岩，T1j4-5
Fig. 2. Characteristics of typical Triassic cores, outcrops and microscopic thin sections in Northeast Sichuan
图 3 嘉四五段 (a、b) 和雷一段 (c、d) 蒸发岩等厚图及岩相古地理图
Fig. 3. The evaporite thickness map and lithofacies palaeogeographic map of T1j4-5 (a, b) and T2l1 (c, d)
图 4 川东北富钾卤水海水蒸发浓缩机制
Fig. 4. Evaporation and concentration mechanism of potassium-rich brine in Northeast Sichuan
图 5 川东北富钾卤水淡水淋滤机制
Fig. 5. The leaching mechanism of potassium-rich brine in Northeast Sichuan
图 6 川东北宣汉地区A井样品C、O同位素和古水温
C、O同位素数据部分引自Zhang et al.（2018）
Fig. 6. C, O isotopes and paleowater temperature of samples from Well A in Xuanhan area, Northeast Sichuan
图 7 川东北富钾卤水深部水岩反应机制
Fig. 7. The reaction mechanism of deep water rock of potassium-rich brine in Northeast Sichuan
图 8 A井原始地幔标准化微量元素蛛网图
Fig. 8. The primitive mantle-normalized trace element spider diagram of Well A
图 9 川东北地区岩石微观特征
a. 普光5井×4（+），井深2 675.2 m，针孔砂屑云岩，铸模孔、粒内溶孔；b. 普光8井×2（+），井深2 366 m，泥粉晶云岩，晶间溶孔；c. 普光11井×10（+），井深3 182.6 m，亮晶砂屑灰质云岩，粒间、粒内溶孔；d. A井×10（‒），井深3 338.6 m，粒间溶孔；e. A井×10（‒），井深3 338.6 m，微裂隙填充及填充物溶蚀；f. A井×10（+），井深3 425.7 m，微晶白云岩，裂缝
Fig. 9. Microcosmic characteristics of rocks in Northeast Sichuan region
图 11 川东北三叠系富钾卤水“次级深凹”聚盐模式
Fig. 11. The "secondary deep concave" polysalt pattern of Triassic potassium-rich brine in Northeast Sichuan
表 1 A井富钾卤水分析结果
Table 1. Analysis results of potassium-rich brine in Well A
元素 浓度（mg/L） K+ 31 961.25 Na+ 88 217.50 Mg2+ 1 016.48 Cl‒ 210 365.11 Br‒ 1 342.51 SO42‒ 418.08 注：深藏卤水工业开采标准参考《矿产地质勘查规范DZ/T0212 4⁃2020》：K+/Cl-为0.5%~1%；Br-为50~60 mg/L. 
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表 2 A井岩心微量元素含量（μg/g）
Table 2. Contents of trace elements in Well A core (μg/g)
样品数 参数 U Ta Sr Th Hf Sm 19 最大值 9 523.81 142.77 11.14 560.55 12.57 11.41 最小值 226.76 29.74 1.09 13.84 2.09 0.46 平均值 3 974.22 74.52 4.56 174.76 4.96 3.71
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表 3 宣汉‒达州地区雷口坡组和嘉陵江组储层孔渗数据
Table 3. Statistical data of reservoir porosity and permeability of Leikoupo Formation and Jialingjiang Formation in Xuanhan⁃ Dazhou district
系 组 层位 孔隙度（%） 渗透率（10-3 µm2) 平均孔隙度（%） 平均渗透率（10-3 µm2） 储层类别划分 三叠系 雷口坡组 雷三段 2.46 < 0.1 2.57 0.1 Ⅰ类：POR≥12%Ⅱ类：6%≤POR < 12%Ⅲ类：3%≤POR < 6% 雷二段 3.02 < 0.1 雷一段 2.24 < 0.1 嘉陵江组 嘉五段 2.26 \ 3.97 0.642 嘉四段 5.05 0.012 嘉三段 2.36 \ 嘉二段 4.21 0.667 嘉一段 3.12 \
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