Overpressure controlling factors for tectonic fractures in near-source tight reservoirs in the southwest Ordos Basin, China

A typical tight sandstone reservoir with a huge reserve of oil has been discovered in the eighth member of the Upper Triassic Yanchang Formation in Jinghe Oilfield, southwest Ordos Basin, China. In this tight reservoir, the tectonic fractures have been proved to be a critical controlling factor for the distribution of "sweet spots". How to accurately describe the distribution of the tectonic fractures, however, is still challenging due to the poor understanding of the driving mechanisms for fracture formation. In this paper, the effects of the adjacent source rocks on the development of tectonic fractures were studied based on field data and theoretical analysis. Our results indicated that the overpressure transferred from the adjacent source rocks can significantly influence the development of the tectonic fractures in the Jinghe Oilfield. This conclusion was drawn according to two main facts. Firstly, the density and distribution of the tectonic fractures present a high correlation with the development of the overpressure of the adjacent source rocks. In the vertical direction, the density of fractures of the target reservoir gradually decreases along with the increase of the distance away from its above source rocks. Secondly, the period of main hydrocarbon charging and the formation of massive tectonic fractures overlaps very well. Thus, as a typical case, it implies that the effect of the transferred overpressure from source rocks should not be neglected for accurately predicting the spatial distribution of tectonic fractures in tight reservoirs in the southwest Ordos Basin. To do that, pore pressure stress coupling is an inevitable aspect and both overpressure evolution and stress regime should be considered as important controlling factors.