Where is the Gas/Water Contact and the Elusive Western Fault?

[petrengr1]

Hession has said that the data from Antelope-4 ST-1 and Antelope-5 support the following:

1. The gas/water contact may be lower than previously thought.

2. The western fault is further west than previously thought.

In the absence of information on the subjects I will speculate a little.

The comment about the gas/water contact is probably based on better quality wire line logs, especially if they had better quality formation (higher porosity) at the depth of the gas/water contact. They have been using -2,214 TVDSS lately but GLJ had used -2,228 meters TVDSS.  I think the GLJ number is probably about where it should be. We have produced gas on a DST from a depth lower than -2,214 meters  TVDSS and perhaps lower than -2,228 meters TVDSS. Antelope-1 ST-1 DST #8 recovered 0.25 Bbl of  43.9 degrees API oil and 150 scf/day of gas from a depth of -2,193meters TVDSS to -2220 meters TVDSS.  Antelope-2 ST-2 DST #14 tested the interval -2,220 meters TVDSS to -2,252 meters TVDSS and recovered 1.4 MMCFD and 23.8 BPD of condensate (17 Bbl/MMCF). Also recovered 158 BWPD but it was thought to be load water and not formation water.

Now what was the basis of Hession saying that Antelope-5 supports moving the fault to the west? Please know that the following suggestions are highly speculative.

If we have a look at the map at   http://tinyurl.com/q2lzmbq page 13 you will see a post-drill map showing two faults to the west. The first fault is the earlier interpretation and is about 1.5 km west of Antelope-5. The most westerly fault shown on this map is the new interpretation and is about 3.7 km west of Antelope-5.

What additional information could they have to support moving this fault to the west? First they have the new gravity survey data shown on page 13. I believe the black line shown on the gravity graphic  is the same as the most westerly fault shown on the map.

I have said in the past that we need as many as three wells that have cut the fault in order to pin down the location of the fault. What I had not thought  of until recently is that some of the wells already drilled might have cut the fault. Remember they are using -2,214 meters TVDSS as the depth of the gas/water contact. Below I will list the TVDSS of the total depth reached on all of the Antelope wells:

Well Name           Total Depth meters TVDSS            Distance Drilled Below GWC

Antelope-1                     -2,515                                          302 meters (989 feet)

Antelope-2                     -2,323                                          109 meters (357 feet)

Antelope-3                     -2,483                                           269 meters (882 feet)

Antelope-4 ST-1             -2,248                                            34 meters (112 feet)

Antelope-5                      -2,307                                            93 meters (299 feet)

Antelope-6 (planned TD) -2,464                                         250 meters (820 feet)

Here is my speculation: Why did Antelope-5 support moving the fault west? I think they expected to cross the fault before striking the Gas Water Contact. Since Hession says Antelope-5 supports lowering the gas/water contact the well must have reached the gas/water contact before crossing the fault. They drilled another 299 feet below the gas/water contact. Did they cross the fault? I do not know but it is a possibility.

I believe it is also possible that Antelope-1 and Antelope-3 might have crossed the fault when drilling below the gas/water contact. If so they could have the fault located in three of the wells. I do not think Antelope-6 will reach the fault at the planned TD because the fault dips to the east and Antelope-6 is too far east to reach the fault at the planned TD.

Looking again at   http://tinyurl.com/q2lzmbq  page 13, if we take the distance from Antelope-5 to the most western fault shown on the map (3.7 km) and the Antelope TD of -2,307 we estimate the depth of the top of  the reservoir where the fault crosses at -1,500 meters TVDSS then the angle of the fault would be about 12 degrees. I think the fault angle could be anywhere between 10 and 20 degrees. If they have the data to prove the location of the fault is where they have drawn it on Page 13 we may be able to use that data and go to certification after Antelope-6 without drilling Antelope-7.

Just a lot of wild guesses with the information I can scrape up.

Have a good evening!!

