Petro-velocity crossection of the Donetsk coal basin

Abstract

Introduction: recently, seismic exploration has gained increasing importance for the detection of hazardous methane accumulations in coal deposits. Reliable identification of such accumulations requires sufficiently accurate knowledge of the propagation velocity of elastic longitudinal waves in the rock (V_p) and its skeleton (V_p.sk), and elastic transverse waves (V_s). Despite extensive research conducted in previous decades, the parameters V_p.sk and V_s remain largely insufficiently studied. Therefore, to provide a petrophysical basis for seismic exploration, a comprehensive study of petrophysical velocity characteristics and their variation across the composite stratigraphic section of the basin has been carried out.

Methodology: the composite velocity section was constructed by stacking partial sections according to the maximum pre-inversion burial depth of rocks (H_max). The parameter H_max was determined based on coal quality indices; V_p was obtained from acoustic logging; V_p.sk was calculated using the Wyllie equation; V_s was derived from correlation with V_p. To substantiate the observed regularities in the variation of petrophysical velocity parameters, data on the structural and mineralogical features of clastic and argillaceous rocks were employed.

Results and discussion: a petro-velocity crossection of the coal basin was developed in the form of a family of curves describing the variation in total porosity, mineral density, secondary quartz content, fractured and cleaved grains, as well as V_p, V_p.sk, and V_s as functions of H_max. The causes of velocity inversion in clastic rocks and the increase in velocity parameters in argillaceous rocks under constant porosity conditions were identified.

Conclusion: a petro-velocity crosssection has been constructed for all granulometric varieties and post-diagenetic transformation stages of coal-bearing deposits in the Donbas region. The factors responsible for post-diagenetic increases in velocity parameters were clarified, namely porosity reduction and strengthening of the rock framework. It was established that velocity inversion at the boundary between stages 2G and 3G is caused by an increase in V_p.sk, resulting from the extension of clastic grain contacts below this boundary. It was also demonstrated that the post-diagenetic increase in V_p.sk of argillites at constant porosity is due to the strengthening of contacts and an increase in the mineral density of clay particles.