Subvolcanic Rock Petroleum System Potential in the South Malang Region, East Java, Indonesia

The South Malang Region is located in the south-eastern part of the Southern Mountain Volcanic Arc; it presents different opportunities for hydrocarbon exploration. The stratigraphy of the study area from old to young consists of Oligocene Volcanic rocks (Mandalika, Watupatok, and Arjosari Formation), Early Miocene Carbonates (Campurdarat and Jaten Formation), Middle Miocene Volcanic (Wuni Formation), Late Miocene-Pliocene Carbonates (Nampol, Oyo, and Wonosari Formation), and Holocene alluvial deposits (Kalipucang Formation). The dominance of volcanic rocks makes this area considered an area without hydrocarbon play prospects. Petroleum system potential is revealed by evaluating and analyzing potential source rock and reservoir rock outcrop samples. The study shows that the Nampol Formation can be considered as a gas-prone source rock, with type III kerogen, total organic content ranging from 3.48 - 26.18 wt%, and possess the potential to produce good to very good hydrocarbons and a hydrogen index ranging from 43 to 86 mgHC/g TOC. Furthermore, rock core analysis and petrographic studies were carried out on the Nampol sandstone where the rock samples showed good reservoir properties. However, the Nampol and Wonosari limestone that was considered as the secondary target for reservoir possesses a low quality of reservoir properties. This study shows that there is a potential for petroleum system existence in the Southern Mountain subvolcanic arc, which is indicated by the presence of source rock and potential reservoir rock as one of the various elements and processes present in a petroleum system.


Introduction
The research area is located in the eastern part of Southern Mountain, more precisely in Malang Regency, East Java Province (Figure 1). The focus of this research is on the Nampol Formation and the Wonosari Formation which are expected to have the potential to become source rocks due to their organic material content, and to become reservoir rocks due to the presence of layers of sandstone and limestone which may have appropriate porosity and permeability properties. Shale, siltstone, calcareous claystone, and coal inserts [1] in this formation are thought to contain organic material that supports the presence of potential source rocks. The objectives of this research are: 1) to investigate the possibility of potential source rock, 2) to identify the potential of sub-volcanic rock as a petroleum reservoir, 3) to obtain the petroleum system potential of the Southern Mountains East Java sub-volcanic arc.

Methodology
Detailed geological mapping was carried out in several selected tracks. Source rock samples taken from outcrops on the surface were analyzed to evaluate organic material content, type of kerogen thermal maturity, and its potential to produce hydrocarbons through TOC analysis and Rock-Eval pyrolysis. In addition, reservoir rock samples were also taken to analyze the quality of their properties through petrographic analysis (qualitative) and rock core analysis in the laboratory (quantitative). Analysis was carried out on 3 samples of source rock and 6 samples of selected reservoir rock with the following information (Table   1).

Regional Stratigraphy
The Southern Mountains of East Java are generally composed of rocks with Neogene to Quaternary age. This area is mostly dominated by volcanic rock and limestone. Rock sequences are arranged based on detailed stratigraphy through selected paths and presented in a stratigraphic column ( Figure 2).
The stratigraphic order from oldest to youngest is as follows:   [1]. Source rocks are rocks that contain sufficient organic material to create hydrocarbons when subjected to heat and pressure over time [3]. Source rocks are usually shales or limestones (sedimentary rocks). A reservoir rock is a subsurface volume of rock that has sufficient porosity and permeability to permit the migration and accumulation of petroleum under adequate trap conditions [4].
part is interspersed with volcanic breccia, lava, and tuff. Based on its fossil content, the Arjosari Formation is thought to be in the late Oligocene-Late Early Miocene. The Arjosari Formation has a fingering relationship with the Mandalika Formation.

