TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe determination of pore geometry and pore aperture size from mercury injection tests is a very helpful technique. Due to the amount of pressure data points that can be taken, a detailed characterization of the capillarity of the rocks can be accomplished. The size and geometry of the pores are related to variability in grain size, sorting and packing, as well as to factors such as the mineralogy and the diagenetic history of the formation. This paper presents the results of the integration of capillary pressure curves, conventional core analysis and petrographical Investigation, in a highly heterogeneous pore geometry, where a bimodal behavior of mercury-injection capillary pressure curves was identified, showing two main types of pore throat, one with a larger flow capacity than the other. This analysis was backed up by thin sections made in adjacent samples, where strong changes in sorting, grain size and increment in the percentage of clay matrix can be seen. The changes in pore geometry observed within such small distances, demonstrate the high heterogeneity of the reservoir rock.An attempt was made to classify the different rock types present in the reservoir, using the Winland (R35) relationship to calculate pore throat radius based on permeability and porosity. Despite the complexity of the pore system, it was found that R35 can appropriately represent the largest portion of the rock volume with the main pore throat radius that contributes to the flow.The fact that the mercury capillary pressure curves grouped by rock type always show the bimodal shape, was taken into account in order to understand the wells production in the reservoir, because the performance of the wells will depend on the volume of each main pore throat radius in the completed zones.

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The Santa Rosa field is located in the greater Anaco area in the Eastern Venezuela Basin. A total of 280 wells have been drilled, 65 with modern porosity logs. This situation makes the reservoir very difficult to be evaluated using only well log classical techniques. In the studied reservoir have been completed 74 wells, the production started in 1950, and a gas injection project was implemented in 1955. With the purpose of validating reserves and optimizing exploitation plans a petrophysical model based on rock type characterization was developed using existing cores, logs and production data. This model was extrapolated to the rest of the wells. Rock type characterization was based on mercury injection capillary pressure data, which allowed the determination of pore throat profiles for each rock type and the dominant interconnected pore system, which corresponds to a mercury saturation of 35% in a capillary pressure curve. An empirical relationship was used to related conventional porosity and permeability with pore throat, and was used to classify rock types. The upscaling procedure from core to logs was accomplished through a relation between gamma ray and neutron logs with core porosity and permeability in a key well. These relations were found to be dependent on rock type, and were used to extrapolate core characterization to those wells with only gamma ray or neutron logs. Porosity obtained from wells with neutron/density logs was compared to porosity obtained from the gamma ray - neutron -core porosity relationship, with very good results. Maps of rock type distribution were used to identify reservoir zones with better quality rock. These maps along with facies maps from the sedimentological study helped delineate the reservoir limits, in order to validate and identify prospective areas for future drilling. The characterization of a reservoir into rock type integrating geological, petrophysical and production data is fundamental for the development of exploitation plans. Introduction Effective petrophysical models require the integration of core, log and production data, in order to understand the variations in properties such as porosity, permeability, capillary pressure, geometry and fluid content of the rock. The Santa Rosa field is located in the Greater Anaco Area in the Eastern Venezuela Basin (Fig. 1). In the study reservoir 7 wells have modern porosity logs and 3 wells have cores. With the purpose of validating reserves and optimizing exploitation plans a petrophysical model was developed. Rock type characterization was based on storage and flow capacity and was estimated from routine core porosity and permeability data combined with capillary pressure results.

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.

Petrophysical analysis of unconventional plays that are comprised of organic mudrock needs detailed data QC and preparation to optimize the results of quantitative interpretation. This includes accurate computation of mineral volumes, total organic carbon (TOC), porosity, and saturations. We used TOC estimation to aid the process of determining the best pay zones for development of such reservoirs. TOC was calculated as a weighted average of Passey’s (empirical) and the bulk density-based (theoretical) methods. In organic mudrock reservoirs, the computed TOC log was used as an input to compute porosity and calibrate rock-physics models (RPMs), which are needed for understanding the potential of source rocks or finding sweet spots and their contribution to the amplitude variation with offset (AVO) changes in the seismic data. Using calibrated RPM templates, we found that TOC is driving the elastic property variations in the Avalon Formation. We determined the layering and rock fabric anisotropy using empirical relationships or modeled in the rock property characterization process because reflectivity effects are often seen in the observed seismic used for well tie and wavelet estimation. A Class IV AVO response was seen at the top of the Avalon Formation, which is typical of an unconventional reservoir. We then performed solid organic matter (TOC) substitution to account for variability of elastic properties and their contrasts as expressed in seismic amplitudes. To complete the characterization of the intervals of interest, we used conventional seismic petrophysical methods in the workflow and found that the main driver modifying the elastic properties for the Avalon shales was TOC; this conclusion serves as a foundation in integrated seismic inversion that may target lithofacies, TOC, and geomechanical properties. Seismic reservoir characterization results are critical in constraining landing zones and trajectories of the horizontal wells. The final interpretation may be used to rank targets, optimize drilling campaigns, and ultimately improve production.