Abstract:

The drilling result shows the distribution and quality of Lower-Middle Ordovician carbonate reservoirs overlaid by Upper Ordovician in the south of Tahe Oilfield are mainly controlled by faults and fractures, with structural open fractures predominantly, drilling-induced fractures secondly.The fracture orientation is closely related to paleotectonic stress field.This paper employs a large amount of FMI drilling data to recognize and analyzes fractures orientation and their origin. It shows that drilling-induced fractures are controlled by present stress field, while the structural open fractures are controlled by paleotectonic stress field.The orientation of fractures is consistent with the maximum principal stress.The predominant orientation of drilling-induced fractures in the south part of Tahe Oilfield is northeast trend, present maximum principal stress is northeast trend, while the structural fractures predominant trend in north-northeast direction and north-east-east direction.Some of north-north-east trend fractures were filled, suggesting they formed earlier than north-east-east fractures and their forming time is corresponding to middle-Caledonian and early Hercynian paleo stress field respectively.

Key words: the south of Tahe Oilfield, Lower-Middle Ordovician reservoir, fracture, FMI imaging logging, tectonic stress field