Surface geochemistry sampling

Providers of services and technologies for surface prospecting are provided in the following tables:

It is very important to choose the appropriate sediment sampling device to obtain the maximum sediment recovery in the light of penetration depth and sediment regime. The sampling equipment should be suited to the seepage activity, local sediment regime, water depth, vessel capabilities etc. Many devices are available to collect shallow marine sediments ranging from a simple gravity corer to the more complicated vibrocorers and Selcore®. Each sediment sampling device has its own water depth and lithology limitations.

The most frequently used sampling tools for sea-floor sediment coring are:

• gravity corer
• piston corer
• vibrocorer
• push coring (shallow coring only)

The push corer system has a spring loaded or mechanical arm that pushes a tube into the sediment. It is applicable generally only to environmental studies as it recovers only the top 0.5–1.0 m of unconsolidated sediment. In deep waters the system is typically handled by use of an ROV. It is rarely used for sampling of shallow cores for geochemical exploration purposes.

Key elements of push coring:

• mainly used to sample for environmental analysis
• spring loaded or mechanical arm pushes tube into sediment
• short cores (0.5–1.0m)
• limited to unconsolidated sediments
• in deep waters it is employed by using a ROV

Locating the corer during deep water coring operations (>1000 m) is very important. There are several acoustic systems currently available to assist in locating the corer while deployed, the most commonly used for geochemical operations is the Ultra Short Baseline (USBL) system. It can provide an accuracy of 5–10 m, depending on the water depth, the specific USBL system, and the operator.

Although the position of the core body relative to the target can be monitored, placing the corer on the feature is another matter; it is very difficult with the current deep water coring systems. Trying to move a weighted corer in water depths of 1500 m or more from a surface vessel is extremely difficult; the corer body will not move in direct response to the vessel movement due to the large distance between the two. To date there is no cost effective system to position the corer directly on the targeted feature.

Obtaining a real time image of the targeted feature will greatly enhance a seabed geochemical program. It provides sufficient detail to delineate the feature and helps to define its relationship to active hydrocarbon seepage. In addition, it provides a well defined target location.

An effective real time seismic system requires minimal set up time and sufficient power to obtain good penetration and target resolution. Hull mounted systems are ideal since they do not require launch and retrieval. A focused beam with sufficient power is required to obtain good imaging in deep water.

The kind and size of vessel required for a safe and efficient marine sampling program depends on several physical and operational factors, such as:

• need for on-site real-time seismic (side scan sonar/shallow seismic etc)
• need for USBL
• selected sampling device/corer type (corer docking device/space on deck)
• length of core barrel (space on deck)
• likely weather/climatic conditions (relevant to vessel design, height of freeboard/ corer docking mechanism etc)
• number of coring/sampling personnel (accommodation)
• distance between survey sites and from port of mobilization (operational range & speed of vessel)
• winch type (high speed winch for deep waters)
• storage capacity (number of freezers accommodated onboard)
• onboard monitoring facilities

Within the market for survey vessels a number of contractors supply vessels of suitable size and shape, equipped with the necessary positioning and communication equipment, and compliant with the oil industry’s strict HSE/QAM requirements.

A minimum coring crew should comprise the following personnel (double crew for 24 h operations):

• contractor’s party chief
• 1–2 technicians for gravity/piston coring
• 1 engineer, 1–3 technicians for vibrocoring
• 1 engineer, 1–3 technicians for Selcore® operation

• sampling should be in the anoxic part of the core – this is particularly important in gas surveys as most degradation of gas, and organic matter in general, takes place in the upper oxygen-rich part of the sediment (the oxic/anoxic boundary may vary with sediment type and climate but is typically ~1.0 m below seafloor)
• samples should be collected from the base section of the core (minimum requirement) or from multiple depths within entire core (preferably at least three sections per core, unless core is <2.5 m long)
• sample visible hydrocarbons in the core
• sample sand horizons in the core
• sample sections of distinct odour (petroleum, H₂S etc)
• sample any visible gas hydrates
• collect replicates, to be held on site until primary samples have arrived at the analytical facilities

Once the sediment has been cored and the corer brought on deck, a series of important procedures must be followed to process the sediments quickly, efficiently, and consistently. The core liner with the sediment sample should be removed immediately and the core cut into sections by the deck staff, avoiding contamination from lubricants and/or exhaust.

