Loading...
Diagenetic Controls on Reservoir Character of the Lower Permian Wolfcamp and Bone Spring Formations in the Delaware Basin, West Texas
Dobber, Arwin Willem
Dobber, Arwin Willem
Citations
Altmetric:
Abstract
Understanding the impact of diagenesis on reservoir quality is key to successful production in unconventional reservoirs. Core samples from the Wolfcamp and Bone Spring formations of the Delaware Basin were studied from five wells in Loving and Reeves counties, Texas. The purpose of this study was to construct a detailed paragenetic sequence of the diagenetic events in order to constrain the timing of fracture opening and fluid flow, reconstruct the burial and thermal history of the Delaware Basin, and evaluate the controls on porosity evolution and mechanical behavior. Petrographic observations, in conjunction with fluid inclusion, stable oxygen and carbon isotope, strontium isotope, and U-Pb geochronological data, revealed a complicated diagenetic history consisting of thirty-eight major diagenetic events. The earliest diagenetic features developed during shallow burial under mildly reducing to methanogenic conditions in the Early to Middle Permian and include the cementation of primary pores by calcite, the formation of concretions and framboidal pyrite, the replacement of micritic material by dolomite, and the formation and mineralization of pre- to syncompactional fractures (Set I). Bedding-parallel veins of calcite ‘beef’ and blocky calcite are confined to the overpressured interval in the Wolfcamp. They correspond to the lowest δ18O values (–7.6 ± 0.6‰ VPDB) and highest Th values (119 ± 11˚C) in this study, indicating formation at deep-burial conditions. U-Pb dating of beef and blocky calcite resulted in ages of 238 ± 38 and 202 ± 68 Ma (at 95% confidence level), which suggest that overpressure has been maintained since roughly the Late Permian to Early Jurassic. Secondary hydrocarbon inclusions along healed microfractures indicate that the main phase of hydrocarbon migration postdated beef and horizontal vein formation. Calcite-filled near-vertical extension fractures (Set II) postdate the bedding-parallel veins, possibly contain hydrocarbon inclusions of primary origin, and yield U-Pb dates of 95 ± 28 and 102 ± 14 Ma (at 95% confidence level). This indicates that the fractures predate Cenozoic deformation and were generated during, and possibly resulting from, the maturation of organic matter. Set II fractures are also characterized by less negative δ18O (–6.6 ± 0.8‰ VPDB) and more negative δ13C values (–0.3 ± 1.5‰ VPDB), reflecting lower temperatures and the increasing influence of 12C-rich bicarbonate in the decarboxylation zone. The occurrence of primary hydrocarbon inclusions in phases postdating the Set II fracture-fills suggests that the main phase of hydrocarbon migration took place during the mid-Cretaceous to early Paleogene at temperatures of ~80–120˚C. Late-stage diagenesis was characterized by the widespread replacement of carbonates by authigenic minerals, the creation of secondary porosity by dissolution, and the emplacement of solid bitumen. These events appear to have been driven by the generation of organic acids and/or carbon dioxide during organic matter diagenesis. Precipitation temperatures estimated from fluid inclusion microthermometric and stable oxygen isotopic analyses reveal a complicated burial history with at least two episodes of burial and unroofing. Maximum burial is estimated to have occurred in the Late Triassic to Early Jurassic when burial temperatures reached as high as ~120–140˚C. These elevated temperatures can be best explained by increased heat flow, with geothermal gradients of up to ~26–34˚C/km. More favorable reservoir quality coincides with areas that were affected by carbonate replacement of siliceous skeletons and were later subjected to dissolution by organic and/or carbonic acids. The project has resulted in a conceptual diagenetic model that, due to its relationship with lithology and host-rock mineralogy, could potentially be correlated with petrophysical logs to predict variations in reservoir quality.
Description
Date
2022-12-31
Journal Title
Journal ISSN
Volume Title
Publisher
University of Kansas
Collections
Research Projects
Organizational Units
Journal Issue
Keywords
Geology, Sedimentary geology, Petroleum geology, Delaware Basin, Diagenesis, Hydrocarbon Migration, Reservoir Quality, Thermal History, Unconventional