Low Resistivity - Structural Clay Reservoir: GOM Case Study
This quick analysis is related to an offshore field located in the Gulf of Mexico. The main reservoirs are made up of Tertiary arkosic sandstones found at a depth varying from 1300 to 2800 meters (4264 to 9184 feet).
The field was discovered in a study performed on a seismic anomaly classified as a flat spot, that is a direct hydrocarbon indicator (DHI). The seismic sections show reflectors of high amplitude whose plane geometry was associated with a fluid contact. Moreover, there are some other reflectors of anomalous seismic character, but lower amplitudes that cannot be classified as DHI. Consequently, drilling was carried out through the entire sedimentary column, including low seismic contrast packages.
The deeper reservoirs have a very complex lithology, with a high content of structural clay, dispersed-clay, plagioclases, K feldspars and carbonates. The mineral complexity, especially structural clay, as well as the high salinity of formation water affect the log readings, causing the formation to show high radioactivity and low resistivity, which in turn, makes their identification and evaluation difficult.
Petrophysical characterization and production tests in the exploratory well confirmed these low resistivity structural clay reservoirs as hydrocarbon producers.
Logs are affected by shaliness and, the effect produced by it, depends on the amount of shale, its physical properties and the way it is distributed inside the rocks. In this case study, the evaluation model took into consideration structural and dispersed clay in accordance with the observations noted from core and cuttings analysis. Structural clay is shown as matrix grains, whereas dispersed clay lodges in the intergranular interstices, and as a result a decrease in porosity occurs. (Fig. below)
The conceptual model illustrated in the figure to the left, shows a lithology made up of structural and dispersed clay, feldspars (Orthoclases and Plagioclases), carbonates and quartz. In addition to the complex lithology, the very high salinity of formation water (129 to 344 Mppm) produces greatly affected well log responses.
Observations derived from seismic interpretation before drilling, as the interpreted saline intrusion allowed predicting a probable regimen with high salinity water. In addition to the expected low resistivity, lithological description from samples taken from drilling cuttings and cores, evidenced considerable percentages of structural clay, which induced a further drop in the formation’s resistivity.
Taking into consideration the aforementioned findings added to the degree of oil impregnation found in the cores and cuttings, as well as chromatography records of hydrocarbon shows from mud logging, a petrophysical model was derived for the exploratory well. This allowed for a successful identification of low resistivity structural clay reservoirs, whose producing capacity cannot be easily detected by way of conventional log analysis.
Having integrated the petrophysical characterization, structural interpretation and seismic anomalies found, followed by production and pressure tests carried out in several intervals of the exploratory well, reserves associated were calculated.
Methods and Procedures
The evaluation process was divided into two main stages: calibration of the lithological model and saturation adjustments. The lithological model calibration consisted of two phases: first, dispersed clay determination was carried out; and then, matrix components were quantified (structural clay, quartz, feldspars and carbonates).
1 - Dispersed clay estimation
Dispersed clay lodges itself in the intergranular spaces, as can be observed in the conceptual model explained before, thus decreasing porosity. Using this model as a basis, it is assumed that low sandstone porosity is a function of high shaliness, thus a dispersed clay log can be calculated from porosity data (logs, cuttings and cores) and other porosity dependent measurements (spontaneous potential and resistivity logs). This calculation must be done using all available lithological data from cores and cuttings.
In the case of some layers, as shown in the figure to the above, (2690-2700 m; 8825-8858 ft), the spontaneous potential (SP) -green line- shows a deflection that provides evidence of porous system connectivity and the gamma ray logs (GR) –blue line- shows high radioactivity. To calculate dispersed clay in this interval, SP was used considering lithological descriptions from core and cuttings, as well as thin section petrography and SEM data.
2 - Matrix components calculation
The calculated dispersed clay log is used as an input for solving the matrix components of the multimineral model, which also included structural clay.
The dispersed clay log calculated from SP in the intervals of interest shows little shaliness, but the GR log readings are high, indicating the presence of radioactive minerals in the matrix. Thin section microscopic analysis demonstrates the presence of structural clay beside feldspars. These components were input into the numerical model and the results obtained show a good correlation between the weight percentages of total clay measured by x-ray diffraction (XRD) and, the total clay (dispersed and structural) as calculated from the numerical model (Figure below).
Average core weight percentages, as determined by XRD, show the presence of clay, up to 76% of total clay, even though core porosities range from 19 to 32% in the same interval. The intergranular porosity of the system can also be appreciated in thin sections. Moreover, the core showed observable high oil impregnation and fluorescence.
The figure to the left shows a high resolution (X1500) view, part of a sample from 2694.17 m (8839.14 ft), with an abundant clay matrix (E8); also the details of an intergranular pore, partially occluded by an illite / smectite (H7) and some other remaining pores.
3 - Fluids calibration
In order to calculate saturations, clay parameters were calibrated using weighted average properties based on clay type data obtained by XRD and SEM. Total clay volume (structural and dispersed) was plugged into the Dual Water equation, taking into consideration the additional effect of structural clay on true resistivity and consequently on hydrocarbon saturation.
Hydrocarbon shows, core and cuttings’ oil impregnation and fluorescence were considered while calculating saturations.
Track 3, in the figure, shows a simple model without structural clay in it and, track 4 represents the adjusted model according to the protocol outlined.
The first model shows a low hydrocarbon saturation, whereas the second one exhibits much higher oil saturation, validated by production tests carried out in several intervals, and chromatographic data logged during drilling (last track on the right).
The methods and procedures used for the evaluation in this study, allowed to visualize and successfully prove the presence of oil-bearing zones whereas it were not previously detected by conventional methods.
Formation characterization together with seismic information, made it possible to estimate reserves associated with reservoirs.