Source rock assessment

Selection of samples is based on well logs and cuttings/core descriptions so that suitable lithologies (usually dark grey/brown claystone/shale or coal) can be targeted. All types of well samples are potentially contaminated by organic drilling mud additives. It is important to know what mud system and coring lubricants have been used so that washing and any other pre-treatment can be optimized in order to provide the most reliable results from subsequent analyses. Core chips/plugs and swc are preferable to cuttings (which may need picking).

Core chips are best obtained from a reasonable distance away from zones of core plugging, because of the potential of additional contamination by plugging lubricants. Plugging specifically for source rock studies is best performed using water as a lubricant.

Outcrop samples can suffer from the effects of weathering, so the deeper the sample below the surface the better. Sub-sampling from as far removed as possible from the exterior surface of the outcrop is recommended.

Cuttings can suffer from contamination by drilling materials and cavings, and a particular source rock unit of interest may represent only part of the depth range of a cuttings sample. Consequently, after washing and drying, visual inspection and possibly picking may be required to ensure the desired lithological unit is analysed.

Initial washing of cuttings can be with water, to remove water-based drilling muds (e.g. polyglycols, lignosulphonates and polyacrylamides), or with the addition of detergents to remove oil-based muds, although further extraction is usually required to remove all organic contaminants. It is useful to note the depths of casing and drill-bit changes when selecting samples.

To summarise, samples should be subjected to the following treatment prior to commencement of the analytical programme:

• washing of cuttings appropriate to drilling mud system
• lithological description and picking of cuttings as may be required
• milling of rock fragments to fine powder
• pre-extraction of milled cuttings and core material depending upon drilling mud

If it is thought that analyses in addition to general screening are likely to be undertaken, it is recommended that sufficient material is obtained during initial sampling, to avoid any influence from inhomogeneity (e.g. with respect to different bags of cuttings being sampled.

The table below provides a guide to sample quantities; outcrops can be considered equivalent to washed cuttings. Available samples from cores are likely to be limited, but it is beneficial if the minimum amounts for washed cuttings can be exceeded.

Basic screening, using cost effective and relatively rapid methods, is usually performed on a large sample set in order to assure adequate resolution of variations within a suspected source unit (see SR selection):

• total organic carbon (TOC)
• Rock-Eval pyrolysis

TOC provides a simple measure of how much organic matter is present, but an indication of the amount and type of hydrocarbons that might be generated requires the addition of Rock-Eval analysis. Rock-Eval also gives an idea of current maturity level.

Correlation of results with wireline logs is recommended.

Total C determinations can also be undertaken, if required.

The broad screening of source rocks by TOC and Rock-Eval provides a general idea about source rock quality and type, but sometimes the results can be misleading, so some form of validation is often worthwhile. This may be achieved by examination of the individual compounds produced during pyrolysis and/or by visual analysis of the organic matter (kerogen).

Based on screening, a sub-set of samples may be selected for more detailed analyses, providing a more complete and reliable indication of source rock quality and maturity:

• pyrolysis-GC
• visual kerogen analysis
• vitrinite reflectance
• composition of extracted bitumen
• kerogen compositional kinetics
• C and S isotopic composition
  

Pyrolysis GC (Py-GC) effectively identifies the individual compounds that go to make up the total hydrocarbon yield (S2 peak) from Rock-Eval, and provides a more reliable indication of the type of petroleum that may be generated.

Visual kerogen analysis determines the type of organic matter present, which also contributes to a knowledge of source rock quality.

Vitrinite reflectance is a standard maturity measurement, which is used to calibrate generation modelling.

The amount of bitumen that can be extracted provides an additional measure of oil potential, and further kerogen quality and maturity information can be gained from analysis of extracted bitumen by SARA analysis followed by gas chromatography linked to a flame ionisation detector (GC-FID), gas chromatography-mass spectrometry (GC-MS) and bulk C isotopic analyses. In particular, oil-source correlation benefits from GC-MS analysis. These molecular distribution methods are also useful for assessing reservoir extracts and oils/condensates too.

Ultimately, for modelling of generation, it may be useful to obtain kinetic data for selected source rock samples, particularly if suitable generic published data are not available.

S content and isotopic composition of kerogen can be useful when assessing the likely depositional environment and potential for early petroleum generation, due to the cracking of S–S and C–S bonds, which are weaker than C–C bonds. It requires isolation of kerogen. Bulk C isotopic composition can aid oil-source correlation and provide indications of kerogen source. More information is provided by the C isotopic composition of SARA fractions from extracted bitumen.