Third surrender: Reservoir Map in Siliciclastic and Lenticular Formations (Well Information)

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Reservoir Map in Siliciclastic and Lenticular Formations

Third surrender: Well Information
Author: Aldo Sardelli  Msc-Geologist
Reviewed by: Eugenia Sardelli Geologist-Specialist

Well Information:

Once the mapping of the unified thickness contour maps has been defined and the contacts delineated at the top of the stratigraphic unit, the next step is to provide the geoscientists with the necessary elements to infer or deduce the lithological configuration of the Stratigraphic Unit (SU) and the location of the different fluids present in said unit, by means of these without needing to go back to review the records of wells. For example, when composite stratigraphic units with more than one interleaving (lens / deposit, etc.) are being mapped, the provision of this information should be able to indicate to the map reader that said unit is characterized by lenticular deposits with the presence of coalescence between all or some of these, the amount of lenses / deposits, and even the lateral variation of sand deposits.

The form and order in which the information in each well must be reflected in the isopach-structural maps is not different from that already known. Table 1. In the first line the name of the well, in the second the depth of the structural, in the third line the values of net sand (NS) are represented, in the fourth those of net oil / gas sand (NOS or NGS), and finally in the fifth the indicative symbology of the perforated completion is placed in the corresponding intervals.

Table 1. Correct way to represent the geological information of the well in an isopach -structural map.

Description                                                                         Example

WELL Name:                                                                   WELL 01

Structural top:                                                                        -0000’

Net sand:                                                                      10’+45’+30’   

Net Oil/Gas Sand:                                                           0’+25’+30’ 

Perforated completion:                                                           =    =  

In this article we will focus on explaining the correct methodology of how to represent the well information corresponding to the third, fourth and fifth lines, that is, in the values of net sand, net oil / gas sand and perforated completion.

1. Net Sand Information:

To correctly graph or capture the values of net sand on a map it is important to know the possible sequences that make up the stratigraphic unit (First surrender). Figure 1 shows a two-dimensional profile view of the possible combinations that could be presented for a stratigraphic unit with 3 stratigraphic levels.

Figure 1. Stratigraphic profile showing the different lithological combinations that can occur in a stratigraphic unit. Sardelli A., 2019.

Figure 1a shows an example where the three (3) deposits that make up the stratigraphic unit have lateral continuity and vertically coalescing. Figure 1b shows the medium to lower deposits arranged coalescently, while the upper deposit is separated from the underlying levels, all of them with lateral continuity. Figure 1c shows lateral continuity of the three deposits, while vertically the upper levels are arranged coalescently and the upper one separated from the overlying deposits. Figure 1d shows the case in which the three deposits are separated vertically, both with lateral continuity. Figure 1e shows separate deposits both laterally and vertically. Figure 1f shows discontinuous deposits laterally, while vertically they are both coalescing and separated from each other. Figure 1g shows the upper and lower lenses separated laterally, while the middle lens has lateral continuity. In the same way, the combination can be presented where all the deposits are vertically separated, while laterally there can be absence, continuity or discontinuity of these as can be seen in Figure 1h. Finally, there may be absence of deposits at some levels of the stratigraphic unit as shown in Figure 1i.

All these lithological configurations obtained from the information provided by the well must be correctly represented on the isopach -structural maps, giving the reader the possibility to identify and interpret them without wasting time by duplicating the work of verifying and / or searching for information. in well logs. Figure 2 shows the disposition of the net sand information in its various lithological combinations explained above (since it is a didactic and idealized model consisting of three levels, letters will be used to represent the value of net sand).

Figure 2. Arrangement of net sand information according to the lithological combination. Sardelli 2019.

One of the basic principles that must be taken into account to correctly map the deposits present in a stratigraphic unit is the way in which the values of net sand are expressed in the well. In the case of coalescing deposits, these must be expressed with a unified value that represents the total thickness of the coalescing deposits; if, on the contrary, the deposits are separated, a net sand value must be placed for each deposit present, followed by the “+” symbol and following a specific order from left to right from shallower to deepest, that is, the shallower deposit in the left position until the deepest deposit is located towards the far right. In the case where the well has not crossed any deposit, the value of "0" shall be placed in the corresponding stratigraphic position. To exemplify it, consider a stratigraphic unit of three (3) levels (deposits) with net sand values of 10 feet, 0 feet, and 20 feet. In this case, the first value corresponds to the first level associated with a sandstone deposit with 10 feet of net sand, the second level is associated with a 100% shale facies (0% net sand); and the last value would be representing the deepest level associated with a sandstone deposit with 20 feet of net sand.

Next, each case represented in Figure 2 will be explained independently. For the first case (Figure 3), according to the information extracted from the well, it indicates that there are three (3) sandstone deposits coalescing at a specific point or area. Within a stratigraphic unit, a total net sand value is placed even when they are presented laterally lenticularly within the same stratigraphic unit. The expression for the case of Figure 1 would be expressed as follows: Well 1: P ’+ R’ + T ’; Well 2: A ’; and Well 3: Z ’+ 0’ + X ’.

