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Identification of Volcanics and its Application for Reservoir Prediction of Bioclastic Dolostone: An Example From Bohai Bay Basin

Abstract

The paleogeomorphic restoration is widely used for reservoir prediction of bioclastic dolostone. Although it is an excellent method due to its fast calculation speed and clear physical meaning but it has a premise that the structure of marker horizon for flattening is true. However, this premise sometimes can not be met where false structural images occur resulted from complex lithology, strong heterogeneity and severe horizontal velocity variation. In this paper the horizontal velocity variation caused by the uneven distribution of volcanics has been identificated to solve the false structural images in the Nanpu depression, Bohai Bay. The Nanpu depression located in the north-west of Bohai Bay and the main reservoir is bioclastic dolostone. The drilled wells revealed that a set of volcanics developed in shallow strata characterized by high velocity and uneven distribution, because of which severe lateral velocity variation and false structural images occur and the reservoir prediction from paleogeomorphic restoration can not truly represent the distribution of bioclastic dolostone. As a result, drilling on the reservoir has led to many dry holes due to reservoir prediction error, the failed wells seriously affect the productive performance of the reservoir. As shown in the paper, an integrated workflow has been established in this study to improve the reservoir prediction accuracy. First, the preliminary map-view distribution of the volcanics is obtained from the normalized amplitude slice: low amplitude is related to explosive volcanic breccia; high amplitude is related to effusive eruption andesite; the translation with medium amplitude is related to mixture rock in varying proportion. Then, the distribution range of volcanics is interpreted based on the mechanism of volcanic eruption and the above results of map-view distribution. Last, the structure is corrected by the thickness of volcanics, which is defined innovatively with the help of interval velocity of volcanic layer, to accurately predict the distribution range of bioclastic dolostone. As a result, the reservoir prediction calculated from the workflow matches the new drilling results very well, so we timely adjust the productive plan to employ potential and to avoid risk. This workflow not only improves the reservoir prediction accuracy but also provides valuable references for paleogeomorphic restoration in the area with false structure images.