Fluid History of Hydrothermal Textures in an Exposed Quarry in Chumphae District, Khon Kaen Province, Northeast Thailand

Abstract

Fractured reservoirs produce gas in the Phu Horm & Nam Phong region of NE Thailand, but the reasons for this production, as well as timing of the fracturing and porosity enhancement in the Permian limestone host are poorly documented and not well understood. Fractured and brecciated analogues to Phu Horn reservoir outcrop were studied within a nearby quarry on the western margin of the Khorat Plateau, near Chum Phae, KhonKaen. There we have exposures of different types of local fault-controlled breccias and more widespread less disturbed non-breccia areas, with distribution patterns controlled by the interactions of tectonic fracturing and fluid flow. Detailed measurements of fracture orientation and density in the quarry show that the scale of fracturing, the fracture trends, and distances between fracture corridors are not unlike similar scale features in Phu Horn field, but the maximum horizontal stress directions are not the same.

In the quarry, the integration of outcrop observations (fracture orientations and mesoscale textures) with microscale laboratory analyses (thin section, XRD and stable isotope determinations) define the timing of fracturing in relation to episodes of tectonically-driven fluid flow. Stable isotopes were collected from a series of matrix blocks and calcite veins in both the brecciated and non-brecciated zones. The integration of the stable isotope measurements in these two zones, with the thin sections collected from the same sample sites, shows that matrix blocks in non-brecciated zones are less altered than matrix in the brecciated zones. Matrix blocks in brecciated zones show evidence of pervasive hydrofracturing, indicated by dense networks of criss-crossing microveins and a re-equilibration in the isotope data. 

The matrix in the nonbrecciated zone preserves the regional burial signal typical of the Permian carbonates of northern and central Thailand. In contrast the micro-veined hydrofractured matrix in the breccias and all the calcite from fractures in both zones show the influence of a carbon-depleted organically-influenced fluid source, tied to a tectonically-driven fluid flow event. This event is later than burial during the Indosinian Orogeny and may be as young as mid Cenozoic. It is not related to modern meteoric karstification.

Based on this new isotopic insight, we now know that there was pervasive hydrofracturing during overpressuredriven deformation in northern Thailand. It occurred in a tectonically-drivenmethanogenically-influenced fluid escape event. It is likely that this fluid event played an important role in the development of fractured reservoirs in the subsurface of northern Thailand.