U-Pb age and formation mechanisms for calcite-filled fractures of the First Bone Spring carbonate debris flows, Delaware Basin, New Mexico

The Kungurian Age (middle Permian) First Bone Spring Formation of the Northern Delaware Basin of southeastern New Mexico and West Texas is comprised of carbonate debris flows and siliciclastic turbidite deposits. Calcite features such as fracture fillings and bioclasts are found throughout the carbonate debris flows. Samples of these features were examined and sampled from the Big Eddy Unit 14 Federal SWD No 1 well provided by ExxonMobil. The fractures found in these deposits were of particular interest because the timing and origin of fracturing events may constrain estimates for subsidence rate and timing of oil generation in the basin. Five major fracturing mechanisms (and timing related to the burial history of the First Bone Spring Formation) considered for this study include: 1) disequilibrium compaction (early), 2) aquathermal expansion (early to late), 3) oil generation (late), 4) clay dehydration (late), and 5) tensional fracturing related to tectonics (variable timing depending on specific tectonic events). In order to evaluate oil generation and clay dehydration as potential fracturing mechanisms, analyses were performed using HAWK rock pyrolysis, X-Ray Diffraction, and Fourier Transform Infrared spectrometry. For assessment of the remaining mechanisms, data were drawn from previous research done in the Delaware Basin. Results from HAWK rock pyrolysis and FTIR analysis indicated significant presence of organic material with the HAWK analysis recording an average TOC at 2.72 wt. % and thermal maturity ideal for oil generation. XRD results confirmed the presence of illite-smectite clays with percentages reaching thirty percent for the carbonate debris flows. These results support the possibility of oil generation and clay dehydration as potential fracturing mechanisms for the First Bone Spring carbonate debris flows. U-Pb ages of the calcite filled fractures and bioclasts were measured using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. The resulting ages reported from this analysis were interpreted and separated into three age populations. Each age population was then placed within a depositional model of the Delaware Basin. Potential fracturing mechanisms assigned to the age populations are based on the burial depths and timeline of geologic events of the basin model. The depositional age of the First Bone Spring was obtained from U-Pb ages of ash bed zircons in the core and was used for a reference date used to compare with the calcite age populations. Calcite Age Population One (comprised of two separate calcite fractures) has measured U-Pb ages of 274. 5±1.8 Ma and 272.5±4.4 Ma which places the formation of this population shortly after deposition (275.0±1.7 Ma, from ash bed zircon U-Pb ages). Due to the near depositional age, the fracturing mechanism of population one may be disequilibrium compaction. Age Population Two (comprised of four separate calcite filled fractures and one fluid-altered bioclast) has a combined U-Pb age of 263.2±1.0 Ma – over ten million years after deposition. This age population thus may record the subsidence of the First Bone Spring Formation into the oil window (substantiated by organic inclusions found in fracture fill). The final age population (comprised by a single sample) had a measured U-Pb calcite age of 249.4±4.8 Ma – close to twenty-five million years after deposition. This final age population likely records episodic oil generation or clay dehydration events as the Formation would have remained buried at sufficient depths for these processes to continue to take place. By anchoring burial history models with U-Pb zircon and calcite ages and relating these ages to geologic events, more substantiated economic and scientific predictions can be made for basins of interest – not only the Delaware Basin.