The Low Reistivity Reservoir Identification And Evaluation Techniques In Bohai Oilfield

Electronic well logging is one of the most significant well log method to discover andevaluate oil and gas reservoir, the theory is that there is more oil and gas in the larger pores,and the smaller pores contain water which called as"co-exist water". In respect that oil andgas is non-electrical material, therefore the resistivity of oil and gas is higher than waterzone. For many decades, we get used to identify oil from water by means of deep laterologand deep induction resistivity log, and interprete formation original water saturationquantatively base on formation resistivity, porosity and pore structure parameters.Therefore, we take great efforts and spend lots of time on the acquiration of formation trueresistivity. When the ratio of oil and water resistivity is larger than 3 to 5 times, a satisfiedwater saturation value could be accquired by means of the above method. However, whenthe oil zone resistivity is equal or lower than water zone resistivity, we could not get asatisfied water saturation values only by means of deep laterolog or deep induction log.A series of middle to large scale oil fields in Neogene formation in Bohai sea havebeen discovered which contain a large number of reserves(Original oil in place) for the last10 to 15 years,nevertheless, there exist a series of great challenges which never beenencountered before. One of the more remarkably challenges is the existence of a largeamount of low resistivity reservoir. The low resistivity reservoir discussed here is the ratioof the oil or gas bearing zone resistivity to the water zone resistivity less than 2 or the oil orgas bearing zone resistivity equal to water zone resistivity. In some oilfields the net paythickness of the low resistivity pay sand is more than 50% persent of the total net paythickness of the whole oil field. For example, the ney pay thickness of low resistivity netpay is nearly 69% persent of the whole Qikou 17-2 oilfield. It is significant to preciselyidentify and quantitatively evaluate and interprete low resistivity oil and gas reservoirquantitatively for the new area exploration, evaluation and development phase of bohaioilfields.This article summarized the main causes, genetic types or classifications andidentification methods of low resistivity reservoir , give a further study on quantitativecalculation of water saturation, and table a proposal of synthesizing spontaneous potential(SP) log, micro resistivity log and deep resistivity log to identify low resistivitypay from non-pay while utilizing conventional well log data. Furthermore, bring forward aquantative method of calculating water saturation by mean of Wax-Smits equation and Jfunctions in low resistivity reservoir. After apply this method to Peng lai 19-3 Oilfield andCao fei dian 11-1 Oilfield, it is considered that this methods are not only effective andpractical for low resistivity reservoir in Neogene formation in Bohai sea, but alsoaccumulate some experiences and recognitions for further study on Bohai low resistivityreservoir.