Second surrender: Reservoir Map in Siliciclastic and Lenticular Formations (Fluid contact)

Important:  The graphics, images and methods have been developed by the author. Its use is exclusively for academic purposes and use in subsequent works without prior consent is prohibited.

Reservoir Map in Siliciclastic and Lenticular Formations

Second surrender: Fluid contact
Author: Aldo Sardelli  Msc-Geologist
Reviewed by: Eugenia Sardelli Geologist-Specialist

In the previous edition, everything related to the theme of the Isopic Maps of net oil sand in siliciclastic and lenticular formations was discussed. In this new edition, everything related to Gas-Oil, Gas-Water or Oil-Water Fluid Contacts, etc. will be discussed, generally finding water deeper than other fluids (oil, gas.) by density characteristic of it. However, there are exceptions to this rule, such as the Orinoco Oil Belt south of the Eastern Basin of Venezuela, where there are records of inverted contacts, that is, the water is more shallow than the hydrocarbon (Water Contact -Oil) associated with the viscosity of the hydrocarbon; However, this topic will not be addressed in this edition.

One of the most important elements when quantifying the amount of hydrocarbon present in the subsoil is fluid contacts, which represent reservoir limits due to the difference in density between the fluids present. An incorrect representation of these on the maps may lead to a miscalculation of the area of the reservoir and, consequently, of the hydrocarbon volumes present in said reservoir.

To map fluid contacts for reservoirs represented by a sandstone deposit and / or several connected sandstone deposits, traditional fluid contact mapping methods propose projecting the contact depth in the structure, and then defining the contact across the map following the structural contours. This is applicable when the depth of the contact is sighted and read in the register, and it is the same at all points (wells) where the contact is present. However, in the case of lenticular stratigraphic units this methodology is not applicable since, considering that the different deposits that constitute a stratigraphic unit have different oil accumulations with fluid contacts at different depths, it could be generated “on estimates” of area and mapping errors, and consequently errors in the calculation of hydrocarbon volumes. An example of this is when you have water wells inside a reservoir. Let's remember that the reservoir map for the purpose of estimating reserves and development plan is made under original reservoir conditions

The methodology presented in this article is based on the observations mentioned above, and which proposes a modification of the traditional method. This methodology will allow a more accurate mapping and estimation of the area of a reservoir and the volume of hydrocarbon present in it.

CARTOGRAPHY OF FLUID CONTACTS

The methodology proposed in this article suggests the consideration of some important elements within which the Original Oil Water Contact - CAPO stands out, which can be interpreted at different depths for each hydrocarbon accumulation present in a stratigraphic unit. Figure 1 shows a didactic case of four hydrocarbon accumulations distributed in three levels. If you want to map the CAPO of the accumulation that is in the lower level using the traditional methodology, the next step is to locate the depth of the contact in the structure of the upper level, thereby incurring the error of mapping a CAPO of a reservoir with a depth "B" referenced to a level A3 in a level A1 (higher) whose accumulations have a depth "A". This would generate an overestimation of the area due to depth difference as illustrated in Figures 2 and 3.

Figure 1. Examples of contacts interpreted at different depths within a Stratigraphic Unit. Sardelli A., 2019

Figure 2. Errors generated when projecting CAPOs with depth at level A3 at level A1. Sardelli A., 2019

Figure 3. CAPO mapping method by coordinate projection and comparison with depth method. Sardelli A., 2019

Modified methodology:

Based on the statement that CAPO is a limit of hydrocarbon accumulation or reservoir represented by a line formed by geo-referenced points, that is, they have an X, Y coordinate, the proposed methodology suggests projecting this limit to the upper level (example: level A1), as it is located in the original level (example: level A3), representing it as a line called “Fluid Contact Limit” without an associated depth (Figure 3). Said Fluid Contact Limit could be represented by the characteristic symbology of the arbitrary limit (Figure 4), although it cannot be ruled out to create a new nomenclature to differentiate it from another limit that can be found in the reservoir.

Figure 4. Profile and plan view of the projected contacts and their representation on the map. Sardelli A., 2019.

Finally and as additional data, it is suggested to label the map by placing the word CAPO, followed by the real depth and the level to which it belongs (“CAPO @”). Figure 5 shows an isopaque map of net oil sand with the projected interpreted contacts. In the case where the contact is inside the unified reservoir again, even if this has been a determining element in the exclusion of water zones, its representation becomes unnecessary.

Figure 5. Isopaque map of net oil sand with the projected contacts and their representation on the map. Sardelli A., 2019.

For the third surrender we will discuss the topic associated with Well Information. Although it seems irrelevant, the way in which the well information is placed on the map is very important for its reading.