Understanding The Petroleum System of North Serayu Basin: An Integrated Approach from Geology, Geophysics, and Geochemistry PDF

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IPA16-63-SG PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Fortieth Annual Convention & Exhibition, May 2016 UNDERSTANDING THE PETROLEUM SYSTEM OF NORTH SERAYU BASIN: AN INTEGRATED APPROACH FROM GEOLOGY, GEOPHYSICS, AND GEOCHEMISTRY Dimas A. R. Prawiranegara* Fauzan Eka Saputra** Fisco Raseno* Ba...

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IPA16-63-SG

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Fortieth Annual Convention & Exhibition, May 2016 UNDERSTANDING THE PETROLEUM SYSTEM OF NORTH SERAYU BASIN: AN INTEGRATED APPROACH FROM GEOLOGY, GEOPHYSICS, AND GEOCHEMISTRY Dimas A. R. Prawiranegara* Fauzan Eka Saputra** Fisco Raseno* Bayu Hary Utomo*** Alfin A. Sani* Agung W. Wibowo**

ABSTRACT The North Serayu Basin is one of two active petroleum systems in Central Java. Wells drilled in this area failed to find economic reserves though there are numerous hydrocarbon seepages in the area. One hydrocarbon seepage occurs in the Watukumpul Area, Central Java. This paper integrates regional geology, detailed geologic data, biostratigraphy, paleostress analysis, potential reservoir characteristics, organic geochemistry and gravity. This paper studies the Watukumpul Area as a possible petroleum system model within the North Serayu Basin. Geologically, the study area has three formations which are the -- Rambatan Formation, Halang Formation, and Basalt Intrusion. Paleostress analyses were performed on 1606 shear fractures in study area, resulting in three interpretations of Shmax and the tectonic evolution. The tectonic evolution began with a Pliocene tectonic phase associated with the north-south compressional Java Wrench System. Further tectonics were associated with more localized compression from the Mount Slamet volcanism. Each element of a viable petroleum system is present in this area. The source rock candidate is derived from shale of Middle Miocene Rambatan Formation. TOC and vitrinite analysis shows a mature to post-mature organic contentup to 0.99%. Source rock could have attained maturity via the appropriate depth of burial and intrusion heat effects. HI analyses show kerogen dominantly type III; meaning the source has terrestrial origins; however, biostratigraphy and sedimentological analyses indicate marine depositional environment. Intraformational sandstones of Halang and Rambatan Formations may serve as potential * University of Jenderal Soedirman ** dr. Bumi Research Group (dBRG) *** GDA Consulting

reservoirs, with the sand-shale ratio up to 100% with visible fractures, dissolution, and intraparticle porosity. The seal is from intraformational mudstone of Halang and Rambatan Formations; with possible Quartenary volcanics are potential seals. Structural traps are postulated in the noted anticlines or thrust faults. Generated hydrocarbons could enter the traps of the toe-thrust anticlines formed in this area (Satyana and Armandita, 2004). Petroleum evidence presented in this study will be useful for future exploration in North Serayu Basin. Keywords: North Serayu, Geochemistry, Geology, Geophysics INTRODUCTION The North Serayu Basin is one of two active petroleum systems in Central Java. Numerous hydrocarbon seeps and one oil field in North Serayu Basin were reported by van Bemmelen (1970). The seepages occur in the areas of Karangkobar, Bawang and Subah, Klantung and Sodjomerto, Kaliwaru, West and East of Mount Ungaran, and Watukumpul. Oil seeps in these areas have been geochemically analyzed and sourced by sediments equivalent in age and facies with the Oligocene Talang Akar Formation in West Java Basin (Satyana, 2015). Comparison of regional stratigraphy of West, Central, and East Java with the occurrence of hydrocarbons is presented on Figure 2. However, a source rock study by Iswahyudi and Widagdo (2009) in the Watukumpul area interprets a potential source rock is derived from the Middle Miocene Rambatan Formation. The present study area is located in the Watukumpul Area (Figure 1); within the locality is a hydrocarbon seep. This is an area of complex tectonics affected by the Java Wrenching System

