鄂尔多斯盆地延安地区本溪组—石盒子组高精度层序地层格架研究

前人对鄂尔多斯盆地上古生界沉积体系和层序地层的认识存在着不同的观点。为消除人工划分地层层序的多解性和不确定性,提高旋回划分、地层对比的精度和准确度,在高分辨率层序地层学理论的指导下,运用地震时频和测井小波分析方法,对位于该盆地东南部延安地区的上古生界本溪组—石盒子组层序地层进行了识别。综合分析野外露头、不同地震频率及单井时频等资料后认为,等时地震反射同相轴具有非频变的特征,共识别出5个等时地震界面。采用不同频率的0°相位Ricker子波与反射系数褶积,建立起合成地震响应正演模型,其中30~50 Hz主频的地震数据能很好地反映长期层序基准面旋回。对测井曲线demy小波进行分析,结合岩心、露头界面及地震等时界面等分析资料,识别出层序地层长期、中期和短期基准面旋回。研究结果表明,该区本溪组—石盒子组可以划分为5个长期基准面旋回、12个中期基准面旋回和22个短期基准面旋回。进而以地震层序框架为约束,开展了井间层序对比分析,建立了等时层序地层格架,为确定该区下一步的油气勘探目标和方向提供了依据。

High-resolution sequence stratigraphic framework of the Benxi-Shihezi Formation in the Yan'an Area,Ordos Basin

There are different views on the Upper Palaeozoic sedimentary systems and the sequence stratigraphy in the Ordos Basin. In order to eliminate ambiguity and uncertainty of artificial stratigraphic sequence classification and increase the precision and accuracy of cycle division and stratigraphic correlation, stratigraphic identification was conducted on the sequence stratigraphy of the Upper Palaeozoic Benxi-Shihezi Formation in the Yan'an area of the southeast Ordos Basin by means of seismic time-frequency analysis and log wavelet analysis, in reference to high-resolution sequence stratigraphy theories. Comprehensive analysis of field outcrops, multiple seismic frequencies and single-well time frequency data shows that isochronal seismic reflection event is frequency-independent and there are totally five isochronal seismic interfaces identified. First, synthetic seismic response forward model was established by using the convolution of 0°-phase Ricker wavelets and reflection coefficients at different frequencies. Long-term base-level cycles were best reflected by the seismic data of master frequency 30-50 Hz. After logging demy wavelets were analyzed, long-, mid- and short-term base-level cycles of sequence stratigraphy were identified by using cores, outcrop interfaces and seismic isochronal interfaces data. Results show that 5 long-term, 12 mid-term and 22 short-term base-level cycles can be identified in the Benxi -Shihezi Formation in this area. Then, well-to-well sequence correlation was performed with seismic sequence framework as the constraint. Finally, an isochronal sequence stratigraphic framework was established, which provides the basis for the determination of subsequent oil and gas exploration targets and directions.