special Focus: ADVANCES IN EXPLORATION

World Oil / september 2011 1

A properly designed survey should include the ability to identify the target features in real time and collect the sediment cores on target below the zone of maximum disturbance. The analysis program should include a full range of hydrocarbons as well as select non-hydrocarbon gases, and interpretation should include a fully integrated geochemical and geological analysis.

ŝŝ MICHAEL A. ABRAMS, Apache Corporation

seabed coring operations for the University of Utah surface Geochemistry Calibration field study conducted in August 2006 over Anadarko’s marco polo field in the Gulf of mexico, Green Canyon block 608.

Surface geochemistry is a relatively routine exploration method used to inves-tigate issues of hydrocarbon generation and charge.1–6 The presence of near-sur-face migrated petroleum provides strong evidence that an active petroleum system is present as well as critical information on petroleum source, maturity and mi-gration pathways.6–8 This article summa-rizes results from the multiphase Surface Geochemistry Calibration (SGC) study conducted by the Energy and Geoscience Institute (EGI) at the University of Utah.9

Multiple methods are applied to col-lect, prepare, extract and analyze near-surface migrated hydrocarbons contained within marine sediments.2,6,7,10–12 Few of these methods have been rigorously examined in both lab and field studies. The discussion below is based on a se-ries of lab and field studies conducted to groundtruth the current methods as well as develop new protocols.9,12–14

PRE-SURVEY PLANNINGPre-survey planning is critical to iden-

tify potential petroleum seep targets, de-

velop a cost-effective real-time seismic program, choose a coring device best suit-ed for local sedimentary conditions with a safe corer recovery system, and organize sediment sample preparation protocols.

Core site selection. Core samples should target migration pathways that contain seismic and/or other evidence of petroleum leakage.15–16 Petroleum from subsurface accumulations, or a mature source rock, will leak to the near surface via buoyancy-driven forces along major cross-stratal breakage (faults and fluid expulsion features) or along major fluid flow pathways (hydrodynamic). The leaking fluids and gases can exhibit seismic expressions (acoustic anoma-lies) and/or seabed morphological fea-tures, depending on the hydrocarbon phase, leakage rates and volume, as well as sediment type. These features include pockmarks (seabed depression), seabed hardgrounds (carbonate), hydrocarbon-related diagenetic zones (HRDZ),17 wipeout zones (also known as gas chim-neys) or acoustic blanking (reflection discontinuity and amplitude loss creat-

Designing a seabed geochemical study

2 september 2011 / WorldOil.com

ADVANCES IN EXPLORATION

ing distortion zones), bottom-simulating reflector (BSR, gas hydrate related), wa-ter column gas anomalies, near-surface bright spots and seismic event pull-downs (signal slowdown due to gas).

Core targets should include a variety of features across the area of exploration interest. In addition, regional reference cores to define the non-seepage sediment geochemical signal will be important, especially in areas where source rock re-working and/or transported signatures could be present.8,18–20 Replicate cores should be collected on targeted features that have the greatest potential to contain near-surface migrated hydrocarbons.

Real-time seismic. Real-time imaging provides greater detail to confirm that the targeted feature is present and its extent, and to better understand its relationship to active hydrocarbon seepage. In addi-tion, the real-time data provides a defini-tive target location. An effective real-time seismic program requires minimal set-up time. Hull-mounted systems are ideal since they do not require time to launch and retrieve. A focused beam and suffi-cient power are required to obtain good penetration and target resolution in deep-water environments.

Sampling equipment. The seabed sampling device chosen should obtain the maximum amount of recovery relative to penetration for the study area’s sediment regime, water depth and vessel capabili-ties. The device most often used to collect seabed samples is a gravity corer, which consists of a hollow tube (barrel) attached to a weight (core body). There are two main types of gravity corers: open barrel and piston.21 Both systems rely on their weight to push a barrel into the seabed.

The open-barrel gravity corer employs a valve system to remove the incoming water, and works best in shelfal to upper-slope silts and very fine grained sands.22 The piston corer is similar to an open-barrel corer, except that it uses a trigger weight to initiate free fall and a stationary internal piston to create a suction effect that assists in maximizing core recovery and minimizing sediment disturbance.23 The piston corer has proven to be a very effective tool to obtain more than 4 m of sediment core in deepwater, fine-grained sedimentary environments.22

The corer launching and recovery sys-tem is also a very important component of a safe and efficient seabed sampling operation that minimizes core sample dis-

turbance. It should be designed to allow for corer free fall with efficient braking, vessel movement during corer operations, fast retrieval to surface in deepwater oper-ations (100–300 m/min.), and safe corer deck recovery in poor sea conditions. Lo-cating the corer during deepwater coring operations (greater than 1,000 m) is also critical. The most common acoustic posi-tioning system used in seabed geochemi-cal operations is the ultra-short baseline (USBL) system, which can provide 5–10 m of accuracy depending on the water depth, USBL system and operator.

