ANALYSIS AND PREDICTION OF ABNORMAL VELOCITY IN PALEOGENE MUDSTONE OF STRUCTURE BZ19-6 IN OFFSHORE BOHAI BAY BASIN
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摘要:
渤中19-6、渤中21/22构造区位于渤中凹陷西南部，其中渤中19-6构造古近系底部砂砾岩层和太古界潜山以及渤中21/22构造古生界潜山均有商业性油气发现。已钻井揭示研究区古近系泥岩段普遍发育异常高压，实测压力系数介于1.2～2.0。在这样的超压背景下，应用传统方法对未钻区块超压泥岩段速度预测存在较大误差。而该段地层速度预测的准确性，对于更深部的砂砾岩层和潜山顶面深度预测及钻井作业安全至关重要。以超压形成机制为出发点，明确本区速度异常的主要控制因素为欠压实作用和生烃作用。随着埋深增加，通过对比压力、速度受欠压实和生烃作用强弱变化的影响，发现在古近系超压泥岩段，欠压实作用对速度影响逐渐趋于稳定，速度的变化主要受控于生烃作用的强弱，并据此建立了速度和影响因素之间的定量关系。根据该方法预测的潜山顶面深度与探井实钻深度吻合度较高，提高了钻井作业的安全性。
Abstract:The structures of BZ19-6 and BZ21/22 are located in the southwest of Bozhong Depression of the Bohai Bay Basin, where commercial oil and gas have been discovered in the glutenite layers of Early Paleogene, the Archean buried hill at structure BZ19-6 and the Paleozoic buried hill at structure BZ21/22. Drilling has revealed that abnormal high pressure generally occurs in the Paleogene mudstone, with pressure coefficient changing between 1.2 and 2.0. The pressure is so high that traditional velocity prediction methods are always invalid for undrilled blocks. Accurate formation velocity prediction is critical important to depth prediction of the top boundaries of the deep glutenite layer and buried hill reservoirs, and to safe drilling. According to the forming mechanism of overpressure, it is found that the velocity abnormals in the area are mainly caused by undercompaction and hydrocarbon generation. Comparing the pressure and velocity affected by the change in undercompaction and hydrocarbon generation, it is found that as the buried depth increased to certain level, the influence of undercompaction on velocity will gradually become stable, and the change of velocity will be dominated by hydrocarbon generation process in the overpressured Paleogene mudstone. Accordingly, the quantitative relationship between velocity and influencing factors is established. Based on this method, the predicted depth of the top of buried hill is in good agreement with the actual drilling data, and the safety of drilling operation is also improved.
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表 1 各层有机质特征对比
Table 1. Characteristics of organic matters
层位 TOC/% S1 + S2/（mg·g−1） 类型 成熟度Ro/% 综合评价 东二下段 0.35～1.78 0.47～12.97 Ⅱ2型为主 0.5～0.8 一般—较好 东三段 0.59～3.89 0.72～18.89 Ⅱ1和Ⅱ2混合型 0.7～1.2 较好—优质 沙河街组 0.43～2.78 0.53～16.37 I和Ⅱ1混合型 0.9～1.2 较好—优质 
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表 2 太古界潜山顶面预测深度与实钻深度对比
Table 2. Comparison between predicted depth and actual drilling depth of Archean buried hill top surface
井名 预测深度/m 实钻深度/m 误差/m BZ19-6-F 4 112 4 121 −9 BZ19-6-G 4 533 4 525 8 BZ19-6-H 4 576 4 564 12 BZ19-6-I 4 411 4 416 −5 BZ19-6-J 4 817 4 828 −11
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