Using Resistivity Log Invasion Profiles, Multiple Porosity Logs, Core and Test Data to Determine Lithology, Porosity and Permeability in Pre-Cambrian Sandstones of Eastern Siberia

Abstract

The Verkhnechonskoye field was discovered and appraised during the late 1970s and early 1980s. One hundred legacy wells and three modern wells have been reevaluated and results used to construct a static model that matched test results and was used to forecast field potential. The Verkhnechonskoye field is currently operated by TNK-BP. Cores in the three modern wells were studied to determine lithology, depositional facies, and mineralogy. Results were applied to crossplots of sonic and neutron log data. These crossplots showed six different lithofacies: basement, weathered-basement, high permeability sand, low permeability sand, salt/anhydrite-cemented sand, and carbonate. Sand was subdivided into relatively high and relatively low permeability sands using the laterolog and microlaterolog, based on the observation that sand with microlaterolog values less than 10 ohm-m were observed to be highly productive on test. To calculate porosity, the neutron, sonic, and core porosity were compared. Preliminary porosity was calculated using a field-wide solution from a combined crossplot of sonic log values and core data. Well by well analysis indicated that the sonic logs often underestimated porosity and that neutron logs tended to match core porosity better. Petrographic analysis confirmed that widespread secondary porosity was not detected by sonic logs. Single-well porosity solutions based on neutron logs were used together with sonic porosity calculations to finalize porosity estimates. For permeability, high and low trends were calculated for sands using the microlaterolog to define high permeability and low permeability sands. A relatively high permeability to porosity relationship was applied to sands with microlalaterolog values less than 10 ohm-m and a relatively low permeability to porosity relationship was applied to sand with microlaterlog values greather than 10 ohm-m.. A significant number of core permeability measurements were available and these points were honored after depth-shifting. Finally, log derived permeability was modified to match average permeability derived from well test results. Identification of multiple lithologies and calibration of porosity and permeability to core measurements and well test results signficantly reduced uncertainty in calculated results. Introduction Western petrophysicists tasked with evaluating Soviet era (legacy) wells in the Eastern Siberian Basin face a problem significantly different from workers in Western Siberia. There Western analysts can adapt Russian logs to customary methods of calculating shale volume, porosity, and water saturation then apply cutoffs to identify reservoir rock and pay. Statistical methods have been developed to convert Russian logs into Western equivalents (Carlston and Cluff, 2006), with an emphasis on generating a pseudo-density log, key to most Western petrophysical approaches. This approach is possible because Western Siberia is not, from a petrophysical standpoint, dissimilar to the moderate porosity shaley-sand setting seen and studied all over the world. The Eastern Siberian Basin, in contrast, offers an unusual and challenging combination of geology, drilling practices, and technology. To get the best possible results for each well, a unique approach has been developed to integrate all available data into a comprehensive answer.