Tectonics

Figure 1 illustrates structural configuration of Northeast Java Basin; from north to the south are Northern Platform, Central Deep and Southern Uplift (Satyana and Purwaningsih, 2003). New data from oil and gas exploration (onshore and offshore) of East Java indicate that two rift basin systems developed during Eocene-Oligocene times (Sribudiyani et al., 2003). The first rift system striking NE-SW and the second system trending E-W direction parallels Rembang-Madura-Kangean (RMK) structural trend (Figure 2).

Stratigraphy

The Paleogene stratigraphy of the Rembang Zone is characterised by rift-related sedimentation. The syn-rift sediments correspond to the lower Ngimbang unit deposited in a lacustrine to marine setting during middle Eocene to early Oligocene. The rift-sagging period is represented by the Kujung Formation, which is composed of limestones and shales of late Oligocene age. The Neogene interval consists of shallow-marine to beach sediments of marls and limestones and sandstones of the Tuban, Ngrayong, Wonocolo, Ledok, and Mundu formations.

Geochemical Analysis

Source Rocks

The results of total organic carbon (TOC) analysis of samples from Rembang-1 and Padi-1 wells reveal that the middle Eocene Ngimbang, upper Eocene CD, Oligo-Miocene Kujung, and lower Miocene Tuban formations, have potential to be sources of East Java oils and gases (Table 1). Results of analysis of samples from Rembang-1 well show that the Ngimbang has poor-excellent source rock potential, with TOC of 0.40-71.88% and generates gas and oil because the kerogen is predominantly type II and III (Figure 3).

Results of analysis from Padi-1 well show that the Ngimbang has good-excellent source rock potential with TOC of 1.27-58.16% and generates gas and oil from dominantly kerogen type II and III (Figure 4).

Source Rocks Maturity

Waples (1985) mentioned that for most kerogens the onset of oil generation is taken to be approximately 0.6% Ro. Peak generation is reached at approximately 0.9% Ro, and the end of liquid-hydrocarbon generation is thought to be at about 1.35%. Maturity based on vitrinite reflectance measurements was determined for source rocks in Rembang-1 and Padi-1 wells. Table 2 illustrates vitrinite reflectance measurements of some source rocks from Rembang-1 and Padi-1 wells, whereas Figure 5 shows plots of vitrinite reflectance (Ro) values versus depth for those wells.

Source Rock to Oil Correlation

Correlation between source rocks and crude oils was based on several techniques, such as biomarker distribution. A visual comparison of triterpane (m/z 191) and sterane (m/z 217) contained in the Ngimbang source rock and some oils shows a close similarity, indicating a good correlation.

Figure 6 illustrates the results of triterpane (m/z 191) analysis, showing high nonhopanoid as oleanane, C₃₀ 17α(H),21β(H)-hopane and C₂₉ 17α(H),21β(H)-30-norhopane. Oleanane is a biomarker derived from angiosperms (flowering plants) whose evolution began from Late Cretaceous time. Comparison of the distribution of triterpane (m/z 191) in oil seep from Kedung Jati and in Ngimbang source rock from Rembang-1 at depth 4901 feet is similar, indicating a good relationship. Furthermore, it can be interpreted from the biomarker distribution that their paleoenvironment was deltaic(?), with organic-matter contribution from marine and higher plants.

Similarity in the distribution of triterpanes was also recognized in the two mass fragmentograms shown in Figure 7, which shows two mass fragmentograms of an oil seep collected from the Galeh area and a source rock of the Ngimbang Formation from Rembang-1 (5500 feet). Again, such a similarity indicates that the crude oil and sediment were deposited in the deltaic(?) area with organic materials derived from marine and higher plants.

Conclusions

The correlation studies suggest that the crude oils found as seep (Kedung Jati and Galeh) in the western part of the Northeast Java Basin were most likely to be derived from the Ngimbang source rocks. An opinion that Ngimbang Formation is possible source rock of the crude oils found in the basin has been expressed by explorationists. This study, based on the geochemical correlation, confirms such an opinion.

Acknowledgements

This study is part of DAW Master’s thesis in the Department of Geology, Bandung Institute of Technology, Indonesia. We would like to acknowledge the management of Lundin Blora B.V. for the supporting data. We also thank BPMIGAS for permission to publish the data.

Selected References

Lundin, B.S., 2004, Petrology of Central American tertiary ignimbrites: M.S. Thesis, University of Rhode Island, 220 p.

Manur, H. and R. Barraclough, 1994, Structural control on hydrocarbon habitat in the Bawean Area, East Java Sea: 23^rd Annual Convention Proceedings, v. 1, p. 129-144.

Satyana, A.H., 2005, Petroleum geology of Indonesia: Current concepts (pre-convention course): Indonesian Association of Geologists 34^th Annual Convention Proceedings, Surabaya, Indonesia, November 2005.

Satyana, A.H, and M.E.M. Purwaningsih, 2003, Geochemistry of the East Java Basin: New observations on oil grouping, genetic gas types and trends of hydrocarbon habitats: Proceedings Indonesian Petroleum Association, 29^th Annual Convention and Exhibition, October 2003.

Sribudiyani, M.N., R. Ryacudu, T. Kunto, P. Astono, I. Prasetya, B. Sapiie, S. Asikin, A.H. Harsolumakso, and I. Yulianto, 2003, The collision of the East Java microplate and its implication for hydrocarbon occurrences in the East Java Basin: Proceedings Indonesian Petroleum Association 30^th Annual Convention & Exhibition.

Waples, D.W., 1985, Geochemistry in Petroleum Exploration: International Human Resources Development Corporation, Boston, 232 p.

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