at a glance
Integrating sedimentology and sequence stratigraphy
Uranium hosted in poorly consolidated sandstone reservoirs separated by mudstone seals in the Chu-Sarysu Basin caused difficulties in predicting where an injected leaching solution will flow. To tackle this problem, a study was designed combining detailed facies logging of cores with wireline‐log calibration and sequence-stratigraphic correlations across a production field.
Integration is key
Tying key depositional surfaces to global sea-level and climatic shifts enable the development of a predictive framework for both reservoir continuity and seal integrity, allowing operators to more reliably map uranium-bearing sand bodies and plan efficient in-situ recovery.
Overview
The Chu-Sarysu Basin (Kazakhstan) holds some of the largest recoverable resources of uranium in the word. The uranium ore is of the roll-front type and hosted in widespread, poorly consolidated sandstone bodies that are interstratified with mudstone intervals acting as regional seals.
Facies analysis of drill cores and stratigraphic correlation were conducted at the scale of a production field to uncover the facies architecture and infer a depositional model for the succession. Further integration of external forcing mechanisms, such as sea-level fluctuations and climate, led to a predictive sequence-stratigraphic framework that improved reservoir and seal distribution models.
Challenges
In the Chu-Sarysu Basin, uranium is extracted through in-situ leaching, a recovery technique where an acidic solution is injected into the orebody, dissolves the uranium as it passes through the deposit, and is pumped to the surface. This technique relies on continuous circulation of the leaching solution, and is achieved only where the uranium ore is present in a porous reservoir located between impermeable sealing layers.
Extensive geological and hydrodynamic modelling is necessary to fully constrain fluid behaviour in such reservoirs, identify zones of remaining reserves, and plan production scenarios. Modelling challenges include the three-dimensional distribution and variability of ore bodies and requires a sequence-stratigraphic approach to predict reservoir heterogeneities and sandstone body pinch-outs.
Solution
A detailed facies analysis was carried out based on stratigraphic cores intersecting the three reservoir bodies and regional sealing intervals across the production field. Graphic sedimentary logs were generated at 1:100 scale and calibrated to their wireline (resistivity) log response. Key stratigraphic surfaces marking drastic changes in depositional environments were identified in cored wells and correlated to uncored intervals.
The resulting stratigraphic framework was deciphered as a product of external factors driving the evolution of the sedimentary system. Allogenic surfaces are expected to propagate outside the production field and, therefore, the spatial and temporal distribution of ore bodies can be predicted.
Results
Genetically related facies were interpreted to represent four depositional environments: i) fluvial channel belts (the primary target for uranium extraction); ii) floodplains and coastal wetlands; iii) marine embayments; and iv) open shelf (the regional seal). The stratigraphic organisation was controlled by eustatic variations in the Peri-Tethys during the Palaeogene: well-documented episodes of sea-level falls likely prompted fluvial incision and the emplacement of extensive channel belts in the Chu-Sarysu Basin.
Contrasts in facies and architectures between the different reservoir bodies was attributed to the global climatic perturbations of the early Cenozoic. Increased atmospheric humidity and seasonality during the early Eocene generated a complex alluvial record enriched in organic debris from which uranium precipitated
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Or to find out more about this case study:
Dillinger, A., Chanvry, E., Bolat, Y., and Fustic, M., 2024. Architecture and history of uranium-bearing Palaeocene-Eocene strata deposited on the eastern margin of the Peri-Tethys (Chu-Sarysu Basin, south Kazakhstan). Sedimentary geology 468 (2024) 106677.