INDONESIAN PETROLEUM ASSOCIATION (IPA) NEWSLETTER, JUNE 2003

GEOCHEMISTRY IN PETROLEUM SYSTEM ANALYSIS : A CRITICAL ROLE

Awang H. Satyana(Exploration BP Migas)

Geochemistry : Crucial but Lacking in Application

Today, people use the concept of petroleum system in evaluating petroleum potential and exploration risks involved in an area of investigation. We have seen that this concept successfully employed. Using this approach, new petroleum accumulations have been discovered and significant reduction of exploration risks has taken place.

Petroleum system concept was developed through petroleum geochemistry. Leslie Magoon and Wallace Dow, two world leading scientists who compiled and developed the concept of petroleum system stated that the ability to identify a petroleum system uniquely depends on geochemical techniques needed to map hydrocarbon shows, and to carry out petroleum-petroleum and petroleum-source rock correlations (Magoon and Dow, 1994). However, generally, one/ones with strong interest and training in geochemistry are seldom. Too much people are interested in reservoir characterization. Geochemistry develops petroleum system but it has been lack of application.

The petroleum system concept was first developed in 1970 (Magoon and Dow, 1994). In the early 1970s, it was generally known, but largely ignored, that traps, reservoirs, seals, source rocks and hydrocarbon migration were all required to make an oil accumulation. Most geologists have known a lot about traps and reservoirs, little about seals, and virtually nothing about source rocks and migration. This contrasts with the knowledge that if we know where the oils came from and how and to where they migrate, we can better predict where they will be found. Dow stated that geochemistry could then be used to high-grade areas in which to concentrate exploration activity, thereby reducing risk.

This article will show the examples of applying geochemistry in : (1) reducing exploration risks – Salawati Basin case, (2) finding new petroleum system – Salawati Basin case, and (3) understanding gas habitat – East Java Basin case. Those cases will show that through geochemistry maturely explored basins can still have promising discovery prospects. Therefore, geochemistry should not be under-employed let alone ignored.

Understanding Charge System of the Salawati Basin : Migration Modeling

A knowledge of the mechanics of migration is important in the general understanding of active charge system, hence in defining areas receiving petroleum charge. Satyana et al. (2000) reported modeling of migration pathways in the Salawati Basin using structural evaluation.

Evidence of migration modeling can reduce exploration risk is best exampled by Koi discovery structure. Koi structure is located isolatedly from classic petroleum accumulation

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of the Salawati Basin. The structure had always been postponed to drill because of difficulty in explaining hydrocarbon charge to Koi. However, the migration modeling study upgraded the prospectivity of the Koi structure. Koi is located in one of the seven identified regional noses of focused migration noses. Figure 1 shows the location of Koi structure at one of the regional noses. Koi was drilled in 1999/2000 and tested 1000 BOPD from Kais carbonates. The well would not be drilled without solution of migration modeling.

Petroleum will tend to move in a homogeneous carrier bed in the direction which has the steepest slope. This is perpendicular to its structural contours, that is, in the true dip direction (orthocontour map). To model hydrocarbon migration pathways in the Salawati Basin, time-structure map of top Kais (present-day structure) and time-isopach map of top Intra-Klasaman to top Kais Formation (paleo-structure at about 3.5 Ma) were used as the basemaps to plot orthocontours.

Evaluation of regional Kais structure map resulted in recognition of some broad Kais regional noses. These broad noses were obtained by simplifying the Kais time structure map through enlarging the contour interval and restoring the contour into the condition of pre-structurization. The study has recognized seven parallel regional Kais structural noses extending from the updip areas of the Salawati Basin plunging into the kitchen (Figure 1). They are, from the southwest to the northeast, : (1) the TBA - TBC, (2) Koi, (3) South Salawati, (4) Matoa - Walio, (5) Moi, (6) Klamono, and (7) Arar regional noses. The presence of regional structural noses within the Salawati Basin has outlined the broad migration pathways. This has caused the unique migration compartments (migration fairway) occurred within the basin. The regional noses have focused hydrocarbon migration. Generated hydrocarbons will first collect and concentrate (focus) around the plunging noses and then flowed updip along the nose.