Figure 3. Example of coalescent sandstone deposits at the center and lenticular towards the ends of the stratigraphic joint. Sardelli 2019.

Figure 4 shows an upper deposit separated from the underlying one, which is coalescing with the lower deposit at the midpoint of the stratigraphic unit. In this particular case the net sand values would be placed as follows: Well 1: P ’+ R’ + T ’; Well 2: B ’+ C’; and Well 3: Z ’+ 0’ + X’ where 0 indicates the absence of sandstone. It is important to emphasize that Well 2 has two (2) net sand values that correspond to the upper deposit and the sum of the middle and lower deposit that are coalescing.

Figure 4. Upper tank separated from the middle and lower deposits arranged coalescingly. Sardelli A 2019.

In the case of Figure 5, the upper and middle deposit are coalescing, while the lower deposit is isolated. The correct way to capture the information in the particular case of Well 2, is to place as a first value the result of the sum of each thickness of both deposits (D ') followed by the net sand value of the lower deposit (E'), remaining expressed as follows: Well 2: D '+ E'. In the case of Wells 1 and 3 located at the ends of the stratigraphic unit, the corresponding net sand value for each deposit would be placed, beginning with the shallowest and ending with the deepest: Well 1: P '+ R' + T '; Well 3: Z ’+ 0’ + 0 ’, indicating with the value of 0 the absence of sand towards the right end of the unit.

Figure 5. Upper coalescing deposits separated from the lower deposit. Sardelli A 2019.

The following case proposes an example of a unit where the deposits are vertically separated from each other, but with lateral continuity (Figure 6). In this case, the expression would be composed of a net sand value for each deposit, ordered from left to right by the shallowest deposit to the deepest, as shown below: Well 1: P ’+ R’ + T ’; Well 2: F ’+ G’ + H ’; and Well 3: 0 ’+ 0’ + Z ’, where the 0 in Well 3 are indicating the absence of sand in the upper and middle position.

Figure 6. Deposits showing lateral continuity vertically separated from each other. Sardelli A 2019.

The following configuration is represented by sand deposits separated both vertically and laterally as shown in Figure 7. In the proposed case, only two of the three wells included in the analysis contacted sands. Applying the rules set forth above, the expression would be as follows: Well 1: I ’+ J’ + K ’; Well 2: 0 ’+ 0’ + 0 ’and Well 3: L’ + M ’+ N’; that is, the corresponding net sand value was assigned for each deposit according to the stratigraphic position (from shallower to deeper) indicating with the value of 0’the non-presence of sands in the wells that did not contact it.

Figure 7. Sand deposits are separated both vertically and laterally. Sardelli A 2019.

Figure 8 shows a case where the deposits represented by each of the three (3) stratigraphic levels proposed have a lateral discontinuity, while vertically a coalescence of the deposits located towards the right end of the unit can be observed. The value of net sand that will be placed for each well will be a combination of the aforementioned, so that where there is separation of the deposits, the characteristic net sand value for each deposit will be placed (starting with the shallowest to the most deep) as is the case of Wells 1 and 2; where there is coalescence or vertical connectivity the result of the sum of each deposit will be placed as is the case of Well 3, and finally the value of 0 'will be placed in cases where the sand has not been contacted by the well as it is the case of Well 4. Thus, the values of net sand will be expressed as follows for each well: Well 1: I'+ J' + K '; Well 2: L’+ M’ + 0’; Well 3: Y’; and finally Well 4: 0’+ 0’ + Z’.

Figure 8. Coalescent and / or laterally separated sand deposits. Sardelli A 2019.

The following example (Figure 9) shows the case where deposits can be presented laterally discontinuously, as in the upper and lower deposits, or continuous as the deposit located towards the middle part of the stratigraphic unit; while vertically, these deposits are separated from each other (not coalescence). In this case, the information will be expressed as follows: Well 1: P’+ R’ + T'; Well 2: F’+ G’ + 0’; Well 3: Q’+ S’ + U’.

Figure 9. Sand deposits with lateral discontinuity in the upper and lower levels and continuity in the intermediate level. Sardelli A 2019.

The same criteria will be used to express the values of net sand in cases where deposits present in a stratigraphic unit disappear totally or partially (Figure 10). In this case, a net sand value will be placed for each stratigraphic level beginning with the value of the upper deposit and ending with the lower one, proposing the absence of sand with a net sand value equal to 0 feet in cases where the well Do not connect the sand. For the wells proposed in Figure 10a the information will be expressed as follows: Well 1: 0’+ W’ + Y’; Well 2: F’+ G’ + 0’; and Well 3: V’+ X’ + Z’; while for the wells proposed in Figure 10b the information will be expressed as follows: Well 1: P’+ 0’ + T'; Well 2: 0’+ 0’ + C’; and finally Well 3: Q’+ 0’ + U’.

