We present a novel approach to correct accessible/fluid-saturated porosity values calculated using mercury injection capillary pressure (MICP) for shale samples. On the basis of recent studies, accessible porosity of shale samples calculated from the MICP test is corrected to consider conformance and grain compressibility. However, we show here that the shale samples experience an additional phenomenon during the MICP test that has not yet been addressed, i.e., compression of inaccessible/unfilled pores. Therefore, we propose a general approach consisting of three distinct corrections to accurately estimate accessible porosity of the shale sample using MICP data: (1) conformance, (2) grain compressibility, and (3) inaccessible pore compressibility. First, we develop a mathematical model to estimate both pore and grain compressibility using MICP data and then calculate accessible porosity using the above-mentioned corrections. In the mathematical formulation, we divide the shale matrix into three constituents: (1) accessible pores, (2) inaccessible pores, and (3) grains. We, then, estimate volume fractions for each stage using MICP test data. Samples from both Barnett and Haynesville shale plays (11 samples for each shale plays) are used to perform our study and validate the hypothesis. Moreover, the impact of newly proposed corrections on petrophysical properties, such as permeability and pore size distribution, is evaluated. Our results suggest that estimated accessible porosity significantly decreases when new corrections are implemented on the MICP test data. Furthermore, the results suggest that inclusion of correction will shift pore size distribution toward smaller pores and can also dramatically reduce permeability estimations down to 2 orders of magnitude smaller than the original values. The outcome of this study can help determine the fraction of accessible porosity for reserve evaluation purposes in shale plays.