Source Rocks

Source rock quality is vital for any conventional or unconventional exploration project. It reveals whether suitable organic matter existed in adequate amounts to generate oil, gas or both in a basin or exist that can be pyrolyzed to yield petroleum. Source rocks include shales, marls, limestones (carbonates) and coals.

TOC (reported in wt %) is analyzed by combustion after removal of carbonate carbon using a Leco Analyzer. The total carbon (TC) and total inorganic carbon (TIC) can also be reported. The TOC can enable you decide on whether to continue an exploration project or not.

This is a rapid method for further assessment of conventional petroleum source rock quality. It provides further information of the amount of free bitumen (S1) in the sample, the kerogen quantity (S2), the kerogen type and the maturity. Parameters reported include:

S1(mg HC/g rock) – hydrocarbons released during isothermal heating at 300°C and measured by the flame ionization detector (FID)

S2 (mg HC/g rock) – hydrocarbons from decomposition of kerogen between 300°C – 600°C with a thermal gradient of 25°C /min

S3 (mgCO2/g rock) – carbon dioxide evolved between 300°C – 390°C and measured by the thermal conductivity detector (TCD).

Tmax (°C) – The temperature at which maximum decomposition of kerogen occurs during pyrolysis.

The measured parameters enable calculation of additional parameters like the hydrogen index (HI), oxygen index and production index that are also reported.
Rock-Eval analysis of cuttings or rock samples can also be used to highlight possible reservoirs and assess likely gas type even before gas samples are obtained.

Visual kerogen enables analysis of macerals in mineral matrix-free source rocks. This allows for determination of the proportion of liptinite, vitrinite and inertinite. The relative proportion of the macerals are characteristic of oil or gas prone source rocks. The colour of the particles like spores are useful indicators of the source rock maturity (spore coloration index/thermal alteration index). Actlabs has the right expertise to provide highly accurate data from visual kerogen analysis including representative photomicrographs.

Vitrinite originates from woody tissues in plants and it shows a linear response to temperature during evolution organic matter, defining maturity of source rocks from the immature to the post mature stage. This classical maturity parameter is very useful in constraining palaeotemperatures for basin modelling. Data from vitrinite reflectance highlights the stage of evolution of organic matter indicating the presence or absence oil and gas in a basin. Actlabs provides reliable vitrinite reflectance data from measurements of at least 50 points (when present) under oil immersion.

For unconventional resources, about 500 g of rock is pyrolyzed to yield oil and gas for assessment of the potential of the source rock using ASTM D3904. Parameters reported from the assay include free water and moisture %, retort water %, oil yield weight and volume, oil density, spent shale% and average molecular weight of gas. The oil yield and gas yield from Fischer assay is strongly related to the generation potential derived from the Rock-Eval.

Molecular Geochemistry

Gas chromatography (GC-FID) provides detailed information on the composition of the organic matter in source rocks. The information is used to assess the organic matter quality. To eliminate interference on parameters useful for complete characterization of source rocks, Actlabs offers high resolution gas chromatography -mass spectrometry (HR-GC-MS). This enables characterization of the kerogen type, conditions during deposition (e.g. redox), maturity, and correlation. Actlabs classic source rock GC-MS package provides data required for complete source rock characterization through our exploration and production packages reporting molecular composition data and ratios including n-alkanes, acyclic isoprenoids, terpanes, steranes (regular, diasteranes, mono and triaromatic) and phenanthrenes.

Other ratios from diamondoids, benzothiophenes, naphthalenes can be reported through a customized package.

Stable isotope data provide insight into the organic matter type contained in source rocks. These data can help in source rock facies delineation, organic matter evolution and source rock correlation to oils and gas.

Actlabs offers bulk C, S and N analyses for conventional and unconventional petroleum source rocks including clastics, carbonates and coals. Carbon isotope analysis for the saturate and aromatic fractions of source rocks commonly used for distinguishing marine and terrestrial sources is also available.

Multielement analyses of source rocks provide data that are useful for source correlation and redox conditions during deposition especially V and Ni. Actlabs offers analysis of multielementsthrough mineralization followed by ICP or ICP-MS analyses.

X-ray diffraction provides data useful for construction of Rock Mineralogy Logs. These logs are useful in understanding vertical changes in lithology useful in identifying reservoir limits, understanding unconventional reservoir compositional variations relevant for designing fracking operations and risk of subsidence and other stability issues for conventional reservoirs.Please see below for details on mineral identification and quantitation of alpha quartz.

Mineral Identification

Most minerals are crystalline and therefore scatter X-rays in a regular, characteristic way dependant on their crystal structure. Each mineral produces a unique diffraction pattern and can be recognized from that pattern like a fingerprint.

Identification of minerals is made by comparing their diffraction patterns with a library of over 17,000 mineral patterns stored in the International Centre for Diffraction Data (ICDD).

The samples for X-ray diffraction analysis are ground or milled to a fine powder and then hand pressed into the sample holder. Approximately 1cm3 of the material is sufficient for rock mineral analysis but smaller amounts can also be accommodated by using a low background holder.

Alpha Quartz

The quantification of crystalline silica phases (Alpha Quartz) in bulk samples is carried out as an extension of the usual quantitative XRD procedures (absorption correction, internal/external standard methods and whole pattern analysis) depending on the sample. Several grams of sample are required.

In cases where industrial applications may cause silica to become airborne, the samples are collected and analyzed on filter membranes, following the guidelines of NIOSH 7500 or OSHA ID-142 Methods for respirable Alpha Quartz.

Determination of concentration of amorphous silica is performed following NIOSH Method 7501. The method uses the property that most amorphous forms of silica transform to cristobalite with heat treatment at 1100°C or 1500°C. After firing, the sample is analyzed for cristobalite following steps similar to Method 7500.

Our new FEG-SEM-MLA automated mineralogy solution can also provide further information on mineralogy for both conventional and unconventional source and reservoir rocks.

Are you drilling in stratigraphic sections where well logs cannot be used to provide clear correlations and that have few fossil markers? LithoChemSM Stratigraphy can easily resolve ambiguous stratigraphic correlations. Actlabs’ LithoChemSM Stratigraphy package provides a complete analysis of the major element oxides plus 7 trace elements in drill cuttings or sidewall cores. A total of 18 geochemical parameters can be used to resolve correlation between wells in ambiguous sections. Actlabs’ analytical technology is not hampered by the problems of “near total” acid digestions used my most laboratories or by semi-quantitative laser-ablation spectrographic analyses. Total analyses are produced accurately and precisely for elements bound in resistate and refractory minerals. All detection limits in ppm.

Microorganism shells have minor element and trace element concentrations that reflect the conditions under which those organisms lived. Salinity, trace element concentration of the water, temperature, oxygen fugacity, nutrients and other factors may affect shell compositions. Chemical alteration of shells during burial and diagenesis can also be estimated based on variations of some trace elements.

Actlabs has developed technology for analysing ostracods, forams or other microfossils (either singly or in aggregate) for 52 trace elements. Detection limits span from percentages to the part-per-billion (ppb) range. This data has been shown to be useful for determining the environment under which the organisms lived. This information can be used to map depositional conditions in sedimentary basins. Listed below are the available analytes and detection limits in parts per million (ppm).

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