Figure 10. Example of a stratigraphic unit with alternating discontinuous and continuous lateral deposits. Sardelli A 2019.

In conclusion, the way in which the geological information provided by the well is expressed on an isopach -structural map allows the geocientist to infer or identify in a reliable and timely manner the configuration of sedimentary deposits present in a stratigraphic unit. That is why it is extremely advisable to use the criteria to express or graph the values of net sand in the isopach -structural maps suggested in this document.

2. Fluid Information:

Figure 11 shows the possible combinations of fluids that can occur within a stratigraphic unit, as well as the amount of sand that is impregnated / saturated with said fluid. The order explained in Table 1 to the beginning of this document, line 3 is assigned to information of the fluids present in each of the tanks that make up the stratigraphic unit, being one of the most important contributions of this methodology and which allows to give the map a real utility when designing and developing an exploitation plan, indicating the presence and type of fluid, location of the water and how it is arranged in the reservoir.

As explained in the first delivery, the presence of water does not limit the quantification of the reserves present in the area. For this, it is necessary to identify the value of net oil sand and graph it on the isopach maps using the methodology explained above, thus excluding the water areas of the isopach-structural map.

Figure 11 shows the possible combinations of fluids that may be present in the deposits of a stratigraphic unit. Considering the sedimentological disposition of the sand deposits, the following cases can be found: all deposits / lenses have or not lateral continuity, isolation or coalescence, and are impregnated with hydrocarbon (Figures 11a, 11b and 11i), when one of the lenses higher this water saturated (Figures 11c and 11d); when water is present only in the intermediate lenses as shown in Figure 11e, 11h and 11k, and finally when water is present in the lower lenses but without being part of the fluid contact (Figures 11c, 11g and 11l).

Figure 11. Possible combinations of fluids present within a stratigraphic unit. Sardelli A., 2019.

The proposed methodology to express the information of net oil / gas sand present in a well is exemplified in Figure 12. Next, each proposed case will be analyzed using letters, since these are theoretical demonstrative models, to express the values of net oil sand.

Figure 12. Examples of the possible scenarios in which the fluids can be found and the correct way to graph them, according to the type of fluid, on the isopach -structural maps. Sardelli A., 2019.

Figure 12 (a) proposes coalescing and impregnated hydrocarbon deposits with a value of net oil sand  equal to the thickness of net sand. In Figure 12 (b) the net sand values are proposing lenticular deposits impregnated entirely with hydrocarbon (Net Sand = Net Oil Sand). Figures 12 (c, e, f, g and h) indicate that, at least one of the three (3) sand deposits present in each stratigraphic unit, is saturated with water. The deduction is derived when a value of 0 is observed in the position of the fluids just below a value other than 0 in the position of net sand thickness.

The examples discussed so far are related to oil impregnated or saturated water deposits, but how to express situations where both fluids are present in the same tank, that is, when in the presence of water-hydrocarbon fluid contact? In this particular case, it is imperative to indicate clearly on the isopach-structural maps of the existence of such contacts. The different combinations of fluid contacts in a stratigraphic unit are shown in Figure 13.

Figure 13. Fluid contact combinations within the same stratigraphic unit. Sardelli A., 2019.

In conclusion, it is imperative to make an assertive reading of sedimentological, lithological, stratigraphic, fluid information, and the location of fluid contacts within the stratigraphic unit (Figure 14) provided by the well registers, and to capture it correctly in the maps that will allow to infer through the isopach-structural maps, avoiding the recurrence of well records, the architecture of the reservoir, the amount of hydrocarbon present and the way it is arranged in the subsoil, allowing the geocientist the decision making on time and properly. In the same way, graphing on the isopach-structural maps, in a correct way the information interpreted from the well registers, will allow to project a plan of exploitation and monitoring appropriate to the development of the deposit (Figure 15).

Figure 14. Disposition of information according to fluid contact. Sardelli 2019.

Figure 15. Reservoir Map of a stratigraphic unit with three sandstone deposits. Sardelli 2019.

3. Perforated completion symbology.

Finally, the correct way to indicate whether or not a sand was open is by placing the “=” symbol just below it. Following the conventional mapping methodology, when placing a total value of net sand even though the stratigraphic unit could be represented by more than one deposit and therefore a open per well symbol, the geocientist would be limited to the tools needed to identify which of the deposits really was open. On the contrary, applying the methodology explained in this article and placing the perforated completation symbol only below the open deposit, the geocientist will be given the possibility to discern accurately, and without the need to resort to the well log, which of the deposits Present in the stratigraphic unit was completed.

The tools proposed along these three deliveries will allow the generation of a unified isopach-structural map with the well information discretized by level (deposit) in order to accurately identify the deposits / lenses that make up the stratigraphic unit, the presence of areas of water / hydrocarbon, possible contacts, and completed deposits. This will allow to know with greater certainty the present reserves and thus project an exploitation scheme that will allow the maximum recovery of said reserves.