Abstract

The Sunda sub-basinal area lies immediately west of the Seribu platform, within the present day Java Sea. Within this region the Baturaja Formation consists of a Lower Miocene transgressive sequence of limestones, secondary dolomites, claystones and shales conformably overlying the Oligocene Talang Akar Formation.Three distinct facies, namely, shelf, basinal and reef are discernable within the Baturaja Formation.

The basinal facies is the deepest and thickest facies and is characterized by low-energy, clay-rich carbonates, both grain and mud supported. The granular fabric was derived from faunal debris and silt to fine grained detrital material, principally glauconite and terrigenous quartz. The faunal elements are dominated by open water planktonic and calcareous benthonic foraminifera along with fragmented pelecypods and echinoderms. Porosity is of the intergranular type and is relatively low. The basinal facies grades up-dip into the stable shelf facies and, locally, into the reef facies.

The shelf facies is characterized by grain supported carbonate, the granular texture having been derived from glauconite and detrital quartz along with various faunal elements. The fine to locally coarse grained biomicrites of the shelf facies range in faunal content from 20% to 50%, echinoderm and pelecypod fragments as well as open water planktonics and the larger forams being the dominant constituents. Carbonates of the shelf facies are of shallow water, inner-sublittoral origin, deposited under low to moderate energy regimes. The bulk of the shelf carbonates are rather argillaceous in nature, the clay content having greatly reduced the porosity of the primary matrix. Occasionally, where the shelf areas were exposed to effects of periodic sub-aerial erosion, meteoric induced solution-vugular porosity was developed, creating a potentially productive reservoir rock.

Where basement controlled platform areas were present, lying favorably within the unrestricted path of current flow, isolated patch/platform type reef growth occurred. These Baturaja bioherms are elongate: possibly due to a prevailing current direction, and sub-circular in plan (average width to length; 1:4). Reefward, the carbonates of the shelf facies increase in both faunal and floral grain content and size. As the inferred depositional energy increases, isolated occurrences of foraminiferal shoals, as well as energy-induced biosparites, are encountered. Coral debris biomicrites formed by reef talus, tightly apron the bioherms and grade inward into the in-situ coralline biomicrites which define the leading edge of active coralline reef growth. Faunal contents range from 50% to 80% with compound corals being the dominant constituents, and to a lesser degree echinoderm and pelecypod fragments, and minor amounts of gastropod and bryozoan debris. Red algae are the dominant floral component. Coral debris biomicrites also fringe the inner edge of the reef top, with faunal content, grain size and depositional energy generally decreasing inwardly, Owing to the highly granular nature and the general absence of interstitial clays, these reefal carbonates demonstrate moderate to high, primary intergranular porosities. The preferential solution effect of ground water within the porous reefal framework has caused dissolution of the aragonite skeletons, creating solution-mouldic porosities as high as 40%. Where extensive leaching of the coralline framework is in evidence, the underlying carbonate is often found to be affected by sparry calcite cementation within the available pore space. This cementation is thought to be the result of precipitation from the supersaturated fluids derived from the leached coral framework. The reef carbonates are also prone to neomorphic recrystallization of the micrite matrix to microspar and pseudospar, which also serves to destroy any pre-existing porosity. This recrystallized carbonate is often quite dense (interval velocities of 12,850 ft./sec. at 3600 ft.) and also fracture-prone. Where fracture systems are developed, this rock type has the potential for being, initially, a highly productive reservoir rock.