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Pore System Characterization of Wolfcamp Lithofacies, Delaware Basin

Abstract

Variability in pore systems between different lithofacies will affect the storage and movability of hydrocarbons in extremely heterolithic formations, such as the Wolfcamp in the Delaware Basin. In this study area, Wolfcamp sediments are broadly organized into thick mixed carbonate-siliciclastic deep-water fans. In general, calcareous material is concentrated toward the axis of these submarine fans, and they become muddier toward the fan fringes, where unconventional reservoir rocks are concentrated. Packages of sediments can be further divided into very thin- to thick-bedded hybrid event beds (HEB), which vary in composition depending upon proximity to axial position within fans. Wolfcamp HEBs are rheologically stratified, with basal packstones, topped by argillaceous calcareous siltstone, calcareous mudstone, and capped by massive organic-rich siliceous mudstone. Understanding the type and character of pores and pore networks within Wolfcamp lithofacies facies, and the distribution of these facies relative to fans, remains fundamental for identifying and high-grading the best landing zones. Samples representative of common HEB facies were selected from three cores, in transect through the frontal fan fringe, the lateral fringe, and off axis from Wolfcamp 100 fans, respectively. Large (up to 1 mm x 1 mm), high-resolution, multi-scale (15-5 nm pixel size) SEM mosaic maps were acquired from ion milled, iridium coated, samples under low voltage conditions. Grayscale SEM images were segmented to label pores, organics, and grains. Pore types (i.e., intergranular, mixed, organic pores) for each pore are estimated using calculated % contact of each pore with grains and organics. This contribution reports pore characteristics of facies that comprise Wolfcamp HEBs. Using pore contacts to approximate pore type, massive mudstone and calcareous mudstone facies contain a higher abundance of “organic” and “mixed” pores; whereas pores within calcareous siltstone faces are mostly inter/intragranular. Pore-scale imaging of Wolfcamp facies suggest pore characteristics vary significantly on a facies by facies basis within HEBs, which when integrated with a facies model, can be used to help understand and predict reservoir potential and distribution.