Determining the Productivity of a Barrier Island Sandstone Deposit From Integrated Facies Analysis

Abstract

Summary Two crossplot techniques, based on core-calibrated resistivity, porosity, and gamma ray (GR) logs, are described that distinguish barrier porosity, and gamma ray (GR) logs, are described that distinguish barrier island sandstones from genetically different nonbarrier sandstones in the Muddy sandstone reservoir in Unit A of Bell Creek field, Montana. The barrier island sandstones are separated further and grouped into two log facies, one consisting of highly productive facies (foreshore, shoreface, etc.) and the other, a less-productive facies (lower shoreface). The distinct crossplot patter for each facies group is due to similar petrophysical properties of each facies group resulting from deposition petrophysical properties of each facies group resulting from deposition within a unique depositional setting. Fluid production results from primary, secondary, and two EOR pilot projects indicate that in the barrier island reservoir at Bell Creek, projects indicate that in the barrier island reservoir at Bell Creek, the distribution of facies, with their characteristic reservoir properties and heterogeneities, dominates primary production, waterflood-sweep efficiency, distribution of residual oil saturation, and the performance of the chemical EOR pilot projects. Introduction The generally prolific production from oil and gas fields in barrier island clastic deposits results from the excellent porosity and permeability of sandstones deposited in relatively shallow, agitated permeability of sandstones deposited in relatively shallow, agitated marine waters. Sediments flanking main barrier sandstone deposits are organic-rich, lagoonal and deepwater, fine-grained deposits that often serve as excellent source beds for petroleum. Process-oriented sedimentological studies have provided better understanding of sedimentary structures, sequence of facies, and other features in different subenvironments that contribute to building barrier island sandstone deposits. Because each barrier island facies (beach, shoreface, dune, etc.) was deposited within a unique setting of wave energy, tidal range, and water depth, the sandstones from each subenvironment tend to have similar petrophysical properties (porosity, permeability, grain-size petrophysical properties (porosity, permeability, grain-size distribution, etc.) at the time of deposition. Davies et al. first demonstrated the remarkable similarity of internal structure and texture in different facies of the modern barrier island in Galveston, TX, with two ancient barrier complexes: one in the Lower Cretaceous of Montana and the other in the Lower Jurassic of England. A subsequent study revealed that major barrier island sandstone facies have recognizable characteristics and may also have some significant variations, depending on local wave conditions and tidal range. Recognizable characteristics of different facies of barrier island sand-stones have been investigated with thin-section analysis. Because of the relative uniformity of depositional processes in each facies, the predictability of fluid production from barrier island reservoirs can be greatly augmented from an understanding of the spatial distribution of thicknesses, flow properties, and geological heterogeneities in each facies. Subsequent to sandstone deposition, such secondary processes as diagenesis or tectonic events may severely affect the distribution of flow properties in different facies. Understanding depositional characteristics, however, leads to understanding the effect of secondary diagenetic processes. In this paper, we describe two crossplot techniques, based on interpretation of log and core data, that can effectively distinguish some of the barrier and associated nonbarrier island sandstone facies. We use these techniques to group facies with similar petrophysical properties, and then we study the different facies petrophysical properties, and then we study the different facies groups for distribution of reservoir properties and geological heterogeneities and variations in thickness and structure. The usefulness of facies interpretation for predicting productivity of sandstones in the different facies is demonstrated by a productivity of sandstones in the different facies is demonstrated by a qualitative comparison of initial primary and EOR production from Bell Creek field with the areal distribution of reservoir properties and sandstone geometry in the different facies groups. A primary objective of this investigation was to demonstrate that fluid injection and production predictions would be greatly improved if barrier island sandstone were divided into a number of facies or groups of facies with each group of similar facies characterized separately, instead of the average petrophysical properties of an entire sand-stone thickness being studied.