Page 125 - The Petroleum System From Source to Trap

5. Applied Source Rock Geochemistry 119

be light-colored on weathered surfaces. Burrows and abundant These criteria must be applied with caution because the rela­
benthic macrofossils indicate poor conditions for preservation tionship between T max and PI vary with kerogen type, thus,
of organic matter. When in doubt, it is better to sample a some highly oil-prone lacustrine kerogens show little change in
candidate source rock for laboratory confirmation. T max during maturation (Espitalie et al., 1985; Huizinga et al.,
Most geochemical logs are based on cuttings, comple­ 1988).
mented by sidewall cores and conventional cores. The All samples containing migrated oil or contamination that
following discussion is directed toward cuttings, although the show S1 � 1.0 mg/g rock are solvent extracted in an ultrasonic
same general criteria for preparation and interpretation apply bath for 30 min using dichloromethane:methanol (9:1). This
to cores and outcrop samples. Each sample should weigh about step is intended to reduce interference by migrated oil or conta­
50 g. Cuttings are best washed at the well site prior to shipment minants in the measurement of Tmax' Sv and other Rock-Eva!
and storage because mudcake can become more difficult to parameters. Not all samples show reduced 5:1 and altered Tmax
remove with time. Cuttings are washed of mud cake using fresh after extraction of contaminants. For example, some diesel cuts
or salt water, and in the process, wet-sieved with a 2-mm top commonly affect only S1 and not S2. The extracted sample is
sieve and a 180-J.l.fi bottom sieve. Cuttings must not be washed transferred onto microfiber filter paper using additional
with organic solvents, which remove soluble components. solvent, placed on a watch glass, and oven dried at 400C for 2
Many particulate additives can be floated off by panning in an hr prior to pyrolysis. If the 51 peak is still �1.0 mg/ g rock, then
evaporating basin. Samples are air-dried at about 40°C. the sample is Soxhlet extracted for 24 hr with
After arrival in the geochemical laboratory, cuttings are dichloromethane:methanol (9:1), dried for 2-3 hr, and
rewashed and described for lithology. Washed and air-dried pyrolyzed.
cuttings are examined using a binocular microscope, and conta­ We recommend that both pyrolysis and TOC be completed
minants, such as walnut hulls, woodchips, metal, and obvious on every sample. Rock-Eva! pyrolysis is more expensive than
caved material, are removed by "negative picking." We do not TOC, so many prefer to complete Rock-Eva! pyrolysis only on
recommend "positive picking," where an inferred representa­ samples exceeding 0.3% TOC. Alternately, TOC can be
tive lithology is selected for analysis from a mixture of litholo­ performed only on samples with 5:1 � 0.05 mg HC/ g rock.
gies in a sample. Positive picking generally results in nonrepre­ Sidewall and conventional cores are better than cuttings or
sentative samples. We do not recommend composite samples outcrop samples for both screening and detailed geochemical
of cuttings from several intervals. Natural mixing of cuttings in analyses. However, caution must be applied where these
the mudstream during drilling is a form of compositing that samples represent organic-rich beds of insufficient thickness to
need not be compounded in the laboratory. When severe, serve as regionally significant source rocks. Sidewall or core
caving or bypass of lithologies on the shaker table can cause samples for source rock analysis should be taken at 15-30 m
problems in interpretation. We have found that natural mixing intervals in fine-grained lithologies. Although not recom­
is reasonably representative of significant rock lithologies. For mended for cuttings, composites of sidewalls or cores at 15-30
example, thin, organic-rich beds that might be sampled by m intervals may be a useful supplement for analyses of isolated
sidewall cores are "averaged" and do not appear as organic­ samples, particularly where organic matter in the source rock
rich spikes on geochemical logs (Figure 5.10). varies nonsystematically with depth (Figure 5.10).
A small portion of the dried cuttings is crushed to fine sand Critical information that should be included with rock
particle size (0.125-0.25 mm). Grinding to a smaller size is not samples includes well name, operator, location, formation, age,
recommended because powdered samples can result in depth, and type of sample. Good quality paper or zip-lock
anomalous Rock-Eva! results, including poor � peak defini­ plastic sample bags are recommended. Paraffin coatings are not
tion, low 52 yield, and erroneous T max values. The crushed recommended for storing rock samples. Maps and lithology
samples (100 mg) are analyzed using Rock-Eva! pyrolysis logs assist in evaluating sample quality and geologic relations.
where every twentieth sample is a rock standard. If very rich in If additives or migrated oil could represent contaminants,
organic matter (> 10 wt. % TOC), sample size is reduced and the samples of these materials are helpful.
sample is rerun to ensure linearity of response. Sample size also
affects Rock-Eva! pyrolysis response (Peters, 1986). Another Oil Sample Preparation
approach that avoids this problem is to dilute an organic-rich
sample with pure carbonate, followed by pyrolysis of 100 mg of Oils are best collected in quart-sized or smaller glass
the mixture (Peters, 1986). containers with teflon cap liners. Oil can leach contaminants,
Samples showing high 5-t values result from (1) potential or such as phthalates, from plastic containers or rubber-lined caps.
effective source rocks or (2) rocks containing migrated oil or Fill only to the shoulder of the container to minie contact of
contaminated by drilling additives. Samples containing oil with the cap. Each sample container should be carefully
migrated oil or drilling additives are readily distinguished from labeled using waterproof ink. Bottled oils do not appear to
source rocks by anomalously high production indices for their undergo biodegradation for several years in storage. However,
level of thermal maturity. Samples that do not meet the metal containers react with emulsified water and can leak.
following criteria (Table 5.3) are assumed to be contaminated Care must be taken in collecting production oils because
by drilling additives or migrated oil: they may represent mixtures from several zones. Surface
exposure diminishes the information content of seeps due to
l
• If Tmax is in the range 390°-435°C, then PI must be �. . evaporation, biodegradation, and possible contamination. It
• If T max is in the range 436°-445°C, then PI must be �.3. may be necessary to dig into seeping rock exposures to obtain
• If T max is in the range 445°-460°C, then PI must be �.4. the freshest possible samples.