Polygonal faults in the Sanzhao sag of the Songliao basin: their significance in hydrocarbon accumulation

Polygonal faults, generally distributed in fine-grained sediments, are layer-bound faults and are important in hydrocarbon accumulation. Using 3D seismic data, we analyzed the plane and profile features of faults developed in the Qingshankou formation of the Sanzhao sag. We identified these faults as having typical features of polygonal faults: 1) layer-bound; 2) normal faults; 3) slight fault displacements and steep in dip angles; 4) multi-directional in strike and 5) a single fault has a short horizontal extension. In addition, these faults intersect each other and form polygons. These polygonal faults are the result from the combined action of compaction, volume contraction and episodic hydraulic fracturing, conditions favorable for oil/gas accumulation. They are the dominant channels for migration of fluids in the Qingshankou mudstone, forming a large number of fault-lithologic oil traps. Polygonal faults improve reservoirs.