Abstract:

The southwest gentle slope belt in Huimin sag is located in the structural slope to the south of Xiakou fault. In the north,it is adjacent to Linnan depression, an important oil kitchen,and is bounded by Xiakou fault zone; it is adjacent to Luxi uplift in the south, Qudi horst and Jiyang graben in the east, and Yucheng depression in the west. Its area is about 500 km². The Paleogene in the gentle slope belt is relatively thin. On the whole, it appears to be a wide and gentle monocline,uplifted in the south and dipping to the north.There are only a few NNE conjugated normal faults with small displacement in it. At the same time, these faults cut through the strata into the basement and pinch out upward in the Paleogene or end up beneath the unconformity.The Cenozoic tectonic evolution in that area can be divided into 4 stages:(1)During the deposition of Kongdian Fm to Es₄,Huimin sag was a simple half-graben fault depression and the southwest gentle slope belt was located on the structural slope of the half-graben. The tilting movement resulted in the whole area to be dipping to the north,and so deposited a wedge-shaped sedimentary sequence composed of Kongdian Fm -Es₄. During this stage, the displacement of Xiakou fault was small,and it could be regarded as accommodating the counter-inclined fault developed on the hanging wall of Linshang fault. (2)During the deposition of Es₃-Es₂,the activity of Xiakou fault was intensive,which became the southern boundary fault of the graben-type Linnan depression. The southwest gentle slope belt located on the footwall of Xiakou fault subsided slowly,and the tilting had apparently been restrained. There were conjugated normal faults with small displacement in the belt. The thickness of the Es₃ and Es₂ were relatively thin and uniform. (3)During the deposition of Es₁ to Dongying Fm,the faults in the southwest gentle slop belt had stronger activity. During the depositon of the Dongying Fm,the tilting made the whole belt to be dipping to the north .(4)During the deposition of the Neogene to Quaternary, Xiakou fault and the faults within the southwest gentle slope belt were basically inactive. As a result, a regional unconformity occurred between Guantao Fm and the underlying formations. In the southwest gentle slope belt near the basin margin,source rock can only be found locally in some well-developed Es₄. The sealing characteristics of Xiakou fault have an important effect on the hydrocarbon migration and accumulation in the southwest gentle slope belt. Xiakou fault is characterized by listric and high-angle plane fault. The smearing coefficient analysis of Xiakou fault indicate that the fault's sealing property becomes poorer with increasing depth. The middle Es₃ and younger formations have relatively good sealing characteristics, while the lower Es₃, Es₄ and older formation have relatively poor sealing characteristics. The Es₄ source rock in the southwest gentle slope belt reached the hydrocarbon-generating threshold (2 600 m) after Dongying Fm was deposited. The hydrocarbon generated from Es₄ source rock on the hanging wall of Xiakou fault can migrate laterally through the fault to the southwest gentle slope belt. Because of the lack of source rock in Es₃ and sequences above it, and the sealing of Xiakou fault, the hydrocarbon generated in Linnan depression have been hampered to migrate to the gentle slope belt. The characteristics of the structural evolution and the hydrocarbon accumulation model set up from the migration condition show that fault-block and stratigraphic traps might be formed in the Es₄ and older formations in the southwest gentle slop belt,but oil and gas reservoirs would hardly be formed in the Es₃ and younger formations.

Key words: structural evolution, sealing property of fault, oil and gas accumulation, Xiakou fault, Huimin sag