Seabed Sampling /Water column data/Offshore geochem analysis
For geochemical exploration‚ the only effective technique is piston coring. But to be effective it has to be done correctly. It is not that simple. Equipment and set–ups can mean the difference between what may seem an acceptable 2–3m recovery and regular 4m+ recoveries. For geochemical exploration the whole aim of the exploration is to obtain samples from beneath a sulphate/methane interface that is within reach below the seabed.
Locating areas within reach of seabed sampling for this interface is not easy. Such locations are most reliably associated with the hydrocarbon seeps which are also typically associated with the authigenic carbonate and the hard grounds. Piston cores are generally limited in their penetration to around 150Kn compressive strength sediments––better than gravity cores‚ but still incapable of penetrating many seep sites.
The set–up and positioning of the piston cores is a delicate and skilled operation as the most likely areas to reach the interface without being foiled by hard grounds is on the edge of the seep. An accurate drop with a well–calibrated USBL system and good vessel positioning is needed.
Many contractors and surveys rely on the principle of “a seep is here somewhere and with a bit of luck we will hit the right place if not on the first drop maybe on the second or third”. This is a costly and often unsuccessful approach.
Until recently without the improved mapping techniques now available‚ this approach could be defended. But now it is simply a waste of money and time and risks poor or no recoveries. Using the x2 data on the first survey carried out with such data targeting the edges of seeps or sometimes holes within seeps resulted in improved success to close to 70% i.e.‚ 7 out of 10 of the cores expected to recover significant quantities of hydrocarbons did recover significant concentrations (significant is in excess of 2‚500ppm alkane gases). This compares with often 15% or at best 20% in surveys using the hit–the–high backscatter–and–maybe–we–will–get–something–back (hit and hope) method.
The rationally targeted coring also provides more effective geohazard data and effective confirmation of environmental mapping plus reducing the equipment damage that is likely with the hit–and–hope strategy.
Setting up the piston core is also a skilled job and our Client Representatives are experienced and knowledgeable and will assist a contractor obtain the best results for the client.
The development of water column data over the last four years has allowed extensive mapping of flares/plumes of gas and oil rising from the seabed. This has revealed how widespread these features are and how closely they are linked to the seeps identified from multibeam data. Recent surveys showed that only 3% of these gas plumes were generated from areas without previously identified seeps. The 97% that was clearly linked to seeps also were confirmed by the x2 coverage to be ephemeral at any precise location. On a second look over the same seabed‚ only 20% were in the same/similar location (within 100m).
Other interesting data have shown how the plumes/flares are focused on seep edges.
The rising behavior of the plumes/flares also allows differentiation between oil seeps and gas seeps and between dirty (oil sheen) bubbles and clean gas (hydrate coated) bubbles.
Offshore Geochemical Analysis– early results and revised targeting reducing no–shows
The advantage of on–board analysis is that it provides the possibility of revising coring plans with the up–to–date knowledge of the geochemistry during a survey‚ without having to remobilize for extra cores. The cost saving of the immediate knowledge allows dropping planned cores‚ and thus saving costs for a prospect area shown to be non–productive. To do this‚ some contractors now provide offshore analysis of the piston core samples. Gas presence may be simple to identify in cores by the association with sulphides (smell) and the disturbance of the cores with gas cuts and bubbles but recognizing liquid hydrocarbon traces is much more difficult and traces are rarely obvious in a fresh core. With offshore labs this means hits can be confirmed in a couple of days for liquids and further cores planned. Offshore sampling (the normal process is three samples from base up) also gives updates on the stratification. Traces can indicate that the sample was taken just too far from the seep source and that the methane/sulphate interface was just too deep at that location‚ suggesting that a re–core could be productive.