and gravity tectonics since Miocene to Pleistocene. Gravity tectonics of North Serayu Basin have been previously published by Prawiranegara and Saputra (2015), Satyana (2014), and Satyana (2007).   METHODS This paper integrates information from regional geology, detailed local geologic data, biostratigraphy, paleostress analysis, potential reservoirs, organic geochemistry, and gravity. Synthesis of these data was used to identify a possible petroleum system play in the study area. STRATIGRAPHY Stratigraphy of the study area is presented on a geological map by Saputra (2015) as seen in, Figures 3 and 4 and on stratigraphic columns (Satyana, 2007, Figure 2 and Figure 5). Three formations are exposed in the study area -- in sequence they are the Rambatan Formation, Halang Formation, and a Basalt Unit. Rambatan Formation Rambatan Formation exposures cover approximately 40 percent of the research area. This formation is comprised of predominantly claystone with interbedded sandstone (Figure 6). The claystone is dark grey, brittle, strongly calcareous with interbed thicknesses ranging from 5 to 100 centimeters. Sandstones are brownish to light grey, moderate hardness, fine to medium grain, well sorted, closed fabric, moderately to strongly calcareous, parallel lamination, cross lamination, wavy lamination, hummocky cross stratification, convolute, and consist of interbed thicknesses ranging from 5 to 15 centimeters. Based on Figure 4, D-D’ a geological section reconstruction the thickness of this unit can reach 314 meters. Micropaleontology analyses were performed on both benthonic and planktonic forams from this unit. Planktonic forams include Globorotalia archeominardi, Orbulina universa, Globorotalia lobata, Sphaeroidinellopsis subdehichens, Globorotalia mayeri, and Globigerinoides trilobuss. Benthonic forams include Robulus sp, Nonionella atlantica Elphidium sp, Eggrella advena, Amphistegina sp, Cibicides sp, Nodosaria Mexicana, and Pyrgo murhina. The presence of these forams indicate this portion of the claystone unit was deposited in middle neritic to outer neritic zones (30-150 meters) with an age no older than N9 and not younger than N13 or Middle Miocene  

(Blow, 1969; Bandy 1967) which is equivalent to Rambatan Formation (Djuri et al, 1996). One sandstone sample had been plotted to Pettijohn's classification (1975) and classified as feldsphatic wacke, while the claystone was classified as mudrocks (Table 1). Lithic Wacke consists of lithic (10%), feldspar in the form of plagioclase (32%), quartz (23%), opaque mineral (9%), and foraminifera (9%). The matrix (28%) is composed of clay mineral and lime mud. The composition plotted in a Q-F-L diagram (Dickinson & Suzcek, 1979) shows that the Rambatan Formation sample was derived from a magmatic arc provenance. Sandstone from the magmatic arc provenance has characteristics of a higher percentage of feldspar mineral than lithic fragments and quartz minerals. Halang Formation Halang Formation exposures covered approximately 55 percent of the research area. This formation is comprised of predominantly sandstones, interbedded with claystones. The Halang Formation was deposited conformably above the Rambatan Formation. Two sandstones are found in this formation are primary sandstone and fragmented sandstone. Fragmented sandstones are found at the bottom of this Formation. Primary sandstones units are characterized by grey color, slightly calcareous, fine to medium grain, well sorted, parallel lamination, wavy lamination, convolute, graded bedding, slump, and consist of interbed thicknesses ranging from 20 to 100 centimeters. Fragmented sandstone are greenish grey, hard, sand to gravel grain, poor sorting, slightly calcareous with interbed thicknesses up to 8 m. The composition consists of hornblende, plagioclase, quartz, andesite lithic, mudclast, and clay minerals. Claystones are dark grey, hard, slightly calcareous with the thickness ranging from 5 to 10 cm. Based on Figure 3, E-E’ a geological section reconstruction the thickness of this unit can reach 632 meters. Micropaleontology analyses were performed on both benthonic and planktonic forams from these units based on Blow (1969) and Bandy (1967) zonation for depositional age and environment. Planktonic forams consist of Globorotalia minardii, Globorotalia siakensis, Sphaeroidinellopsis subdehichens, Globoquadrina baroemoensis, and Globorotalia miocenica. Benthonic forams consist

