川东北早-中三叠世聚盐环境及海水浓缩成钾模式

川东北地区中下三叠统海相含蒸发岩地层是近十年来油气勘探开发的重点区域,但对其中的蒸发岩沉积序列研究较少,对含钾蒸发岩系的分布规律还不清楚。本文以气钾兼探为指导思想,在钻井岩心、野外露头和大量钻测井资料的基础上,开展了川东北中-下三叠统层序岩相古地理、蒸发岩分布特征和成钾条件研究,结果显示:嘉陵江组沉积时期,蒸发岩明显受三级海平面升降变化的控制,主要发育于高位体系域,即层序SQ1高位期沉积的嘉二段和层序SQ2高位期沉积的嘉四、五段;雷口坡组沉积时期则主要受上扬子区域构造体制转换的控制,盆地整体抬升及周缘古陆或古隆起的形成,造成该时期总体为相对局限的沉积环境,海侵体系域和高位体系域的蒸发岩分布没有明显差别,都有膏盐层发育,且厚度都较小;川东北早-中三叠世在克拉通浅水环境碳酸盐岩局限台地、低纬度温暖干旱气候、海平面下降的联合控制下,海水向中心浓缩咸化,在宣汉-达州地区形成浓缩咸化中心,是最有利的找钾地区。

Evaporatic environment and the concentration model of potash in the Early-Middle Triassic, northeastern Sichuan Basin

The Lower-Middle Triassic marine evaporatic-bearing sequences in the northeastern Sichuan Basin received much attention in the petroleum exploration. However, little research was focused on the evaporatic sequences and thus the distribution regularities about potash-bearing sequences are poorly constrained. Based on drilling core, outcrop, and logging data, sequence-based lithofacies-paleogeography, evaporite distribution and conditions for potash formation of the Lower-Middle Triassic in northeastern Sichuan Basin were performed. The results show that evaporite of Jianglingjiang Formation was affected by third order sea-level change obviously and developed mainly in the high system track, corresponding to T₂j² of the SQ1's HST and T₂j⁴⁺⁵ of the SQ2's HST; The evaporites of the Leikoupo Formation was controlled by tectonic regime change of the Yangtze Block leading to basin uplift integrally and formation of palaeo-land or palaeo-uplift, then restricted environment was formed. Therefore, evaporite distribution bears no obvious difference in TST and HST. Under combined control of carbonate restricted platform of craton shallow marine environment, warm and arid climate in low latitude and falling of sea level, sea water concentrated and salinized centripetally. Xuanhan-Dazhou area is the salinization center, as well as the favorable exploration zone for potash salts.