CLUSTER ANALYSIS OF THE BALAKHANY VIII RESERVOIR UNIT WITH SPECTRAL GAMMA RAY LOGS AZERI-CHIRAG-GUNASHLI FIELD, OFFSHORE AZERBAIJAN
Surek, Melahat Asli 1988-
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The objective of this research was to discriminate the source of reservoir sediments coming from different paleo-river systems, both in space and time, to infer the provenance of the sediments. Spectral gamma ray logs were used to identify different compositional suites of sediments vertically and laterally that could affect the multiple sources of Balakhany VIII sands of the South Caspian Basin (SCB) and the impact of those compositional suites on reservoir quality. Three radioactive elements, uranium, thorium, and potassium, all related to different mineralogical suites, were used to test how these affect effective porosity and permeability. The Lower Pliocene Productive Series containing thick fluviodeltaic rocks are the major hydrocarbon reservoirs in the Azeri-Chirag-Gunashli field. Basin isolation, uplift of the basin margins, and fall of base level resulted in the discharge of several river systems into the SCB; most notably the paleo-Kura, paleo-Volga, and paleo-Amu-Darya. The Balakhany Suite is part of the Productive Series, and the Balakhany VIII, which is the focus of this study, differs from other sub-units by the prevalence of fluvial sediments facies. Cluster analysis was used to understand the distribution of spectral gamma ray log elements. Vertical and lateral clusters from different wells among the study area were used to infer the different petrofacies and the source of the reservoir sediments. It is important to understand the mineralogical preferences of Th, U, and K in relation to deposition, transport, weathering, and diagenesis. In contrast to previous studies, the results of this study show the dominant sediment contributors are the paleo-Amu-Darya and paleo-Uzboy Rivers, rather than the paleo-Volga as previously though. The dominant systems brought sediments from the eastern side of the study area from outcrops of sediments within the Balkhan, Kopet Dag, Pamir and Tian-Shan mountain ranges. Major petrofacies are established by the clustering routines. Results of 13 wells giving consistent results for sands and all lithologies showing dominant sediment contribution from eastern side. Sand-rich petrofacies A having better reservoir qualities are observed mostly in the western side of the study area, and is derived from N-S draining paleo-Volga. The shalier petrofacies B, having less but good porosity and permeability averages and higher K average, is derived from eastern mud-dominated sediment contributors, the paleo-Amu-Darya and paleo-Uzboy rivers.