Diagenetic Evolution and Isotopic Characteristics of a Clast Breccia Reservoir in Nang Nuan Field, Chumphon Basin, Gulf of Thailand

Abstract

Hydrothermal karst in the Pre-Tertiary Ratburi limestone plays a significant role in the formation of reservoir in the Nang Nuan oil field, offshore Thailand. Hydrothermal karstification is considered the most likely mechanism to explain reservoir geology in the Nang Nuan field, (Heward et al., 2000). However, the exact controls on the reservoir properties and the style of hydrothermal alteration or diagenetic evolution are still uncertain. 

This research focuses on subsurface data from Nang Nuan wells and generates a carbon-oxygen isotope set of plotfields to identify the effects of ongoing diagenesis in fracture fills and the breccia matrix. Distinctive features of the breccias are documented and defined using rock textures and then integrated with thin section microscopy, stable isotope signatures and XRD determinations. Drilled cores and cuttings from 4 wells are selected for isotopic analysis and classified by lithology into 6 groups to differentiate diagenetic events in the Nang Nuan field.

Isotopic characteristics of diagenetic features in Nang Nuan field show two main trends: one follows a regional burial trend for the Ratburi limestone, similar to the burial trend seen in reservoir carbonates in Sinphuhom field in NE Thailand; the other trend implies a separate mixing trend. There are mixing trends in both Nang Nuan and Sinphuhom fields defined by significant decreases in _δ¹³CPDB values that relate to deeply-circulating telogenetic fluids. But the _δ¹⁸OPDB value range is different in Nang Nuan as it shows a vertical trend tied to increasing negative carbon values, with a limited and hotter temperature range in its oxygen values compared to Sinphuhom. Sinphuhom reservoir carbonates show a downward sloping trend tied to decreasing negative oxygen values that are related to exposure to ongoing shallower and cooler fluid crossflows that in turn tie to uplift and increasingly shallow meteoric waters. The difference in mixing trend between the two fields implies Nang Nuan experienced a shorter deeper (warmer) fluid event that would have allowed further porosity loss after the porosity-enhancing hydrothermal event. The differences in telogenetic fluid styles mean that Nang Nuan field will perhaps show lower proportions of connected permeability needed to allow lateral crossflows of reservoir fluids between matrix and veins, compared to Sinphuhom field.