EAGE Shallow Anomalies Workshop

The Differentially Normalized Method of Electrical Prospecting (DNME) was initially developed by the Russian company SGRPC and deployed commercially already in 2002. It uses a towed controlled electromagnetic source and a towed receiver cable for hydrocarbon detection, by recording and analyzing the Induced Polarization (IP) effects at relatively shallow depths. It differs fundamentally from conventional CSEM. While CSEM targets resistivity changes in the reservoir and overburden, IP measures mineralogical changes that are normally high above the reservoir. These smaller depths of investigation allow for a cost-efficient, towed system to be applied. The IP measurements can even be deployed simultaneous with seismic reflection data acquisition. Our geoelectric methods have been successfully applied both onshore and offshore for derisking hydrocarbon prospects.

Recent exploration and production activities in the Cenozoic Southern North Sea (SNS) deltas, proved the economic potential of shallow gas resources, especially if located near existing infrastructures. Nonetheless, shallow gas production is still limited due to a lack of insight in the petroleum system, especially with respect to the relation between the anatomy of the delta and charging/trapping conditions. In order to mature the shallow gas play, a multidisciplinary workflow was applied to the SNS delta that involves 1) the reconstruction of the internally complex delta body, 2) a combined deterministic/stochastic approach to make reservoir property predictions, 3) evaluation of the HC origin, and 4) a grain-size based method to predict the seal-integrity of the sealing clay layers. The results present the first steps towards de-risking the shallow gas play in terms of trapping geometry, seal capacity, sourcing and migration. The presented workflow is applicable to areas were limited exploration data is available, but where critical production data is (still) missing.

Gravity coring and dredging has been used to obtain extensive seabed sampling of potential hydrocarbon seep sites and escarpments offshore in Arctic frontier basins. Sampling of subcropping strata and thin overburden sediments provide consistent information on ages of the strata and the nature of potential active hydrocarbon systems. Our results are the first to document active hydrocarbon systems in the Baffin Bay, the northeast Greenland shelf, and on the southern Jan Mayen Ridge.

Geochemical surveys will tend to show ambiguous results and will remain tarred as “unreliable” unless there is skilled interpretation of the shallow petroleum system and objective selection of seafloor targets. It is also essential there is careful navigation to the seafloor target. Doing some analysis on board the vessel in the field will also increase confidence that the intended target is sampled.

Offshore oil seep detection using satellite Radar is an increasingly established method used to assist in the reduction of exploration risk in frontier basins or for re-evaluating previously explored areas. The presence of seepage slicks implies that an active source is present and that there are also leaking traps within the basin. Over the last 20 years a global database of offshore seeps has been developed consisting of ~14,000 potential seepage slicks and over 17,500 Radar scenes. Seep information is most useful when integrated with other geochemical and geophysical datasets. Seepage can be intermittent and drift caused by currents and local weather conditions can make sampling slicks for geochemical analysis difficult. Near-real time methods have been developed for guided geochemical slick sampling.

Hydrocarbon traps in the Hammerfest Basin and in the western part of the Haltenbanken area frequently leak at discrete positions, largely where faults intersections meet the top reservoir surface. We have identified such locations, and investigated to what extent they can be related to soft amplitude anomalies in overburden rocks. It was concluded that leakage-related amplitude anomalies could be identified in the overburden, and that they could be related to vertical leakage from the traps in most cases. However, their seismic expressions vary significantly. This, and the observation that a large number of amplitude anomalies in the overburden rocks have other causes, implies that prediction of hydrocarbon water contacts from seismic anomalies in the overburden can be successful in some cases, but will be highly uncertain in others.

Shallow geophysical anomalies at Nyegga on the continental slope off Mid-Norway were discovered in the 1980’s. The anomalies were first recognized to be associated with a regional BSR and acoustic ‘bright spots’ associated with the scar of the Storegga Slide. Subsequently, surface depressions, mounds, and sub-surface columnar features, ‘pipes’ (chimneys) were discovered. ROV surveys documented large crusts and blocks of methane derived carbonates, irregular shaped pockmark craters, and an exotic fauna which included giant sea spiders, ophiurids, stalked crinoids, basket stars, and various kinds of fish. The academic research at Nyegga culminated with the acquisition of geochemical samples, high-resolution 3D-seismic tomography (2006) and P-cable 3D-reflection seismic data. In summary, the data suggest that there is both pervasive and focused fluid flow occurring at Nyegga. The migrating fluids involve thermogenic hydrocarbons. However, despite there being abundant topographic, geophysical, and geochemical evidence of active seepage, no water column ‘flares’ have so far been reported from the area. Analysis of buried and surfacing ‘chimneys’ suggests that they first form by vigorous escape of over-pressured fluids followed by continuous micro-seepage. The most likely forcing of the fluid escape flux variation seems to be associated with former cyclic glaciations.

Analysis of a 2D seismic dataset from the South Falkland Basin reveals a large number of different amplitude anomalies and fluid flow structures in the Mesozoic and Cenozoic fine-grained successions. These amplitude anomalies are associated with 1) bottom simulating reflection (BSR), 2) enhanced reflections (ERs), 3) pipes, 4) disturbed signal zones (DSZs) and 5) flat-spots. The prominent BSR extend within the central part of the South Falkland Basin and is associated with gas hydrate stability zone (GHSZ). The ERs are interpreted to be gas charged layers and associated with a free gas zone. The pipes are observed as columnar regions of vertical disturbance and they are located above structural highs. The DSZs are located in the deeper portion of the basin and are characterised by stacked amplitude anomalies. These DSZs are located above structural highs. Some flat-spots have been identified and analysed. These flat-spots are consistent with AVO class III response. The hydrocarbon potential of the basin has been recently confirmed by an important gas condensate discovery (Darwin). This discovery suggests that the observed anomalies and fluid flow structures, in association with their acoustic responses, are direct hydrocarbon indicators (DHIs) and potentially linked to a hydrocarbon plumbing system.

Carbonate platforms are remarkable for their longevity and can experience different geotectonic settings that affect fluid flow systems. The Malta carbonate platform shows both extinct and active fluid flow systems that reflect changing geotectonic settings.

Natural gas seepages are of great interest for the petroleum industry, both for prospecting and environmental reasons. Gas flares originating from gas seeps can be fairly easily identified by multibeam water column data, but further investigations require observations from close range. ROV investigations provide high detail, but are time consuming and costly. AUVs (autonomous underwater vehicles) have proven to be very useful for such investigations, traditionally carrying sidescan sonars, multibeam echosounders and other equipment. Synthetic aperture sonars are now emerging as a new tool, giving unrivalled resolution. Used in combination with high resolution colour photos, methane sniffers and other sensors, we can now get detailed documentation over large areas within limited time.