Each core section should be taken to the core preparation laboratory immediately for sub sampling and processing, in accordance with the pre-survey specifications.

• retain the liner from the core barrel
• measure core length to ensure sufficient penetration
• extrude sediment for geochemical sampling from core liner
• cut off base part for geochemical samples
• describe sediment if required
• collect geochemical sample(s) in clean cans/bags
• conserve geochemical samples
• preserve remainder of core
• if the analytical program is only focused on analysis of extractable HC, samples can be sampled in alumina foil (dull side to samples) and zip bags
• cans should be flushed with inert gas (i.e. N₂) before sealing, and then stored lid-down
• backup/replicate samples should preferably be retained – and stored separately
• samples should be stored at a minimum of −20°C (both onboard the vessel and in the laboratory) to reduce bacteriological activity

The amount of sample, volume of water and headspace are important factors in optimising the extraction of small concentrations of migrated hydrocarbons from marine sediments. The type of water added and inert vs air headspace are also important factors.

Currently, each contractor has their own preferred method: EGI use equal volumes of sediment sample, water and gas headspace; the Texas based TDI Brooks International’s procedure is based on a 500 mL compression lid, non-coated, metal can, with approximately equal volumes (170 mL) of sediment, clean seawater, and nitrogen headspace flush, together with the addition of sodium azide ad a biocide; whereas the Norwegian SGS/Geolab Nor uses a similar size can (500 mL) but containing ~375 mL of sediment and no water or inert gas headspace flush.

Compression metal cans appear to be the most cost efficient and suitable for most sampling (so long as analyses are performed soon after), although Isojars provide more reliable sealing and do not rust.

Non-volatile hydrocarbon samples

The ideal container for HMW hydrocarbon sediment samples is a clean glass jar. Unfortunately breakage is an issue during transit, so they are not the best containers for remote area seabed geochemical surveys. Plastic bags are more practical for wet marine samples, however, contaminants from the plastic bags include oleamide, fatty acids (C₁₆ and C₁₈) and Irganox (an antioxidant), as well as various plasticisers. These components do not impact the overall interpretation but may appear as major peaks during screening GC analysis. The oleamides and irganox will be present in total extract GC chromatograms even if a non-polar extraction solvent is used and no derivitizing agent applied to the sample.

One way to get around the plastic bag contaminants is to wrap the sediment sample in aluminium foil prior to placing in the plastic bag. This will keep the sediment separate from the plastic bag reducing potential contamination issues. Not all aluminium foil is sufficiently clean to use for sediment sampling. Some commercial aluminium foil may contain coating to prevent sticking which will affect the solvent extraction results. The recommendations for the collection and onboard processing of the non-volatile (HMW hydrocarbons) samples are as follows:

• Collect at least 250 mL of sediment using clean spatulas;
• Place the sediment on clean aluminium foil, lightly wrap sediment sample in aluminium foil;
• Flatten the sediment sample into a pancake making sure there are no air pockets;
• Place the flattened sediment sample package in a labelled standard plastic bag (Ziploc);
• Sample numbers should be written clearly on sample bags with a permanent marker and a paper tag placed inside the bag inscribed with the sample number.

The most common methods currently used by the industry to obtain an ocean surface slick sample are simply collecting a surface water sample directly into a receptacle or placing an absorbent material such as Nybolt fibre (silicone based absorbent material) or Gore-based systems such as the AGI Universal Sampler.

The simple water sampling method is limited to relatively robust slicks, since collecting sufficient ocean surface petroleum in thin microlayer sheen has proven to be very difficult (Abrams et al. 2002). The following section summarizes the most common material/equipment used for surface sampling.