hANdLING ANd PROCESSINGOnce the corer has been retrieved, a

series of important procedures must be followed to process the sediment samples quickly, efficiently and consistently. The core liner with the sediment should be re-moved immediately, taken to the wet lab-oratory, and cut into core sections by the deck staff, avoiding contamination from lubricants or vessel exhaust fumes. Three sections per core should be collected at designated depths with replicates. There are two types of geochemistry samples collected, each requiring special protocols and sampling containers. The volatile hy-drocarbons (C1 to C12) and non-hydro-carbon gases require special handling, since these sediment gases and volatile liquids can be lost relatively easily during the sediment handling and preservation process. The most common container used for the collection of volatile gases is a 500-ml compression-lid non-coated metal can. The higher-molecular-weight hydrocarbons (C12 and up) are more sta-ble at surface conditions and, thus, do not require special handling.

A volatile hydrocarbon sample will in-clude a consistent volume of unprocessed sediment in a proper storage container with water and inert gas (helium) or air headspace in a mix of equal parts sediment sample, water and gas headspace.12 Addi-tional processing is required to prevent post-sampling microbial activity. The method most commonly used involves the addition of antibacterial agents such as sodium azide followed by deep freez-ing. Research indicates that super-satu-rating the water with salt and thoroughly mixing sediment with the salt-saturated water is also effective to minimize bacte-rial activity.14 In addition, the higher sa-linities decrease the water solubility such that more of the volatile hydrocarbons

will partition to the vapor phase (con-tainer headspace) relative to the dissolved phase (water-sediment mix), assisting in the headspace extraction process.12

The preservation of non-volatile, high-molecular-weight hydrocarbon samples involves placing about 500 ml of unpro-cessed sediment in an aluminum foil square using clean spatulas to prevent con-taminations from hand cleaning materials or moisturizers. The sediment sample is tightly wrapped in the foil square, flat-tening the sediment sample and making sure there are no air pockets. The sample is then placed in a labeled standard plastic sealing bag and deep frozen until analysis.

SAMPLE ANALYSISThe screening analytical program is

conducted on all sediment samples to identify those samples with anomalous-gas, gasoline-range and/or high-molecu-lar-weight hydrocarbons, as well as non-hydrocarbon gases (CO2, N2 and O2).

The most common method to exam-ine seabed migrated gases includes con-ventional can headspace analysis. This procedure examines the sediment gases contained within the pore space, either dissolved in the pore waters (solute) or as free gas (vapor).10 The amount of gas depends on pore water salinity, in-situ temperature and pressure, and gas type (relative amounts of non-hydrocarbon components and methane vs. wet gases: ethane, propane, butane and pentane). This non-mechanical procedure uses high-speed shaking to release vapor-phase interstitial sediment gases into the can headspace. The SGC laboratory and field calibration studies demonstrated that conventional interstitial headspace gas extraction, when conducted properly, will provide sediment gas compositions and compound-specific isotopes similar to the charge and reservoir gases.12

The bound gases are believed to be attached to organic and/or mineral sur-faces, entrapped in structured water, or entrapped in authigenic carbonate inclu-sions. They, therefore, require a more rig-orous procedure to remove.2,11 The SGC research project demonstrated that the adsorbed and ball-mill-bound gas meth-ods can introduce a systematic fraction-ation resulting in gas and isotopic com-positions different from the charge and reservoir gases.8,12 Previous studies have shown that, in some cases, the bound sed-iment gases can provide reliable informa-

World Oil / september 2011 3

ADVANCES IN EXPLORATION

tion on the subsurface petroleum-related gases. It is the author’s opinion that car-bonate inclusions were formed as part of the shallow sediment alteration process, which entrapped near-surface migrated gases in these unique cases.16,24–25

The gasoline-range hydrocarbons are molecules with five to 12 carbon atoms, and are derived from thermogenic pro-cesses associated with a mature source rock. These moderate-boiling-point hy-drocarbons are volatile and migrate along key oil migration pathways. They, there-fore, require a more advanced method to extract and analyze. Two methods are currently available to evaluate these hydrocarbons: the Gore Module, devel-oped by W. L. Gore and Associates, and headspace solid-phase microextraction (HSPME).14 The Gore Module method examines C2 to C20 using a module con-structed of ePTFE (polytetrafluoroethyl-ene), sorbent-filled collectors and thermal desorption coupled with mass spectrom-etry. HSPME uses a fused silica SPME assembly followed by thermal desorption gas chromatography analysis. Both meth-ods provide important information on a group of hydrocarbons rarely examined in most seabed geochemical surveys.

