川东北嘉陵江组嘉四-五段杂卤石地震识别及属性预测方法

杂卤石作为一种重要的钾盐矿产,在川东北地区下三叠统嘉陵江组嘉四-五段(T₁j^4-5)分布广泛,而T₁j^4-5埋藏深,杂卤石横向分布不稳定,利用地震技术对杂卤石进行预测是本区钾盐勘探的必要需求。而杂卤石夹杂在低速盐膏岩和高速膏云岩互层的地层中,一般单层较薄,在地震剖面上反应弱,地震识别困难。本文通过井震联合对比和地震正演分析,总结了杂卤石的地震响应特征,认为杂卤石由于形成机理的特殊性,其一般位于厚层盐膏层的上部,因此在地震上具有低频、强波谷背景上的高频、弱振幅的特征。在低频地震剖面上,杂卤石对应低频、强波谷的部位;而在高频地震剖面上,杂卤石对应高频、弱波峰的部位。在此基础上,通过分析传统的层属性在杂卤石预测上的难点和制约因素,提出了利用杂卤石在高、低频地震体上的幅频差异特征,通过体属性运算预测杂卤石分布的技术方法,并取得了较好的预测效果。该方法可为其他地区杂卤石的地震识别和预测提供借鉴。研究结果认为,杂卤石在纵向上主要发育于T₁j^4-5第2旋回盐膏层的上部,平面上主要在盐膏层塑性变形增厚的部位富集,呈条带状展布。

Seismic identification and attribute prediction of polyhalite in the 4th and 5th members of the Jialingjiang Formation in Northeast Sichuan

Polyhalite, an important source of potassic mineral, is widely distributed in the 4th and 5th members of Lower Triassic Jialingjiang Formation (T₁j^4-5) in Northeast Sichuan. But T₁j^4-5 is deeply buried and the lateral distribution of polyhalite is unstable. Therefore, it is necessary to use seismic technology to predict the distribution of polyhalite. However, the polyhalite is mixed in the interbedded strata of low-velocity salt-gypsum and high-velocity gypsodolomite rocks, and the single layer is generally thin with weak response in seismic section, hindering seismic identification. In this paper, well-seismic tie comparison and seismic forward modeling were used to analyze the seismic response characteristics of polyhalite. We observed that polyhalite generally lies in the upper part of thick salt-gypsum layer due to particularities in its formation mechanism; as a result, its seismic response spectrum featured high-frequency and low-amplitude wave on the background of low-frequency, high-amplitude wave trough. More specifically, polyhalite corresponds to low-frequency, high-amplitude wave trough in low-frequency seismic section, and to high-frequency, low-amplitude crest in high-frequency seismic section. On the basis of this differential seismic response of polyhalite and further analysis of the limitations and constraints of traditional layer attributes in polyhalite prediction, we proposed a new method to predict the distribution of polyhalite through volume attribute computation and obtained a better prediction result. It showed that polyhalite is mainly developed in the upper part of the second cyclic gypsum-salt layer of T₁j^4-5 in the longitudinal direction; on the plane and mainly in areas thickened by plastic deformation, polyhalite is enriched in banded distribution. Our study provided a reference for seismic identification and prediction of polyhalite elsewhere.