Lower Cretaceous lacustrine carbonate–siliciclastic rocks (including siltstones and mudstones) form a unique tight oil reservoir in the Erennaoer Sag in the Erlian Basin of northeastern China that is characterized by various carbonate cements and volcanic material. Based on integrated analysis of the petrology, mineralogy, and carbon and oxygen isotope geochemistry, the origin, compositions, and evolution of carbonate cements and pore fluids are determined. Five types of carbonate cements are identified, from oldest to youngest: penecontemporaneous micritic-crystalline ferroan dolomite-1; eogenetic micro-crystalline ferroan dolomite-2; sparry, micro-crystalline ferroan calcite-1; mesogenetic micro-crystalline non-ferroan dolomite-3; and calcite-2. The evolution of the pore fluids also controlled the compositions and distribution of the carbonate cements of the lacustrine mudstone during different periods. This may be advantageous to predict the distribution of high-quality reservoirs. The geochemistry of the paleo–pore fluids in these mixed lithological reservoirs are more complicated. Thus, the origin and distribution of pore fluids and their influences on carbonate cementation, fluid–rock interaction, and reservoir quality are complicated and certainly worth studying for hydrocarbon exploration. In addition, the evolution of the pore fluids of the Arshan Formation have experienced different salinity fluid environments under alkaline to weak acid conditions.