Interesting fact about these noses is that all existing oil and gas fields within the Salawati Basin are located within these regional noses. Within the Matoa-Walio regional nose are located most oil/gas fields of the Salawati Basin and this major nose have accommodated almost 70 % of the basin’s hydrocarbon reserves. The areas flanking the regional noses are low/ synclinal areas. Interestingly, most dry wells of the Salawati Basin are located in these areas. This because migration pathway will away from structurally low areas and such areas will not receive petroleum.

Presence of faults are also important to provide conduits for migration. The Salawati’s fault trends were evaluated and it was discovered that the trends of faults play significant role. The interplay between structural noses and fault trends appears to control the focus of hydrocarbon migration within the Salawati Basin. High efficiency migration takes place in an area where the fault trends are parallel with the structural noses. In this way, the faults enhance the migration through structural noses.

Proper understanding of the migration pathway can give advantages in considering future strategic exploration efforts. The study has identified the areas with active charge system. It provides the reasoning for the distribution of both proven hydrocarbon accumulations and dry wells within the Salawati Basin. The modeling also provides the tool to reduce migration risk for undrilled prospects.

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Identification of New Active Pod of Salawati’s Source : Clues from Biomarkers

Salawati Basin, West Papua, has been explored for tens of years using single petroleum system of the Miocene Klasafet/Kais : Kais (!). Other petroleum systems are indicated both older and younger in stratigraphic setting than that of the Klasafet/Kais : Kais. Satyana (2001) summarized the identification of the new system called the Pliocene Lower Klasaman : Upper Klasaman (.). The knowledge that the new identified source of Lower Klasaman has generated hydrocarbons is based on geochemical evaluation of the oil seepages.

In the petroleum system of Klasafet/Kais : Kais it is known that oils in the Salawati Basin show increasing maturity to the north. Figure 2 shows this maturity profile. This relates to the increasing depth of the Klasafet/Kais sources northward until they reach the dry gas window right to the north of the WIR structure in the Salawati Island. WIR-1A well discovered dry gas in Kais reservoir contaminated by around 20 % CO2. CO2 gas was resulted from thermal breakdown of the Kais lime sources entering the dry gas window in this area.

To the north of WIR area at the southern border of the Sorong Fault Zone, there are many oil and gas seepages. The presence of oil seepages is confusing since if they came from the Klasafet/Kais sources they should be dry gases. The oil seepages were analyzed geochemically and it is known that the oils were generated from early to middle maturity oil window. The Klasafet/Kais sources in this area are within dry gas window, therefore they are impossible to source the oil seepages. The overlying Lower Klasaman shales are within oil window and have been considered as the sources of the oil seepages. Numerous reverse faults deformed the Lower Klasaman and the occurrence of the seepages are mostly associated with these faults. The faults basically do not deform the Klasafet/Kais level. This confirms that the seepages were sourced by the Lower Klasaman shales, a new proven source has been identified. This will never be known without geochemical examination of oil seepages.

Maturity of oil seepages hold a clue for the new source identification. This is based on ratios of phenanthrene (m/z 178) and methylphenanthrene (m/z 192) biomarkers of aromatic oil fraction. As maturation progresses there is a systematic increase in the proportion of the 2- and 3-methylphenanthrenes, due to rearrangement reactions to the more thermodynamically stable structures. At higher temperature, demethylation reactions become more important, resulting in a reversal of the above trend, with lowering of the proportion of the 2- and 3-methylphenanthrenes, together with an increase in the proportion of phenanthrene. Consequently, different empirical correlations of the methylphenanthrene indices (MPI) are applicable and correlated with vitrinite reflectance (called as Rc or calculated vitrinite reflectance). MPI increases as maturity increases.

Oil to source rock correlation was made using carbon-13 isotope and alkane/saturate biomarkers of terpane (m/z 191) and sterane (m/z 217) applicable to oil seepage samples and source samples of Kais, Klasafet, and Lower Klasaman. The results were that the oils have better correlation with the Lower Klasaman shales than with the Kais/Klasafet samples.

Gas Habitat of the East Java Basin : Applying Gas Geochemistry

Entering the 21st century, gas consumption of the East Java province increased rapidly. Gas discoveries of the contractors should be developed immediately and gas exploration should be enhanced. Knowledge on gas habitat will support this. Satyana and Purwaningsih (2002) provided the regional account on gas geochemistry of the East Java Basin.

