Lamellation fractures in shale oil reservoirs: Recognition, prediction and their influence on oil enrichment

Lamellation fractures, as important reservoir space and fluid flow channels, have significant effects on reservoir quality and hydraulic fracturing engineering in shale oil reservoirs. Core observation and this section reveal that there are three types of fractures identified, including tectonic fractures, lamellation fractures, and abnormal pressure fractures. Lamellation fractures are parallel along with the weak bedding (laminae) planes, and they can appear as open, partly sealed, and fully filled. Morphology of lamellation fractures surface is various, such as continuous, intermittent or pinched even or bifurcated, bent, or straight. Conventional petrophysical log suites are used to recognize lamellation fractures. The results indicate that lamellation fractures result of a positive deviation from deep and shallow resistivity logs, and they are associated with low gamma ray values. In addition three porosity logs are not sensitive to the lamellation fractures in shale oil reservoirs. According to the image logs, borehole-wall structural interpretation is used for detecting geological objects and determining their strike, dip, and dip direction. Geological objects types and their distribution are determined in a single well. The results show that the strata have a low dip angle with an average value of 1.5 degrees. The maximum horizontal stress refers toward the EW direction, which is coinciding with the direction of the regional compressive stress field. The strike of lamellation fractures is consistent with the bedding planes and their dip angle is low in the majority value of 1.5 degrees. Furthermore, the relationships between lamellation fractures and reservoir quality are clarified according to petrophysical experiment analysis including mercury injection capillary pressure (MICP), twodimensional nuclear magnetic resonance (2D NMR), and physical property analysis. As a result, the development of lamellation fractures favors the connectivity of the pores and throats, and the closer to the lamellation fractures, the larger the diameter of the pores. Thus, lamellation fractures are favorable for high reservoir quality of shale oil reservoirs. In addition, the charge of oil and gas in the early stage during hydrocarbon generation and expulsion plays a crucial role in the preservation of lamellation fractures. Recognition and prediction of lamellation fractures are therefore vital in sweet spots prediction and mechanism of oil accumulation analysis in shale oil reservoirs.