The origin and accumulation model of crude oils from oil reservoirs Chang 9 and Chang 10 in the Yanchang Formation of the Ordos Basin

In the lower parts of oil reservoirs Chang 9 and Chang 10 of the Yanchang Formation are oil-bearing layers newly found in oil exploration in the Ordos Basin. Based on GC, GC-MS analyses of saturated hydrocarbons from crude oils and source rocks, reservoir fluid inclusions and BasinMod, the origin of crude oils, accumulation period and accumulation models are discussed in combination with other petroleum geology data in this paper. The result shows that (1) there are two different types of crude oils in oil reservoir Chang 9 in the Longdong and Jiyuan regions: crude oils of type I(Well D86, Well A44, Well A75, Well B227, Well X62 and Well Z150) are mainly derived from the Chang 7 source rocks (including mudstones and shales) and distributed in the Jiyuan and Longdong regions; those of type II(Well Z14 and Well Y427), are distributed in the Longdong region, which are derived from the Chang 9 source rocks. Crude oils from oil reservoir Chang 10 in the Shanbei region are mainly derived from the Chang-9 source rocks; (2) there are two phases of hydrocarbon filling in oil reservoir Chang 9 in the Jiyuan and Longdong regions and oil reservoir Chang 10 in the Shanbei region: The first phase started at the early stage of J₂z. The process of hydrocarbon filling was discontinuous in the Late Jurassic, because of the tectonic-thermal event in the Ordos Basin. The second phase was the main accumulation period, and hydrocarbons began to accumulate from the late stage of J₂a to the middle-late of K₁, mainly at the middle-late stage of K₁; (3) there exist two types of accumulation models in oil reservoirs Chang 9 and Chang 10 of the Yanchang Formation: source rocks of the reservoirs in oil reservoir Chang 9 in the Jiyuan region and oil reservoir Chang 10 in the Shanbei region, the mixed type of reservoirs on the lateral side of source rocks and source rocks of the reservoirs in oil reservoir Chang 9 in the Longdong region.