Characteristics and resource prospects of tight oil and shale oil in Ordos Basin

The Changqing Oilfield Company has extensively and efficiently developed reservoirs with ultra-low permeability between 0.3 mD and 1 mD.Based on the practice of petroleum exploration and development in Ordos Basin,the tight oil herein refers to the oil that accumulates in oil shale or interbedded tight sandstone reservoirs adjacent to source rocks and with surface-air permeability less than 0.3 mD.Generally,this accumulated oil has not yet experienced large-scale and long-distance migration,and thus its accumulation includes tight sandstone and tight shale oil reservoirs.The tight oil of Yanchang Formation mainly develops in a semi-deep to deep lake zone,typically in the oil shale and tight sandstone of Chang-7 interval and the tight sandstone of Chang-6 interval of the central lake.Tight oil in Ordos Basin is characterized by a wide distribution,superior conditions of source rocks,tight sandstone reservoirs,complex pore-throat structures,poor physical properties,high oil saturation,better oil properties and low pressure coefficient.Wide development of nano-sized pore-throat systems is one of the fundamental characteristics for the oil and gas continuous accumulation of tight oil reservoirs.Most of the connected pore-throat diameters of tight sandstone reservoirs in Yanchang Formation are greater than the critical pore-throat diameter,gratifying the need of oil and gas to migrate in tight reservoirs.Reservoirs of the tight oil in Yanchang Formation can be divided into 3 different types depending on the contact relationship between tight reservoirs and source rocks,i.e.compact massive sandstone reservoirs,sand-shale interbedded reservoirs and tight reservoirs in oil shale.Tight oil is widely distributed in Chang-6 and Chang-7 intervals of Yanchang Formation in Ordos Basin,and it is preliminarily forecasted that the total resources of tight oil in Ordos Basin are about 30×108t,of which the shale oil resource in Chang-7 interval exceeds 10×108t,while the tight sandstone oil resource in Chang-7 and Chang-6 intervals amounts to about 9×108t and 11×108t,respectively.Therefore,tight oil resources are the realistic oil replacement resources that can ensure Changqing Oilfield to achieve an annual output of 5 000×108tons of oil equivalent and a long term stability of oil production.