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

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.

A Fresh Look at Thin-bed Prospectivity Within Conventional Shallow Water Clastic Plays; Implication for Significant Bypassed pay

“…This may originate from the lesser segregation between pure shale and pure sand end members as compared to deepwater counterparts plus less "perceived" connectivity of individual sand lamina as well as connectivity between thin-bed packages. However, there are numerous documented studies that show that thin sand beds in these environments have comparable reservoir quality to that of the conventional thickerbedded facies (e.g., Henderson et al 2006). Same observation was made in this study when bed thickness was plotted against rock quality index (RQI) computed from core data (i.e., core porosity and permeability) (Fig.…”

Effects of erosional scours on reservoir properties of heterolithic, distal lower-shoreface sandstones

“…The proportion of sandstone can readily be measured in core, and potentially indirectly deduced in wireline log data using careful petrophysical analysis (e.g. Henderson et al, 2010). However, the thickness of scour-fill sandstones is unlikely to be a parameter that can be robustly measured in core and wireline-log data (e.g.…”

Characterization of effective permeability in heterolithic, distal lower-shoreface sandstone reservoirs: Rannoch Formation, Brent Group, UK North Sea

“…10), although, with due care, cored intervals may be used to calibrate wireline-log responses in uncored wells (e.g. Worthington, 2000;Henderson et al, 2010). 3Compare the thickness of sandstone scourfills in the outcrop analogue with mudstone-interbed thickness in cored reservoir successions, in order to interpret thick, laterally extensive mudstone barriers.…”

“…Such intervals are common in many wave-dominated deltaic reservoirs, but it is difficult to characterise their effective properties and potential productivity (e.g. Worthington, 2000;Henderson et al, 2010;Baillie & James-Romano, 2010). Core plugs sample an unrepresentatively small volume, typically from either a sandstone bed or a mudstone interbed, with the former giving measured values of vertical-to-horizontal permeability ratio (kv/kh) of 0.01 to 0.4 (e.g.…”