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A total of 33 gas data have been compiled for this study derived from gas seeps and gas discovery wells. Geochemical gas data consist of hydrocarbon (C1 to C7+) and non-hydrocarbon (CO2, H2S, N2) gas compositions and isotope data of carbon (C1-C4, CO2) and of deuterium (D). Gas geochemistry study shows that based on the gas composition and carbon-13 and deuterium isotopes, the East Java natural gas can be divided into three genetic types : thermogenic associated and non-associated gas, biogenic/bacterial gas, and mixed biogenic-thermogenic gas. Thermogenic gases have wet gas composition of methane 73-94 % and ethane plus 6-27 %, isotopes of carbon-13 methane are -39.8 to -33.84 ‰ and isotopes of deuterium -152 to -145 ‰ indicating a wet thermogenic origin. Biogenic gases have dry gas composition dominated by methane from 99.5 to 99.8 % and ethane plus below 0.5 %, isotopes of carbon-13 methane are more negative (lighter) than -60 ‰. Mixed gas occurred by depth-selective accumulation with shallow biogenic and deep thermogenic origin.

Thermogenic gas sources are similar with those of oil sources, i.e. from middle Eocene Ngimbang to early Miocene Lower Tuban shales/coals. Biogenic gas sources are the middle Miocene to Plio-Pleistocene shales and coals of the Tawun, Wonocolo, Mundu, Paciran, and Lidah Formations.

Non-hydrocarbon gas of CO2 was evaluated regionally and found that significant high CO2

gas content (25-79 %) occurs in the fields with Kujung-Tuban carbonate reservoirs located at the Cepu High area. Carbon isotope of CO2 show ratios of -5.17 to 4.85 ‰ indicating that the origin of CO2 was from thermal destruction of carbonate rocks (deeper reservoir or source) within the adjacent basinal kitchen at the temperature above 100º C. High CO2 gas content also occurs in the offshore East Java areas adjacent to Muria and Lasem volcanoes and the Bawean Island. Origin of CO2 gas in the offshore areas is different with that of the onshore area. The habitats of East Java’s gases are discussed in their relationships with oils. Four trend of habitats are recognized : Ngimbang, Kujung, Ngrayong, and Mundu Habitats. Existing and future potentials of the East Java’s oils and gases are located within these trends (Figure 3).

Closing Remarks : Integration of Geology-Geophysics-Geochemistry

The foregoing discussions confirm the roles of geochemistry in reducing exploration risks of prospect, identifying new petroleum system, and defining hydrocarbon habitat. In a well-run exploration division, geochemistry should have its place as good as geology and geophysics have had. In discussing a prospect, the pertinent geochemical questions should be asked (Hunt, 1996) : Where are the source rocks ? Have outcrops or well samples of the source rocks been analyzed ? What is their maturity ? Have they generated enough hydrocarbons to fill the trap being considered ? Where are the migration pathways to the structure ? What are odds that the structure contains oil or condensate or dry gas or CO2 or water ? How good is the seal ? Are hydrocarbons currently escaping to the surface ? Offshore, can you see gas chimneys over the structure on the seismic profiles ? Are there any seeps in the area ? Have oil samples been analyzed ? Obviously, any geochemical application that can reduce dry hole/discovery ratio will pay for itself many times over. Using established geochemical methods in prospect evaluation should therefore be part of every prospecting procedure. All the data available from whatever source – geological, geophysical, or geochemical – should be put together to reach a sound decision.

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REFERENCES

Hunt, J.M., 1996, Petroleum Geochemistry and Geology, 2nd ed., W.H. Freeman and Co., New York, 743 ps.

Magoon, L.B. and Dow, W.G., 1994, The petroleum system in Magoon, L.B. and Dow, W.G., eds, The Petroleum System – from Source to Trap : AAPG Memoir 60, p. 3-24.

Satyana, A.H., Salim, Y., and Demarest, J.M., 2000, Significance of focused hydrocarbon migration in the Salawati Basin : controls of faults and structural noses, Proceedings IPA, 27th Annu. Conv. & Exhibition, p. 513-530.

Satyana, A.H., 2001, Identifying new petroleum system of the Salawati Basin, West Papua : exploration opportunities in mature basin, Proceedings Lomba Karya Tulis II Pertamina Directorate of Exploration and Production.

Satyana, A.H. and Purwaningsih, M.E.M., 2002, Geochemistry and habitats of oil and gas in the East Java Basin : regional evaluation and new observations, Proceedings IAGI (Indonesian Association of Geologists), 31st Annu. Conv., p. 68-102.

AHS/ipa june 2003/11 may 2003

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