of Robulus sp, Amphistegina lessonii, Elphidium sp, Quinquelaqulina sp, Planularia australis (Chapman), Pyrgo murhina, Uvigerina peregrine, and Pseudorotalia schroeteriana. The presence of these forams indicate Halang Formation was deposited in middle neritic to outer neritic zone (30 -200 meters) with age no older than N13 and not younger than N20 age (Late Miocene to Early Pliocene). According to the sandstone classification from Pettijohn (1975), two samples have been classified as lithic wacke sandstone. One sample has been classified as mudstone. Lithic wacke consists of lithic (25%-39%), plagioclase (15%-18%), quartz (3-10%), opaque mineral (3-8%), pyroxene (10%), hornblende (10-15%), lime mud (2-8%). The matrix (32–33%) is composed of silt and clay sized material (Figures 8 and 9). The composition plotted in a Q-F-L diagram (Dickinson & Suzcek, 1979) shows that the Halang sample was derived from a magmatic arc provenance (Figure 10).

In this paper, the authors used a simplified crosssection method to reconstruct the position because the flank and plunge of the folds are likely to change. It causes less accuracy recording the classification of folds. Fault Analysis Fault structure can also be interpreted through chaotic and extreme dips and position of stratigraphic layers. Ten faults were interpreted in Watukumpul area as seen in Figure 11. 1. Medayu Fault Medayu Fault is located near Medayu Village. This fault is classified as Right Slip Fault (Rickard, 1972) with NNE-SSW trend. This fault cuts Rambatan and Halang Formations. This is a second-order fault that formed during the second phase of tectonic deformation of geological structures in the study area. The fault is illustrated in Figure 6c.

Basalt Unit 2. Bodas Fault Basalt Unit covers approximately five percent of research area. Basalt is characterized by black color, hard, and afanitic. Based on E-E’ geological section the Basalt is a volcanic plug. Stratigraphic and geological structure analysis shows the age of intrusion is Pleistocene (Djuri et al, 1996). This is equivalent to second phase of North Serayu volcanism that caused the formation of Quarternary volcanoes.

Bodas Fault is located along Bodas River and Pagelaran Village. This fault classified as Reverse Left Slip Fault (Rickard, 1972) with NE-SW trend. This fault cuts Rambatan and Halang Formation. It formed during the first phase of geological deformation in the study area and is a first-order fault. 3. Longkeyang Fault

STRUCTURE AND TECTONICS Folds and faulted structures in the field are identified by map lineaments of morphological appearances, ridges, hills, and rivers. However, there is also evidence of fault movement by the offset appearance of the lithology in the study area. Evidence in the field are expressed in the form of brecciations, microfolds and shear fractures. Geological structures of Study Area are illustrated in Saputra's (2015) structural map (Figure 11). Fold Analysis In the study area there are folds of both anticlines and synclines with an axis direction dominantly North-South and cut by faults. The folds were demarked such as Suru Syncline, Pedagung Anticline, Longkeyang Syncline, Medayu Anticline, and Jatingarang Syncline (Figure 11).

 

Longkeyang Fault is located on the river of Longkeyang Village. This fault is classified Right Slip Fault (Rickard, 1972) and is oriented NE-SW. This fault cuts Rambatan and Halang Formations. This fault formed during the first phase of the process of formation of geological structures in the study area and is a second-order fault. 4. Gunungbatu Fault Gunungbatu Fault is located from Wanarata Village to Gunungbatu Village. This fault is classified as Right Slip Fault (Rickard, 1972) with NE-SW orientation. This fault cuts Rambatan and Halang Formations. It was formed during the first phase of geological deformation in the study area and is a second-order fault. The fault is illustrated in Figure 6d.