I wrote the above message back on 2-2-16 where I was speculating on which wells might have crossed the fault based on their location and how deep they were drilled. If I may I would like to change what I said above about the possibility of them knowing where the fault is based on having three wells that might have crossed the fault.  As a matter of fact none of the Antelope wells have crossed the fault. How do I know this? Let’s have a closer look at http://tinyurl.com/q2lzmbq page 13. If you look at the cartoon on the lower right side of this slide you will see that they show where they drilled Antelope-5. The lower dotted line indicates where they think the fault is. The well was drilled 93 meters below the gas/water contact but it never reached the fault. Since Antelope-5 is the well drilled fartherest to the west none of the other  wells have crossed the fault either. So they are back to depending upon the seismic and gravity interpretation (shown of the left side of page 13) to determine where they think the fault “might” be.

What might we learn from this information? First I doubt that the chart on the right side of page 13 is to scale but, just for fun, let’s assume that it is. Based on this I have estimated that they think the fault is 539 meters below the present TD of Antelope-5. How can we use this information to determine where the western fault might be?

First have a look at wikipedia   http://tinyurl.com/zpwnlzq . Quote: “If the angle of the fault plane is low (generally less than 20 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust.”

What we have here is an overthrust fault so we might expect the angle to be less than 20 degrees from the horizontal.

Now let’s fast forward to the presentation that they gave us on May 13, 2016 http://tinyurl.com/j6fu99b page 7. You will note that the map has changed since the one shown on June 9, 2015 http://tinyurl.com/q2lzmbq page 13. You see two faults to the west. You must understand that there is only one fault but this map is showing two possible interpretations of where this one fault may be located. Since June 9, 2015 they have moved both faults toward each other making the distance between the two interpretations much smaller. Using my trusty ruler I have made some “back of the envelope” estimates of how far west of Antelope-5 they have placed the location of Antelope-7 and how far west of Antelope-5 they have placed the location of the two new interpretations of the fault’s location. Now they obviously believe the correct location of the fault is the one that is the most westerly. If the eastern fault interpretation is correct then Antelope-7 will be a dry hole since Antelope-7 is to be drilled west of where that interpretation shows the fault to cross the top of the limestone pay zone.  So, of course, they do not believe that interpretation to  be correct if they are about to spend up to another $100 million drilling Antelope-7.

Based on my estimate of how far the fault is below Antelope-5, the distance from Antelope-5 to the eastern fault, and the top of the formation sub sea depth where the fault crosses the top of the formation as shown on their map I have estimated that the fault angle would have to be about 36 degrees for this interpretation to be correct. That angle looks to be too steep for this interpretation to be correct. So let’s do the same calculation using the distance for Antelope-5 to the western fault interpretation and the sub sea depth where their map shows this fault to cross the top of the formation. Using this information I have estimated the fault angle to be about 19 degrees which looks more reasonable for a thrust fault. So this westerly fault interpretation appears to be more likely to be correct.

Now let’s do this calculation once more using the distance from Antelope-5 to the Antelope-7 location and the sub sea depth their map shows at that location. Since we have found that this pay zone is up to 2200 feet thick (670 meters) at some of the Antelope wells we would like to know if Antelope-7 will cut the fault and how much of the pay zone we will drill through before we get to the fault. We know that the best of the pay zone is in the top half of the zone so let’s just see what the angle would be if we have half of the pay zone (670/2) in the well bore when we get to the fault. Using this information I have estimate the fault angle to be about 20 degrees or about the same as I got for the western fault. If the western fault location interpretation is correct we should see the top half of the normal pay zone at the Antelope-7 location. Of course that is where we have been finding the high porosity limestone and dolomite. Their map shows that the Antelope-7 location is in what they are calling the “slope” which I think is their way of saying the formation at this location is likely to be of lower quality than the other wells that have been drilled. If the drilling of the well proves to be similar to what I have described above I think we should see some high porosity limestone and dolomite based on the results of all of the Antelope wells that have been drilled so far.

If the fault angle is less than 19 -20 degrees the fault will be further to the west and we will see a thicker pay zone in Antelope-7 before crossing the fault.