High-molecular-weight hydrocarbons (C12 and up) are examined using solvent extraction followed by whole-extract gas chromatography (GC). Comparison of different extraction solvents indicates both n-hexane (low polarity) and dichloro- methane (DCM, higher polarity) are suit-able for marine sediment petroleum hy-drocarbon screening.13

Published studies demonstrate that surface geochemical signatures can vary with sediment size, type and organic con-tent.2,4,6 The lithology type provides non-geochemical information that may be criti-cal in interpreting the geochemical results.

When anomalous high-molecular-weight hydrocarbons are found using ex-tract gas chromatography screening, fur-ther molecular characterization is needed to understand the anomalous hydrocar-bons origin. Gas chromatography-mass spectrometry (GC-MS) provides detailed molecular information on biological mark-ers, allowing correlation of surface seeps to subsurface oils and/or source rocks.

Sediment-extracted gas composition by itself will not provide sufficient infor-mation to determine anomalous gas ori-gin. Compound-specific isotopic analysis using isotope-ratio (IR) GC-MS can de-

termine gas maturity and source facies, as well as potential mixing with in-situ derived gases and secondary alterations.24

dISCUSSIONThe above information is based on the

SGC laboratory and field studies, which provided a framework to develop tech-nically sound and safe seabed geochemi-cal surveys. Examination of a worldwide geochemistry database with post-well re-sults demonstrated that most failures (dry

holes in areas where surface geochemistry identified thermogenic seepage) were re-lated to three key issues: improper sample collection, problematic analyical proce-dures and incorrect interpretation.

Collecting samples on target (within the petroleum seepage zone) and at least 3–4 m below the water-sediment inter-face is critical to ensure that the sample contains migrated hydrocarbons and to avoid near-surface alteration effects from the zone of maximum disturbance.8,15

4 september 2011 / WorldOil.com

ADVANCES IN EXPLORATION

The SGC lab and field studies dem-onstrated how analytical procedures may impact the surface geochemical results. The adsorbed and ball-mill-sediment gas extraction methods provided gas com-positions with elevated gas wetness and incorrect compound-specific gas isotopes relative to the charge and reservoir gases. It was concluded that this was most likely related to fractionation resulting from sample transfer and sediment washing.12 Thus, much caution is required with bound-gas sediment-extraction data. Is-sues of field and lab contamination were also identified in the SGC research proj-ect. The analytical procedure and con-tract lab must be chosen with great cau-tion to ensure that the results are real and not an artifact of the lab and procedure.

Lastly, misinterpretation is one of the most significant reasons for surface geochemistry failures. The SGC study indicated that the misidentification of variable background noise and reworked source rock or seepage as true signal was a relatively common error. In addition, few geochemical studies integrate the sea-bed geochemical results with basin geol-

ogy. Mapping thermogenic hydrocarbon seeps (oil and gas) relative to key cross-stratal migration pathways via fluid flow modeling and seismic attribute analysis provides an effective petroleum systems evaluation tool to better understand the seepage relative to subsurface hydrocar-bon generation and entrapment. Fluid flow modeling, seismic attribute evalua-tion (mapping vertical noise trails) and surface morphology analysis are indepen-dent non-geochemical ways to interpret near-surface geochemical anomalies and how they may relate to subsurface hydro-carbon generation and entrapment.

The introduction of new analytical procedures such as HSPME and the Gore Module to evaluate seabed gasoline-range hydrocarbons will provide an additional hydrocarbon measurement not currently used in most seabed geochemical surveys. Most recently, a new microbiological sur-vey method has been tested in the Gulf of Mexico to evaluate microbial communi-ties related to seeping hydrocarbons.26 These microbial communities inhabiting prolific hydrocarbon seeps can be charac-terized by culture-independent DNA pro-

filing of 16S rRNA genes. Microbial pro-filing may prove to be an important tool in areas of low or inactive seepage.

ACKNOWLeDGmeNtsThis article was prepared from AAPG 947643 presented at the AAPG Annual Conference and Exhibition held in Houston, April 10–13, 2011. Thanks are due to the SGC research project’s industry supporters: Anadarko, Geosci-ence Australia, Statoil, Petrobras, Shell, ConocoPhillips, Eni, Wintershall, Nexen, BHP, Hunt, Total and Marathon. TDI-Brooks, Fugro, Gore Surveys, Taxon Biosciences, the University of California and the University of Victoria all provided support during the EGI research project. Also thanks are due to the research staff at the University of Utah—Nick Dahdah, Eva Francu and Janice Erickson—who provided assistance during the various phases of the SGC research project.

WEB EXCLUSIVE: For the complete literature cited, see this article at WorldOil.com.

MIChAEL A. ABRAMS is manager of Geochemistry for Apache Corporation. prior to working with Apache, he was senior research scientist for the University of Utah’s energy and Geoscience Institute (eGI)

and senior research Geochemist with exxon production research Company. Dr. Abrams has over 30 years’ experience in petroleum exploration, production and research providing integrated petroleum geochemical services. / michael.abrams@apachecorp.com