Abstract
Fractures are the main seepage channels in tight reservoirs, and they affect the distribution of high-quality reservoirs and the enrichment of hydrocarbons. Fine logging identification of fractures in the strongly heterogeneous continental tight oil reservoirs of the Upper Triassic Yanchang Formation in the Ordos Basin is a hot and difficult point in the field of petroleum geology. In this paper, taking the Yanchang Formation as an example, a fractal model was constructed to identify fractures in tight oil reservoirs using a large number of cores, conventional and micro-resistivity imaging logging data. The results show that high-angle, vertical and bedding fractures are mainly developed in the Yanchang Formation tight sandstones. There is a negative correlation between sand body thickness and fracture development degree. The fracture sensitivity parameters were used to construct a coupled fractal fracture index. The fractal model incorporates logging information from natural gamma, acoustic wave time difference, rock density, and shallow lateral resistivity. In addition, the constructed fracture fractal index realizes the functions of multi-conventional logging information fusion, which can effectively identify fracture development segments in sandstone. According to the statistics, the fracture identification rate is 83.3%. The study also found that with the increase of sandstone brittleness index, the fracture index has a “S” shape increasing trend. Therefore, the content of brittle mineral components in sandstone is an important factor affecting the development degree of natural fractures, and fractures are more likely to occur in high brittle, thin sand bodies. The highly brittle framework minerals have strong stress-supporting capacity, which can keep fractures open by resisting high overlying loads.
Highlights
In recent years, continental tight sandstone is considered to be another new hot spot in the global unconventional oil and gas exploration and development after shale gas (Dong et al, 2018; Chen et al, 2021)
In the fracture-developed segment of the target layer, the natural gamma value is mostly higher than 72 API, the shallow detection resistivity is generally less than 72 Ω m, the rock bulk density is less than 2.42 g/cm3, the neutron porosity is greater than 26%, and the acoustic wave time difference is greater than 245 μs/m (Figure 8)
The fracture index FI is lower than the threshold value of 1, indicating that the fractures are relatively underdeveloped; and the brittleness index of mudstone is usually lower than 60% (Figure 12)
Summary
Continental tight sandstone is considered to be another new hot spot in the global unconventional oil and gas exploration and development after shale gas (Dong et al, 2018; Chen et al, 2021). In recent years, a large number of core-scale fractures have been found in the tight sandstones of the Yanchang Formation in the marginal areas of the Ordos Basin. Most of these fractures are vertical fractures with shearing properties, and their vertical penetration depths are mostly in the range of 1–5 m. There are few systematic studies on the fine logging evaluation of the tight oil reservoirs in the Yanchang Formation in the western Ordos Basin. In this paper, taking the Yanchang Formation as an example, a fractal model was constructed to identify fractures in tight oil reservoirs, combining a large number of cores, conventional and micro-resistivity imaging logging data. This study can provide guidance for the prediction of tight oil sweet spots in similar areas
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