Nybolt fibre absorbent material has limitations due to the sampling process. It has to be in direct contact with the oil in order to collect an adequate amount of hydrocarbons and a representative sample. The amount of seep material that can be collected is therefore limited by surface area.

AGI Universal Samplers can be used for sampling oil slicks on water. They are constructed of GORE-TEX ePTFE® (polytetrafluoroethylene) and filled with sorbent and have previously been used as passive soil samplers. Since ePTFE has a chemically inert microporous hydrophobic structure, it allows vapour phase transfer of hydrocarbons but not water.

The sorber modules are analyzed by thermal desorption coupled with gas chromatography – mass spectroscopy. Potentially they can sample the full range of hydrocarbons from C₂ to C₂₈₊, although recovery declines with decreasing vapour pressure (i.e. increasing b.p.).

The General Oceanics (GO) samplers were developed in conjunction with the U.S. Coast Guard Marine Safety Laboratories for spill collection, transportation and geochemical identification. The Model 5080 device is made up of a highly porous TFE-fluorocarbon polymer net which is reputed by the manufacturer to capture up to 10 times the volume of oil compared to the traditional water sampling methods.

The polymer net is passed through the surface ocean oil slick using a pole and disposable ring to collect a petroleum sheen sample. The net is removed from the disposable ring and handle and placed in a 4 fl oz glass jar with Teflon lined cap for storage and transportation.

The Equilon-SEPCO Sheen Sorbent Sampler was collaborative design by Equilon and SEPCo (US Patent Number 6,237,431 B1). It uses chemically treated tetrafluoroethylene, polypropylene or polytetrafluoroethylene strips, with a large surface area, which float and do not disperse the sheen on contact, according to Equilon studies.

The strips are chemically clean and do not interfere with the geochemical fingerprinting. The sampler is wrapped in a cover which dissolves on deployment, and the maximum sorbent material surface area is then exposed to the slick using a rod and reel system, after which it is placed in a special container for shipping. The sorbed petroleum is recovered using standard solvent extraction prior to analysis.

The Dembicki Teflon Squid is a teflon bundle which is placed in a slick using a rod and reel system similar to the AGI and Equilon-SEPCO samplers. The sampler is placed in a glass vial and kept cool or frozen until analysis, when standard solvent extraction is used to recover the hydrocarbons.

Ocean surface skimmers are designed to collect thin petroleum layers on the ocean surface. Most surface skimmers have been designed for the collection of large volume oil spills. The Marine Sciences Institute at the University of California Santa Barbara (UCSB) has designed a system to collect thin petroleum sheens. It requires very calm conditions but can sample a large area (which provides an average composition across a slick).

A rotating metal drum collects the thin layer of petroleum, which is then moved into a holding tank. The technique has not been assessed for potential hydrocarbon compositional fractionation effects, and there is a potential issue with thorough cleaning of the drum on-board between sampling of different slicks.

These are generally fairly small devices which are easy handled and portable. They are based on a spring loaded or mechanical arm which pushes the coring tube into the sediment. They work in most terrains, but are limited to unconsolidated sediments and short cores (0.5–1.0 m), and are not routinely used for sampling soil. They allow high sampling frequency and are thus cost effective.

Manual augers are the most frequently used coring devices onshore; they are easy to transport (in a back pack or in a van) and handle. There is a variety of designs, but they are typically screwed/twisted into the soil by man power.

They work in most terrains and permit high frequency sampling, but are restricted to soft sediments/soil and short cores (<1 m). In contrast, hydraulic or mechanical hammer devices are large and will normally need to be transported in a van. They will also require an external energy source in the form of electricity or pressurized air for actuation of the core barrel hammer.

