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Example 3: Identifying Source Rocks on Seismic Data
Tomography lacks the resolution to capture the velocity decrease associated with a source rock and geochemists usually identify source intervals based on sampling. In this example, the source rock has not been identified as significant, but rock physics models suggest a decrease in impedance in the main generation window. Amplitude response of a source rock that ties with generation is used to locate the source interval below a large Class 111 AVO anomaly.
The section shows the present day transformation rotio faro Type 11-111 source rack based on the temperature history from the basin model.
Lines in the section are the geologic interpretation of the section. In an integrated basin simulator, the temperature history is modeled from a discretization of the geologic model using an energy balance and moss conservation. These models require boundary conditions at the sides, base and the top and ore calibrated to present day temperature data in wells drilled in the area. The graph shows the full stack amplitude extractions on two horizons near heavy block line in the section. The graph also shows the transformation ratio of a Type 11-111 source rock based on the temperature history. The source interval amplitude correlates with the transformation ration but the shallow horizon does not. Matava et al. (2020) shows that hydrocarbon generation delays compaction which decreases the acoustic impedance of the source rock relative to the surrounding non-source rock sediments.
Plan view extraction of transformation ratio on multiple 2D regional lines. The closed polygon outlines an approximately 650 sqkm Class Ill AVO anomaly <1.5 km shallower than the source interval horizon. The amplitude anomaly is on the edge of the source rock maturity window.
Wireline log data {red} and the Dry Rock Approximation to the Xu-White rock physics model {black} fit to the log data. The velocity decrease and the porosity and Poisson ratio increase are often associated with source rock intervals. The dashed line in the acoustic velocity plate is the tomographic velocity. The unconformity is the same as the thick line in the maturity cross section. At this location the section is immature. The source interval at 4500 m was tied to the seismic data and carried through the volume for the analysis.
Identification of source rocks on seismic data is possible because of the density difference between kerogen and hydrocarbon fluid and also because the phase transformation occurs over a relatively large temperature range.
Burial also compacts the sediment and the schematic shows that the porosity is only affected during the main phase of generation. Rock physics models shows that generation can be modeled as a change in porosity and seismic data can be used to identify these processes. However, it is rare to observe these on 3D data volumes because of the size required and 2D regional data is better suited for this analysis.
Exceptions occur in complicated structural settings where salt locally affects the maturity. These cases are generally production settings.
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