The determination of sedimentary environment and associated energy in deep-buried marine carbonates: insights from natural gamma ray spectrometry log

It has always been challenging to determine the ancient sedimentary environment and associated energy in deep-buried marine carbonates. The energy represents the hydrodynamic conditions that existed when the carbonates were deposited. The energy includes light and chemical energies in compounds and kinetic energy in currents and mass flow. Deep-buried marine carbonates deposited during the Ordovician depositional period in the eastern Tarim Basin result from a complex interplay of tectonics, sedimentation, and diagenesis. As a result, determining the ancient sedimentary environment and associated energy is complex. The natural gamma-ray spectrometry (GRS) log (from 12 wells) is used in this paper to conduct studies on the sedimentary environment and associated energy in deep-buried marine carbonates. The findings show that the values of thorium (Th), uranium (U), potassium (K), and gamma-ray without uranium (KTh) in a natural GRS log can reveal lithological associations, mineral composition, diagenetic environment, stratigraphic water activity, and ancient climatic change. During the Ordovician, quantitative analysis and determination of sedimentary environment energy are carried out using a comprehensive calculation of natural GRS log parameters in typical wells (penetrating through the Ordovician with cores and thin sections) of well GC4, well GC6, well GC7, and well GC8. The results show that GRS log can determine different lithology associations in typical wells than a sieve residue log. Furthermore, cores and thin sections can be used to validate the determination of lithology associations. Based on the determination of lithology associations, the lithology associations that reflect the sedimentary environment and associated energy can be analyzed in a new approach. Furthermore, the sedimentary environment energy curve derived from a natural GRS log can reveal hydrodynamic fluctuations during depositional periods, which will aid in the discovery of carbonate reservoirs, establishing sequence stratigraphic frameworks, and the reconstruction of sea-level changes in the future.