Genesis of cenomanian gas reservoirs (Yamal-Nenets autonomous district)

The authors used the proprietary methodology based on an integration of pyrolysis results and material balance to evaluate oil and gas generation potential of the Organic Matter making a part of Lower Cretaceous deposits and Bolshekhetsky formation sediments. The studies were conducted on core samples taken from one well in the Medvezhy and two wells in the Urengoi fields. The specific volumes of the naphtides formed and the pore space of time-equivalent reservoirs are compared. The following conclusion was drawn: the amount of hydrocarbons generated by the Lower Cretaceous deposits is insufficient for formation of oil and gas accumulations in these sediments, and especially for gas pools formation in Cenomanian deposits. The obtained results of evaluation of the Jurassic sediments oil and gas generation properties showed that they are the main oil and gas source rocks, while the Lower Cretaceous deposits are predominantly reservoirs. The authors discuss the options of biogenic and abiogenic mechanisms of formation of methane that filled Cenomanian sediments. To substantiate the organic genesis of methane, the results of investigations of foreign scientists showing the possibility of methane generation at the expense of the methoxy groups in lignite (brown coal) were used. Abiogenic mechanism of methane production is based on the results of investigations by volcanologists, as well as data on isotopic signature of carbon, which is a part of methane in Cenomanian pools. Evidence has been presented to prove the possible presence of abiogenic (endogenous) methane in Cenomanian sandstone. Based on tectonophysical modelling using optical-polarization and tectonosedimentation methods, the possible mechanism of gas reservoir formation in Cenomanian sediments of the Pokur Formation is reconstructed. Predicted zones of compression and decompaction in modelled sediments are identified. The authors recommend that the method of seismic data integration with tectonophysical modelling carried out using them should be applied to predict hydrocarbon reservoirs