关键词: 钾盐, 杂卤石, 地震识别, 地震属性预测

Abstract: 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.

Key words: sylvite, polyhalite, seismic identification, seismic attribute prediction