First surrender: Reservoir Map in Siliciclastic and Lenticular Formations (Isopacos Maps)

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

First surrender: Isopaque Maps
Author: Aldo Sardelli  Msc-Geologist
Reviewed by: Eugenia Sardelli Geologist-Specialist

1.- Introduction:

The main objective of a reservoir map is to represent the total oil volumes that is contained in a specific stratigraphic area or unit. It represents, in a bi-dimensional plane, the geological structure, the interception of the faults and the sedimentological events happened in that stratigraphic unit. 

For a proper reading or interpretation of a reservoir map, it must have every single needed element to infer more accurately the actual situation of the basis, included structural and isopach contours, faults, stratigraphic boundaries, basic well and borehole information, etc. The objective of this document is to show the appropriate technics to generate, using the most effective and technical methodology, a reservoir model that allow the most assertive quantification of the total reserves represented in the interpreted geological model. It is important to highlight that, the uses of unsuitable or erroneous methodologies can lead to an incorrect reserves calculation in particular situations, such as the case of non-oil sandstones deposits intercalated in an oil stratigraphic unit, the water-oil fluids contacts, among others. In addition, the final reserves model will allow defining the type of exploitation that must be carried out for a good management of the reservoir. 

2.- Premises to make unified maps 

The main objective of a reservoir characterization is quantify the whole oil that is present in a specific stratigraphic unit and its distribution, which allow evaluating the best suitable or optimal method for recovering it to obtain an economic benefit. However, before explaining the recommended methodology it is important to mention the premises that should be considered: 

1.- Coalescence is not a limitation; it may or may not be. 

2.- Intercalations of sandstones deposits saturated by water and oil sandstones deposits are not a limitation. This methodology allows excluding water sandstone deposits from the total quantification.  

3.- The water/oil contacts’ depth is not a limitation; it recovers its accumulation boundary function while doing a unified model. 

4.- The type of the oil must be the same for the entire stratigraphic column that will be mapped. 

5.- The gas would be a limitation depending of its position within the basis. 

6.- It is strongly advisable to define the stratigraphic units based on stratigraphic studies to maintain a coherence between the stratigraphic correlation and the sedimentation basin. 

3. Methodology

3.1 Unified isopach map (sum of sandstones)

To map a stratigraphic unit with intercalations of sandstones deposits and particular sedimentation orientations, it is important to establish the lateral extension of the deposits and know, using conventional petrophysical analysis, the content of oil in that unit. (Figure 1). It is important to mention that this methodology does not replace the petrophysical model, which is done to evaluate the properties of the rock and the fluids. 

Once the geometry of the sandstones deposits is defined both laterally and vertically, the mapping strategy should be evaluated. This strategy will depend on the deposits’ geometry, and the followings are some examples of it: coalescent sandstones, separated, with lateral extension or with lateral short extension, which are shown in the following images and which were generated and validated using raw data. 

Figure 01. Illustrative models of distribution of reservoir rock in the area to be mapped. Sardelli 2019

The reservoir rock could be represented by different patterns, such as coalescent (Figure 1a) and compound (Figures 1b and 1c) patterns. In the case of compound models or patterns, the geometry of the deposits could change both laterally and vertically, that is, having a coalescent pattern in the southern part of the deposit and lenticular towards the north of it and vice versa. The Figure 1b shows an example in which the deposits are coalescingly arranged in the lower part of the stratigraphic unit and lenticular towards the top, while the Figure 1c shows a deposit coalescingly arranged towards the top and lenticular towards the base.  Another characteristic pattern is defined by the coalescence of several deposits arranged along a stratigraphic unit with intercalations laterally separated as it is shown in the Figure 1d. In addition, there are deposits that show a lenticular stratigraphy with sandstones deposits through the entire area as it is shown in the Figure 1e, and some in which the lateral continuity is limited with no sandstone deposits crossing the stratigraphic unit, as it is shown in the Figure 1f.

The final map will result from the sum of all deposits present in a stratigraphic unit (Figure 2). Regardless of the available information used to do the maps either considering the sum of the defined deposits or from well information, the most important input that must be considered is the geometry of the sedimentological deposits, that is, the way in which it is distributed laterally or vertically, which allow mapping it more accurately. It is important to mention that, there are currently specialized software, which allow the specialists applying those process in an automatic way; however, the quality of the results will depend of the technical knowledge applied by the specialist.

Figure 02. Projection of all net oil sand of levels A1, A2 and A3 to an A1-3 plane. Sardelli 2019

Analyzing the “sum of the sandstones deposits in a stratigraphic unit” principle in detail, it is important to understand the process of summing and contouring process to do unified maps, which starts with the assumption of the stratigraphic levels. (Figure 3). It is done by joining coincident points with X, Y geographic coordinates, and then, adding the obtained value for each one to generate a new “unified value” (Figure 4). Then, the contouring principle is applied in order to obtain the iso-values or isopach contours for each stratigraphic level of the stratigraphic unit. Finally, a total isopach map is generated by adding the isopach map obtained for each stratigraphic level. 

Figure 03. Projection of all oil levels in the Mapped Stratigraphic Unit. Sardelli 2019

Figure 04. Principle used for the construction of the unified map isopaques. Sardelli 2019

For the methodology exemplification, the following hypothetic names will be used:  A1, A2 and A3 (Figure 5). The final modeled geometry of the reservoir will be defined by every external boundary of each individual accumulation, which, from a plan view, would be represented by the “shadow” of the entire overlapping oil area. The Figure 6 shows the reservoir total isopach map as result of the sum of the existing sandstones deposits isopach maps.

Figure 05. Isopacos representing all net oil sand contained in the mapped stratigraphic unit. 

Sardelli 2019

Figure 06. Final Isopacos map representing all net oil sand contained in the mapped stratigraphic unit. Sardelli 2019

In next articles we will talk about:
-Contacts of fluids.
-Well Information