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Charging of Oil Fields in Qionghai Uplift: Oil Generated From Multiple Kitchens and Source Rock Intervals


Qionghai Uplift (QU) is an important oil producing region. It is sounded by two generative kitchens both of which contain two sets of mature source rocks. The multiple kitchens and source rock intervals complicate the charging of oilfields in QU. The origin of oil in QU is controversial and explanations are various and therefore restrict exploration. This paper discusses the origin and charging direction of oil fields in QU by using biomarker distribution and pyrrolic nitrogen compounds for 18 source rock and 17 oil samples. The two sets of source rocks has distinct biomarker assemblage indicating they has different sources of organic matter and depositional settings, which allow the identification of two oil classes. Oil samples from Eastern Qionghai Uplift (E-QH) have similar biomarker assemblages and have been proven to be generated from the Oligocene Enping Formation (E3e, 33.9-30 Ma) in WCA. Whereas, oil samples from Western Qionghai Uplift (W-QU) possess characteristics of both E3e and Eocene Enping Formation (E2w, 56-33.9 Ma), indicating it was derived from both source rocks. Studies have shown that oil in W-QU was derived from the WCB. It is generally acknowledged that oil in remote uplifts possess much more E2w characteristics than that of oil in depression center. However, the fact that oil from W-QU has more contribution from E3e than WCB, which is contradictory with previous studies that believed the oil in QU was only expelled from WCB, is an evidence that oil in W-QH also received contribution from the WCA. The oil found in Qionghai uplift is more mature than the oil found in WCB, which also indicates WCA make a contribution to the oil in W-QU. The secondary migration direction of hydrocarbon is verified by the variation of absolute and relative contents of pyrrolic nitrogen compounds. This distribution pattern is also proven by using a simple threedimensional model called PathWay, which assumed petroleum expelled from source rock and migrated vertically up to the top of reservoir (also the bottom of seal rock), then migrated to structural culminations. They all indicate that the E3e source rock dominated the oil found in E-QH and had a recognizable contribution in W-QH. Secondary migration of petroleum plays an important role in petroliferous basins and is a practical problem in oil and gas exploration. We think the combination of geological, geochemical data and basin modeling technique can makes the hydrocarbon migration study more reliable.