Formation Conditions and Mechanism of Oil Reservoir in Maling Oilfield, Ordos Basin

Key words: 
• Ordos basin

Abstract: Ordos basin is a large inner continental sedimentary basin in China. Mesozoic crude oils in the basin are derived from nonmarine source rocks, so that it has also been used as a typical model of a nonmarine sedimentary basin generating oil. Maling oilfield is located in the southwestern Yishan slope of Ordos Basin. It was discovered in Lower Jurassic Yanan Formation in 1970 and is one of the earliestdiscovered oilfields in Ordos Basin. It also is typical representative of oil reservoir in land fluvial deposits. However, previous geochemical studies of oils from Maling oilfield are limited and the mechanism of hydrocarbons accumulation for this oilfield has not been well known. Understanding the formation law of this kind of oil reservoir is very important for petroleum exploration and production in fluvial deposits. We collected the crude oil samples from different regions of Maling oilfield and source rocks in the Ordos Basin, and analyzed biomarker compounds in the oils and source rocks. In this paper, the formation conditions of oil reservoir in Maling oilfield, such as source rock, reservoir rock, oil migration, traps and accumulation, were systematically studied, and its forming mechanism was discussed dynamically. The results show that crude oil of Maling oilfield was derived from the Yanchang Formation source rocks. Reservoir rocks are mainly the Low Jurassic Yanan Formation fluvial channel sandstones, while coal seam, carbonaceous mudstone and mudstone constituted the cap rock. Hydrocarbon vertical migration was controlled by charging position and migration window. The lateral migration pathway of crude oil is sandbodies of river channels. The force for the vertical migration was likely the capillary pressure and the abnormally high strata pressure. There were five types of traps. 