Research paperCharging history of Paleogene deep gas in the Qibei sag, Bohai Bay Basin, China

- The geochemical characters of E2s3, E2s2 and E2s1 source rocks are analyzed.
- Natural gas in research area is cracking gas in high mature stage from E2s3.
- Two charging periods exist and the second period in the center is stronger.
- Gas charging history is restored and oil-gas inversion phenomenon is clarified.
- It provides a deeply understanding of deep gas accumulation processes and mechanisms.

The Qibei sag (QS), at the western region of the Bohai Bay Basin (BBB), eastern China, has made significant progress in the deep gas exploration recently, indicating that the deep gas reservoirs have good exploration prospects. The deep gas reservoirs within the second member of the Shahejie Formation of the Paleogene (E2s2) are buried at approximately 5000 m. The sources and hydrocarbon accumulation mechanisms were investigated by analyzing the regional structure, integrating geologic and geochemical parameters of source rocks and natural gas. Analyzing results show that, in the QS and adjacent areas (QSAA), three sets of source rocks (E2s1, E2s2, E2s3) were developed, which differ in organic chemical characters, but have abundant organic matters in fair-good standard. Affected by burial history, E2s3 source rocks have entered into the high mature-over mature stage and E2s1 source rocks are mainly in mature stage. The methane content of natural gas (78%∼91%), the methane carbon isotope value (−41.3‰∼−34.6‰) and the positive carbon isotope sequence (δ13C1<δ13C2<δ13C3<δ13C4) indicate that the deep gas is organic origin and mainly from thermogenic gas. The relationship between light hydrocarbon character parameters and thermal evolution degree indicates that the deep gas in the QS is mainly from E2s2 and E2s3 source rocks of the sag. Since the different hydrocarbon generation history, there exist two hydrocarbon accumulation periods in QS, and the early period is characterized by oil accumulation, which occurs at the shallows, such as the Binshen6 block; the late period is characterized by gas accumulation, which happens in the depth mostly, such as the Binshen22 block. The different hydrocarbon generation history results in the relative enrichment of gas in the depth and the phenomenon of oil-gas inversion.