黄257井区叠前纵波方位各向异性裂缝分布预测

Late Triassic fracture detection with seismic azimuth anisotropics in Huang 257 survey,Ordos Basin

鄂尔多斯盆地姬塬地区黄257井区延长组地层埋深较大,储层较致密,天然裂缝在改善储层物性方面起到了一定作用。为了研究该区裂缝特征及其影响因素,基于长庆油田采集的60km2三维叠前宽方位角地震资料,利用基于纵波各向异性的相对波阻抗属性开展了延长组裂缝密度和方向的预测。研究表明:(1)叠前相对波阻抗属性预测的裂缝方向与叠后相干、倾角属性分析的断裂方向一致,但深、浅层裂缝方向有变化。长6层和长71小层裂缝主要为北西西向,长72、长73、长8及长9层裂缝方向主要为近南北向。此变化与燕山期和喜山期不同方向应力场作用及长72和长73小层泥岩的塑性变形有关;(2)裂缝的密度整体较低,结合实际测井资料,认为研究区裂缝密度的大小受砂体含量的控制,此控制作用在100m左右的地层段尺度比在30m左右的小层尺度更明显。试油资料分析结果显示,裂缝相对发育区,砂体的含油性受油层厚度和裂缝密度共同影响。以上实践表明,受地震分辨率限制,叠前纵波各向异性裂缝方向和裂缝密度预测的地层单元不宜太薄,目的层厚度大于λ/2(约60m)时预测的结果才更可靠。

Yanchang formation in the Huang 257 area, Ordos Basin is deeply buried and tightly compacted sandstones and its natural fractures play a definite role in improving reservoir property. In order to research fracture characteristics and its effects on reservoirs, prestack impedance of P-wave anisotropic are utilized to predict fracture density and direction on 60 km23D seismic data. The following observations are obtained: A. Fracture direction predicted by prestack impedance is consistent with that predicted by poststack coherence and dip, The fracture direction is mainly NWW spreading in Chang 6 Mem and Chang 71, Mem, but N-S spreading in Chang 72, Chang 73, Chang 8, and Chang 9 Mem. The fracture direction is parallel or perpendicular to the fault direction, and might be resulted in by different stress fields in Yanshan and Himalayan periods or local plastic deformations of Chang 72and Chang 73shale; B. The fracture density in the area is low. According to logging data, fracture density in the area was controlled by sand body thickness and oil content. It is more obvious in 100 m thick strata than 30 m thick strata. Well oil test data show that sand body oil-bearing property is affected by reservoir thickness and fracture density in developed fracture area. Our practice shows that strata unit thickness to be predicted with the proposed approach should not be too thin due to the limited seismic resolution. If target thickness is more than 1/2 wavelength (60 m), prediction is reliable. © 2017, Editorial Department OIL GEOPHYSICAL PROSPECTING. All right reserved.