Integrated Petrophysical Model, C4 and C5 Reservoirs, VLA-6/9/21 Area, Block 1, Lagomar, Lake Maracaibo, Venezuela

Abstract

Reservoirs C4 and C5 belong to the VLA-6/9/21 area of the Lagomar Segregation, and are located in the East Flank of the Block I, Lagunillas field, Lake of Maracaibo Basin, Western Venezuela. The C sands correspond to the Misoa formation of Eocene age. For the present study, 25 wells of the area were selected, 12 of which belong to a special Integral Field Laboratory, and have enough information that allowed the generation of a model that can be extrapolated to other wells in the field. Using all this information, certainty maps for each reservoir were constructed. Pore throat radius was estimated from porosity and permeability data from conventional core analysis, along with capillary pressure data. Winland's equation, an empirical relationship between porosity, air permeability and pore throat radius corresponding to a mercury saturation of 35%, was used for reservoir C5. In 1992, Winland's work was modified by Pittman, and for reservoir C4, the R45 Pittman equation was applied. Cementation and saturation exponents (m, n) for C4 and C5, were determined from core analyses. The resistivity of the formation water (Rw) was estimated from chemical analyses and by the SP curve method. Clay volume (Vsh) was obtained using Larionov's tertiary rocks model, which correlated well with X-Ray difraction analyses. Once the key wells were evaluated using the Waxman-Smits water saturation model, which resulted in a good match between core and log data, the evaluation was extrapolated to the other new and old wells of the area. Five different rock types were identified: Mega, Macro, Meso, Micro, Nanno, and were used to construct cross-sections showing their lateral distribution. Flow units were identified using the Modified Lorenz Plot and the Vertical Flow and Storage Diagram. Rock type areal distribution maps were also constructed, and have an excellent agreement with the sedimentary facies maps obtained from the sedimentological study. Geological Information Reservoirs C4 and C5 belong to the VLA-6/9/21 area of the Lagomar Segregation, and are located in the East Flank of the Block I, Lagunillas field, Lake Maracaibo Basin, Western Venezuela, as shown in Fig. 1. Both reservoirs lie in a tectonic block bounded by normal faults in all directions, separating them from other reservoirs in the same area. Their limits are: North and East: East Fault, West: Icotea and Atico Faults and South: a fault that separate them of VLA-8 area, (Fig. 2). The Misoa formation of Eocene age is subdivided in members B (B1 to B9 sands) and C (C1 to C7 sands) based on the characteristics of the electric well logs. The present study was carried out in sands C4 and C5. Available Information For the present study a total of 25 wells were selected, twelve (12) of which belong to a special Integral Field Laboratory, and have enough information that allowed the generation of a model that can be extrapolated to other wells in the field. Five wells have core analysis, reporting porosity values and permeability, but only two of them have complete special analysis in reservoirs C4 and C5. Wells that were completed in these reservoirs have detailed production data, production logs and chemical analysis of water samples. Information of sedimentary environment exists, because facies areal distribution maps coming from previous studies1.

Predicting Permeability From Well Logs in Carbonates With a Link to Geology for Interwell Permeability Mapping

“…[5][6][7][8] This is accomplished either in cored wells with directly measured porosity and permeability or in uncored wells using separate porosity and permeability well log models. Nevertheless, the model can be used to predict permeability if the pore-throat radius has been independently estimated or if a rock type based on the pore-throat radius has been assigned.…”

“…If variations in the flow-zone indicator or pore-throat radius could be independently estimated laterally within the rock-fabric layers of each HFC in a reservoir, using methods applied in wells, 8,9 then permeability could be modeled in 3D just as well with the aid of these parameters as with the rockfabric number. However, there is no method to measure these properties between wells, and there is as little information about their relationship to carbonate stratigraphy as there is for permeability itself.…”