Abstract:

The western slope of Songliao basin is an important oil sands metallogenic belt in China. In order to further clarify the characteristics of Cretaceous oil sands resources in the central and southern parts of the area, the basic geological survey work has been implemented with five oil sands investigation wells and one hydrological investigation well in the Dagang area of Jilin. Based on core observation, geophysical log interpretation, and oil content test of core samples, a comprehensive geological study was carried out. The results show that, the oil sands of the Cretaceous Yaojia Formation are located at a depth from 175 m to 208 m, and the lithology is mainly siltstone and fine sandstone. The oil content level of the core reaches oil immersion grade, with a maximum oil content of 16.02% and an average oil content of 8.23%. The oil sand formation logging curve is characterized by low natural gamma value, high acoustic velocity value, medium to high resistivity and low density. The oil sands formation is divided into four sub-layers (S1, S2, S3, S4) and oil content characteristics are analyzed. Vertically, the oil content is non-homogeneous and increases gradually with depth. The average oil content of S3 and S4 in the lower part of the reservoir is 10.80 % and 11.40 %, respectively, reaching the grade of rich ore. The effective thickness of each sub-layer varies widely in the vertical direction, with a maximum value of 3.10 m and a minimum value of 1.20 m. The average effective thickness of the lower S3 and S4 sub-layers is 2.90 m and 1.90 m respectively, which is obviously better than the upper S1 and S2 sub-layers. On the scale of oil-sand development, the distribution area of S3 sub-layer is the largest, and the area of sand body with effective thickness≥2 m is about 270 m×160 m, and the thickness center is located in the southeast of the well area, and the thickness is thinned from the thickness center to the northwest; The distribution area of S4 sub-layer is the second largest, and the area of sand body with effective thickness≥2 m is about 250 m×120 m, its thickness center is located in the middle of the well area. In terms of planar spreading, its distribution area is large in the southeast and northwest directions, and the distribution area in the middle zone is slightly smaller, with a nearly dumbbell shape, the thickness thins from the thickness center to the north-east and south-west directions. On sand deposition characteristics, controlled by the sedimentary evolution of the regional shallow water delta, high-quality oil sands are mostly deposited in the fine sandstone of the divergent river channel, especially the S3 and S4 sub-layers located in the lower part of the reservoir have obvious deposition patterns of multi-phase superimposed river lateral accumulation sand bodies. In general, the S3 and S4 sub-layers have high oil content, large effective thickness and considerable spreading area, and are the main reservoir of high-quality oil sands in the area.

Key words: oil sands, oil-bearing characteristics, Songliao basin, western slope, Dagang area, Upper Cretaceous, Yaojia Formation