Welcome to Petroleum Geology Forums

Hi all,

I am currently working in a field where the charging mechanism is still have question mark, we have a model that can--in a way--explain on why hydrocarbon (gas) is there. Being a new petrophysicist with no background in geology, can anyone help on literatures that can explain the basic of petroleum system, especially charging mechanism?

From the petrophysics side, the saturation from the log doesnt match the cap pressure measurement (more than 15% saturation units higher on the log Sw; i tried already to cut up measurement artifacts, such as bed boundaries, etc)... the guess is the--presumably--isolated sands were not fully charged because if it does, then the log derived Sw will reasonably agree with the cap pressure measurement... correct me if im wrong..

just something to discuss...

What I understand from your message is that your log derived Sw is higher (meaning more water) than the capillary pressure derived Sw. This usually happens for many reasons.
The log derived Sw is very much dependent on the resistivity log, formation water resistivity, porosity, cementation exponent, saturation exponent and the saturation equation used. There are many variables with associated uncertainties which affect the Sw based on logs.
On the other hand, the Sw derived from capillary pressure data is purely based on the bouyancy force of the hydrocarbon which replaces the water from the formation during the drainage cycle. There are also a lot of parameters which can affect the computation of Sw from capillary pressure namely, hydrocarbon density, wettability of the formation, formation porosity and permeability, height above the Free Water Level. To compute Sw from capillary pressure data, you must have a good estimation of the formation permeability based on correlations, transforms or empirical equations, etc. The accurate location of he FWL will also have a big impact on the Sw derived from capillary pressure.
In my experience, the Sw from log tends to be more pessimistic (higher Sw) than the Sw from capillary pressure, especially in shaly reservoirs.

yes kokokyi, the log sw is higher than the cap pres measurement. the problem is i believe with the log sw (we have good archie core coef measurement, several water samples, and formation properties effects on log), yes we have uncertainty in Rw (the biggest), but still it will not shift that much (shift to cap pres sw).

and for the cap pres sw, the measurement is quite robust, since we have use the same method for other reservoir (different age), and it matched reasonably well....

in this particular field, we have no FWL , pressure data suggested isolated sands, thats why i was thinking of immature charging from the kitchen itself, which can explain the pressure scatter and mismatch between sw log and cap pres...

any suggestions?

we are planning to appraise the field further, but we have limited data now..
thanks kokokyi and all for the response..

If you do not have a FWL, you cannot use the approach of computing Sw from capillary pressure. For this method to work, you must know where to hang your Saturation Height Function. However, if you are confident that your log derived Sw is robust and accurate enough, you can estimate the possible FWL by matching your Sw curve from capillary pressure with the Sw from log. Most petrophysical software, which has a capillary pressure module, will allow the estimation of FWL by Sw curve matching. If you don't have it, you can still do it by iteration using a spreadsheet.
Sw from capillary pressure method also does not work in overpressured reservoirs.

If you are not going to build a static model, you do not need to generate a Saturation Height Function to populate the model with Sw. You can estimate the average Sw for the reservoir of interest and use a tank model to compute hydrocarbon volumes.

yes, i am not currently using it for computing sw, sorry for the misunderstanding, i was only matching the shape of the log sw curve with the cap pressure....

from what i interpret from the log vs cap pres sw; the only way its matched is if its in the transition zone/close to FWL (because of high log sw); but there is no indication of transition zone in the logs as well....

current static model are using the one value of Sw as per your suggestion... but i am just hoping to link what we have on cap pres measurement and the log saturation to explain the charge mechanism or relate to the geology story of the field....

in addition, we have the pressure scatter all over as well indicating separate compartments which are not talking to each other...

thanks kokokyi.. appreciate your response....