Textural and Mineralogical Characteristics of Chicontepec Reservoirs in Mexico; Keys to Understanding Oil Productivity
Authors: Granados-Hernandez Juan Carlos , Fisher Quentin J 
Published: March 2015

Abstract
A number of subsurface rock samples from two oilfields of the Chicontepec reservoirs in Mexico were analysed through a series of descriptive techniques that included RCA, MICP, XRD, optical microscopy, NMR and SEM. The textural and mineralogical characteristics of these samples were carefully described and later linked to their porosity-permeability values. A number of rock types, with different textures and permeability-porosity ratios, were recognised from this approach. Rock-type definition in this work resulted from the integration of: the distribution of detrital and authigenic minerals based on petrographic descriptions;the mineral identification and quantification from XRD analysis;the pore-throat size from mercury-injection data; andthe routine porosity-permeability data.
The relative abundance of quartz and calcite play an important role in controlling the Chicontepec reservoir-rock quality. A minor variation in such abundances generally exerts a severe impact on their petrophysical features. Higher quartz-to-calcite ratios generally resulted into greater permeability-porosity values. Moreover, sandstones with coarser and well-sorted detrital grains normally have higher quartz contents, demonstrating that textural and mineralogical characteristics can be linked to the petrophysical features of these rocks. Additionally, beyond what has traditionally been believed from these reservoirs, the authigenic clay fraction seems to not being exerting great influence in controlling rock-quality.
The results from this work provide a new model that can be used for improving the characterization of the Chicontepec reservoirs. In particular, mineral content and textural features, as it is demonstrated in this study, may be employed as a predictive tool for evaluating these challenging reservoirs.

Introduction
Chicontepec reservoirs contain nearly the 36% of country's total reserves (SENER, 2014) and represent one of the most important producing plays in Mexico. The Chicontepec project is under intensive development strategy and currently produces a stabilised output of 46,000 BOPD (CNH, 2014).
The low-oil production from these reservoirs, which averages 20 BOPD -per operating well- has generally been believed to be the result of the fine-grained, low-permeability fabric of the rock. However, the low productivity of these reservoirs should be understood in a wider framework, that accounts for their depositional (e.g. sedimentary environment, texture), diagenetic (e.g. authigenic cementation, hydrocarbon emplacement) and dynamic characteristics (e.g. drive mechanism, connectivity of sand units, wettability). These factors need to be analysed in detail to understand their individual contribution to fluid flow, and in this way propose strategies to maintain/increase oil production. This work is focused on the textural and mineralogical characteristics, which control a significant portion of the rock quality.
The Chicontepec reservoir is a mainly litharenite composed of a high proportion of calcium carbonate, present in the rock as detrital fragments and authigenic calcite cement. Additional constituents are usually a variable amount of quartz, rock fragments and authigenic clay. Reservoir quality varies from moderate to poor, which in conjunction with the drive mechanism (i.e. solution gas drive), typically results into low productivities, high declination rates during primary production and a limited hydrocarbon recovery. Primary recovery (i.e. natural oil production by solution gas drive) represents less than 5% of the oil-in-place volume (Gachuz-Muro, 2009).