Publications

A detailed fracture characterization of Saraburi Limestone outcrops was made in a Muak Lek quarry and a Chumphae quarry in order to develop a facies-based understanding of fracture distribution in the exposed Permian limestones of Thailand. This study is the first stage in developing an analogue understanding for fracture distributions in subsurface limestone reservoirs in Thailand and elsewhere. The work integrated petrographic, stable isotope determinations of outcrop samples with mapping and photographic documentation of fracture properties in the two areas. Three spectral gamma ray profiles were measured and integrated with photographic data to construct a pseudo-FMI log made along the gamma pseudowell traverse. This FMI construction assumes a microresistivity contrast is present for each fracture within the section of interest. In both quarries, there are two lithofacies associations that have distinctive fracture characteristics. Factors influencing the differences in fracture density between the facies are, grain size, bed thickness, and elastic modulus. For example, in the Muak Lek area, it clear that fracture density has a strong match to the host rock type, the fine-grained lithofacies, with their thinner beds and smaller elastic moduli, show higher fracture density and smaller apertures compared to the coarsergrained lithofacies. In contrast to Muak Lek, the Chumphae outcrop, because of pervasive diagenetic-silica cementation, shows similar fracture densities across the various lithofacies. In summary, study of outcrop analogs to fractured Permian carbonates, which constitute potential reservoirs in the subsurface of central and northeastern Thailand, show that the fracture density and fracture aperture are responses to variations in the mechanical strength. In some diagenetic situations these relationships are resolvable in a gamma log, in others they are not. This has significant implications when a gamma log is used to cluster FMI-based fracture observations in wells penetrating subsurface platform carbonates.