The Niobrara Formation of north‐east Colorado, USA, has anomalously negative δ18O values compared to all other Cretaceous chalks. These unique δ18O values have been attributed to elevated heat flow and/or freshening of the Cretaceous Western Interior Seaway. This work utilises clumped isotopes of calcite (Δ47), peak burial temperatures estimated from pyrolysis data, and strontium and neodymium isotopes of carbonate to re‐evaluate the origin of the calcite's 18O‐depletion. Peak temperatures indicate lateral variability in geothermal gradients of ca 20°C/km at the tens of kilometre scale, and corroborate prior studies proposing locally elevated palaeotemperatures. Greater insight is provided by numerical models of calcite recrystallisation and oxygen isotope evolution that are constrained by measured Δ47‐derived temperatures, calcite δ18O values and inferences from the 87Sr/86Sr and εNd values. The models indicate that (1) sea water in the seaway had normal marine δ18O values of −1 (VSMOW) except on the eastern margin of the basin where some freshwater dilution yielded −2 to −3‰ (VSMOW) water, and (2) the main driver of the anonymously negative calcite δ18O values was a semi‐open hydrologic system that provided a few percent by pore volume of meteoric groundwater derived from post‐Laramide recharge into the basin. Minor contributions were a Laramide‐aged heat pulse related to the underlying Colorado Mineral Belt, the thermal insulating effects of now eroded coals, and a small flux of compaction‐driven Cretaceous sea water evolved by smectite dehydration. However, those three factors alone were insufficient drivers of the calcites' 18O depletion. High burial temperatures are interpreted to have caused clumped isotope reordering in at least one well, but those temperatures cannot yield the observed calcite δ18O values. The study illustrates the unique attributes of the Niobrara's diagenetic system that results in its anomalous δ18O values, and reaffirms the value of clumped isotopes in unravelling the diagenetic history of chalk systems.
Oil and gas wells in Colorado had a circular spacing regulation that required wells to be spaced 745 ft (227 m) in diameter apart. Some oil and gas wells in the Piceance Basin experience hydraulic communication, which is when the hydraulic fracturing of one well damages the production of a neighboring well. During hydraulic communication, formation water enters the producing borehole at the front of the pressure wave and must be removed before the well is brought back into production. Several wells that have experienced communication have come back on line, but months later, have not recovered their original production rates. It is known that when communication occurs in this scenario there is an increase in the casing pressure of the pumping well of 400 psi (280 meters of head). During hydraulic communication, the bedrock formation experiences long term damage. To test this hypothesis, MODFLOW is utilized to model a generalized two-well system in which a producing well and hydraulic fracturing well are approximately one mile (1,610 m) apart. Modeling is conducted to test how geologic heterogeneity in aquifer conductivity and specific storage, and engineering parameters including well spacing and hydraulic fracturing injection rates affect the change in hydraulic head between the injection well and the pumping well. By changing these parameters in a sensitivity analysis, it is possible to determine which have the greatest effect on hydraulic communication. Overall, the systems with lower hydraulic conductivity, higher specific storage, and increased well spacing have a decreased probability of well-to-well communication in the Piceance Basin. Hydraulic communication in the Basin is due to geologic heterogeneity of the area. However, for impacted wells, hydraulic communication can cause both unexpected shortterm expenses and permanent economic loss. It is therefore critical to understand the appropriate geological parameters for communication minimization, despite a fairly limited number of occurrences thus far. Additionally, a better understanding of hydraulic communication between wells will provide new knowledge for Colorado regulators to reevaluate their current spacing regulations.
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