珠江口盆地陆丰13-1油田恩平组产层电阻率下限计算方法及应用

Calculation and application of resistivity lower limit of pay zone of Enping Formation in Lufeng 13-1 Oilfield,Pearl River Mouth Basin

珠江口盆地陆丰13-1油田的主力产油层段古近系恩平组储层属于典型的低电阻率产层,其产层电阻率下限不清给油水层判别造成了很大困难。在储层参数研究的基础上,对比了常用测井解释模型中含油饱和度与地层电阻率的关系,选择Indonesia公式从测井解释的角度理论上分析了影响地层电阻率大小的因素,提出了恩平组不同物性条件下及各个深度点处产层电阻率下限的计算方法。结果表明:①储层泥质含量越高、孔隙度越大、地层水电阻率越低、含水饱和度越高,则产层电阻率值越低。②恩平组在储层最差物性条件下,产层的电阻率最高下限为4.6Ω·m;在储层平均物性条件下,产层的电阻率平均下限为3.2Ω·m;在储层最优物性条件下,产层的电阻率最低下限为2.5Ω·m。③在不同的深度点处,油层、油水同层和水层的电阻率测井值与下限计算值之比分别为大于0.85、0.7~0.85和小于0.75。

Low resistivity pay zone has low resistivity contrast with sandstone and adjacent mudstone.They are widespread in the global petroleum basins,and play a critical role in increasing petroleum production in China as well.The sandstone reservoir of the Paleogene Enping Formation is the main oil producing zone in Lufeng 13-1 Oilfield,Pearl River Mouth Basin.The pay zones are typical low resistivity reservoirs because the resistivity of the pay zone is usually lower than that of the up and below mudstone layers:resistivity of pay zone is about 2~8Ω·m,and resistivity of nonreservoir mudstones is about 5~20Ω·m.Even though these reservoirs have been exploited for nearly 30 years,the resistivity lower limit of pay zone is still uncertain.It makes great difficulties in discriminating oil and water in reservoir.Based on the study of reservoir parameters,the relationship between oil saturation and formation resistivity in common logging interpretation models are compared in this paper.Then the Indonesia formula is optimized to theoretically analyze the influencing factors of reservoir resistivity including shale content,porosity,formation water resistivity and water saturation.Calculation method of resistivity lower limit under different physical properties and at different depth points is proposed.The results show that the high shale content,well porosity,low formation water resistivity and high water saturation would result in the low resistivity of the reservoir.The resistivity lower limit is a dynamic parameter:in different physical property condition,the resistivity lower limit is different.Under the worst physical property with shale content of 35%and porosity of 12%,the maximum resistivity lower limit of reservoir is 4.6Ω·m;under the average physical property with shale content of 10%and porosity of 18%,the average resistivity lower limit of reservoir is 3.2Ω·m;and under the best physical property with shale content of 3%and porosity of 22%,the minimum resistivity lower limit of reservoir is 2.5Ω·m.The average resistivity lower limit is proved by production data.At different depth points,the resistivity lower limit is calculated based on corresponding reservoir parameters.The ratio between resistivity logging values and resistivity lower limit of oil,water-oil and water layers are>0.85,0.7~0.85 and<0.75 respectively.The differentiation criterion is applied in perforated intervals and its validity and accuracy are proved by the oil test result.The proposed method would be beneficial to identify oil and water layer fastly.