Fracture Analogue Modeling and Fluid Evolution, Central Thailand: A Tectonically Driven Fluid System

Abstract

The ultimate aim of this study is to develop better relevant local diagenetic analogs for the fluid and structural evolution system that created the economic porosity in producing carbonate reservoirs in the subsurface of northeast and southern Thailand.  This is done using field observations tied to detailed petrographic and stable isotope determinations focused on exposures in a quarry in Nakhon Sawan Province, central Thailand. There outcrop studies show the folds and thrusts in the Permian limestone underwent complex deformation related to the Indosinian Orogeny (Late Permian to Early Triassic) and that some fractures within this structural grain were reactivated during Cenozoic time. The deformation history evolved mainly under the influence of (approximately) NNE – SSW directed compression. The largest thrust faults in the study area trend SE-NW, and dip 30° – 40°. Fault bend fold and duplex structures are also present. Two types of fractures are present: parallel to sub-parallel of bedding and inclined or sub-vertical of bedding. Fracture sets can be grouped into two main categories: a) Bedding-parallel fractures with WNW– ESE trend, and b) inclined or sub-vertical fractures with bedding showing a NNE– SSW trend. These fractures progressively developed during folding and thrusting in the core zone of the deformation. The high-angle fractures in the fault zone are truncated by a late stage subhorizontal pressure solution cleav-age, which probably developed during loading by a now eroded higher thrust sheet.  Stable oxygen and carbon isotope signatures in calcite veins indicate that they are related to and evolv-ing tectonic history. Group I defines a C-O plot field that indicates lower burial temperatures, it is considered to reflect a cooler, likely earlier, part of the mesogenetic burial trend. Group I is especially apparent in the matrix across the quarry and is also associated with the development of widespread early fractures and stylolites. Group II is interpreted as a late burial stage calcites precipitated in what was a hotter and perhaps deeper sub-surface environment. Group III is an intermediate set of C-O values possibly derived during mixing of deeply circulating meteoric fluids in the early stages of telogenetic uplift. The bicarbonate in the pore fluids precipitat-ing Group III calcites is strongly influenced by dissolution of rock material in a deep stagnant phreatic zone. Recent speleothem samples give slightly variable oxygen numbers in an uplift trend with consistently more negative carbon values than Group III, and so defines the isotope cluster that is Group IV. Group IV samples come from zones of carbonate soil in karst, speleothems and slickensides calcites. The Group IV plot field de-fines a mixing trend where the CO2 and consequent bicarbonate in the precipitated calcites are strongly fluid influenced during fluid-rock interactions in the zones of active phreatic and vadose circulation. Much of its negative carbon signature indicates precipitation from rainwaters that have moved through zones of soil gas, that is, flow in the upper meteoric carapace to the uplifted sediments....