Chrono-Litho: A key step in any petroleum system study

Despite not being based on the most high-tech analytical data, our chrono-lithostratigraphic dataset is still a very powerful tool to guide petroleum system studies, especially for understanding the organization of the stratigraphic frameworks and characterizing the spatial and temporal distribution of the source-rock and reservoir sequences of the Brazilian sedimentary basins. The backbone of the feature is the definition of the chrono and lithostratigraphic patterns described in the wells, with the resultant lists of formation tops and ages providing a framework for the process-response relationship between the stratigraphic architecture of the basins and their respective reservoir distribution and hydrocarbon generation potential.

We are not aiming to review the entire depositional framework of the Brazilian basins, but to target the stratigraphic architecture in packages where reservoirs and source rock can be correlated and compared in terms of lithotype and geochemical signature. Based on the recognition of markers found in the well reports and composite logs, our expert team has constructed a table of the stratigraphic zoning used to categorize the formations, along which the geochemical and lithologic characteristics are plotted in graphs such as the natural series. The source-rock sections were studied in great detail to build the geochemical architecture of the most important basins, while the chrono-stratigraphic tops are supported locally by geochemical data and/or petrophysical characteristics. We currently have more than 270 wells with lithostratigraphic interpretation that help piece together the sedimentary puzzle of the various basins. 

We are especially interested in providing the layers with similar source-rock characteristics. The task is quite challenging as distance increases and sedimentary environments change along the basins, but studying co-genetic basins offers powerful insights into rock type and deposition styles.

Another extremely useful application of the chrono-litho dataset is the construction of stratigraphic frameworks for petroleum system modeling. Figure 1 shows an example from our recent study of the deep waters of Sergipe-Alagoas Basin, where the chrono-litho of the wells was defined and tied to the seismic. The sedimentary package of Sergipe-Alagoas has variable thickness across the basin and most of the wells in the deep and ultra-deep waters do not reach the rift section.

Figure 2 shows an example of a Natural Series Plot in Sergipe-Alagoas, constructed by interlinking the geochemical and chrono-stratigraphy datasets. The deepest stratigraphy section is observed in well 1-BRSA-978-SES (Figure 1), which is the deepest well that crossed the transitional and rift sediments in the southern portion of the basin. The mega rift sequence is represented by the Neocomian and Barremian lacustrine facies of the Barra de Itiúba formation and the fluvial and deltaic facies of Penedo and Rio Pitanga. The correlated Aptian seismic horizon was mapped and used in the calibration of the basin modeling, which was integrated with a comprehensive review of the geochemical rock and oil data from several wells. The 3D PSM results calibrated to the seismic framework showed that the lacustrine Barremian to Aptian calcareous black shale and marls present thick sections with high hydrocarbon source potential (Figure 2). Although the deep waters have never been drilled, the calibration with advanced geochemistry analyses of diamondoids, CSIA-Bh and QEDA from nearshore and deep-water oils and stratigraphic studies indicates that the source-rock intervals have been buried deep enough to achieve peak to late-stage generation in the depocenters located in the deep and ultra-deep areas (seismic-based map in Figure 2).

Figure 1 – Maps of the top of the Muribeca Formation based on wells (western part) and PSDM seismic (eastern part). Well 1-BRSA-978-SES (highlighted in the map and with the composite log to the right) is the deepest well that perforated the rift section in the deep waters of the basin. The defined chrono-litho of this well, together with those of other shallower wells, were used to map the seismic framework. The basin modeling results corroborate the existence of deep source rocks in the deep waters, increasing the potentiality for new discoveries in the basin.
Figure 2 – Example of Natural Series Plot in the Sergipe-Alagoas Basin, constructed by interlinking the geochemical and chrono-stratigraphy datasets.

Lastly, it is important to remind that chrono-litho dataset can be easily downloaded so the key stratal stacking patterns can be applied to basin modeling studies or for genetic interpretation of the geochemical plots of the natural series, among other uses. There are actually two ways to access the litho data in the BPS Geodatabase. The first is via the natural series tool in the rock geochemistry map layer, where the lithostratigraphic data can be exported together with the rock geochemistry exports of each individual pyrolysis parameter or with all the parameters. In addition, it can also be exported using the chrono-lithostratigraphy layer of the stratigraphy data tables of any selected wells, or individually through our Well Evaluation dashboard.

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