The Origin Of Chang 2 Reservoir Group Low-resistivity Oil-bearing Layers In Renshan Area In Ordos Basin

Sandstone reservoirs with low porosity,permeability and oil saturation often have absolute or relatively low resistivity,but it still can form industrial oil flow,called as “low-resistivity reservoirs”.Because of the difficulties in the identification of low-resistivity reservoirs in traditional logging curves,it leads to the confusion or misjudgment when to interpret the oil or water layers in oil exploitation and development.In this paper,the Chang 2 reservoir group of Yanchang Formation in Renshan area of Zhidan Oilfield,Ordos Basin,is taken as the research target,aiming at the characteristics of low resistivity of Chang 2 reservoir group.For the characteristics of low resistivity in Chang 2 reservoirs,the regional stratum situation,local structure,sedimentary facies and reservoir features are analyzed to discuss the identical features and mechanism of low resistivity reservoirs.A variety of methods are applied to distinguishing oil and water layers.The overlapping method of logging curves and the lateral comparison method are proved to be the most effective ways to identify the oil-bearing layers in Chang 2 sandstones.The research results show that the diagenetic modifacation of pore structure,salinity of formation water,too far away from the center of source rocks,drilling fluid invasion and structural amplitude are the reasons leading to the formation of Chang 2 low-resistivity oil layers.Among these factors,far from hydrocarbon accumulation center,invasion of drilling fluid and structural amplitude are the most critical reasons of Chang 2 low-resistivity oil layers.