5. Pasir Fault Pasir Fault is located along Pasir Village to Suru Village. This fault is classified Right Slip Fault (Rickard, 1972) with NE-SW trend. This fault cuts Rambatan and Halang Formations. It formed during the first phase of the process of formation of geological structures in the study area and is a third-order fault. 6. Gapura Fault Gapura is located on the Gapura Village. This fault is classified as Right Slip Fault (Rickard, 1972) with E-W orientation. This fault cuts Halang Formation. It formed during the first phase of the process of formation of geological structures in the study area and is a first-order fault. 7. Majakerta Fault Majakerta Fault is located in the Majakerta Village. This fault is classified as Right Slip Fault (Rickard, 1972) with NW-SE trend. This fault cuts Rambatan and Halang Formations. It formed during the first phase of the process of formation of geological structures in the study area and is a first-order fault. 8. Gunungbatu Thrust Fault Gunungbatu Thrust Fault is interpreted as backthrust fault located along Pasir Village to Pagelaran Village. This fault is classified to Right Thrust Slip Fault (Rickard, 1972) with N-S orientation. This fault cuts Rambatan Formation. It formed during the first phase of the process of formation of geological structures in the study area and is a third-order fault. 9. Igir Bodas Fault and Candi Fault Igir Bodas Fault and Candi Fault are implied thrust faults because the support is inferred from chaotic and extreme of dip and position of stratigraphic layers (geological section of F-F' and G-G' Figure 4). These faults have a trend of a Northwest-Southeast orientation that cross the Rambatan Formation.

sorted by paleostress parameters (Table 2). Three stresses (3-Stress horizontal maximum with different orientations) are plotted graphically with interpretative Sh-max trajectory map (Figure 12). The three orientations of Stress Horizontal maximum (Sh-max) are: 1. Stress Horizontal Maximum (Sh-max) NNESSW Shear fractures data formed by stress with NNESSW orientation found in the location of the FTA. 2, 4, and 5, Medayu subset 1, FTA. 67 East Kalijurang subset 2, FTA. 78 Belik subset 2, and FTA 101 Majakerta subset 4 is located in Halang Formation. Based on the analysis of all NNESSW stress data found the maximum stress direction (σ1) average is 1º / N 13º E, intermediate stress (σ2) 23º / N 282º E, and minimum stress (σ3) 67º / N 105º E. The ratio of R stress is 0.77 and R’ stress regime is 2.77. Therefore, the regime stress of NNE-SSW stress is pure compressional (R' 2.77). 2. Stress Horizontal Maximum (Sh-max) NESW Shear fractures data formed by stress with NESW orientation spread across 13 locations. Based on the analysis of all the data stresses NE-SW found maximum stress direction (σ1) average is 01º/ N 051º E, stress intermediate (σ2) 82º/ N 313º E, and minimum stress (σ3) 8º/ N 141º E. The ratio of R stress is 0.64 and R’ stress regime is 1.36. Therefore, stress regime of NE-SW stress is pure strike-slip (R' 1.36). 3. Stress Horizontal Maximum (Sh-max) WestEast Shear fractures data formed by stress with W-E orientation and spread across 12 locations is the last stress. Based on the analysis of all W-E stress data, we found maximum stress direction (σ1) averages is 02º/ N 091º E, intermediate stress (σ2) 88º / N 255º E, and minimum stress (σ3) 01º / N 000º E. The ratio of R stress is 0.64 and R’ stress regime is 0.4, the stress regime of W-E stress is pure strike-slip (R' 1.36). Gravity Analysis

