GEOLOGICAL GENESIS OF LOW-RESISTIVITY OIL LAYERS OF MINGHUAZHEN FORMATION IN QINAN FAULT STEP BELT

In order to better predict the distribution pattern of low resistivity oil layers of the Lower Minghuazhen Formation in the Qinan fault step belt, genetic mechanism and controlling factors of the layers were studied on the basis of core observation, thin section analysis and laboratory testing data. Results show that the genetic mechanism of the low-resistivity oil layers in the Qinan fault step belt depends upon six factors: the low structural amplitude, the complex pore structure, the additive conductivity of clay minerals, the thin sand-shale interbedding, the natural water flooding and the drilling fluid pollution. The geological origin of low resistivity reservoirs includes three aspects: tectonics, sedimentation and diagenesis. Frequent and intense faulting activities lead to natural water flooding and the formation of low amplitude structures. The frequent change in the ratio of accommodation space and sediment supply leads to the formation of the reservoir consisting of thin sand-shale interbedded deposits with complex pore structures. The weak diagenesis of the early diagenetic stage leads to the formation of additive conductivity of clay mineral and drilling fluid pollution. Influenced by the above-mentioned three factors, the low resistivity oil layers of lower Minghuazhen Formation in the Qinan fault step belt mainly develop in the weak hydrodynamic zones of the low amplitude structural areas with connection to faults.