泥岩压实研究中有机质导致声波时差异常的定量校正方法

A quantitative method for revising abnormally high sonic data in rich-organic rock during compaction study

高丰度有机质导致的声波时差异常高值不能客观反映地层孔隙度的变化,定量研究有机质对压实曲线的影响才能减小压实研究结果的不确定性。从泥岩压实研究的原理出发,构建考虑干酪根的岩石等效体积模型,提出有机质引起声波时差增量的校正公式;以鄂尔多斯盆地南部镇泾地区为例,讨论压实曲线有机质校正方法的实用性及可靠性。结果表明：烃源岩层段高丰度有机质往往造成压实规律的较大误差,压实研究中应当消除有机质对声波时差的影响;镇泾地区泥岩压实曲线经过校正后,正常压实趋势线斜率减小约30%～55%;长7段底部页岩欠压实幅度显著降低,最大埋深时期的剩余压力约为3～5 MPa,相比于校正前降低5～8 MPa,反映出以往研究可能过高地估计了长7段古超压的幅度;利用校正后声波时差计算的泥岩孔隙度与实测孔隙度吻合性较好,且计算的流体压力与数值模拟结果一致,证实提出的定量校正方法能够有效地消除研究区有机质造成的声波时差异常,可作为鄂尔多斯盆地及其他地区压实研究的借鉴。

Abnormally high values in the sonic data of the source rocks cannot objectively reflect normal variations in porosity of the study of mudstone compaction. In order to eliminate the uncertainty of compaction measurements, quantitative corrections need to be introduced. In this paper, by using the principle of mudstone compaction, the organic rich rocks are divided into four parts by modifying the rock volume model of Wyllie equation： rock matrix, kerogen, pore and pore fluid. A correction formula for acoustic time increment caused by organic matter is derived. Taking Zhenjing area as an example, using the ωTOC and measured porosity data in Zhenjing area, the practicability and reliability of compaction correction method for organic matter are discussed. The results show that the high content kerogen leads to markedly large errors in the compaction results, therefore reducing these errors in the sonic data becomes very important. After applying the correction, the compaction coefficient decreases by 30% -55%, and the uncompacted belt amplitude drops significantly. The mud overpressure of C7 shale calculated based on the equilibrium depth method is 3-5 MPa, comparable to the results of numerical basin modeling. The pressure values are 5-8 MPa smaller than that of without correction, indicate that previous studies may have overestimated the abnormal pressure of Yanchang Formation. Based on the Wyllie equation, the calculated porosity using the corrected sonic data is consistent with the measured porosity. The results suggest that the quantitative correction method can eliminate the abnormal sonic data due to the presence of organic matter, and can be effectively applied to other area of Ordos Basin and other basins.