Geochemical characteristics of lacustrine source rock: An insight of Banuwati Formation from Sunda and Asri Basins, Indonesia

The Sunda and Asri Basins are petroliferous in Indonesia. Despite producing billions of barrels of oil equivalent since their initial production, research on the source rock organic geochemistry of these basins remains limited, particularly concerning the sapropelic kerogen of the syn-rift Banuwati Formation. A comprehensive study of the Banuwati Formation in the Sunda and Asri Basins was conducted using kerogen and biomarker data. Various techniques, such as carbon analysis, Rock-Eval pyrolisis, vitrinite reflectance (Ro), gas chromatography (GC), and gas chromatography-mass spectrometry (GC-MS), were employed on samples from six wells. The Banuwati Formation exhibits source rock with good to very good quantities. Biomarker analysis offers a novel interpretation, suggesting that the Banuwati shale source rock was deposited in an oxic-suboxic environment. The formation is primarily composed of Type I organic material, characterized by the presence of biomarkers such as C26 tricyclic terpane, C30 4-methylsterane, and a possible botryococcane peak, indicative of a lacustrine setting. Maturity assessments based on Ro and Tmax values show conflicting results. Tmax values imply maturity, while Ro values suggest immaturity. This discrepancy is likely due to Ro suppression, a phenomenon often observed in Type I kerogen samples.


Introduction
The Sunda and Asri Basins are back-arc basins located close to each other at southeast of Sundaland (Figure 1).The exploration has been actively conducted since the 1960s in the Sunda Basin, and then several fields have been found to mainly contain oil since the 1970s in the Sunda Basin.A successful exploration in the Sunda Basin was then followed by another one in the Asri Basin.
The Sunda and Asri Basins are prolific basins with numerous hydrocarbon discoveries with total cumulative production of more than 1700 million-barrel oil equivalent (status until the year 2023).Several authors have evaluated the petroleum system in these producing basins [1]- [5].Nevertheless, none of them conducted an in-depth study of the geochemistry of source rock and simultaneously appraised both the Sunda and Asri Basins.As few exploration wells have penetrated the basin area in Indonesia, this study gives an ideal opportunity to gain knowledge about the organic geochemistry features of the initial sediment basin-fill, the Banuwati Formation.Therefore, the primary objective of this study is to be the first to provide a comprehensive description of the source rock organic geochemistry of Banuwati Formation in the Sunda and Asri Basins, Indonesia.

Geological background
The Sunda and Asri Basins are categorized with Cenozoic sediments filled.The geological setting is described by several authors [2]- [10].Tectonostratigraphically, the formation of the Sunda and Asri Basins could be divided into a period of pre-rift, syn-rift, and post-rift (Figure 1).The pre-rift, basement is pre-Tertiary and consists of various non-sediment rocks such as granodiorite, granite, slate, schist, and marble.In the Eocene/Early Oligocene (Paleogene), due to tensional stresses, rifts were formed by 1245 (2023) 012009 IOP Publishing doi:10.1088/1755-1315/1245/1/012009 2 predominantly north-south oriented faults in the eastern part of the basins.In the syn-rift phase, the first sediment filling deposited overlying the basement rock, characterized by sedimentary rock with conglomerate, shale, sandstone, and alluvium fan deposits to the basin margin, known as the Banuwati Formation.This formation is characterized by lacustrine shale deposits containing rich organic material.The maximum thickness of the Banuwati Formation is approximately 1500 and 1000 meters in the Sunda and Asri Basins, respectively.Overlying the Banuwati Formation are Zelda and Gita Members of the Talangakar Formation in the Late Oligocene -Early Miocene.The Zelda and Gita Members are characterized predominantly by the fluvial to the deltaic system with lithology succession of sandstone, siltstone, shale, and interbedded coal.The sandstone of the Talangakar Formation was important as the main reservoir.The Baturaja Formation, predominantly consisting of limestone lithology in marine settings, was deposited overlying the Talangakar Formation due to sea-level rise.The post-rift, during the Middle Miocene, the Gumai Formation deposited and consisted of claystone and minor sandstone.The Gumai Formation served as a regional seal rock.Further, the Airbenakat and Cisubuh Formations were deposited in the Late Miocene-Holocene.

Figure 1.
The study area is in the Sunda and Asri Basins, Indonesia.Both basins are characterized by Paleogene half-grabens with a north-south major fault trend in the eastern part (figure 1a, modified after Doust and Noble [5]).The Banuwati Formation was deposited in the Paleogene syn-rift (figure 1b, modified after Prayitno et al., [2]; Sukanto et al., [4]).