Paleostress Analyses Paleostress analyses data processing of shear fractures using Optimization Rotation Methods (ORM) were performed on 1606 shear fractures and  

Based on residual gravity map (Figure 13) and 3D Inversion (Figure 14), the Study Areas are classified to low density (negative anomaly) and high density (positive anomaly) areas. Low density areas are

characterized by blue to green colours. While high density areas are characterized by green to red colours. The density of high density area ranging from 0 mGal to +11 mGal, while low density are ranging from 0 mGal to -14 mGal. The negative anomaly may associated with low density rocks and geological structures or fluids composition (Telford et al, 1990). The high anomaly may derived from high density rocks such as a Basalt plug in the study area. The residual gravity map and the modeling shows abrupt density gradations which indicate two dominant trends of gravity which are NW-SE and NE-SW. These trends are appropriate with the main geological structure which illustrated in geological map such as NW-SE anticline and NE-SW fault. However, north-south section of gravity map show low density area distribution from SW to center of study area which associated with low density rocks or fluids. Correlation between gravity and geological map shows the seepage occurs at anticlinal structure with positive anomaly (red colour) in the center of study area. Tectonic Evolution

gravity effects, and the vertical force of magma rising to the surface occurred repeatedly causing imbalances of rock load resulting in faults and folds from the crater to the outer areas of the volcano or foot of volcano. This tectonic event is called gravity tectonics caused by volcanism. Conceptual model of volcanic gravity tectonics was published by Bronto (2013) in Figure 16. Folds, tear faults, and fault reactivation were identified in the Watukumpul Area as resulting from this tectonic event (Figure 17 and Figure 18). Phase III Tectonics (W-E) Phase III Tectonics developed in the Pleistocene, influenced by the total stress effect of gravity tectonics caused by the volcanism of Mount Slamet in the form of inflation and subsequent deflation. In this phase the back thrust faults and the formation of new tear faults were generated (Figure 19). Generally, gravity tectonics are associated with thrust faulting (Fossen, 2010). However, in some cases, particularly in the study area, gravity tectonics formed a back thrust fault, due to stress compression on rocks that lead to changes in a reversal of the stress. The gravity tectonics affect extended to -3721 meters below sea level shown by the presence of high and low areas on gravity analyses such as syncline, anticline, and back-thrust fault at this depth.

Tectonic framework of Study Area was controlled by Java Wrenching System during the Pliocene and volcanic gravity tectonics (Saputra, 2015). The interpretations were based on the paleostress analyses.

PETROLEUM SYSTEM

Phase I (NNE-SSW)

Source Rock

Phase I developed in Pliocene influenced by PlioPleistocene tectonics associated with change of subduction order due to Sundaland rotation and accompanied by shortening or compression. In this period, magmatism and volcanic activity in Central Java ceased (Hussein et al., 2013). Faults that developed in Phase I are the strike-slip faults of the wrenching system on Java based on the Pure Shear concept by Moody and Hill (1956). This phase formed Majakerta Fault, Gapura Fault, and Bodas Fault illustrated in Figure 15.

Source rock evaluations were performed by Iswahyudi and Widagdo (2009) on one mudstone sample and seven shale samples from the Rambatan and Halang Formations in the Watukumpul area. The highest value of Hydrogen Index (HI) analysis is 237 indicating a kerogen type III; meaning the source is derived from a terrestrial environment (Figure 20). TOC analysis found 0.99% - 0.18% showing that the organic content of the sample is poor to fair (Table 3). Tmax analysis 449 - 455º C and Vitrinite Reflectance (%Ro) are 0.55 – 0.68 indicating the source rock is mature. However, several samples showed %Ro is 2.37 (Table 4) indicating the source rock is post-mature. Such source rock maturity could be attained via depth of burial (Satyana and Armandita, 2004) and intrusion effects.

Phase II (NE-SW) Phase II was influenced by Slamet volcanism during the Pleistocene. The generation of this volcano was associated with...