Abstract The study of sedimentary pyrite morphological characteristics provides useful information on depositional environments and early diagenic processes and can be used as an indicator of redox conditions in ancient lake bottom waters. The results of this study in the Songliao basin in northeastern China show that euhedral crystals and framboids are the dominant pyrite forms in the mudstones of the first member of the Qingshankou Formation (K2qn1). The framboidal size distribution indicates that during the deposition of K2qn1, redox conditions in the bottom water fluctuated from oxic–dysoxic to euxinic–anoxic to oxic–dysoxic. The presence of euxinic–anoxic bottom water suggests that an anoxic event developed in the Songliao ancient-lake and continued for 196 ka. The development of an anoxic bottom environment was the most important factor in hydrocarbon source rock formation in the Songliao Basin.