Coal and shale distributions on half-graben slopes: A case study of the Eocene Pinghu Formation in Xihu Sag, East China Sea

Abstract

Steep and gentle slopes in a half-grabben basin often form oil source rocks of coal and shale. However, which source rock is dominant and what the relation is with the slope type and sedimentary facies are not clear. The Pinghu Formation in Xihu Sag is a half-graben basin with coal seam as the main source rock. Five condensate reservoirs have been found in its west slope, with several wells and 3D seismic data, which makes it possible to study the relationship between slope types and source rocks. We have used logging data, lithologic identification statistics, and seismic inversion to reconstruct the sedimentary facies and paleoslopes in the western Xihu Sag and have analyzed the variations in the proportions of coal and shale in the study area and their relationships with the facies, slope types, and subsag. The results indicate that the upper slope contained a higher proportion of coal than the lower slope. In contrast, the lower slope contained more shale than the upper slope. Coal is dominant in the delta plain and front facies, whereas shales are dominant in the prodelta facies. Although the proportions of coal and shale vary among the southern hinged margin, the central escarpment margin, and the northern hinged margin, the thicknesses of the coal and shale increases northward, independent of the rift margin type. These variations may be related to the filling of the delta from north to south during the deposition of the Eocene Pinghu Formation. These results also suggest that there were no substantial differences in the depositional slopes and water depths of the hinged and escarpment margins. The abundance of shales in the lower slope and at the center of the depression suggest that, in addition to coal, shales should have contributed to petroleum generation in Xihu Sag.