Experimental Studies on Dense Carbonates - An Analysis of Elastic Properties Dependent on Pore Structure

The Ordovician carbonates of the Yingshan formation in the Tarim Basin present a complicated pore structure due to diagenetic and secondary structures. Seismic elastic properties (e.g., velocity) depends on porosity as well as pore structure. We estimated the average specific surface, average pore-throat radius, pore roundness, and average aspect ratio of carbonate rocks from the Tazhong area. Measuring results show that high P-wave velocity samples have small average specific surface, small average pore-throat radius, and large average aspect ratio. Diversity of pore structure of dense carbonate samples result in fluid-related velocity variability. For large or small average specific surface, the pore structure of the rock samples becomes uniform, which weakens squirt flow and minimizes the residuals of ultrasonic data and predictions with the Gassmann equation. When rigid dissolved (casting mud) pores coexist with less rigid microcracks, there are significant P-wave velocity differences between measurements and predictions.