Chemostratigraphic Delineation of Sedimentary and Igneous Lithologies During Drilling without MWD Measurements

Abstract The identification of igneous basement rocks from ditch-cuttings samples can be challenging, due to cuttings sizes and textures generated by PDC-bits and turbine drilling. Thus, basement rocks are sometimes mistakenly categorized as fine-grained sedimentary lithologies. With the absence of measurement while drilling (MWD) logs, basement identification becomes very challenging. Advanced mudlogging data, i.e. X-ray fluorescence (XRF; elements) and X-ray diffraction (XRD; mineralogy), are capable of overcoming this challenge when understood properly. Geochemical data (XRF) are used to model sedimentary lithologies (element ratios, proxies, and simulated gamma ray logs). The data are further linked to underlying mineralogy (XRD and element:mineral proxies). Geochemical indices and provenance discrimination function analysis logs and diagrams indicate different rock types, e.g. sedimentary versus igneous. Both, geochemistry and mineralogy are then matched to standard igneous rock compositions. Additional methods, if available, such as petrography of cuttings, can be integrated and increase confidence in the chemostratigraphic interpretation. Post-drilling analyses, e.g., wireline logs, image logs, and sidewall cores will verify the results or refine the model. Standard mudlogging cuttings geological descriptions from a well drilled without MWD support were ambiguous and incapable of identifying basement rocks while drilling. Well-site generated XRF and XRD data interpretations (chemostratigraphy) indicated different sedimentary lithologies, as well as penetration of mafic igneous rocks. Petrography on cuttings thin-sections agrees with the geochemical/mineralogical findings. Subsequently, a bottom core (diorite) was cut and the well was terminated. Post-drilling petrography and lab-based XRD on sidewall cores, as well as wireline logs confirmed the modelled lithology types and interpreted basement rocks from the cuttings samples. This geochemical approach has successfully been applied to several wells (with and without MWD) encountering both, mafic and felsic igneous basement rocks. Challenges, however, are encountered with the presence of a weathering zone above the basement complex or sedimentary rocks derived from an adjacent basement complex, which show very similar geochemical signatures. Nonetheless, scanning electron microscopy petrography on cuttings verified the initial chemostratigraphic interpretation. The identification of basement rocks while drilling without (risk minimalization or cost optimization) and even with MWD measurements is often challenging, if not impossible. Interpretation of advanced mudlogging XRF/XRD data (low risk, cost-efficient) lowers the uncertainties in recognizing the penetration of basement rocks that agrees well with post-drilling analyses and thus assists in determining the well’s terminal depth (TD).