Abstract: High oil production has been achieved from the mud-rich reservoir of Baikouquan formation in multiple wells of Wellblock Ma-18 of Mahu sag, which contradicts the traditional understanding of "poor oil-bearing property of mud-rich intervals". To study the formation mechanism of shrinkage pore in mud-rich reservoir and its influences on the reservoir, micron-CT scanning technology and digital core calculation method are used to study the 3D pore structures in mud-rich reservoirs and the results show that the permeabilities of rock samples with developed shrinkage pores are much higher than those of the samples with undeveloped shrinkage pores. In-situ electronic probe technology is used to analyze the components of mud matrix in Baikouquan formation and the results indicate that during the transformation of potassium- and silicon-rich, iron- and magnesium-poor mud matrix to mica minerals, shrinkage pores occurred. The analysis on pore content, porosity, permeability and formation test production show that the porosity and permeability of the mud-rich interval with developed shrinkage pores shows an increase trend. Combined with the sedimentary analysis, it is considered that the mud-rich reservoir in the Wellblock Ma-18 deposited in an underwater distributary channel. The comprehensive analysis shows that the shrinkage pores can not only expand reservoir spaces but connect with other types of pores, then to improve the storage capacity and permeability of the mud-rich reservoir. Favorable sedimentary environment, good lithology and unique configuration of reservoir spaces are the main reasons for the high quality mud-rich reservoir, and the shrinkage pore is the key factor. Based on the above understandings, good formation test results have been gained in the mud-rich reservoir identified by using logging chart, which provides a new idea and direction for potential tapping in the Mahu area