K oil reservoir

in Oilfield-A

Iraq

is a typical porous carbonate reservoir. It is characterized by complex pore structure

wide range of resistivity and complex oil-water contact（OWC）. In order to establish the reliable criteria for fluid typing and ascertain the OWC

firstly

the study of pore structure has been taken as a starting point

and the relationship between resistivity and elevation has been taken an insight into. Then a new criteria for fluid typing and OWC for each sub zone have been established by incorporating well testing and production dynamics. Lastly

the lateral variation of OWC and its genesis is analyzed based on structural evolution and hydrocarbon accumulation history. The results are as follows. ①Pore structure of the K oil reservoir shows strong longitudinal stratification characteristics

and the amount of mold pores and interparticle pores as well as effective pore-throat radius increases upwards. ②Vertical variation in pore structure is the main controlling factor of the identification criteria for oil and water layers and OWC variations for each sub layer. The layers dominated by micropores have small pore-throats radius

high displacement pressure

high OWC and low reservoir resistivity threshold

while the layers dominated by interparticle pores and mold pores are the opposite. ③The OWC is curved laterally

and slopes eastward along the long axis of the structure in the same small layer. Along the short axis of the structure

the contact is arched

that is the north and south margin is lower than the structure crest

and the lowest contact is in the northeast margin. ④The determination of the curved OWC has been verified by the reservoir interpretation of 85 vertical wells and the production dynamics of more than 260 horizontal wells. This study provides not only an effective solution for water control and stabilizing oil production but also a guideline for further optimization of field development plan and trajectory adjustment of horizontal wells.