DEPOSITIONALAND DIAGENETIC CHARACTER OF CARBONATE SEDIMENTS ATTHE LEVEL OFA BASEMENT HIGH, OFFSHORE THAILAND: A NEW EXPLORATION PARADIGM FOR THE REGION?

Abstract

Is it possible to have a Tertiary-age carbonate reservoir in the Pattani Basin? This has long been an unanswered question. Especially when it is compared to other Tertiary Basins in similar tectonic settings across South East Asia, where there are many producing fields hosted in Tertiary reef limestones. So, this question was thought to have been finally answered, when a subsurface section that encompassed sediments above a basement high in the central Pattani Basin, Gulf of Thailand, was shown to 1) possess massive limestone characteristics in wireline logs and, 2) was located in a region where 2D seismic indicate geometries similar to that of a carbonate build-up. However, both seismic and wireline are indirect methods; the final answer is thought to lie in an advanced analysis of drill cuttings selected from a number of wells in the area. An analysis of the stable δ13C - δ18O isotope signatures of the cuttings, tied to a detailed petrographic and XRD study of the same chips, shows that the “reef-like” feature is not a Tertiary-age carbonate buildup. Rather, the drill cuttings show the two distinct diagenetic types linked to; 1) a basement section or 2) an overlying section. The basement section, including the distinctive reef-like feature, is a complex lithology responding to variable hydrothermal diagenesis in heavily-cemented calcareous (calcite - ferroan dolomite) mosaic of Permian siliciclastics and dolomitic-limestones. The isotope signatures in the basement complex indicate a number of these basement lithologies experienced a similar burial diagenetic intensity (and thermal evolution) to onshore outcrops of Ratburi limestone. Moreover, the δ 13C - δ 18O isotope signature in the carbonate-cemented siliciclastics in overlying Lower Miocene section show two diagenetic responses: 1) siliciclastic cements produced in a shallow burial sulfate-reducing environment, 2) Carbonate lithoclasts retaining the burial signature of their Permian  precursor. It seems conditions in the Pattani Basin never allowed marine reef formers to flourish. Isotope signatures in the basement indicate a potential for fracture and karst reservoir formation during the Oligocene syn-rift stage. Then there were places where the basement high was subaerially exposed and fault conduits could carry undersaturated meteoric waters into the carbonates, so generating zones of enhanced secondary porosity