Controls on pore systems and surfaces of the
carbonate-rich Eagle Ford Formation

Abstract

In all types of reservoirs, understanding the pore system is crucial to quantifying and
predicting multiphase fluid storage and flow. The prediction of the fluid storage and flow
helps to decipher the volume of the reservoir of interest, how easily accessible it is and
how recoverable its reserves are. In the hydrocarbon industry, the focus of many recent
studies is towards reserves in tight reservoirs. Improvements in the extraction methods
such as the introduction of horizontal wells and hydraulic fracturing have indeed
rendered these reserves economically viable.
However, quantifying the pore system of fine-grained reservoirs is hugely challenging
due to the mineralogical and textural heterogeneity at the microscale and the subnanometer
to micrometer size of pores. In this work, the pore system and pore surfaces
of the Cretaceous Eagle Ford Formation are characterised by analysing a set of 25 samples
from outcrops and six wells with maturities of R0 0.4-0.5%, 0.9% and 1.2%. The aim of this
work is to establish an analytical workflow for the characterisation of the pore system in
tight reservoirs, also by highlighting the importance of a multi-disciplinary approach,
often neglected.
The set of samples was analysed using a varied range of techniques; X-Ray Diffraction,
optical microscopy, Energy Dispersion X-ray spectroscopy (EDS), Scanning Electron
Microscopy (SEM) and micro-CT scans were used to reconstruct the mineralogical and
textural framework in which porosities occur. Petrographic studies show that the organic
matter (OM) is a marine type II kerogen and that microfacies vary from finely laminated
foraminiferal mudstones to packstones. SEM-EDS and Cathodo-luminescence (CL)
techniques were used to reconstruct mineral paragenesis and OM evolution. SEM and
Backscattered-SEM (BSEM) high resolution maps identified different pore types and
showed how pores change with maturity. At R0 0.4-0.5% the main porosity types are
interparticle, enclosed within the coccolithic matrix, whereas at R0 1.2% spherical OM
pores smaller than 20 nm are more frequent, related to the thermal maturation of the OM.
Pore sizes were calculated using a combination of SEM, N2 and CO2 gas adsorption and
Mercury injection Porosimetry (MICP). Immature and oil window samples present pores
larger (~2-100 nm) than samples in the gas maturity window(~1-40 nm). MICP analyses
indicate a connected pore system in all the samples. Focussed Ion Beam (FIB)-SEM volumes show that at R0 0.4 to 0.9%, the pore system is connected through interparticle
pores, whereas at R0 1.2%, the connectivity occurs through pore throats < 10 nm.
Environmental SEM (ESEM) observations and Chemical Force Microscopy (CFM) studies
at the nanoscale show that surface wettability depends on chemical variations of the fluid
interacting with the pore surfaces, also validated with AFM-IR analyses, and on the pore
surface mineralogy. AFM-IR studies also identified in situ chemical changes between
different organic molecules and between the same organic molecules at increasing
maturities. In summary, this work brings to light the necessity to use a combination of
physical and chemical methods to define the parameters affecting the pore system and its
evolution with time. Moreover, the use of state-of-the-art methods such as the AFM-IR
has allowed to validate previous theories on the organic molecules behaviour and to
suggest a new approach for further studies at the nanoscale of rock surfaces.