Characterization of organic matter fractions in an unconventional tight gas siltstone reservoir

Ardakani

Akai

et al. 2020

Sci Rep

Self Cite

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...

Section: Resultsmentioning

confidence: 99%

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

Ardakani

Akai

et al. 2020

Sci Rep

Self Cite

“…7). Rocks along these stratigraphic trends generally have lower organic matter content (dominantly solid bitumen) than adjacent regional strata1617, and therefore likely have lower total sorptive capacity. Low total sorptive capacity along these stratigraphic trends would have led to reduced sorptive retention time and further acceleration of methane migration.…”

Section: Discussionmentioning

confidence: 99%

“…Basin modelling suggests that the Montney tight-gas fairway passed into the thermogenic hydrocarbon generation window ∼80–90 Ma (million years ago)13. Present-day total organic carbon, typically in the range from 0.25 to 4.0 wt%, is virtually all in the form of solid bitumen/pyrobitumen14151617. The solid bitumen represents a previous oil phase that partially filled the pore network of Montney siltstones during hydrocarbon charging1617.…”

mentioning

confidence: 99%

Secondary migration and leakage of methane from a major tight-gas system

Wood

2016

Nat Commun

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Tight-gas and shale-gas systems can undergo significant depressurization during basin uplift and erosion of overburden due primarily to the natural leakage of hydrocarbon fluids. To date, geologic factors governing hydrocarbon leakage from such systems are poorly documented and understood. Here we show, in a study of produced natural gas from 1,907 petroleum wells drilled into a Triassic tight-gas system in western Canada, that hydrocarbon fluid loss is focused along distinct curvilinear pathways controlled by stratigraphic trends with superior matrix permeability and likely also structural trends with enhanced fracture permeability. Natural gas along these pathways is preferentially enriched in methane because of selective secondary migration and phase separation processes. The leakage and secondary migration of thermogenic methane to surficial strata is part of an ongoing carbon cycle in which organic carbon in the deep sedimentary basin transforms into methane, and ultimately reaches the near-surface groundwater and atmosphere.

“…In the second case, primary-oil solid bitumen is derived from the early maturity crude oil that is expelled and possibly migrated further into the tight and/or conventional reservoir rocks (e.g., Lower Triassic Montney tight gas reservoir 13,18,19,51 ). Crude oil is a colloidal cocktail that can readily dissociate into different fractions (including asphalt) and that these fractions then mature differently with increasing thermal stress.…”

Section: Diagenetic Solid Bitumen (Degraded Bituminite)mentioning

confidence: 99%

Genesis of solid bitumen

2020

Sci Rep

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This paper presents a new schematic model for generation and timing of multiple phases of solid bitumen throughout the continuum of organic matter maturation in source and tight reservoir rocks. Five distinct stages in the evolution of solid bitumen are proposed: (1) diagenetic solid bitumen (or degraded bituminite), which is not a secondary maceral resulting from the thermal cracking of kerogen. Instead it is derived from degradation of bituminite in the diagenesis stage (Ro < 0.5%); (2) initial-oil solid bitumen, is a consolidated form of early catagenetically generated bitumen at the incipient oil window (Ro ~ 0.5–0.7%); (3) primary-oil solid bitumen is derived from thermally generated bitumen and crude oil in the primary oil window (Ro ~ 0.7–1.0%); (4) late-oil solid bitumen (solid-wax) is derived from the waxy bitumen separated from the mature paraffinic heavy oil in the primary- and late-oil windows; and (5) pyrobitumen, which is mainly a non-generative solid bitumen, is evolved from thermal cracking of the remaining hydrocarbon residue and other types of solid bitumen in the dry gas window and higher temperature (Ro > 1.4%). This model shows concurrence of multi-populations solid bitumen with oil, bitumen, and other phases of fluid hydrocarbon residue during most of the maturity continuum.