Dolomites in SE Asia — varied origins and implications for hydrocarbon exploration

Abstract

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.

Reservoir quality, textural evolution, and origin of fault-associated dolomites

“…Recent research reveals clear differences in depositional and diagenetic characteristics of equatorial humid-zone carbonates as compared with their more intensively studied subtropical arid-zone counterparts ( Wilson, 2002). However, because of the paucity of detailed data, the processes and controls on dolomitization of southeast Asian carbonates are largely unknown, and the models invoked have had poor data constraints (Carnell and Wilson, 2004). Spatial associations of dolomitization and karst features have been noted in southeast Asia, but are poorly explained (Carnell and Wilson, 2004).…”

“…However, because of the paucity of detailed data, the processes and controls on dolomitization of southeast Asian carbonates are largely unknown, and the models invoked have had poor data constraints (Carnell and Wilson, 2004). Spatial associations of dolomitization and karst features have been noted in southeast Asia, but are poorly explained (Carnell and Wilson, 2004). Mixing-zone (Sun and Esteban, 1994) and mudrock dewatering mechanisms have commonly been inferred.…”

“…Although carbonates are major reservoir units in Indonesia, and dolomitization is known to be patchy (0 locally to 100%), but widespread, the relationship between dolomitization and reservoir development remains poorly known (Carnell and Wilson, 2004). The most complete study of a fractured and fault-associated dolomite reservoir is of the Jurassic Manusela Formation on Seram, eastern Indonesia (Kemp, 1992;Nilandaroe et al, 2001), where an estimated 40 MMBO have been discovered.…”

Unravelling the depositional origins and diagenetic alteration of carbonate breccias

“…The minor occurrence of micro-dolomite rhombs along dissolution seams is indicative of Mg ions being locally sourced for dolomite cementation (Ali, 1995;Carnell and Wilson, 2004;Machel, 2004;Madden and Wilson, 2012;2013).…”

Combination and distribution of reservoir space in complex carbonate rocks

“…Thus, under tectonic stress, the reservoir interval is more likely to form structural fractures due to fracturing (Aqrawi et al 1998;Carnell and Wilson 2004;He et al 2012;Moutaz et al 2010). Fractures account for 71 % of the reservoir space in the reservoir interval but 29 % of the space in the nonreservoir interval.…”