New Insights Into the Understanding of Sand Injectite Complexes, Using Advanced Log Data, Ultradeep Resistivity Inversions and Outcrop Field Observations

A sand injectite complex forms when the internal pore pressure of the parent sand body exceeds the pressure in the surrounding mudstones, leading to hydraulic fracturing and remobilization of the sand, which moves horizontally (as sills) and vertically (as wings and dykes) into the surrounding formations. Field A, located in the Norwegian North Sea, is part of a sand injectite complex sourced from sand-rich turbidite deposits. While the exact cause of sandstone injection in Field A is unclear, similar injectite complexes occur within the offshore basins of the United Kingdom and Norway, indicating they are common within the geological record. Key challenges in accessing this hydrocarbon reservoir include significant variations in rock properties over short distances and poor seismic imaging of thin sills and dykes within the injectite complex due to the resolution of the available seismic data and the inherent complex geometries, which may hide important pay zones. A recent drilling campaign took place in Field A, where three lateral production wells targeted different sand bodies within the injectite complex. This case study demonstrates the use of advance logging-while-drilling (LWD) data to further understand the internal facies architecture of the injectite geobodies. The integrated approach used in this study included three main steps, comprising real-time log interpretation, ultrasonic image interpretation, and estimation of the net thickness, area, and volume of identified geobodies. By using an advanced LWD data set and integrated interpretation workflow, it was possible to map sand injectite geobodies using ultra-deep azimuthal resistivity (UDAR) and conventional log data. High-resistivity intervals correspond to hydrocarbon-bearing sills or dykes. However, low-resistivity intra-sill and intra-dyke mudstones also occur. Ultrasonic image log interpretation showed three different types of image textures within the injectite complex that can be linked to core and outcrop observations: laminated to non- laminated intervals that correspond to overburden mudstones; non-laminated, massive sand intervals with significant high-frequency borehole spirals that are interpreted to be sill and/or dyke sands; and intervals that contain angular clasts that are interpreted to be injectite breccia, which form when mudstone clasts become entrained in the injected sands. The reservoir quality of injectite breccia is highly dependent on the amount and distribution of the mudstone clasts. In addition, the interpretation was further integrated with 3D ultra-deep resistivity inversions, which identified three types of high resistivity features: U, V and W shapes), could also be interpreted as conical shaped and irregular based bodies. Lateral, high-angle discordant contacts, high- angle faults, and high-angle fractures were also interpreted. This study provides an understanding of the internal architecture and lateral geometrical connectivity in an injectite complex, from micro-scale (image log) to meso- scale (1D and 3D UDAR) and macro-scale (outcrops). This work will be used in future modeling exercises to aid in the planning of wells within injectite complexes and to improve geosteering.