Quantitatively Unlocking Lithofacies Utilizing High Resolution LWD Image, Core and Petrophysical Data for a Tectonic-Controlled Fan Delta Formation, Bohai Bay Basin, Offshore North China

Abstract

Abstract Lithofacies plays an important role in controlling reservoir quality, distribution, and prediction. Understanding and exploring the best reservoir lithofacies within tectonic-controlled fan delta formations can be challenging, as the multiple depositional episodes and lithofacies variation cannot be clarified by conventional logs alone. Quantitative characterization of high-resolution lithofacies variation, formation heterogeneity, grain-size, porosity distribution, and the vertical sedimentary sequences is critical for predicting the best reservoir facies and their spatial distribution across the basin. The target area is confined by cross boundary faults and the formation comprises stacked, multi-layered, sharp-based conglomerates and sandstones with rapid vertical variation in facies. Mud logging typically provides only a very basic lithology profile due to the biased sampling and mixing of grain sizes during cuttings retrieval. The integration of high resolution LWD resistivity image data and core data are employed to characterize and refine the fan delta lithofacies. Firstly, grain size and its variation are quantitatively calculated from the high resolution LWD resistivity image with supporting information from cuttings. Then, the sedimentary structure (bedding fabrics) observed on the image are combined with this information to characterize the lithofacies. In total, eight lithofacies are sequentially classified from coarsest to finest: massive conglomerate, laminated sandy conglomerate, laminated gravel fine sandstone, cross-bedded sandstone, massive sandstone, amalgamated sandstone, slumped sandstone and laminated siltstone. Finally, the petrophysical properties (effective porosity determined from conventional logs) of different lithofacies is analyzed and compared to demonstrate reservoir quality grade and, from a geological perspective, the factors controlling the reservoir quality grade of the lithofacies are determined. The result shows the cross-bedded sandstone and amalgamated sandstone lithofacies hold the best reservoir quality; lesser reservoir quality is observed for the laminated siltstone and massive sandstone lithofacies, followed by the slumped sandstone and laminated sandy conglomerate. The poorest reservoir quality is associated with laminated conglomeratic fine sandstone and massive conglomerate. Quantitatively characterizing lithofacies through both grain size content and sedimentary structure allows the understanding of vertical depositional sequences of lithofacies and their development and aids the prediction of reservoir distribution laterally. By utilizing such information from both the geological fabric and corresponding petrophysical properties of the formation, identifying lithofacies with particular reservoir qualities can be a highly efficient means of making decisions on developmental strategies, including optimization of testing-zone selection and future well planning and placement.