How to Build New Interpretation Concept using Dynamic Data: A case Study in Carbonate of Upper Cibulakan, North West Java Basin, Indonesia

Bravo Structure is one of the Pertamina EP gas producing field in North West Java Basin region. This structure located at Subang about 10 km southeast from City Subang. The objective of this structure is a vertical compartment of carbonate due to diagenetic effect and transgression. This evident cause shale deposited and divide multi-layer carbonate reservoir. The carbonate lies at Cibulakan Formation and has three reservoirs: Z-14, Z-15, and Z-16 limestone. Since each reservoir has different pressure data and CO2 contain, so it becomes challenges to make new interpretation and to know those compartments between reservoirs. To look up geometry distribution detailed, this structure is reinterpreted using 3D seismic (2014) and new concept depositional environment based on wells correlation and core analysis. Generating seismic interpretation using seismic inversion and attribute. Amplitude envelope and instantaneous frequency are calculated to obtain sweetness. Both of them are built by 3D volume seismic. High magnitude from amplitude envelope is used to characterize geometry distribution of carbonate while instantaneous frequency determines low frequency because of gas distribution. The result of this study suggests vertical and lateral carbonate distribution characteristics Meanwhile sweetness attribute can determine gas contain in each layer Z-14 and Z-15. Vertical distribution of Z-14 layer about 140 m (porosity 7-11%) and Z-15 about 70 m (porosity 8-13%). Finally, interpreting carbonate both Z-14 and Z-15 as shelf margin that new concept gives a chances to develop this structure and optimize reservoir management.


Introduction
Bravo structure is one of PT Pertamina EP gas producer field in North West Java Basin region. It is part of Subang Field which located southeast from Subang city. Based on Regional Geology, it lies at Pamanukan High formed as result of bed rock lifting with a north-south orientation and it is part of Pasir Putih sub-basin system. This high is surrounded by fault system which separates Kepuh low at East and Pasirbungur low at West [1].
This structure has three main wells which are: Bravo-01, Bravo-02, and Bravo-03. Among those wells data, the objective of this structure is vertical compartment of carbonate due to diagenetic effect and transgression. Those (Figure 1).

Figure 1.
Well data correlation shows different pressure and CO2 contain each layer. The results of the processing conducted are the conceptual geological model built from acoustic impedance volume and sweetness. Acoustic impedance inversion can show carbonate characteristic associated with the porosity of reservoir. Sweetness succeeds to confirm stratigraphy feature. The high magnitude of amplitude envelope can describe the character of geometry distribution of carbonate reservoir. Instantaneous frequency expected to describe fluid distribution that trapped in the carbonate reservoir. The relation between acoustic impedance and sweetness used for creating a conceptual geological model of carbonate build up that refers to Bubb & Hatlelid concept [2].

Seismic Inversion Acoustic Impedance
Acoustic impedance inversion is a process to get reflection coefficient value from the seismic record that will be used to determine quantitative rock property such as acoustic impedance value. Inversion method for this study is the model based. It needs an early impedance model (initial model) from well log data, which are velocity log and density log [3,4]. Parameters used for inversion are background value 14,806 gr/cc.m/s, 40% model weight and 40x iteration in the inversion process.

Sweetness Attribute
Sweetness attribute is the empirical algorithm to identify reservoir existence in an area based on seismic data that can reveal high amplitude value and low frequency. The ratio between amplitude envelope and the root of instantaneous frequency can build it. This attribute will identify feature which has general energy change of seismic data [5].  To acquire acoustic impedance value distribution pattern laterally for carbonate Z-14 and Z-15, slicing with window width 10ms along the horizon Z-14 and Z-15 is done (Figure 4). The result can be used for facies interpretation of build up carbonate depositional environment based on rock property value, which is porosity. Low acoustic impedance value distribution that relates with porous carbonate has porosity value around 7-11% for Z-14 and 8-13% for Z-15. Refers to core analysis data (Table 1), depth 1935.5 m (Z-14) and depth 2080 m (Z-15), lithology description shows that carbonate facies is packstone with coral, algae, and foram contains. So that interpreted low acoustic impedance zone associates with shelf-margin facies (Figure 7).   Finally, interpretation result using acoustic impedance value and sweetness distribution, the conceptual geological model built for describing the geological condition ( Figure 6). Acoustic Sweetness attribute can describe a carbonate trap that contains fluid (gas) at 1935 depth with reservoirs thickness around 131-152 m. Acoustic impedance distribution, sweetness, petrophysical analysis and core can build a conceptual geological model of carbonate Z-14. Shelf-margin facies generated from acoustic impedance value distribution, petrophysical analysis, and core, whereas patch reef, shelf, and open marine facies determined by sweetness value distribution.