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It has not been clearly understood whether the hydrocarbon generated from the argillaceous sourcerocks of the Lower Cambrian Qiongzhusi Formation could upward pass through the regional cap rocks of the Lower-Middle Cambrian gypsum rocks and accumulate in the Middle-Upper Cambrian Xixiangchi Group reservoir, namely the cross-formational hydrocarbon accumulation, in the eastern Sichuan Basin. Therefore, the studies on the reservoir solid bitumen in the Xixiangchi Group and the sealing performance of the regional cap rocks of the Lower-Middle Cambrian gypsum rocks were carried out, and the cross-formational hydrocarbon accumulation of the Cambrian oil and gas and its significance for natural gas exploration were discussed. It was shown by synthetic studies that the reservoir solid bitumen in the Xixiangchi Group mainly originated from the argillaceous source rocks of the Lower Cambrian Qiongzhusi Formation. The sealing integrity of the regional cap rocks of the Lower-Middle Cambrian gypsum rocks was damaged to some extent by the reactivated basement faults and the large nascent faults in the Middle Permian to Middle Triassic though it had good sealing performance in geological times, which could provide convenient vertical pathways for the Cambrian cross-formational hydrocarbon accumulation in the eastern Sichuan Basin. Three evolutionary stages were built up about the Cambrian hydrocarbon migration and accumulation as follows. Stage 1 is the formation of the Cambrian pre-salt early paleo-reservoirs in the period ofthe Late Silurian to the Early Permian (S3-P1), stage 2 is the one of the Cambrian cross-formational hydrocarbon accumulation in the period of the Middle Permian to the Middle Triassic (P2-T2), and stage 3 is the formation of the Cambrian late paleo-gas reservoirs in the period of the Late Triassic to the Early Cretaceous (T3-K1). The cross-formational hydrocarbon accumulation is an important feature of hydrocarbon accumulation in multi-cycle superimposed basins, and the reactivated basement faults and the large nascent faults in the period of the MiddlePermian to the Middle Triassic are important clues to the Xixiangchi Group primary paleo-gas reservoirs. Identification and evaluation of the Xixiangchi Group primary paleo-gas reservoirs will be the key points and the main challenges to natural gas exploration in the study area.

It is difficult to distinguish reservoirs and non-reservoirs based on the rock's physical properties. The quality of post-stack seismic data in the loess plateau is poor, which makes it a technical bottleneck for the prediction of reservoir of the Upper Paleozoic, and the exploration in the southern Ordos Basin was then constrained. Block evaluations have indicated that the northeastern part of Zhenjing district has a good potential for exploration, with the second member of Permian Shanxi Formation and the first member of Xiashihezi Formation as the main target beds. The results of 3D seismic facies analysis in Zhenjing area are consistent with the regional sedimentary facies studies on provenance and the distribution of sand bodies, indicating that seismic data can be used for the prediction of reservoir. The distribution of sedimentary facies of a series of seismic forward modeling showed that the sand bodies distributed with a certain distance and thickness in the second member of Shanxi Formation and the first member of Xiashihezi Formation formed recognizable seismic reflections. This vertical spatial combination of stratigraphic lithology is consistent with the depositional model of regional superimposed and thick single-type sand bodies, which is the typical form of reservoir-caprock combination that has been discovered in the surrounding gas reservoirs. The reservoir prediction based on the optimal number of wave crests from the forward simulation results confirmed that the superimposed and the thick single-type sand bodies of the second member of Shanxi Member were distributed in SW-NE directions. Furthermore, the picking and interpretation of co-phase axis and the structural mapping of seismic profile were carried out, which directly delineated the distribution range and burial depth of lithologic traps in the sand bodies of the second member of Shanxi Formation, and provided drilling targets for exploration and deployment in this area.