RE: Reservoir continuity14 Jan 2023 01:14
@Dave - reasonable questions, unfortunately not simple answers!
First, mud gas analysis would be very unreliable, especially if you are trying to detect what would probably be very small chemical differences. As Jimmy has said, most of the gas has originated from the same place, so you are left with looking for very minor secondary changes. During drilling it's difficult to tell exactly where it has come from, if gas from different levels has mixed (eg shallow biogenic, deeper thermogenic), or how it has interacted with mud & air. It would be more reliable to use free flow gas from discrete reservoir levels during flow testing, but even this may not provide reliable or useful data.
Reservoir continuity is also rather misunderstood. In an ideal world, you might get one big laterally continuous sheet of reservoir sands. When you look at a cross section of a borehole, this is easy to imagine - but the borehole is really just a visualisation in one dimension. Turbidites in particular are complex - @Caterham7 did an informative post on this about a week ago - you should rely on what he has to say, he has a lifetime of practical experience in this area, I have not :-)
Now imagine the reservoir in two dimensions - it is a bit like a river delta, with not only sheets, but also ribbons and banks of sand (reservoir) interspersed and overlain with mud (seal). If you put in two or more boreholes, you may see the same strata in some, but in others it may be absent. Next three dimensions - lots of these complex deltas stacked on top of each other. Lastly, we bring in the fourth dimension - time. While all this depositioning and repositioning is going on, erosion is also taking place, often removing lateral barriers of mud, or else removing overlying mud layers. These processes can either connect or disconnect reservoirs, both laterally and vertically.
For the MOU-fan, from the seismic data and examining the cores from previous boreholes, it would appear that most of the thickest reservoirs are likely to extend laterally across most of the fan, so should be in communication, but there is no guarantee that everything is connected - there could be an outlier left after erosion, for example. It is also very likely that there may not be communication between the multiple stacked reservoirs that we are hoping to intersect, particularly since we have seen a range of different pressures across the various formations.
Couldn't think up any useful culinary analogues, perhaps a pot of different kinds of pasta - if the lasagne sheets or spaghetti are touching, we have reservoir connection, but not if sauce between them? I'm too old/tired/sober for this.