Samples and methods
For this study, six offshore wells were selected, including SADE-1, SFAN-1, SMON-1, SLAS-1, and SZEL-14 in the Sunda Basin and ADEL-1 in the Asri Basin.The data on source rock samples from those well were obtained and compiled from the current operator.The rock samples were analyzed using conventional geochemical methods.Detail descriptions of the procedures and analyses can be found in the works of several authors [11]- [14].The laboratory analyses included TOC (Total Organic Carbon), Rock-Eval pyrolysis, vitrinite reflectance (Ro), gas chromatography (GC), and gas chromatographymass spectrometry (GC-MS) such as m/z 123, m/z 191, and m/z 217.The geochemical data available for this study, focusing on the Banuwati Formation in the Sunda and Asri Basins, are limited and presented in table 1.The simplified flowchart of research methodology shown in Figure 2.
3 Rock geochemical data is selected and validated to produce an accurate interpretation using geochemical parameters such as TOC and S2 (organic compounds generated by cracking of the kerogen in the Rock-Eval pyrolysis) to maintain the quality and validity of the data [12], [15].Drilling activities from wells focus of the study do not use oil-based mud (OBM) but water-based mud (WBM).All rock samples used in this research are free of hydrocarbon contamination from the drilling operation.Thus, further evaluation of the source rock is conducted with high confidence.
Table 1.The total numbers of Banuwati Formation data from each geochemical laboratory analysis.

Results and discussions
The evaluation of the source rock includes the evaluation of the quantity of organic material, the quality of the organic material, the maturity of the source rock, the biomarkers and interpretation.
The rock geochemical studies are done with good-quality data.Some researchers who have conducted a geochemical study of rocks in the Tertiary basin in western Indonesia, for example [16] in the Central Sumatra Basin use a TOC threshold value (screening) of 1% and S2 of 2.5 mg/g source rock.This study uses a TOC threshold limit value of 1% because this value is the threshold limit for a sedimentary rock categorized as good to be a source rock [12], [17].The S2 threshold limit used in this study has a higher value, 5 mg/g of source rock because this value is the threshold limit of source rock having a good quantity and quality of organic material [17].The numbers of data after screening using a minimum TOC value of 1% and S2 minimum value of 5 mg/g are shown in table 1.

Source rock quantity
The Banuwati Formation core sample is only available from SFAN-1 and SMON-1 wells and shows excellent visual source rock characteristics (Figure 3).Based on classification from Dembicki Jr. [17], the Banuwati Formation in the Sunda Basin has a good to a very good amount of organic material with a TOC value of 1.87% to 7.12% and an S2 value of 5.34 to 94.16 mg/g source rock.
The Banuwati Formation in the Asri Basin exhibit a high quantity of organic material, with a TOC value of 3.29% to 3.83% and an S2 value of 24.44 to 34.55 mg/g source rock.Notably, one sample from ADEL-1 well in the Banuwati Formation reached the coal category, with TOC and S2 values of 58.32% and 183.8 mg/g source rock, respectively (Figure 3).

Source rock quality
The quality of organic material is determined using Rock-Eval pyrolysis data from Banuwati Formation.High hydrogen index (HI = S2/TOC x 100) ranging from 167.96 to 673.84 in the Sunda Basin and 315 to 902 in the Asri Basin, suggest a predominance of Type I kerogen with good oil-prone quality.This classification is based on the work of Dembicki Jr [15] (Figure 4).

Source rock maturity
The maturity of organic material in sedimentary rocks is determined by vitrinite reflectance (Roa parameter for determining the maturation of organic matter in fine-grained rocks using a microscope) and the Tmax of Rock-Eval pyrolysis analysis (Tmaxa thermal maturity parameter based on the temperature at which the maximum amount of pyrolyzed/S2 is generated from the kerogen in a rock sample).The Banuwati Formation has entered the maturity window to the peak of maturity in the Sunda Basin, with a Tmax value ranging between 440 o C and 463 o C. In comparison, this formation has just reached the oil window in the Asri Basin with a Tmax value of 428 o C to 440 o C.However, the Ro values from the Banuwati Formation in the Sunda and Asri Basin indicate that this formation is still immature (Ro < 0.6%), with one sample having reached the peak of maturity (Ro = 0.66%).This difference in maturity can be attribute to two factors.First, the high Tmax value from the Banuwati Formation could result from the samples' low quantity of organic material [12].Second, vitrinite suppression could cause low Ro values [18].The first factor is unlikely to occur because the sample used has been geochemically selected, leaving a sample with a good quantity of organic material.Vitrinite suppression is the most likely explanation, given that it occurs in samples with high HI values (HI > 400).Some samples of the Banuwati Formation in the Sunda and Asri Basins have HI values of more than 600 in the SMON-1 and SLAS-1 wells, even reaching 900 in the ADEL-1 well.Therefore, the Ro values of samples with high HI values are corrected on the graph from [18].After correction, the Banuwati Formation matured, ranging from the peak of maturity to the end of maturity, with Ro values between 0.7% and 1.17% (Figure 5).The Banuwati Formation also exhibits matured source rocks, as shown by the high Ts (Trisnorneohopane) over Tm (Trisnorhopane) from rock sample (Figure 7).

