Multi-scale approach to reservoir characterization of the Wolfcamp A, Delaware Basin, Texas, A

Abstract

A multi-scale reservoir characterization on the fine-grained (< 62.5 microns) siliciclastic lithologies of the Wolfcamp A was completed in the Texas portion of the Delaware Basin. The depositional and diagenetic controls on the fine-scale variations in reservoir rock properties were investigated from basin to pore-scale. This research is comprised of three parts: 1) a sub-regional basin-scale characterization of the sedimentological, reservoir quality and stratigraphic complexities, 2) an investigation into the compositional and diagenetic controls on the pore network systems of the identified reservoir facies, and 3) insight into the geologic controls on silica cement textures, and the implications on mechanical and petrophysical properties. As no published literature exists on the Wolfcamp A in the Delaware Basin, the sub-regional basin-scale investigation relied on a high-resolution dataset to provide a fundamental geological foundation for subsequent studies. A detailed facies analysis was based on eight Wolfcamp A cores (1,737’), integrated with bulk mineralogy, source rock, and geochemical analyses, and further supplemented with thin-section petrography. The Wolfamp A is comprised of nine fine-grained silicliclastic and four carbonate facies. These facies were associated with a suite of different depositional processes that include: carbonate debris flows, high and low-density carbonate and siliciclastic turbidity currents, transitional flows, dilute turbulent wakes and hemipelagic suspension settling. Eight of the nine siliciclastic facies were defined as potential reservoir rocks, based on TOC values (> 2 wt.%), and mineralogical composition and consistency, the result of the different depositional processes and subsequent early diagenesis. Despite sedimentological and compositional variability, the pore-size distributions and volumes for all reservoir facies are strikingly similar, characterized by unimodal distributions, (2-200 nm), within the coarse meso-to fine macropore range. Pervasive, pre-compaction silica cement altered the reservoir facies to have similar storage potential and conduits for hydrocarbon transport. The compressibility and permeability of the different depositional reservoir facies respond variably to changes in confining stress. The variable rock behavior is diagenetically controlled by the relative volumetric abundances of pre-compaction silica cement textures. The silica cement textures and their abundances are depositional facies-specific and are related to the geologic factors that control the primary grain assemblages and early diagenetic pathways for silica precipitation. Although pore size distributions and volumes are similar for the reservoir facies, the pore networks of the interbedded facies will respond differently during hydrocarbon production and associated pressure decrease. Knowledge of these rock property behaviors provides insight to assist completion effectiveness and potentially improve reservoir production.