Identifying and Quantifying Thin-Bedded Pay (Part A): Log Characteristics and Reservoir Quality

Abstract

Abstract Thinly bedded reservoirs are common throughout the Columbus Basin, but have traditionally been overlooked as productive targets. The presence of interbedded shales suppresses resistivity values within the thin sand beds (<2 feet thick), resulting in low-resistivity log signatures that do not meet conventional pay cut-offs. Core data and advanced high-resolution log analysis are required to accurately evaluate reservoir quality. In the absence of such data, thin bedded pay can still be identified on conventional logs as low resistivity intervals and a modified petrophysics workflow can be used to estimate gas-in-place. An example is taken from a recently drilled well where 180 feet of conventional core was acquired together with an advanced logging suite over a thinly bedded interval. The resulting petrophysical analysis indicates lower water saturation than predicted by conventional logs, which is independently supported by wellsite plug measurement. Core data also demonstrates that the porosity and permeability distributions in the cleaner, thin sand beds show similar ranges to conventional "thickly bedded" sands, irrespective of bed thickness. Porosity and permeability decrease in the more heterolithic zones (with increasing silt content) as would be expected. These results endorse the concept that reservoir quality of thin sands is comparable to that of thicker sands, although this may not be evident from conventional log analysis. As a result, a well-by-well screening was carried out for a number of fields in the Columbus Basin to identify and classify potential thin bedded intervals based on typical log signatures. The reservoir properties of these intervals can then be evaluated using a modified petrophysics approach based on calibration to the core and advanced logging technology. This workflow has proved to be an important tool for the identification of thin bedded intervals, highlighting opportunities for targeted surveillance and providing an inventory of potential targets for future access.