Basin and petroleum system modeling is a work to tracks the evolution of a basin through time as it fills with fluids and sediment, and how its generate hydrocarbon as well as other contents. With the new technology and algorithm enable us to run the model as a map based and cell based.
Basin and petroleum system modeling comprise of two main works: model building and forward modeling. Model building encompasses a structural model construction and the deposition chronology and physical properties identification of each layer. Forward modeling performs calculations on the model to simulate sediment burial, pressure & temperature changes, kerogen maturation and hydrocarbon expulsion, migration and accumulation.
One of the contributions of basin modeling for geologist is the ability to run petroleum system simulation entirely from source rock until timing and migration. With advance of technology nowadays, Basin Modeling software could take into account all 5 elements of petroleum system with ease in a run.
The modeling requires several main input parameters such as depth and thickness of the source rocks, stratigraphic units, Total Organic Carbon (TOC), Hydrogen Index (HI), temperature gradient, heat flow received from well reports, seismic, well logs, and previous publications. Those data should be integrated with some geological knowledge such as regional basin evolution, tectonics, stratigraphy and source rock depositional model to build a more comprehensive model.
As well as static model which commonly used in field development project, Basin modeling work also depend on the calibration data, where more data calibration will result in more reliable output. In theory, collecting and correcting the calibration data taking 70 percent of the total work.
The first thing to do is to collect temperature data from well header, well test report and geochemistry data from the well report into the excel tabulation. Then adjustingthe data. Currently, there are a lot of methods to correct the temperature of BHT, which are commonly used by geologists are using Horner method. Geochemistry calibration data generally be obtained from the pyrolysis result, and often found from the biomarker study, if there is an oil sample analysis performed in the well.
One of the very important calibration data is Biomarker. Biomarkers are complex molecules derived from formerly living organisms. Biomarker found in crude oils, rocks, and sediments show little change in structures from their parent organic molecules, or so-called biogenic precursors (for example, hopanoids, and steroids), in living organisms. Biomarker concentrations are relatively low in oil, often in the range of several hundred ppm. Biomarkers are useful because they retain all or most of the original carbon skeleton of original natural product.
One of the usefulness of Biomarker is for Oil-Source rock correlations. Oil source correlations are based on the concept that certain compositional parameters of migrated oil do not differ significantly from those of bitumen remaining in the source rock.
Detailed oil-source rock correlations provide important information on the origin and possible path of migration of oils that can lead to additional exploration plays. For instance, the exploration nearby field like in South Sumatra, a company that is looking for opportunity in the near field exploration will certainly make an analogy of their leads and prospect on the block with the nearest field, of course with the same play, one of the basic question would be “is whether the source rock coming from the same kitchen with the source rock which feed the field?” Or not? That question, for sure is a great challenge for a geologist or a geochemist, and will certainly affect the result of kinetic model and the maturity of source rock
In here, the best approach is with the help of biomarkers, another supporting data is doing field geology observation, search and investigate the seepage, if exists. Ideally the surface manifestation of oil seepage both on land and seabed (from seabed coring) connected with the seismic data. After we get the oil sample and do the GC-MS of oil seepage, then the next step is conduct oil to source rock correlation with the biomarker from nearby field. This method has the limitation particularly the effect of biodegradation. However, some compounds will be very resistant to biodegradation like C27 Ts and Tm hopanes and tricyclic terpanes (C19 to at least C54). The C28 and C29 tricyclic were used extensively in correlations of oils and rock extracts (Seifert et al., 1982; Seifert and Moldowan, 1981) So the oil-source rock correlation is still possible to do and quite reliable.
The second problem is exploration away from the field or exploration in frontier area, with the limitation of data available and the demands to produce a conclusive result, pushing for more in-depth source rock analysis.
One of the example of Exploration in south Sumatra,usually the source rock modeled in Talang Akar Formation and Upper Gumai Formation, this because of the lack of the wells penetrated Lemat/Lahat formation. Regionally Lemat/Lahat formation in some places is a lacustrine environment. Then with biomarkers analysis will certainly help to trace the origin of source rock, the depositional environment and also the maturity.
Analysing the Triterpanes peak on m/z 191 (measured using SIM/GCMS) could be used as a first step to identify the origin of the source rock. Why triterpanes? Because some tricyclic terpanes are terrigenous indicators (Noble, 1986). Terpane fingerprints reflect source rock depositional environment and organic matter input. Because bacteria are ubiquitous in sediments, terpanes occur in nearly all oils, and oils from different source rock deposited under similar conditions may show similar terpane fingerprints. Oleananes presence can also be used as an indicator the presence of terrestrial environments as an input to the source rock. Because Oleanane are produced by angiosperms (flowering land plants) and its presence can used as marker for geologic age.
Shortly if the first observasion above has found (presence of oleananes), then it can be concluded about the possibility of terrestrial depositional environment for the source rock. Next question would be: these Oleananes are found both in fluvio-deltaic system and lacustrine system. When linked with the formation, two candidates will appear, weather derived from Talang Akar Formation which is regionally are fluvio-deltaic or from Lemat/Lahat which is lacustrine. Although in some areas, Lemat/lahat Formation is a concept to consider as a source rock, but in some area are proven. The next step is to Plot Ts/Tm versus Oleanane/C30 Hopane, where the plot is fairly accurate because it can show the marine influence, such as organic sulfur, dibenzothiophene/phenanthrene, and homohopane ratio during diagenetic. Low value of olenane/C30 hopane and higher value of ts/tm is one of strong indication of lacustrine environment as a source rock depositional environment.
After get the positive result from both step above on m/z 191 ,we could preliminary conclude the presence of lacustrine depositional environment , Then for the next is doing quick analysis on peak m/z 183 , in m/z 183 we could see the presence of botryococcane (figure 1).
Figure 1. botryococcus braunii (source: Wikipedia)
Its presence is the sign of lacustrine environment , while their absence are meaningless, Because Sumatran oils contain very low steranes and have similar triterpene distributions, these method are useful for distinguished various lacustrine Sumatran Oil (moldowan et al., 1982, Metzger and laegeau 1999). After we get the 3 analyses done (Oleanane , ts/tm plot and presence of botryococcane) , Then we conclude that the source rock are come from lacustrine environment which is Lemat/Lahat Formation, even though there are no wells penetrated that formation. This concept could drive the exploration concept and plays to more advance.
Direct implications to basin modeling is the source rock characteristics or organofacies in kinetic and the maturity of the source rock which could be used as a calibration when we run the thermal modeling.
A short exercise above is only a small part of the usefulness of biomarkers in the Basin Modeling work. For the better result, we could integrate another calibration like chromatographic fingerprints, carbon or sulfur stable isotope ratio, or the relative contents of vanadium and nickel (V/Ni) to make a more robust model.
Series#2 – March 2017
Author : Leo Kokoh – Repsol