Organic Geochemical Characteristic of Crude Oils From Orange Graben, South Sumatra Basin

Lemat Formation in the Orange Graben in the South Sumatra basin has been considered as syn-rift sediments consist of fluvio-lacustrine sediments, create source rock with fluvio-lacustrine characterize. Based on existing publications, showing that the oils from Orange Graben have fluvio-deltaic characterization and generated by source rock from Talangakar Formation with fluvio-deltaic depositional environment. This paper emphasizes geochemistry methods. Source rock analysis, consist of 26 samples for carbon isotope and 14 samples for biomarker, while oil analysis, consist of 15 samples for carbon isotope and 19 samples for biomarker. Characterization has been based on qualitative and quantitative data. Qualitative data comprise evaluation based on chromatograms and mass-fragmentograms, whereas quantitative data consists of a series of cross-plots, eg. cross plot of carbon isotope δ13C saturates - aromatics, distribution of C27-C28-C29 sterane, Pr/nC17-Ph/nC18, Pr/Ph-Pr/nC17, carbon isotope δ13C saturates-Pr/Ph, Pr/Ph-total hopane/total sterane, and ratio of C26/C25 (tricyclic). Based on geochemical result of analysis, source rocks of Lemat Formation in the Orange Graben interpret as source rock with fluvio-deltaic characterize, have terrestrial influence while Talangakar Formation in the Orange Graben interpreted as source rock with deltaic characterization, having marine and terrestrial influence. Both of them , consist of humic kerogen. Whereas, oil samples in the Orange Graben interpreted as oilwhich is generated by source rock with fluvio-deltaic characterization, having terrestrial influence, in anoxic-suboxic-oxic conditions, consisting of humic kerogen. Correlation result between source rocks and oils in the Orange Graben, indicating that fluvio-deltaic oil type in the Orange Graben are correlate with not only the source rocks of Talangakar Formations but also with source rock of Lemat Formation.  


Int r oduct i on
South Sumatra Basin is a potential and m ature basin for hydrocarbon source. There are a number of sub-basins w hich is potential as the hydrocarbon source in this basin. Orange Graben is one of sub-basin on t he South Palem bang Sub-Basin, form ed in Upper Cretaceous to Low er Tertiary, in extensional phase, this is the beginning of sedim entation in this area.
Lem at Form ation in Orange Graben has been considered as syn-rift sedim ents consist of terrestrial sedim ent (fluvio-lacustrine), creating source rock w ith fluvio-lacustrine characterization. W hile based on existing publications, show ing that oil from Orange Graben have fluvio-deltaic characterize and generated by source rock from Talangakar Form ation w ith fluvio-deltaic characterization. This is interesting to study further, especially about possibility of source rock w ith fluvi olacustrine characterization in research area. This study em phasized to source rocks of Lem at and Talangakar Form ation in Orange Graben, South Palem bang Sub-Basin, South of Sumatra Basin.

Locat i on of St udy Ar ea
The study area located in Orange Graben, South of Sumatra ( Figure 1).

Resear ch M et hods
The basic source rock analyses com prise screening, kerogen typing, and m aturation. Further analysis, such as correlation betw een source rock and crude oil as w ell as crude oil to crude oil, has been done by using m ore sophisticated techniques, i.e. carbon isotope and biom arkers. Source rock analysis, consist of 26 sam ples for carbon isotope and 14 sam ples for biom arker, w hile oil analysis, consist of 15 sam ples for carbon isotope and 19 sam ples for biom arker. Characterization has been based on qualitative and quantitative data. Qualitative data com prise evaluation based on chrom atogram s and m ass-fragm entogram s, w hereas quantitative data consists of a series of cross-plots, eg. cross plot of carbon isotope δ 13 C saturates-arom atics, distribution of C 27 C 28 C 29 sterane, Pr/nC 17 Ph/nC 18 , Pr/Ph Pr/nC 17 , carbon isotope δ 13 C saturates Pr/Ph, Pr/Ph total hopane/total sterane, and ratio of C 26 /C 25 (tricyclic). The result s of t his study expected could explain the character of source rocks and oil in the Orange Graben, also to find out the possibility of lacustrine source rock existence and determ ine the correlation bet w een source rocks and oils in this area, so can be know n w het her Lem at Form ation source rocks also have contributed to produce oil in this area or not. In addition, to provide a new opportunity in the exploration of hydrocarbons in the Orange Graben w hich considered as a m ature and potential basin for hydrocarbon.

Regi onal Geology
The evolution of the South Sumatra Basin started in the M esozoic Era (Pulunggono et al., 1992) (25) as a back-arc basin ( Figure 2). Back-arc basins in Indonesia are generally hydrocarbon prolific basins. The m echanism involved in the basin form ation is general rifting, w hich form ed a graben or half graben. This basin w as form ed in tw o phases, i.e. the rifting phase occured during the Paleogene and the sagging during the Neogene. The rifting phase is characterized by the thinning of crust form ing Graben or half Graben, w hereas the sagging phase is characterized by stable t ectonics, and even a change in tectonic com pression, w hich finished w ith volcanic activities that are show n by the occurrence of tuff that appeared in the Air Benakat Form ation. The rifting phase w as filled by the Lahat Group that is comprised of the Benakat and Lem at Form ations, w hereas the sagging phase w as filled w ith sedim ents from the Talangakar   and C 29 , source rocks of Lem at and Talangakar Form ations and oils in Orange Graben. From this picture it appears that the source rocks of Lem at and Talangakar Form ations and oils affected by terrestrial m aterial in anoxic suboxic until oxic conditions, but m ostly in high oxic conditions. It is usually believed that C 27 steranes derive m ainly from phytoplankton and m etazoa, w hereas C 29 steranes m ainly originate from terrigenous higher plants (Huang and M einschein, 1979 (19); Volkm an, 1986 (33)). From the cross plot of sterane distribution C 27 , C 28 , and C 29 , show Lem at Form ation derived from estuarine to terrestrial environm ents, w hereas Talangakar Form ation and oils derived from m arine, estuarine, and terrestrial environm ents. Based on t hese data, oil in the Orange Graben interpreted originated from fluvio-deltaic source rocks and has a correlation w ith Lem at and Talangakar Form ations in Orange Graben.

Conclusi on
Source rocks of Lem at and Talangakar Form ations and oils in Orange Graben consists of humic kerogen and terrestrial and m ixed m aterial. Source rocks of Lem at and Talangakar Form ations and oils in Orange Graben, is not derived from a lacustrine sedim ents, affected by terrestrial m aterial in anoxic -suboxic until oxic conditions, but m ostly on high oxic conditions. Besides, its also looks like Lem at Form ations derived from estuarine to t errestrial environm ents, w hereas Talangakar Form ation and oils in Orange Graben derived from m arine, estuarine, and terrestrial environm ents. Lem at Form ation and oils in Orange Graben show terestrial pattern, w hereas Talangakar Form ations show m arine and terrestrial pattern. These data indicate Lem at Form ation interpreted as fluvio-deltaic sedim ent, w hereas Talangakar Formation having m ore m arine characteriztion than Lem at Form ation. Oils in the Orange Graben interpreted originated from fluvio-delta source rocks, has a correlation w ith Lem at Form ation and Talangakar Form ation in Orange Graben.
Acknow ledgem ent s W e w ould like to thank the m anagem ent of Directorate General of Oil and Gas and M edco EP for their perm ission to publish t his paper.