FRACTAL CHARACTERIZATION OF SILTY BEDS/LAMINAE AND ITS IMPLICATIONS FOR THE PREDICTION OF SHALE OIL RESERVOIRS IN QINGSHANKOU FORMATION OF NORTHERN SONGLIAO BASIN, NORTHEAST CHINA

Abstract

Cretaceous Qingshankou Formation [Formula: see text] in the Songliao Basin, the largest petroliferous basin in East China, has favorable geological conditions for the shale oil reservoir. Although the first member of Qingshankou formation [Formula: see text] has the maximum value of the organic matter content (total organic carbon (TOC)) comparing with other members, most of the layers are less than 1[Formula: see text]cm. This limits the application of surface seismic survey and logging methods. To explore “sweet spots” of the shale oil and to evaluate resource potential, it is of great significance to determine the thickness and distribution of silty beds/laminae in the thick shale system. In this study, the observed data of the outcrops, cores, and thin sections of [Formula: see text] are used to analyze the characteristics of reservoir space in silty laminae. Distribution model of silty beds/laminae was established by using fractal method. The results show that the silty beds/laminae have large particle size, poor sorting, and relatively developed reservoir space. The pore sizes in silty laminae vary greatly, and the diameters are mostly larger than 50[Formula: see text]nm. The connectivity between pores is good. Although the thickness of single silty bed/laminae mostly ranges from 1[Formula: see text]mm to 6[Formula: see text]mm, the average layers per meter (layers/m) of silty bed/laminae range from 2 to 64. This contributes the cumulative thickness 2.73–31.79% of the total thickness of the whole shale reservoir. According to the fractal analysis, the silty beds/laminae have the uniform fractal characteristics in the decimeter, centimeter, and millimeter levels. And the fractal dimension keeps the scale invariance. The layers and thickness of the thin silty laminae, calculated from Number–Size (NS) fractal model, are in good agreement with observations. This indicates that the fractal-based NS model is an effective method to estimate the parameters of the silty laminae, such as layer number, cumulative thickness, and the ratio of sand to formation. This method provides a reliable reference for the “sweet spot” exploration.