In areas covered by heavy vegetation (woodland, rainforests etc.), deserts, tundra or other remote areas with limited access for conventional sampling equipment, remote sensing or passive sampling technologies can be used for surface geochemical exploration. There is a wide variety of methods ranging from satellite imaging to adsorbents and probes. The most frequently applied technologies are:

• Synthetic Aperture Radar (SAR)
• Airborne Laser Fluorescence (ALF)
• microbial prospecting (for oil and gas)
• Gore™ Technology probes
• PETREX^® Technology
• soil vapour sampling
• active adsorbent gas-sieve analysis

Synthetic Aperture Radar (SAR) and Airborne Laser Fluorescence (ALF) have become standard tools for petroleum system validation in offshore frontier exploration areas. SAR and ALF sensors use radar to build an image from satellite/plane-borne detectors. Incident radar waves are emitted at an oblique angle to the ocean surface and the backscatter signal is controlled by the state of the sea surface. The dampening of ocean surface capillary waves by oil slicks, natural films, biological material or physical processes such as currents and winds will produce an anomalously low backscatter. Because not all wave dampening features are related to petroleum seepage, contractors use several criteria to rank anomalies:

• repeatability (temporal)
• shape (dog-leg, blob or tadpole)
• dimensions
• thickness
• relationship to subsurface and near-surface features

In certain geological settings the measurement of hydrocarbons to evaluate petroleum seepage may not be effective, and near-surface microbial populations have been used as an alternative for many years. The current near-surface microbiological methods include GeoMicrobial’s Microbial Oil Survey Technique (MOST) and MicroPro GmbH Microbial Prospection for Oil and Gas (MPOG).

Both methods assume there is a direct relationship between the hydrocarbon concentrations in near-surface sediments and particular microbial populations, so that elevation of the latter can be used as a reliable indicator of light hydrocarbon leakage from individual petroleum accumulations.

The microbial prospecting technology is based on detection of anomalies in microbial distributions in soil samples and has proven effective even where complex reservoir geology is involved. It has the advantage that sampling is simple (limited equipment is required), environmentally friendly and unaffected by local disturbance.

The method has been used to distinguish between oil reservoirs, gas reservoirs and oil bearing structures with a gas cap. It is not influenced by fractures, overlying salt or other geological structures.

Areas of application include both onshore (e.g. permafrost and desert) and offshore sites. Sampling is carried out onshore using a hand auger at a depth of ~150 cm, whereas offshore gravity/vibration corers or a grab sampler are used to obtain sediment from ~30 cm below the seafloor. ~100 g of soil/sediment is stored in airtight sterile sampling bags for preparation and analysis at the laboratory.

A new DNA based assay method called SARD (serial analysis of ribosomal DNA) has been developed, which appears to have potential but needs further testing before adoption as a routine surface geochemical method.

The PETREX^® technology is in general terms quite similar to that of the AGI Universal Sampler, in that an adsorbent is used and analysis is by thermal desorption MS. However, because the adsorbent is charcoal, care has to be taken with placement and environmental conditions to avoid deactivation by moisture.

In the PETREX^® sampler the activated charcoal is fused to a ferromagnetic wire within a glass tube. For soil sampling, the tube is buried in a shallow hole for up to 2–3 weeks, open end downward, whereas for surface sediment sampling the tube can be placed in a gas permeable but water-tight plastic bag. During this period the samplers will gradually adsorbed gas/vapour migrating to the surface until equilibrium between the gases and the charcoal is reached.

As with the GORE technology, the PETREX^® system is easily handled and installed with simple hand tools and is environment friendly.

This less well known exploration technique is used by a limited number of contractors (mainly based in the USA and Canada). It is based upon driving a sampling system 10–20 cm below ground level, where a vacuum system is used to lift 1 L of soil vapour to the surface and across a gas sieve, so is confined to onshore prospecting.

The gas sieve is small, light and easily shipped equipment which collects and concentrates C₁–C₁₈ hydrocarbons present in the soil vapour, allowing direct analysis of these compounds. Each sampling event requires about 7 min. and an individual gas sieve.

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