Understanding of Barents Sea blocks 7017, 7018, 7117 and 7118 has been transformed following a recent project carried out by PGS together with DIG Science. It identifies multiple strong prospects close to and far more exciting than the Big Dipper, using a model created using DIG Science technology applied to PGS 3D GeoStreamer data, calibrated to rock physics properties supplied by PGS’ rockAVO Barents Sea Atlas.
These four blocks lie in the area surrounding the much-hyped Big Dipper prospect in the Barents Sea. This is an area that previously suffered from lackluster illumination and attracted little interest. Imaging was based on 2D legacy data from 2011 and 2013, with no good quality prestack data.
PGS acquired a GeoStreamer survey here in 2017, with coverage that extends from the Senja Ridge in the north over the Tromsø and Harstad Basins, the Troms-Finnmark Fault Complex and on to the Finnmark Platform. The imaging is excellent, however, inconclusive drilling results in nearby wells meant the industry needed more data to be convinced of the prospectivity. PGS’ VP Sales for Europe Chris Watts proposed the dataset to a digitalization team within PGS that is integrating QI expertise with real-time 3D rock physics modeling, in collaboration with external partners DIG Science, to see if they could extract the information required to derisk the area and generate prospects worth drilling.
A paper on this project by Avseth et al will appear in First Break September 2020?
Although many oil and gas companies regarded this corner of the Barents Sea as risky and under-explored, the wells that have been drilled, while low on HC content, confirmed good reservoir-quality Paleocene sandstones in the Torsk formation, with porosity levels beyond 30% and permeability of 600 mD.
Overcoming the Challenges
Unfortunately, the Paleocene sands are very sparsely distributed, so finding them at all in this large province, let alone ones with sufficient hydrocarbon saturation, is rather like identifying the proverbial needle in a haystack. With few wells, there are very few control points. Though on the positive side, the Torsk shale is a reliable cap rock.
PGS project manager Laurent Feuilleaubois explains that among the challenges was agreeing how to describe the location: “This is a very complex area, as the Harstad Tromsø Basin lies at the boundary between the Atlantic Margin and the better-known Barents Sea. That presents some fundamental questions, like do we expect uplift to stop at the fault complex, or might it also affect the basinal area? How can we take this into account and build the rock physics model, especially with such poor well control?”
To derisk the area, the team had to go beyond the conventional methods in order to ascertain if the location had been uplifted and eroded. Their model would have to consider the uplift. Using FWI velocities to ensure good lateral and vertical resolution, they managed to define where the boundaries lie, and separate areas in the survey with no uplift from those with up to 1500 meters of net erosion. There were very different targets in each. In the uplifted areas, they were able to identify good Jurassic targets within the tilted fault blocks, while areas with no uplift had good and plentiful Paleocene sandstones, some of which are contained within large 4-way dip closures.
The third risk was the presence of a viable source rock. Was it too deep and burnt out? Or was the kitchen still working? The legacy 2D data did not come close to imaging that would allow conclusions about this.
The main proven source across the area is the Hekkingen Late Jurassic shale. On the 2017 GeoStreamer 3D data, the Base Cretaceous Unconformity can be identified, and the source rock can be found. Located in the late oil and gas window, within the basin and close to the platform, this can charge good Jurassic and Paleocene reservoirs. So, using the PGS 17011 3D dataset, the PGS-DIG team was able to derisk two new elements of the petroleum system in the area.
But what about the risk associated with the uplift? What was required to convince the skeptics? To assess and predict reservoir quality, the team used all information available. They ran Full Waveform Inversion on the GeoStreamer seismic data to estimate net erosion and constrain burial history, then modeled the reservoir quality using real-time rock physics combined with the latest rockAVO Barents Sea Atlas, and ran sensitivity analyses to quantify associated uncertainty to uplift, sedimentological and geological changes. The team then applied technology from Dig Science to quantify prestack seismic observation. The model results were striking.
Tore Hansen COO and Per Avseth CTO at Dig Science predict excellent reservoir quality in Paleocene sands and very good reservoir quality in the Jurassic, different places in the survey.
“Using GeoStreamer prestack data we were able to directly correlate elastic responses with our model,?indicating the presence of reservoir, hydrocarbons, and the probability of commercial reserves in the Paleocene. We were surprised at how much Paleocene sandstone we can observe in the data, and in areas with mapped charge from mature high-quality source rock. The evaluated opportunities can potentially open up this new play for the southwest Barents Sea,” they say.