Factors controlling the formation and evolution of source rocks in the Shahezi Formation, Xujiaweizi fault depression, Songliao Basin

Abstract

The types and quality of source rocks in the Shahezi Formation are the key factors affecting the distributions of various deep gas reservoirs in the Xujiaweizi fault depression in Songliao Basin. To clarify the quality differences and origins of different types of source rocks in the Shahezi Formation, this study reconstructed the sedimentary and water environment, determined the controlling effects of fault activity, sedimentary facies, and paleo-sedimentary environment on the quality of various source rocks, by making full use of seismic, logging, core, organic geochemical and element geochemical analysis. The results show that two types of source rocks developed in the Shahezi Formation, namely, mudstones and coals. The mudstones have a relatively high abundance of organic matter, which consists of type-II kerogen and partial type-III kerogen, and are concentrated in Sha-I Member. The coals have a high abundance of organic matter, which consist of type-III kerogen, and are mainly distributed in Sha-II Member. During the deposition of Sha-I Member, intense fault activity formed arrow, deep-water lacustrine basins with high salinity and strong reducibility on the downthrow sides of faults. During the deposition of Sha-II Member, fault activity progressively weakened, and the areas of lacustrine basins enlarged to their maximum values and became wide, shallow-water basins with low salinity and low reducibility. The development of source rocks was controlled by fault activity, sedimentary facies, and paleo-sedimentary environment. Fault activity formed accommodation space on the downthrown sides of faults for mudstone accumulation, thus determining mudstone thickness. The sedimentary environment controlled the organic matter input and determined the distribution of mudstones and coals. The paleo-sedimentary environment, which consisted of paleo-salinity, as well as paleo-water depth and redox conditions, affected the accumulation and preservation of organic matter and is the main controlling factor for the quality difference of various source rocks in the Shahezi Formation.