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Quantitative Description of Different Facies Belts of Paleogene Volcanic Rocks in BZ349 Oilfield, Bohai Bay Basin


BZ349 oilfield drilled in 7.2m to 280.9m of volcanic rocks in Paleogene oil-bearing intervals, evaluation of volcanic rocks is the first time in Bohai Bay Basin. Volcanic rock lithologies are identified by cuttings, sidewall cores and thin sections, lithology chart is built by different well logging responses to different volcanic rocks, and thus five kinds of lithologies including basalt, dolerite, tuffaceous sandstone, tuff and sedimentary tuff are quantitatively recognized. Based on well-to-well correlation, three dimensional seismic forward model is built to study volcanic mechanism. The result shows six volcanic facies in the study area, that is, volcanic channel facies, overflow facies, explosive facies, extrusive facies, secondary volcanic facies and volcanogenic sedimentary facies. Two eruption patterns are determined according to different facies combination, central vent eruption represented by 1 well area and crack-central vent eruption represented by 6 well area. It's necessary to be evaluated that volcanic channel facies and secondary volcanic facies have an impact on reservoir boundary and thickness. Quantitative description standards are established by the differences in petrophysical and geophysical characteristics of different volcanic facies. Volcanic channel facies in seismic profiles show characteristics of middle-high amplitude and disorder discontinuous reflection. With acoustic forward modeling technology, describing standards taking trough-formation disconnections as imaging boundaries are established. Thus, 14 volcanic channel facies are depicted, with areas of 0.03 km2 to 0.16km2 and diameters of 110m to 660m, which are verified by well 8. Secondary volcanic facies in seismic profiles show characteristics of strong amplitude and continuous reflection. Under the guidance of amplitude classification theory and based on relationship between amplitude and thickness obtained from volcanic rock wedge forward model, quantitative descriptions of volcanic rock thickness based on seismic response characteristics are conducted. 17 secondary volcanic facies are described, with thicknesses of 12.3m to 45.2m and areas of 0.52km2 to 5.37km2. The results are verified by well E1 and it turns out that the accuracy rate is 31.2% higher than the conventional subtraction method. The quantitative description of volcanic channel facies and secondary volcanic facies can lay foundations for reserve evaluation and development well deployment.