Experience of applying the petro-velocity crossection of the Donetsk coal basin to the search for abnormal fracture-type methane accumulations
DOI:
https://doi.org/10.17308/geology/1609-0691/2026/1/84-95Keywords:
coal basin, petro-velocity crossection, abnormal fracture-type methane accumulation, total porosity, fracture porosity, velocity characteristicsAbstract
Introduction: abnormal methane accumulations, which pose a significant hazard during coal mining and represent a potential unconventional source of methane, are often overlooked during geological exploration but can be identified through seismic surveys. To effectively apply seismic methods, it is necessary to have background velocity parameters – characteristics of undisturbed water-saturated rocks — which can serve as a baseline for detecting gas-bearing fracture-type reservoirs through reductions in these parameters. Background petrophysical-velocity parameters for all stages of post-diagenetic rock transformation are represented in the petrophysical-velocity section of the coal basin. Therefore, in order to provide seismic exploration with the necessary velocity data, a study was conducted on the use of this section for detecting abnormal methane accumulations.
Methodology: the research methodology involves extracting background velocity values from the petrophysical-velocity section and adjusting them to correspond to the conditions of fractured and gas-saturated rocks. Calculations of velocity characteristics for rocks in various states were based on the results of petrophysical and seismic studies of tectonic faults, as well as data obtained from a high-pressure chamber simulating natural rock conditions.
Results and discussion: formulas were developed for calculating velocity characteristics of rocks under different combinations of fracturing, water saturation, and gas saturation. Using the hanging wall of the Grigoriev Thrust as an example, the procedure for applying the petrophysical-velocity section to construct a physical-geological model of an abnormal methane accumulation is demonstrated. The plausibility of the proposed model was confirmed by comparing it with the results of acoustic logging of fault zones. It was established that the reduction in longitudinal wave velocity caused by fracturing amounts to 36–40% in the model, and 30–50% according to acoustic logging data.
Conclusion: a petrophysical approach is proposed for identifying abnormal fracture-type methane accumulations based on the propagation velocities of elastic longitudinal and shear waves. The approach relies on the background physical property values derived from the petrophysical-velocity section and their transformation according to the effects of fracture porosity and gas saturation. The plausibility of the physical-geological model of an abnormal methane accumulation is confirmed by the consistency of its results with acoustic logging data from fault zones. The petrophysical-velocity section constructed for all stages of post-diagenetic rock transformation in the Donbas enables the identification of abnormal fracture-type methane accumulations across the entire coal basin.
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References
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