Knowledge of structure, composition and texture, the three fundamental attributes of sedimentary deposits, is necessary for a robust analysis and interpretation of depositional environment. In the absence of core information; borehole images, "elemental-capture spectroscopy" and NMR logs all contribute to understanding the structure, composition and grain-size attributes. Grain-size is considered the most important textural parameter because it reflects the processes and energy levels active at the time of deposition. Early studies show that depositional processes and environment can be inferred from size distributions (Krumbein1934 and Sahu1964).These inferences are based on deductions and experimental data and deliver acceptable accuracy. Although Krumbein (1941) shows other textural (e.g. shape, roundness, roughness) and compositional information enhances certainty.).
Building on early work, we present a methodology and examples where NMR derived grain-size-distributions are used to infer depositional environment. In contrast to spot-core analysis, NMR provides a continuous along hole profile of grain-size-distributions. This study is the first to utilize a continuous grain-size-distribution profile. The study was conducted blindfolded, without initial reference to core studies, to test the robustness of the methodology.
By using statistical parameters from grain-size distributions that are characteristic of depositional environments, and applying Sahu (1964) linear-discrimination functions, depositional systems for two wells were inferred to be shallow marine-to-fluvial deltaic. Linear-discriminate analysis of geostatistical variables showed bimodal distributions of sediments dominated by fine-grain sands and silts. The studied sandstones were concluded to be mainly fine grained, moderately to poorly sorted, fine skewed, mesokurtic, leptokurtic occasionally platykurtic in nature.
Our conclusion was "shallow-marine deposition" and was compared and confirmed with core-driven studies as "shallow marine". In summary, profiles of grain-size-distributions from NMR logs provide important information about changes in depositional energy level from which we infer depositional setting and reservoir quality. A precise depositional environment interpretation is crucial for optimal/economical field development.