This data release contains Rock-Eval pyrolysis, organic petrographic (reflectance), and X-ray diffraction mineralogy data for subsurface Mesozoic rock samples from the eastern onshore Gulf Coast Basin (primarily Mississippi and Louisiana). Samples were analyzed in support of the U.S. Geological Survey (USGS) assessment of undiscovered petroleum resources in the Upper Cretaceous Tuscaloosa marine shale and evaluation of shale gas prospectivity in the Aptian section of the Mississippi Salt Basin. The U.S. Geological Survey will assess undiscovered petroleum resources in the Upper Cretaceous Tuscaloosa marine shale in 2018. A three-year research program to support the assessment included sampling and analysis of several hundred Mesozoic rock samples from the onshore eastern Gulf Coast subsurface, primarily in Mississippi and Louisiana (Enomoto and others, 2017). Samples were analyzed using multiple analytical approaches, including programmed pyrolysis (Rock-Eval), organic petrography analysis including vitrinite reflectance, and X-ray diffraction mineralogy. Also included herein are related data from rock samples in the Mississippi Salt Basin that were collected during previous investigations into shale gas prospectivity in the Aptian section (Enomoto and others, 2012; Hackley and others, 2014a,b; Valentine and others, 2014).