Abstract
Oils in the Maling oil field, Ordos Basin, are produced from fluvial deposits within the Yan-9 and Yan-10 subsections of the Yanan formation. Through the analysis of pyrrolic nitrogen compounds and fluid inclusions, the oil charging site, migration direction and pathway, and timing were interpreted in relationship with the sedimentary features and the high pressure observed in the Mesozoic sequence. The spatial distribution pattern of the fractionation of neutral nitrogen compounds in the crude oils suggested two possible charging positions in the studied area. The ratios of the isomeric nitrogen compounds increase toward the center of the study area from both northwest and southeast directions of the region, indicating that the lateral charging directions were likely from both the southeast and the northwest toward the center of the oil field. The isomeric ratios of pyrrolic nitrogen compounds in the oils from different area of the Maling oilfield increase with the decrease of depth from Yan-10 to Yan-4+5 subsections, showing vertical migration of oil to the reservoirs. The homogenization temperature of fluid inclusions in the reservoir rocks exhibited two peaks, reflecting the two stages of oil migration and accumulation. The migration pathways include fluvial sand bodies and unconformity surfaces. The migration driving force was likely the capillary pressure and the abnormally high strata pressure in the Mesozoic sequences.
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