Offshore Mexico Cretaceous and Jurassic carbonates are typically brecciated, dolomitized, low porosity fractured reservoirs. These reservoirs are very important resources in Mexico and are petrophysically extremely difficult to interpret using common analysis methods and traditional data.
Reservoir porosity sometimes exceeds 10 percent; but often reservoir pay zones are less than 7 percent due to the different types of intrinsic pore geometries. "Archie m" may vary from 1.3 to greater than 4. Resistivities in non-productive and productive sections can be very similar due to the low porosities. Nuclear magnetic resonance (NMR) logs are considered questionable in many areas due to the low porosity and other conditions. Compounding the challenges with interpretation, this region of the Gulf of Mexico is considered a HPHT (high pressure and high temperature) environment.
Core-log integration requires a working knowledge of how the borehole tools respond to lithology, reservoir fluids and the limits of vertical resolution and depth of investigation.
These challenges require PEMEX petrophysicists to understand geology, pore geometry, mineralogy, diagenesis and stress information before developing an integrated porosity model. Delivering a simple lithology interpretation is a starting point, but the complexity of these reservoirs requires a rock-based integrated lithology model that is calibrated to a core description, lab based mineralogy measurements and petrographic analysis. Permeability predictions are no longer simple linear regressions, but should include a minimum of three components (lithology, porosity and stress) and are often extended to advanced statistical methods. Log based saturations for reservoir models are rarely based on simple electrical rock properties and constant log parameters. Saturation determination has evolved into considering various pore geometry models (1–4 systems), complex wettability, capillary properties and integrated deterministic petrophysical rock types. Results are confirmed by using customized saturation height models.
Results from three fields show uncertainty is reduced by using deterministic Petrophysical Rock Typing (PRT) methods and that the key to success is determining if there is a relationship between initial water saturation obtained from wellbore analysis and predicted pore throat size using various techniques. The method requires access to high quality wellbore data and pore geometry based special core analysis (high pressure mercury injection porosimetry, thin sections, mineralogy, petrographic analysis, electrical rock properties and wettability data). Nevertheless some wells do not have core data available. In that situation the use of cuttings provides key reservoir calibration data.
Through the development of these petrophysical characterizations, several traditional methods have been modified and used on several fields. These "new practical methods" will be highlighted and presented.
PEMEX is using these methods to identify value added opportunities in exploitation, exploration and production.