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Method and Data-Acquisition Workflow for Rock-Type Definitions in the Lower Carboniferous Resource Play, NW England

Abstract

The Lower Carboniferous shales of Northern England are currently under the early stages of exploration. Lithological heterogeneity, both lateral and vertical, is a major challenge to prediction of reservoir and completion design. We here present a method and a workflow for data acquisition and effective characterization of rock types based on data collected from two newly drilled exploration wells. The two exploration wells penetrated the Namurian section before terminating in the Visean Carboniferous Limestone Supergroup. A comprehensive data-acquisition program was carried out to fully characterize the lower Namurian section and the underlying prospective Visean limestone facies. The wireline log-suite acquired included elemental spectroscopy logs, dipole sonic and image logs, in addition to standard wireline logs. In both wells, sections of the Namurian stratigraphy were cored and a comprehensive work program was conducted including shale source-rock specific petrophysical core analysis, geomechanical studies and source-rock evaluation as well as XRD and XRF data acquisition. XRD and XRF measurements on rotary side wall cores and cuttings were also conducted. Use of rigorous mathematical solutions allows us to simplify the range of lithologies and to develop data-derived rock type characteristics. This approach, combined with facies analysis and depositional system modelling, also allowed us to predict rock properties and to selectively sample the formations. A total of 10 rock types were defined ranging from calcareous, quartzose/siliceous, mixed and to argillaceous types. The rock types exhibit a strong stratigraphic distribution. The distribution reflects facies changes related to different depositional systems and the large-scale controls on those systems. Understanding this relationship can assist in the future with subsequent lithostratigrahic correlations and enhanced horizontal well placement, once additional data are acquired on existing and new wells. The prospective parts of the stratigraphy that have optimal properties including high TOC and brittleness have been related to specific rock-type ranges. The essential compositional characteristics for these rock types have been defined, facilitating identification via multi-element fingerprinting. Future work will continue to improve the model and develop a robust framework for assessing lithological heterogeneity and geomechanical properties of the Lower Carboniferous resource play.