The Paleoproterozoic Barney Creek Formation (BCF; McArthur Basin, Australia) is one of the oldest active hydrocarbon systems on Earth with oil and gas shows present within organic-rich intervals (up to 7 wt. % total organic carbon). We combine bulk geochemical analyses, pore-space characterization, and high-resolution electron imaging techniques to characterize the evolution of the BCF pore system with maturity. A thermal gradient from the pre-oil window (0.48% calc. Ro) to gas window (1.01% calc. Ro) shows a progressive change in pore networks with the loss of organic-hosted pores dominant in thermally immature samples to porosity increasingly associated with the mineral matrix with thermal maturity. Precipitation of fine-grained, high surface area cements reduced porosity within the oil window, whereas feldspar and dolomite dissolution and creation of secondary pores increased porosity within the gas window. The abundance of feldspar grains (up to 50%) provided a significant potential for secondary pore formation as well as a source of silica and clay cement. This study identifies a first-order linkage between the chemically reactive sediments that are key to the reservoir properties in the BCF and a provenance and/or weathering intensity conducive to supplying fine-grained, mineralogically immature sediments during deposition. These findings likely apply more broadly to other chemically immature mudrocks typical of Precambrian age sediments or Phanerozoic settings subject to limited chemical weathering.