Re-Assessing Exploration Potential and Targets on the Vast Monocline of Northern Ordos Basin, North-Central China, through Oil-Source Correlation and Secondary Migration Studies — An Example from Qing-Ping-Chuan Field

50 oilfields have been producing from Triassic Yanchang Formation (900-1400 m thick, covering 230,000 km2) on the vast monoclinal slope in northern Ordos Basin, China. Production is mainly from 5 of the 10 producing sandstones in upward Chang 8, 6, 5, 4, and 2 members in stratigraphic traps. Future exploration is hampered by great variability of oil saturation among and within individual sandstones, poor understanding of source rock distribution and quality, and secondary migration history. Our study focuses on the mature 700-km2 Qing-Ping-Chuan Field in the easternmost of the monocline to assess distribution of high-quality source rocks and secondary migration pathways to predict future exploration targets. Early discoveries are 134-210 m deep and recent ones are deeper in different parts of the field. Previous studies suggested oil migrated from distant basin center via sandstone carriers and fractures. Faulting and fractures are rare to absent in the field. Thus, source rock distribution and migration pathways are key to predict oil accumulations. Source-oil correlation can identify source signature of oils. Analysis of 23 mudstones indicates TOC of 0.11-11.44% (deep-buried mudstones have TOC >0.5%), dominantly type II2 kerogen with minor type III, and R0 greater than 0.7% (maximum at 1.21%). The results suggest the mudstones are good (in Chang 4, 5, and 6) to excellent (Chang 7) source rocks in peak oil generation. Furthermore, group composition, organic carbon isotope composition, and typical biomarkers of hydrocarbon extracts from mudstones and crude oils were compared. Oil-source correlation indicates that oil in Chang 4, 5, and 6 has a mixed source, largely from mudstones in Chang 7, some from Chang 6, 5 and 4. As a result, oil from the source rocks migrated first vertically into sandstone carriers, then along channel-fill sandstone belts up-dip the monocline and accumulated in high-porosity reservoirs. Migration path was short and local, not long and distant as previously suggested. Last, migration-related biomarkers were compared with nitrogen-bearing compounds in crude oils. The correlation identified several areas along migration pathways that may have been charged and are future exploration targets. Our study provides new concepts and methodology to re-assess the exploration potential and targets in many other mature fields on the monocline of Ordos Basin.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California