鄂尔多斯盆地西部古峰庄地区三叠系延长组长7—长9段裂缝特征及油气意义

天然裂缝对鄂尔多斯盆地三叠系延长组低渗透油藏的勘探开发具有重要的作用。盆地古峰庄地区延长组油气资源丰富，具有良好的勘探前景，但有关裂缝研究成果较少，对其分布规律和活动时期认识不清。通过古峰庄地区延长组长7—长9段岩心、成像测井、碳—氧同位素、流体包裹体、岩石声发射及构造应力和断裂活动分析，对其天然裂缝的特征、形成期次及成因进行研究，并探讨了裂缝对油气成藏的作用。古峰庄地区长7—长9段天然裂缝较发育，尤以长7段最发育；以高角度和垂直裂缝为主，走向以NEE向为主，NE向次之，少量NW和NWW向；裂缝以未充填和方解石半充填为主，有效性较好；裂缝面较平直，可见油侵、油迹、油斑等明显的油气运移痕迹。长7、长8、长9段氧同位素均值分别为-15.8‰，-17.5‰，-18.7‰，裂缝的形成深度分别为1 675，2 338，2 785 m；由流体包裹体均一温度峰值90~100 ℃，120~130 ℃，换算得裂缝形成深度范围在1 750~2 000 m，2 500~2 750 m。该区长7—长9段裂缝活动具有多期性，可划分为印支期、燕山期和喜马拉雅期，燕山期和喜马拉雅期为主要活动时期。不同时期发育的有效性裂缝改善了研究区低渗透储层的物性，为长7—长9段等多层系—深层系油藏的形成提供了重要运移通道和储集空间，并影响着油井的产能。

Fracture characteristics and hydrocarbon significance in the Chang 7 to Chang 9 members of Triassic Yanchang Formation of Gufengzhuang area,western Ordos Basin

Natural fractures play an important role for the exploration and development of low-permeability reservoirs in the Triassic Yanchang Formation of the Ordos Basin. Abundant hydrocarbon resources have been found in the Yanchang Formation of the Gufengzhuang area, showing good exploration prospects. However, only limited research results on fractures have been obtained, and the distribution law and active period are both unclear. Based on the analyses of core, imaging logging, carbon-oxygen isotope, homogenization temperature of fluid inclusion, acoustic emission, tectonic stress and fracture activity, the characteristics, formation stages and causes of natural fractures in the Chang 7 to Chang 9 members of the Yanchang Formation in the Gufengzhuang area were studied, and corresponding indications for petroleum accumulation were discussed. Natural fractures [JP+1]are well-developed in the Chang 7 to Chang 9 members of the Yanchang Formation, especially in the Chang 7 member. High-angle and vertical fractures are ubiquitously developed, mainly of NEE orientation, followed by NE, and finally NW and NWW. The fractures are generally unfilled or semi-filled by calcites, providing effective pathways for hydrocarbon migration. The fracture surface is flat, and obvious traces of oil migration such as oil invasion, oil stain and oil spot can be observed. The average values of oxygen isotopes of the Chang 7 to Chang 9 members are -15.8‰, -17.5‰ and -18.7‰, indicating that the factures were formed at depth of 1 675, 2 338 and 2 785 m. According to the homogenization temperature peaks of fluid inclusions of 90-100 ℃ and 120-130 ℃, the formation depth of fractures is 1 750-2 000 m and 2 500-2 750 m. The fractures in the Chang 7 to Chang 9 members were active during multiple stages, including the Indosinian, Yanshanian and Himalayan stages, with the Yanshanian and Himalayan as the main active stages. Effective fractures developed in multiple stages have improved the physical properties of low-permeability reservoirs in the study area. They provided important migration channels and storage spaces for the formation of multi-layer and deep-layer reservoirs such as Chang 7, Chang 8 and Chang 9, and affected the productivity of oil wells.