鄂尔多斯盆地西北缘奥陶系乌拉力克组裂缝孔隙表征

Fracture pore characterization of the Ordovician Wulalike Formation in the northwestern margin of the Ordos Basin

鄂尔多斯盆地西北缘奥陶系乌拉力克组是中国北方海相页岩气勘探评价的重点领域。通过对西北缘多口评价井开展裂缝孔隙多方法识别和定量评价,探索揭示乌拉力克组下段裂缝发育特征、分布规律和形成主控因素。研究认为:①盆地西北缘乌拉力克组下段普遍发育开启状顺层缝,单条缝或断续或连续分布,缝宽一般为0.1~0.2 mm,缝间距一般为0.5~14.0 cm。裂缝发育段在核磁标准T_(2)图谱中多显双峰或多峰特征,在深浅电阻率测井曲线上普遍具有正幅度差双轨特征。②裂缝段长度和裂缝孔隙度区域变化大。在南部探区马家滩—古峰庄区块,裂缝发育程度自西向东总体呈减小趋势。在中部探区上海庙区块,裂缝极为发育,裂缝段连续厚度普遍在20 m以上,裂缝孔隙度平均值在1.3%以上。在北部探区铁克苏庙区块,裂缝发育程度总体较中区和南区差,自西向东同样呈减弱趋势。③富含钙质和硅质的高脆性页岩段与燕山期以来的冲断褶皱与滑脱作用相配置,是产生乌拉力克组裂缝孔隙的关键控制因素。受不同区块构造样式和活动特征影响,不同探区裂缝发育程度存在显著差异。

The Ordovician Wulalike Formation in the northwestern margin of the Ordos Basin is a key area for exploration and evaluation of marine shale gas in northern China.In this paper,through the multi-method identification and quantitative evaluation of fractures and pores in multiple evaluation wells in the northwestern margin of the basin,the development characteristics,distribution rules and main controlling factors of fractures in the lower section of the Wulalike Formation were explored and revealed.Three preliminary conclusions were obtained:(1)Open bedding fractures are generally developed in the lower section of the Wulalike Formation at the northwest margin of the basin.Single fractures are either intermittent or continuous in steps.The width of the fractures is generally 0.1-0.2 mm,and the spacing of the fractures is generally 0.5-14.0 cm.The fracture development section is characterized by double or multiple peaks in the nuclear magnetic resonance standard T_(2) spectrum,and generally has the double track feature of positive amplitude difference on the deep and shallow resistivity logging curves.(2)The fracture section length and fracture porosity vary greatly.In the Majiatan-Gufengzhuang area in the southern exploration area,the fracture development degree is generally decreasing from west to east.In the Shanghaimiao area in the central exploration area,fractures are extremely developed,the continuous thickness of fracture section is generally more than 20 m,and the average value of fracture porosity is more than 1.3%.In the Tiekesumao area of the northern exploration area,the fracture development degree is generally worse than that of the central and southern areas,and also shows a weakening trend from west to east.(3)The highly brittle shale section rich in calcareous and siliceous is matched with the thrust fold and detachment since the Yanshanian period,which is the key controlling factor for the formation of fracture pores in the Wulalike Formation.Affected by the structural style and activity characteristics of different blocks,the development degree of fractures in different exploration areas is significantly different.