Abstract

In the area of the TWP and Busher fields, approximately 15% of the wells were logged with old electric logs (SP-LAT-LN-SN-ML). In order to determine net pay thickness and to calculate volumetric oil reserves for the Cross Cut pay in these two fields, log analysis of the wells with old electric logs should be included.

The old electric logs were digitized and loaded into a computer program called Old Electric Log Advisor (OEA). Because corrections for bed thickness and the method of R_t determination are a function of tool spacing, electrode spacing of the Lateral (LAT), Long Normal (LN), and Short Normal (SN) logs were required as input. Porosities were calculated from both the Microlog (ML) and the Short Normal log, and volume of clay was determined from the Spontaneous Potential (SP). Before the calculation of net pay and hydrocarbon pore-feet thickness, the log was displayed like a modern log with the SP in track #1, the SN-LN-R_t in track #2 (logarithmic scale), and the Porosity in track #3. The reasons for re-displaying the old log data like a modern log were to observe and compare the old log data with nearby modern logs, and to observe resistivity invasion profiles (SN-LN-R_t). When the porosity and R_t values determined from the old log data were comparable to the modern log data, net pay and hydrocarbon pore-feet thickness were determined.

In the example presented there are two thin (<10 feet) Cross Cut sandstones, both of which contain hydrocarbons. However, using net pay cut-offs of Vcl<15%, Porosity>8%, and S_w<50%, only the upper sand contained net pay. The lower sand is too shaly and has low porosity.