Lithofacies, Depositional Environment and Stratigraphic Architecture of the Helvetian Reservoir Units, Western Getic, Romania

Abstract The Helvetian Formation represent the Upper Burdigalian deposits, along the Getic Depression, in Romania. It is marked by a transgressive marine phase and represents the main reservoir interval in a southern Romanian field. The scope of this paper is to understand the geometry and architecture of the Helvetian reservoirs, due to the high vertical and lateral variations of alternating thin sandy layers with shale intervals. Around fifty (50) wells were used, with a total of 143 m of core through Burdigalian to Sarmatian periods, only one well with 21 m of core well preserved, to allow a detailed facies analysis. This core reveals a number of lithofacies which control the reservoir distribution in the area. The sedimentological conceptual model is based on lithofacies characterization, vertical facies distribution, all supported by core and log data. Modern analogues are included to forecast the architecture and geometries of the depositional elements. Twenty-one (21) descriptive lithofacies are recognized for the entire cored section. Overall, seven (7) depositional sub-environments are distinguished: alluvial/fluvial deposits, lacustrine, distributary channel fill, mouth bar, interdistributary area, shoreface and offshore. The most relevant reservoir facies are related to wave dominated mouth bars and distributary channel deposits. Using AVA Clastics database, the elements, such as mouth bars, have an average width of 3800 m, long of approximately 6,300 m, with a thickness around 3.5 m. Carbonates, chlorite, feldspars and bioturbation have modified reservoir properties, but roundness and sorting, grains dissolution and local fractures contribute to enhancing the reservoir quality. The porosity spans between 3 to 22% and the permeability up to 309 mD. The 3D facies model was used to build depositional facies maps which are relevant to illustrate the depositional elements architecture and their distribution in the field. The facies concept provides a predictive distribution of reservoir properties, which often correlates with production history in the area. Reservoir data, along with well logs and core data, helped building a facies evolution, in order to reveal the optimal field development scenarios. This is a key element of the control of the oil and gas saturation distribution per reservoir units. The study brings new approach when analyzing mature fields, integrating old and new data and predicting stratigraphic architecture from sedimentological concept to facies model. Lithofacies, depositional environments and body geometries of the Helvetian reservoir units are guide to further development of the field.