Depositional Environment Interpretation Using NMR and Core Data

Abstract This study is based on data from Gulf-of-Mexico (three wells) and West Siberia, Russia (two wells). We find a clear correlation between the NMR signal and depositional environments that also were identified on conventional log diagrams and in cores. Key sediments associated with the regressive bar and coastal fluvial sand sequences (distributary mouth bar, distal bar, delta-front, and pro-delta facies) were defined by predominant type and distribution of porosity and recognized on NMR diagrams. Corresponding distribution of total and effective porosity, permeability and amount of capillary bound water characterized the major depositional environments. Seven rock types (lithofacies) were defined using statistical analysis of porosity/permeability laboratory data, SEM and thin-section description and mercury injection results; four of them were clearly correlated to various shapes of NMR response. The mixed type of porosity (associated with kaolinite cement) and interbeds with carbonate concretions (hardstreaks) were not recognized. The dissolution of feldspar grains in distal mouth bar facies results in significant enhancing effective porosity and permeability. An abnormally height porosity values (higher than 26 pu.) were formed as the result of feldspar and rock fragments total dissolution. Partial dissolution produces a micro-porosity that contains bound water. Results from both processes were recognized on NMR T2 distribution diagram. A dual porosity system was also identified on porosity-permeability cross-plot as two parallel areas. NMR signals characterize the spectral porosity distribution and therefore could be used for various facies differentiation. It provides a unique information on diagenetic alteration especially about total or partial grains dissolution. Best results on quantitative interpretation could be achieved by combining analysis of core data (SEM) and NMR logs.