7) Oyo Formation (Tmo)
The Oyo Formation is a collection of clastic limestones whose formation is in-

Regional Geological Structure
In Java, there are three dominant structural patterns [5]. First, the structural pattern with a northeast-southwest direction (Meratus direction) was formed in the Late Cretaceous-Early Eocene age. Second, the structural pattern in the north-south direction (Sunda direction) was formed in the Early Eocene-Early Oligocene age. Third, the structural pattern with a west-east direction (Javanese direction) was formed at the Late Oligocene age by compression forces from the subduction in the southern part of Java Island (Figure 3).

Potential Source Rock
The results of TOC analysis (Table 2) on the source rock samples in the study area showed TOC values of 3.48 -26.18 wt% were categorized as good to very good in generating hydrocarbons referring to [7] and [8]. The cross-plot of TOC values was carried out on the values of S 1 + S 2 to determine the total generating potential of hydrocarbons that could be produced by each sample. The plotting results show that the total hydrocarbon potential that can be produced for the sample "LP01" is not good; "LP62" is fair, and "LP05" is good ( Figure 5).  The results show that the total hydrocarbon potential that can be produced for the code sample "LP01" is not good; "LP62" is fair; and "LP05" is good.
The diagram of the content of Total Organic Carbon (TOC) versus Potential Yield (PY) (Figure 6) shows the potential for hydrocarbons in the study area which is indicated by the level of richness of organic material content. This diagram shows that two samples from the Nampol Formation showed good organic material quality with TOC values of 3.48 wt% and 4.01 wt% and PY of 1.52 and 3.66 mgHC/g, respectively. Another sample showed very good organic material quality with a TOC value of 26.18% and a PY value of 19.55 mgHC/g. The three samples tend to form gas (gas prone) with one of the samples, code LP 05, possess the possibility to become a potential or effective source rock.
Based on the hydrogen index (HI) and of S 2 /S 3 values, it can be inferred that the type of organic material in the three samples is Type III kerogen (Figure 7). This kerogen contains humic organic matter derived from woody plants containing cellulose from land plants (equivalent to vitrinite in coal) [9].
Vitrinite reflectance (VR) and Rock-Eval Pyrolysis analysis were carried out on rock samples to determine the thermal maturity. From the results of the analysis of vitrinite reflectance in rock samples, values of <0.5% Ro are obtained ( Table 2). According to the classification of source rock maturity based on vitrinite reflectance value [10], the rock samples are categorized as thermally   "Immature" to become hydrocarbon source rocks (Table 3). Meanwhile, the results from Rock-Eval pyrolysis showed that the maximum temperature when breaking the three samples' kerogen are ranged from 408˚C to 430˚C. According to the Tmax classification and its relationship with the maturity level of the source rock [11], the rock samples are categorized as "Immature" source rock. (Table 3). Table 3. Guidelines for quantity, quality, and maturity of source rock determination.

Qualitative Analysis (Petrographic)
In the qualitative analysis of porosity, we used ImageJ software. This software has a feature to display microscopic images which then will be analyzed to obtain porosity percentage. The stages of this analysis start from giving blue dye fluid to a petrographic thin section. This liquid will fill in the pores so the pores in the rock thin section will be colored blue. Then, the plate of a thin section is displayed on a microscope. Then the image of the thin section will be taken and displayed on the ImageJ software. The analysis steps using the software are 1) open image, 2) change the image type to 8-bit, 3) adjust threshold, 4) analysis measuring. The porosity percentage results will be obtained afterward (Table 4).
After getting the porosity value, the classification is carried out using the porosity classification [12] (Table 5).    (Table 6).

Conclusion
Based on the results obtained, it can be concluded that black shale and coal in the Nampol Formation are immature source rocks but have the potential to produce hydrocarbons where at the peak of their maturity, it can be predicted its tendency to produce gas. Meanwhile, from the quantitative and qualitative analysis of the porosity and permeability of rocks in the Nampol Formation, it can be concluded that the sandstones of the Nampol Formation have more potential as a reservoir than the clastic limestone units. This study shows that there is a potential for petroleum system existence in the Southern Mountain subvolcanic arc, which is indicated by the presence of potential source rock and reservoir rock as one of the various elements and processes present in a petroleum system.