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Carbon Capture Utilization and Sequestration (CCUS) entails sequestration of carbon dioxide (CO2) which is injected into underground geologic sinks. Critical to the success of sequestration projects is detailed site characterization that includes reservoir assessment and evaluation of potential spill points such as intersecting faults and plugged and abandoned (P&A’d) wells, prior to the injection of CO2. Current offshore CO2 detection and monitoring falls into two basic categories: geophysical methods and CO2 testing methods at existing wells. Geophysical methods: are critical for the evaluation and identification of potential spill points;can image the opening of natural fractures as pressure increases during CO2 injection;can map the CO2 plume as it moves across the field. However, geophysical methods do not answer the critical question: is CO2 actually leaking? Rather, these methods highlight where CO2 may be leaking. The ability to determine if subsurface structures have adequate seal and whether those seals remain leak-proof is difficult since the majority of offshore CO2 monitoring methods are performed at or in existing wells. What happens when there are no wells in close proximity to potential spill points? CO2 leakage could be missed. A new approach has been developed with passive geochemical sorbers which have been used onshore for reservoir seal evaluation and CO2 monitoring for some time. This new approach involves attaching passive geochemical sorbers to the bottom of Ocean Bottom Seismic (OBS) nodes or Autonomous Underwater Vehicles (AUVs) (Figure 1). The weight of the OBS node/ AUV pushes the geochemical sorber into the seabed floor, which is then left in-place for approximately 3 - 4 weeks. This provides time for the acquisition of high resolution OBS imaging as well as for CO2 molecules, CO2 tracer molecules, and hydrocarbon molecules to concentrate on the geochemical sorbers. The end product is high resolution seismic data with ultrasensitive CO2, CO2 tracer, and hydrocarbon seepage data acquired simultaneously from co-located sites. The ability to overlay these disparate data sets allows for improved understanding of faults, natural fractures and CO2 leakage to a high degree of accuracy. Two onshore case studies will be presented to illustrate the efficacy and potential of the tandem deployment mechanism. A site characterization survey in northwestern Oman demonstrates the ability of passive geochemical sorbers to identify and map elevated hydrocarbon signatures along certain fault segments, showing that an associated reservoir was unsuitable for sequestration purposes. A CO2 monitoring program in Algeria presents a case in which 3D seismic data indicated CO2 injection had activated a deep fracture zone several hundred meters wide, extending ∼150 m above the reservoir. While CO2 tracers were detected in some production wells, the surface geochemical survey detected no CO2 tracers above spill points or around injection wells. The tandem technologies provide a unique capability to measure CO2 directly over potential spill points, not miles away. Furthermore, once CO2 injection has begun and changes occur in the subsurface, this system allows for movement and redeployment of seismic and geochemical sensors directly over potential spill points. Additionally, passive geochemical sensors can be placed around P&A’d wells during the site characterization studies to determine the potential for leakage due to improper plugging or cement deterioration that may occur over time.

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Salt is one of the most effective agents for trapping oil and gas. As a ductile material it can move and deform surrounding sediments and create traps. However, effective sealing of reservoirs for movement of hydrocarbons along breaching faults or fracture swarms (i.e. macroseepage) is a completely different mechanism than the molecular movement of hydrocarbons through grain boundaries and microfractures as found in microseepage. Forum Exploration chose to evaluate the applicability of passive surface geochemistry for mapping hydrocarbons in their onshore West Gebel El Zeit lease due to difficulties in seismic imaging through salt and anhydrites sequences. Two economic producing wells had been drilled in the lease, but due to compartmentalization and complexity in the area, three dry wells had also been drilled. Target formations included the Kareem Formation at ∼2,700 m and the Rudeis Formation at ∼3,000 m.The geochemical survey encompassed 100 passive geochemical modules. Passive samplers were also deployed around two producing wells and one dry well. Calibration data generated positive thermogenic signatures around the two producing wells in contrast to the background or baseline signature developed around the dry well. The Rudeis Formation calibration signature ranged from ∼nC5 - ∼nC9 while the Kareem Formation calibration signature ranged from ∼nC6 – nC12. This suggested the Rudeis calibration signature was lighter than the Kareem. This correlated with independent API gravity testing on produced oil samples (41o API gravity oil for the Rudeis, 35o API gravity oil for the Kareem).A post-survey well, Fh85-8, was drilled based on combined geochemical and seismic data results. The well was an oil discovery, with initial production of 800 BOPD. The evidence presented in this Gulf of Suez example shows that microseepage can occur through salt sequences. As such, ultrasensitive passive surface geochemical surveys provide a powerful tool for derisking salt plays.