Biomarker and interpretation
The biomarkers were evaluated based on n-alkanes, isoprenoid, bicyclic alkane, terpane, and sterane components.Each saturated biomarker component was identified by comparing the retention times and mass spectra of the monitor ions for bicyclic alkanes (m/z 123), terpanes (m/z 191), and steranes (m/z 217).
Normal alkanes and acyclic isoprenoids (e.g.pristane, Pr; phytane, Phy) are represented by saturated hydrocarbons between nC5 to nC38.The acyclic isoprenoid composition is used in interpreting redox conditions [19].All Banuwati rock samples from Sunda and Asri Basins used in this study show that pristane is consistently higher than phytane.The higher pristane content over phytane in all samples indicates that the Banuwati Formation was deposited in a suboxic-oxic environment.However, the ratio of pristane to phytane must be used cautiously because this parameter is influenced by several factors other than the redox conditions, such as organic matter contribution, thermal maturity, and postexpulsion alteration [20], [21].The oxic-suboxic environment was also demonstrated by the low molecular weight steranes, such as C21 sterane (pregnane) and C22 sterane (homopregnane), which are lower than normal sterane on the extended m/z 217 fragmentogram [22] (Figure 7) and the presence of C29 norneohopane (Cy) on the m/z 191 fragmentogram [20] (Figure 7).Interestingly, Banuwati rock samples have a moderate 17α(H),21β(H),28,30-bisnorhopane (BNH) peak, the origin of which still needs to be understood.The BNH biomarker is frequently found in source rocks deposited in anoxic conditions [20].Despite this explanation, our interpretation differs and provides novelty interpretation based on biomarkers from the existing interpretation that suggests the crude oils in Asri Basin primarily came from an anoxic environment [4].The Banuwati rock samples, dominated by shale sediments, exhibit lower C34 homohopane than C31 and lower C29 hopane than C30 hopane [20], [23].The biomarker that supports the deposition of the Banuwati Formation in a lacustrine environment is the C26 tricyclic terpane, which is higher than the C25 tricyclic terpane, as shown in figure 7 [20], [24].The presence of 18α(H)-oleanane (OL) in the m/z 191 fragmentogram indicates the presence of higher plant inputs into the Banuwati Formation.However, the small amount of oleanane present in the Banuwati Formation suggests only minor inputs of higher plants, such as Angiosperms (Figure 7).Other evidence of higher plant material contributing to the organic material in the Banuwati Formation includes the presence of the bicyclic alkanes (e.g.8β(H)-drimane and 8β(H)-homodrimane) compound in the m/z 123 fragmentogram, which are related to the contribution of higher plants [25] (Figure 8).
Another supporting for the lacustrine environment of the Banuwati Formation is the high the alkane peak after nC22 in rock samples, which exhibit relatively high waxy properties [26].The gas chromatogram data show carotenoid compounds, which are usually associated with algae [27].One of these samples also displayed an indication of botryococcane, associated with lacustrine algae, in figure 8 [28], [29].The dominance of algae as a constituent of the Banuwati Formation is also demonstrated by the predominance of C27 sterane compared to C28 and C29 on the m/z 217 fragmentogram [30], [31].
The presence of C30 4-methylsterane further supports the deposition of the Banuwati Formation in a lacustrine environment, as shown in figure 6 [32].The high diasterane levels also suggest a clay-rich environment, rather than a carbonate one.

Conclusions
This study concludes that the syn-rift packages of the Banuwati Formation in the Sunda and Asri Basins are abundant in source rock, with good to very good quantities.Biomarker analysis reveals a novel interpretation, suggesting that the source rock was clay rich and deposited in an oxic-suboxic environment.This challenged the prevailing notion that anoxic conditions are necessary for substantial hydrocarbon production.The Banuwati Formation primarily consists of algae or Type I organic material and displays excellent kerogen quality and oil-prone characteristics.Distinctive biomarkers indicate deposition in lacustrine environments.However, the mechanism by which oxic-suboxic source rocks produce significant hydrocarbons in these basins was not evaluated in this study.Further research is needed to explore these mechanisms and validate biomarker-based interpretation.

Figure 2 .
Figure 2. The simplified flowchart of research methodology.

Figure 3 .
Figure 3.The cross plot between S2 and TOC values shows that all samples analyzed have a good to very good quantity of organic matter (classification based on Dembicki Jr. [17]).

Figure 5 .
Figure 5.The presence of vitrinite suppression in the Banuwati Formation sample with a high HI value causes a low Ro value.Therefore, the value of Ro is corrected with the re-digitized graph of Lo [18].

Figure 9 .
Figure 9.The sediment extract of the Banuwati Formation from the Asri Basin shows higher pristane (Pr) than phytane (Phy), the possible presence of Botryococcane and Carotenoid.