Abstract:

In order to evaluate the sealing ability of the Lower Cretaceous mudstone caprock in the D basin in Chad, the thickness, porosity, and break-out pressure of the mudstone caprock are obtained by using well logging data; the relationship between break-out pressure and interval velocity, and the relationship between mud-to-formation ratio and seismic attributes are analyzed; the plane distribution of break-out pressure and mudstone thickness are described; and the evaluation standard for the grading of the mudstone caprock is established by taking break-out pressure and mud-to-formation ratio as primary evaluation parameters. The results show that the mudstone in the Kedeni formation in the D Basin is classified into Type Ⅰ and Ⅱcaprocks, and the mudstone in the Doba formation is Type Ⅱ and Ⅲcaprocks. The sealing ability of the mudstone caprock in the Kedeni formation is stronger than that in the Doba formation. On plane, the mudstone caprocks in the northern steep slope zone and sags have the best sealing abilities, and are ranked as Type Ⅰ and Ⅱ, followed by the mudstone caprocks in the central low-amplitude uplift zone and the southern gentle slope zone, which is dominated by Type Ⅱ and Ⅲ, and the worst mudstone caprock is in the northeastern transition zone, which is Type IV. The evaluation results are in good agreement with the actual drilling results, indicating that the evaluation method is effective and provides a good reference for mudstone caprock evaluation in similar basins with relatively low exploration degrees.

Key words: D basin, break-out pressure, sealing ability, mud-to-formation ratio, mudstone caprock, plane distribution, porosity