Comprehensive Evaluation Method of Shale Oil Sweet Spot Based on Grey Correlation Analysis: A Case Study of Bonan Sag in Bohai Bay Basin

The geological conditions of shale oil in China, which is dominated by continental deposits, are complex and changeable, and the evaluation factors of geological engineering sweet spots are multiple, which brings difficulties to the comprehensive evaluation of shale oil sweet spots. With Bonan Sag in Bohai Bay Basin as an example, nine evaluation indexes were selected based on the grey correlation theory, including shale thickness, total organic carbon (TOC), vitrinite reflectance, porosity, pyrolysis free hydrocarbon content, crude oil density, crude oil viscosity, formation pressure, and brittleness index, and correlation analysis and weight optimization among these evaluation indexes were carried out to calculate a comprehensive evaluation index, so as to realize the comprehensive, quantitative, and accurate assessment of shale oil sweet spots. The results revealed that high values of the comprehensive evaluation index for shale oil in Bonan Sag were mainly concentrated in the XYS9, Y182, and Y187 well areas, as well as in the southern part of the Y283 well area and the surrounding vicinity of the L42 well area. The overall comprehensive evaluation index gradually decreased in the south and north directions. The comprehensive evaluation index had a good correlation with the single well production, which allowed for a good matching of shale oil production in Bonan Sag. The overall decline in the comprehensive evaluation index of the Well BYP5 from top to bottom, aligned with the trend of production contribution rates monitored by tracers, demonstrating a significant correlation between the two. The research indicate that after the completion of a shale oil horizontal well, by calculating the comprehensive evaluation index, the sweet spots of the horizontal section can be evaluated promptly and accurately to guide the fracturing measures of the shale oil horizontal well.