Evolution of the Early Miocene Carbonate: Baturaja Formation in Northwest Java Basin, Indonesia

The Oligocene-Miocene carbonate successions around the world are fundamental targets for hydrocarbon exploration and production. New findings indicate the importance of the red algae and large benthic foraminifera (LBF) association for this geological time, proposed as the main carbonate producers worldwide, accompanied by the scattered and small coral colonies. This skeletal domination also occurs in the Baturaja Formation, one of the prolific reservoirs in the Northwest (NW) Java Basin, Indonesia. This Early Miocene unit was deposited on a ramp setting governed by a north-south fault system. Previously, a classic barrier-reef to lagoon model, was applied to this formation, with corals as major carbonate factory which is a common setting for modern environments. The Early Miocene is characterized by global conditions of warm temperature and higher carbon productivity and higher CO2. The abundance of the red algae and LBF association might be related to their ability to adapt to these specific conditions. Contrastingly, corals tend to have difficulties in the euphotic zone and to build rigid and wave-resistant reefs to sea level in this setting. A regional paleogeography and depositional model are proposed for the Baturaja Formation using this skeletal association scheme. The ramp configuration also was controlled by the pre-existing topography and might be responsible for the facies variety across the ramp, facies distribution, stratigraphic architecture, and geometry of its carbonate mounds. The Baturaja Formation reservoir porosity have been known to be influenced by meteoric diagenetic processes, especially in its carbonate mound settings. Evidences of this meteoric dissolution events occur as dissolution voids and 13C-depleted intervals. Subaerial exposure episodes are indicated by the negative-shifts of δ13C. Acidic meteoric water flushing and soil derived CO2 are responsible for the dissolution of mainly ellipsoidal red algae and perforated LBF, that produced extensive moldic porosity with a fabric selective dissolution texture. This dissolution was then followed by precipitation of calcite cement while retaining its moderate strontium content. Meanwhile, pore spaces resulted by a possible burial dissolution are inferred by higher CO2 contaminants in the reservoir intervals at the well located at the deeper inter-mound depositional setting; an area unaffected by meteoric dissolution events. Mantle degassing- and magmatic-derived CO2 are likely the sources for this gas. The upward pathways for this corrosive CO2 are facilitated by deep-seated faults to the basement and intrusion-related faults formed in the area. Non-fabric selective dissolution texture is resulted by burial dissolution processes and it cuts across all the carbonate components. The burial dissolution porosity generation suggests to the shifting of the hydrocarbon exploration play from the well-known carbonate mound to the deeper inter-mound depositional target. Red algal-LBF packstone-grainstone facies with good moldic and vuggy porosity are expected. Meanwhile, fieldwork in the Mason County, central Texas, on the Early Cretaceous Fort Terrett Formation shows that the subtidal to peritidal succession consists of mud-dominated facies with minor intervals of grainy bioclastic facies. Dolomite beds are observed throughout this formation, in addition to the matrix-replaced dolomite layers in the Hensel Formation. Dolomite formations in these units resulted from the progressive development and recrystallization of the dolomite type. Interconnected fenestral porosity is proposed to have had a significant impact on initial permeability within the muddy succession of Fort Terrett Formation and provided the pathways and conduits for Mg-rich dolomitizing brines. A density head driven-mechanism, generated by dense hypersaline fluids from an evaporating lagoon, was responsible for the downward movement of the dolomitizing fluids