PORE SYSTEM EVOLUTION AND FLOW UNIT OF MIOCENE CARBONATE RESERVOIR, NORTHWEST JAVA BASIN, INDONESIA: A SIGNIFICANT EFFECT OF DOLOMITE AND DOLOMITIZATION IN A WARM BASIN

Baturaja Formation is one of a very prolific reservoirs in Northwest Java Basin, Indonesia; a back arc setting with anomalous high geothermal gradient (30⁰ to 50⁰C/km) and heat flow which may have accelerated diagenetic transformation such as recrystallization, dissolution or dolomitization. This unit consists of coral-rich carbonate buildups developed on paleobasement highs, large-benthic foraminifera and red algae packstone-wackestone and muddy carbonates deposited on deeper part of the basin during a number of sea-level cycles in Early Miocene. Dolomite is present in offshore part of the basin and can be divided into three phases of dolomitization: sulphate reduction from organic material oxidation, mixing zone and methanogenic-derived dolomitizations. In addition, dedolomitisation is particularly common in the upper portion of the formation. The formation has been affected by several diagenetic processes impacting its porosity and permeability and, therefore, production and recovery on the fields. Generally, production zone is at the upper part of the formation which has porosity ranges from 10% to 15% and average permeability of 46mD. A complete dataset of well-logs, core samples, and 3D seismic data from an oil and gas field of the onshore of the basin are being analyzed to identify facies distribution, diagenetic evolutions in conjunction to reservoir properties development and structural control on the development of the formation.

As dolomite is reported in the formation, research questions are submitted on how dolomite was occurred, its distribution and evolution as it is unstable and can change to calcitized dolomite in meteoric realms. We propose that dolomite forms in early to shallow and deep burial dolomitizations. Early stage dolomite is affected by sea level fluctuations while lower portion of the formation could have dolomite cementation in the form of saddle dolomite as a product of deep burial dolomitization that occluded good initial porosity-permeability. This late stage deep burial dolomitization may have controlled by thermal convection model as it has higher temperature in the basin. Saddle dolomite as the result of this type of dolomitization at that tectonic setting could be precipitated at a shallower depth due to that high heat flow compare to the other type of basin.