Abstract: Since the major oil reservoir of the BZ Oilfield in the Shanan Sag, the bioclastic dolostone in the 1st Member of the Shahejie Formation, was put into production, there has been large contradiction between the actual production and the initial geological estimation. In some of the well areas, there is large potential to increase oil reserve and production. Based on the data from oil production performance, such as the data of pressure, output and PVT experiment, using the method of general material balance calculation, we found that the A4 well block has the largest rolling exploration potential.Through detailed description of palaeogeomorphology and paleo-ecological environment of the bioclastic dolostone in combination of the distribution pattern of the bioclastic dolostone reservoir, a new reservoir space distribution pattern of the bioclastic dolostone of A4 well block was established. The prediction is proved by the actual drilling results, and a newly proved reserve of nearly 4 million tons is identified, which will certainly ensure the sustainable development and investment profit of the BZ Oilfield. The solution is expected to be an example to be referred in progressing exploration and development in both the domestic and offshore oilfields under similar geological conditions.