STRUCTURAL DEVELOPMENT OF THRUSTS, FOLDS AND FRACTURES IN QUARRY EXPOSURES OF PERMIAN LIMESTONES, PAK CHONG, NAKHON RATCHIMA PROVINCE, CENTRAL THAILAND: IMPLICATION FOR FRACTURED RESERVOIR DEVELOPMENT IN NE THAILAND

Abstract

Detailed field study and laboratory analysis show Permian carbonates in Pak Chong, Nakhon Ratchasima Province are deformed into folds and thrusts by N - S compressional stress during the burial stage (Indosinian Orogeny). This created a series of calcite-filled E - W fractures, and related features, via transpressional stress. After that, tectonic reactivation during the Cenozoic created a set of N - S fractures. These N - S fractures are widespread at the reservoir scale and tend to be partially to completely open fractures.
This structural evolution, as mapped in the field, is supported by stable isotope measurements (carbon and oxygen) of vein-filling calcites, which show a clear temperature and time-relation separation along the same burial trend into: Group I: cooler and shallower; defined by pervasive fluid crossflow in the matrix, driving ongoing calcite cementation and isotope re-equilibration in the matrix and in early fractures, and, Group II: hotter and likely deeper; indicates fault- and fracture-focused calcite precipitation. By this burial stage the matrix is largely impermeable due to pervasive calcite cementation and stylotitisation, which had shut down matrix fluid flow, so that fluid flow was confined to tectonically activated fractures and faults.

This temperature-related bipartite isotope trend refines the earlier isotope burial trends established by Ampaiwan (2011) and Thanudamrong (2011), who studied structural and diagenetic evolution in the same folded and thrusted formation (Khao Khad Formation), but at different locations in Central Thailand. In combination, this study and the previous two, have now established a reliable stable isotope-derived burial trend in Central Thailand (made up of more than 250 isotope pairs). It can be used as a yardstick to test and compare other isotopic analyses conducted in Permian carbonates of Thailand. Much of the earlier work in Thailand has interpreted stable isotope values in various spar cements as indicators of meteoric or mixing zone diagenesis.

The model of structural evolution, fracture development and sequential calcite precipitation, as defined in this study, can now be used to refine our understanding of fluid flow evolution in fractured reservoirs hosted in Permian carbonates in gas and oilfields; such as Sin Phu Horm and Nam Pong gas fields in NE Thailand and Nang Nuan field in offshore Thailand.