Core versus cuttings samples for geochemical and petrophysical analysis of unconventional reservoir rocks

Abstract

core samples from petroleum wells are costly to obtain, hence drill cuttings are commonly used as an alternative source of rock measurements for reservoir, basin modelling, and sedimentology studies. However, serious issues such as contamination from drilling mud, geological representativeness, and physical alteration can cast uncertainty on the results of studies based on cuttings samples. this paper provides a unique comparative study of core and cuttings samples obtained from both vertical and horizontal sections of a petroleum well drilled in the canadian Montney tight gas siltstone reservoir to investigate the suitability of cuttings for a wide range of geochemical and petrophysical analyses. the results show that, on average, the bulk quantity of kerogen or solid bitumen measured in cuttings is comparable to that of the core samples. However, total organic carbon (toc) measurements are influenced by oil-based drilling mud (OBM) contamination. Solvent-cleaning of cuttings has been shown to effectively remove OBM contamination in light, medium, and heavy range hydrocarbons and to produce similar kerogen/solid bitumen measurements to that of core samples. Similarly, pyrolysis methods provide an alternative to the solvent-cleaning procedure for analysis of kerogen/solid bitumen in as-received cuttings. Microscopic study substantiates the presence of significant contamination by OBM and caved organic and inorganic matter in the cuttings, which potentially influence the bulk geochemistry of the samples. Furthermore, minerals in the cuttings display induced micro-fractures due to physical impacts of the drilling process. These drilling-induced micro-fractures affect petrophysical properties by artificially enhancing the measured porosity and permeability. Recent advances in horizontal drilling and hydraulic fracturing provide the opportunity for economic hydrocarbon extraction from the tight rocks such as organic-rich mudrock and fine-grained siltstone. The reservoir quality of unconventional tight mudrocks is highly influenced by the deposited minerals and organic matter (OM) macerals present as well as their alteration by diagenetic and thermal maturity processes 1-4. Reservoir and source quality characterization of unconventional tight mudrocks is often conducted on readily available drill cuttings samples using various organic geochemical, petrographic, and petrophysical techniques. However, analytical results of cuttings are often compromised by the use of oil-based drilling mud (OBM), caved fragments from shallower strata, and physical impacts of the drilling process. While drill-core samples are preferred for rock analysis, obtaining such samples from the horizontal section of wells is difficult, and routinely collected cuttings samples 5 are alternatively used for conducting geochemical and petrophysical analyses. The objective of this study is to compare the organic geochemistry, petrology, and petrophysics results of core samples versus drill cuttings obtained from closely correlated depth intervals in a...

Oil contamination of sediments by freeze-drying versus air-drying for organic geochemical analysis

“…1 ). This is similar to Rock–Eval results on petroleum source rock samples that contain heavy hydrocarbons and bitumen due to lower maturity or are contaminated by drilling additives (Abrams et al, 2017 ; Jiang et al, 2019 ; Sanei et al, 2020 ).

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Sulphur isotope stratigraphy of drill cuttings and stratigraphic correlation of Permian-Triassic evaporites

“…The δ 34 S evap correlation presented in this study is surprising in that the 42/28-2 sediments have been exposed and saturated with drilling fluids: unfortunately, there is no record of what drilling fluids were used when coring. Samples can be fully covered in fluids during the drilling process and washing them away entirely is difficult (Kubo et al, 2016), thus potentially impacting the integrity of sample geochemistry (e.g., Stuckman et al, 2019;Sanei et al, 2020). δ 34 S evap values in drill cuttings are vulnerable to contamination by drilling fluids, which can be characterised by a range of different chemical compositions (Ball et al, 2012).…”

“…However, recent studies have demonstrated the validity of drill cuttings for carbon isotope stratigraphy, in part through comparison with δ 13 C records from cored intervals and outcrop (Metzger et al, 2014;Eldrett et al, 2021). Sanei et al (2020) demonstrated that with sufficient cleaning of drill cuttings, total organic carbon (TOC) values are comparable to those directly obtained from cored intervals. To our knowledge, only a single study has demonstrated the suitability of borehole cuttings for sulphur isotope stratigraphy (Cao et al, 2016), however, the authors did not demonstrate the validity of drill cuttings for evaporitic lithologies.…”

Multi‐elemental chemostratigraphy of Triassic mudstones in eastern Svalbard: Implications for source rock formation in front of the World’s largest delta plain

“…However, mudstone successions are rarely cored during wellsite operations and source rock studies are thus commonly based on drill cuttings (Mansour et al, 2020; Rosenberg et al, 2021; Silva et al, 2017). While drilling mud contamination may pose a serious issue for organic geochemical characterisation of cuttings (Sanei et al, 2020), its influence on the elemental assemblage is generally negligible (Craigie, 2018). Elemental studies of drill cuttings have been shown to be highly useful in mapping chemostratigraphic zones (Craigie, 2015; Wright et al, 2010) and for multilateral well steering of shale plays (Hildred, 2012; Hildred et al, 2011; Zhang et al, 2019).…”