Carbonates in SE Asia range in age from Palaeozoic to Recent, but are most important as reservoirs in the Neogene where they comprise a major target for hydrocarbon exploration (e.g. Batu Raja Formation, South Sumatra, Sunda and Northwest Java basins). Carbonates of pre-Tertiary, Palaeogene and Neogene age all show a strong diagenetic overprint in which dolomite occurs as both cementing and replacive phases associated with variable reservoir quality. This paper reviews published data on the occurrence and types of dolomites in SE Asian carbonates, and considers the models that have been used to explain the distribution and origin of dolomite within these rocks.Pre-Tertiary carbonates form part of the economic basement, and are little studied and poorly understood. Although some, such as in the Manusela Formation of Seram, may form possible hydrocarbon reservoirs, most are not considered to form economic prospects. They are best known from the platform carbonates of the Ratburi and Saraburi groups. in Thailand, and the oolitic grainstones of the Manusela Formation of Seram. The Ratburi Group shows extensive dolomitization with dolomite developed as an early replacive phase and as a late-stage cement.Palaeogene carbonates are widely developed in the region and are most commonly developed as extensive foraminifera-dominated carbonate shelfal systems around the margins of Sundaland (e.g. Tampur Formation, North Sumatra Basin and Tonasa Formation, Sulawesi) and the northern margins of Australia and the Birds Head microcontinent (e.g. Faumai Formation, Salawati Basin). Locally, carbonates of this age may form hydrocarbon reservoirs. Dolomite is variably recorded in these carbonates and the Tampur Formation, for example, contains extensive xenotopic dolomite.Neogene carbonates (e.g. Peutu Formation, North Sumatra) are commonly areally restricted, reef-dominated and developed in mixed carbonate-siliciclastic systems. They most typically show a strong diagenetic overprint with leaching, recrystallization, cementation and dolomitization all widespread. Hydrocarbon reservoirs are highly productive and common in carbonates of this age. Dolomite is variably distributed and its occurrence has been related to facies, karstification, proximity to carbonate margins and faults. The distribution and origin of the dolomite has been attributed to mixing-zone dolomitization (commonly in association with karstic processes), sulphate reduction via organic matter oxidation, and dewatering from the marine mudstones that commonly envelop the carbonate build-up.Dolomite has a variable association with reservoir quality in the region, and when developed as a replacive phase tends to be associated with improved porosity and permeability characteristics. This is particularly the case where it is developed as an early fabric-retentive phase. Cementing dolomite is detrimental to reservoir quality, although the extent of this degradation generally reflects the abundance and distribution of this dolomite.Dolomitization is also inferred to have influenced the distribution of non-hydrocarbon gases. This is best documented in North Sumatra where carbon dioxide occurs in quantities ranging from 0 to 85%. There are a number of possible mechanisms for generating this CO2 (e.g. mantle degassing), although the most likely source is considered to be the widely dolomitized Eocene Tampur Formation that forms effective basement for much of the basin. High heat flows are suggested to have resulted in the thermogenic decomposition of dolomite with CO2 produced as a by-product.

Onset of rifting, andflooding by marine waters, occurred in the late Oligocene in the G u y of Aden and southern Red Sea. The northern part o f t h e Red Sea may have been a largely continental rift at this initial stage, but continued rifting established marine conditions throughout the system by the early Miocene. Episodic isolation of the Red Sea system, leading to evaporite deposition in some basins, commenced in the mid-Miocene and over two kilometres of salt had accumulated in most Red Sea basins by the end ofthe Miocene.Re-esta blish men t of persistentlym arin e conditions occurred in the Pliocene, and m arin e recharge is now sufficiently high to permit vigorous carbonate build-ups in shallow-water areas.elastic sediment textures suggest that marginal escarpments, which jirst developed during the onset ofrifting, were strongly uplifted in the Pliocene-to-Recent period. Subsidence of basin floors seems to have been particularly rapid during the period dominated by salt deposition. I f eruption of sea-floor basalts in the axial r$ areas is excluded, volumetrically important volcanism is centred on the present Afar triangle area, and is confined to the Oligocene and early Miocene. The amount of contemporary volcanic dkbris in the sandstones is consequently not particularly high. Some sandstones in northern Ethiopia, Sudan and Egypt do contain abundant acidic volcanic clasts, but these are derived from the Proterozoic basement and cause less diagenetic reservoir damage than contemporary glassy volcanic ash.Sandstones deposited in freely-drained alluvial fan settings are characterised by early diagenetic kaolinite, whereas those of sabkha and marginal-marine settings tend to show relatively early diagenetic chlorite. Those alluvial fan sandstones which were subsequently invaded by reduced pore waters espelled~from the basin axis, and those in the basin axis, ojen developed later diagmetic chlorite.

No abstract

No abstract