PORE STRUCTURE OF MUDROCKS AND ITS EVOLUTION WITH THERMAL MATURITY: A CASE STUDY OF THE MARCELLUS SHALE

The pore structure of mudrocks has a strong impact on the storage capacity, the produced fluid type, and the porosity and permeability of shale reservoirs. With the help of ion-milling technology and a combination of secondary electron scanning electron microscopy (SE-SEM) and backscatter electron scanning electron microscopy (BSE-SEM), we investigated the microtexture of the Marcellus Shale. 32 core plugs from 3 wells in West Virginia and Pennsylvania composed of 5 samples from Mahantango Formation and 27 samples from Marcellus Formation. For each sample, 10 to 15 SEM images were digitalized to get a more comprehensive understanding of them.

Different types of pores developed within the inorganic matrix (IM), and organic matter (OM) pores can be classified into five types according to their shape and relative location with the inorganic matrix: spongy, bubble, complex, discrete, and crack-like. OM with the same thermal maturity can develop into different shape and size, and some OM has no pore development. So, for the OM experienced with the same thermal history, the compositional difference controls the development of OM pores.

SEM-visible porosity doesn’t correlate with percentage of organic matter present. It does have a positive correlation with the thermal maturity. Higher thermal maturity (Ro ranges from 2.59% to 2.89%) correlates with a higher SEM porosity (1.04%~5.34%), whereas lower thermal maturity (Ro ranges from 1.37% to 1.46%) associates with a lower SEM porosity (0.3%~1.8%).

Pore size varies significantly in mudrock reservoirs. The feret diameter of OM pores ranging from 10 nm (resolution-limitation) to 720nm. Pores developed in the inorganic matrix, on the other hand, can be as big as several microns. Although most of IM pores were destroyed during the burial diagenesis, the remaining IM pores can still have significant storage capacity given their relatively larger size.