Abstract: The Donghetang formation in southwestern Tahe oilfield in Xinjiang is a clastic reservoir which appears negative distinction of deep and medium resistivity between oil and water zones. The dual induction and array induction logging data from typical wells are analyzed. The factors of mud filtrate invasion are classified as static factor and dynamic factor. The mechanism of mud filtrate invasion is studied. The calculation of formation resistivities in the case of different porosity, water saturation and mixed water resistivity shows that the negative distinction of resistivity is resulted from that the increase of formation resistivity caused by high mud filtrate resistivity and poor petrophysical property is larger than the decrease of formation resistivity caused by high water saturation and poor oil-bearing property. It is concluded that oil and water layers should not judged only by difference between deep and medium resistivities or by presence of lower resistivity zone, besides, combining the difference between mud filtrate salinity and formation water salinity with petrophysical property, oil mobility, etc., should be required.

Key words: Xinjiang, mud filtrate, filtrate-invaded zone, induction logging