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

Key words:  sonic data  compaction curve  organic content  shale  Zhenjing area

Abstract:

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 ωTOC 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.