RE: prospects24 Sep 2020 17:23
"I remember Dr.Trice saying oil could not be fed from below in an F.B. reservoir so it is hard to see how you could have a low zone of oil then a zone of water then oil in the main reservoir?"
As usual its the casual use of words that leads to misunderstandings.
Yes that is true that in fractured basement reservoirs the oil cannot come from DIRECTLY below, there is no source rock below to provide the oil. It can and does come from a depth deeper than the reservoir, as oil is lighter than water and finds its way to the shallowest accumulation point. In this case the source rock is deeper but its on either side of the basement ridge so it has to travel both laterally and upwards. The reason for CA talking about the thickness is that they are referring to what is often called the Transition Zone, that is the zone where the oil goes from a high saturation say 90% plus oil and 10% or less water over a distance to to a place where the saturation of oil is much lower or the oil is completely absent. In a Sandstone reservoir this transition zone can be from a few feet to 10's of feet depending on the reservoir type and quality, a long transition zone in a sandstone reservoir can mean a few things, either a poor reservoir or perhaps a reservoir that has subsequently leaked and so the OWC has moved higher, leaving behind some residual oil.
In basement reservoirs its a function of the interconnectivity of the fractures in the rock, this is not easily predictable nor easily measurable. So the migrating oil coming in from the sides and travelling upwards can get caught in "dead end fractures" that don't actually connect up into the main reservoir fracture network (think of a cul de sac). These will still show oil beneath the main OWC but are likely to be minor , indeed this is perched oil (!) within what is essentially the acquifer, just as the so called perched water was supposedly a small volume of water trapped within the oil column. Perched water is far more difficult to explain as the oil being lighter that water would in effect displace the water downwards through bouyancy.. perched water is not impossible, just unusual. The other explanation for oil below the contact is that the trap has at some point leaked and the OWC has risen leaving behind some residual or locally trapped oil. My view is that CA are confusing what they think is a long transition zone (unusual) with oil shows from either perched oil or a paleo oil column. Either way the evidence is pretty strong that the OWC is at the new level and the stuff below is really just "noise". They are entitled to their opinion of course, but I'm not sure many professionals would agree or bet money on their case being correct.
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