Buried-hill play, Jizhong subbasin, Bohai Bay basin: A review and future propespectivity

Abstract

As one of the six subbasins in the lacustrine Bohai Bay basin, the Jizhong subbasin is characterized by a dominance of the petroleum reserves located in buried-hill traps of Paleozoic and Proterozoic marine carbonates, particularly the latter. This paper documents the revitalization of exploration of the buried-hill play and discusses other play fairways not previously considered through use of case studies. Discovery of the largest field, Renqiu field, in 1975 led to the establishment of the buried-hill play. In this play, oil derived from the Paleogene lacustrine source rocks charged into and accumulated in the underlying Proterozoic marine carbonate reservoirs. A number of buried-hill fields were discovered within a time span of ca. 10 yr in the subbasin. With more and more large buried-hill structures at depths of less than 5000 m (<16,400 ft) being drilled, the annual reserve addition showed a rapid decline until 2006. The application of new technologies including reprocessing of merged three-dimensional seismic data, improved logging, and testing techniques, together with innovative exploration ideas, made it possible to revitalize the buried-hill play. The exploration success lies with the focus change from targeting the conventional shallow to moderately buried hills to the unconventional buried-hill pools, which include deeply buried hilltop, hillslope, and intrahill pools. Case studies of one conventional buried-hill field (Renqiu field) and three unconventional buried-hill fields (Chang-3, Wengu-3, and Niudong-1 fields), together with modern geological investigations, indicate that there still exists significant exploration potential for the buried-hill play in the Jizhong subbasin. The potential largely lies with the unconventional buried hills. The Baxian, Wuqing, and Baoding depressions are favorable fairways for the deeply buried hill pools, represented by the Niudong-1 field. The Wenan slope is the favorable fairway for the intrahill pools, represented by the Chang-3 and Wengu-3 fields.